This PDF file contains the front matter associated with SPIE Proceedings Volume 10103, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

One of the main limitations for realizing high-performance time-domain terahertz imaging and spectroscopy systems is the low responsivity and narrow bandwidth of the existing pulsed terahertz detectors. In this work, we present a high-responsivity and broadband large-area terahertz detector that incorporates a two-dimensional array of plasmonic nanoantennas fabricated on a low-temperature-grown GaAs substrate. By using a large-area device architecture, large optical spot sizes can be used, mitigating the carrier screening effect at high optical pump powers. Using a large-area device architecture also makes the device less sensitive to changes in optical and terahertz alignment. The two-dimensional array of plasmonic nanoantennas is designed to offer a broad terahertz detection bandwidth. It is also designed to enhance optical absorption in close proximity to the nanoantennas by exciting surface plasmon waves. This allows drifting a large portion of photo-generated electrons and holes to the nanoantennas in presence of an incident terahertz pulse, offering high responsivity levels. We experimentally demonstrate detection of terahertz pulses with more than 5 THz bandwidth with high responsivity and signal-to-noise ratio levels exceeding that of electro-optic detectors. Such terahertz detectors would play a critical role in realization of the next generation time-domain terahertz imaging and spectroscopy systems.

Recently, intensity correlation measurements have been reported for the first time in the Terahertz range, where a time-domain version of a Hanbury Brown Twiss setup based on electro-optic sampling was employed. This technique proved its usefulness for fundamental studies of photon correlations of bunched (thermal) and Poissonian (coherent) light, but not only so. Also in practical applications, it has been employed to determine the temporal emission pattern of Terahertz Quantum Cascade Laser based Frequency Combs, which are very promising devices for future highly integrated spectrometers. The key parameter of this technique is its short temporal resolution. Up to date, the technique still does not provide the necessary sensitivity for exploring the yet vacuous regime of single photons in the terahertz. In this work we present our recent efforts for increasing the sensitivity of electro-optic sampling, by means of cryogenic cooling and novel organic materials for the Terahertz range. In particular, we present a novel device for collinear electro-optic detection, which features a high-aspect ratio antenna on a quartz substrate with a plasmonic gap filled by electro-optic molecules.

As the ambition behind THz quantum cascade laser based applications continues to grow, abandoning free-space optics in favor of waveguided systems promises major improvements in targeted, easy to align, and robust radiation delivery. This is especially true in cryogenic environments, where illumination is traditionally challenging. Although the field of THz waveguides is rapidly developing, most designs have limitations in terms of mechanical stability at low temperatures, and are costly and complicated to fabricate to lengths > 1 m. In this work, we investigate readily available cylindrical metal waveguides which are suitable for effective power delivery in cryogenic environments, and explore the optimal dimensions and materials available. The materials chosen were extruded un-annealed and annealed copper, as well as stainless steel, with bore diameters of 1.75, 2.5, and 4.6 mm. Measurements were performed at three different frequencies, 2.0, 2.85 and 3.2 THz, with optimal transmission losses <3 dB/m demonstrated at 2.0 THz. Additionally, novel optical couplers are also presented and characterised, with the ability to change the beam path by 90° with a coupling loss of just 2.2 dB whilst maintaining mode quality, or thermally isolate sections of waveguide with a coupling loss as low as 0.5 dB. The work presented here builds on previous work¹, and forms a comprehensive investigation of cryogenically compatible THz waveguides and optical couplers, paving the way for a new generation of systems to utilize THz QCLs for a host of low-temperature investigations.

Radiometric discriminator is the simplest device for comparing the radiometric brightness temperature of two following spots of antenna beam on the observed remote scene. In this case, the real signal from discriminator can be similar to the radar imaging in form of the bright points based on the difference between the metal object and the background of an absorbing or semi-absorbing environment. To realize the function of finding objects, the scanning antenna is symmetrically decided into two parts. This signal arises only when the two parts generate different levels of brightness temperatures because of a target captured in alternative pixel scanning along the observed scene. In this paper, the device named discriminator is proposed, and the operating principle and possible application is introduced for the object with Stealth coating.

We present a readout circuit for 1 × 64 Nb₅N₆ microbolometer array detector. The intrinsic average responsivity of the detectors in the array is 650 V/W, and the corresponding noise equivalent power (NEP) is 17 pW/√Hz. Due to the low noise of the detector, we design a low noise readout circuit with 64 channels. The readout integrated circuit (ROIC) is fabricated under CMOS process with 0.18μm design rule, which has built-in bias and adjustable numerical-controlled output current. Differential structure is used for each pixel to boost capacity of resisting disturbance. A multiplexer and the second stage amplifier is followed after the ROIC. It is shown that the ROIC achieves an average gain of ~47dB and a voltage noise spectral density of ~9.34nV/√Hz at 10KHz. The performance of this readout circuit nearly fulfills the requirements for THz array detector. This readout circuit is fit for the detector, which indicates a good way to develop efficient and low-cost THz detector system.

Multiple-input multiple-output (MIMO) imaging systems in the terahertz frequency range have a high potential in the field of non-destructive testing (NDT). With such systems it is possible to detect defects in composite materials, for example cracks or delaminations in fiber composites. To investigate mass-produced products it is necessary to study the objects in close to real-time on a conveyor without affecting the production cycle time. In this work we present the conception and realization of a 3D MIMO imaging system for in-line investigation of composite materials and structures. To achieve a lateral resolution of 1 mm, in order to detect such small defects in composite materials with a moderate number of elements, precise sensor design is crucial. In our approach we use the effective aperture concept. The designed sparse array consists of 32 transmitters and 30 receivers based on planar semiconductor components. High range resolution is achieved by an operating frequency between 220 GHz and 260 GHz in a stepped frequency continuous wave (SFCW) setup. A matched filter approach is used to simulate the reconstructed 3D image through the array. This allows the evaluation of the designed array geometry in regard of resolution and side lobe level. In contrast to earlier demonstrations, in which synthetic reconstruction is only performed in a 2D plane, an optics-free full 3D recon- struction has been implemented in our concept. Based on this simulation we designed an array geometry that enables to resolve objects with a resolution smaller than 1mm and moderate side lobe level.

We have measured the terahertz (THz) conductance of a 23 quintuple layer thick film of bismuth selenide (Bi₂Se₃) and found signatures for topological surface states (TSSs) below 50 K. We provide evidence for a topological phase transition as a function of lattice temperature by optical means. In this work, we used THz time-domain spectroscopy (THz-TDS) to measure the optical conductance of Bi₂Se₃, revealing metallic behavior at temperatures below 50 K. We measure the THz conductance of Bi₂Se₃ as 10 e²/h at 4 K, indicative of a surface dominated response. Furthermore, the THz conductance spectra reveal characteristic features at ~1.9 THz attributed to the optical phonon mode, which is weakly visible at low temperatures but which becomes more prominent with increasing temperature. These results present a first look at the temperature-dependent behavior of TSSs in Bi₂Se₃ and the capability to selectively identify and address them using THz spectroscopy.

We report an on-going temporal characterization project and method on a shot-to-shot basis for free electron laser pulses by using THz photoelectron spectroscopy at the European XFEL facility. Shot to shot FEL time jitter and pulse profile information can be reconstructed by resolving the photoelectrons energy spectra in an external THz field. Laser based THz generation and optimization, photoelectron generation and detection are described. Further considerations of the temporal resolution based on proposed photoelectron lines are presented and intuitive simulations are made to demonstrate the feasibility of such technique.

Terahertz (THz) waves have been actively studied for the applications of astronomy, communications, analytical science and bio-technologies due to their low energy and high frequency. For example, THz systems can carry more information with faster rates than GHz systems. Besides, THz waves can be applied to imaging, sensing, and spectroscopy. Furthermore, THz waves can be used for non-destructive and non-harmful tomography of living objects. In this reasons, Schottky barrier diodes (SBD) have been widely used as a THz detector for their ultrafast carrier transport, high responsivity, high sensitivity, and excellent noise equivalent power. Furthermore, SBD detectors envisage developing THz applications at low cost, excellent capability, and high yield. Since the major concerns in the THz detectors for THz imaging systems are the realizations of the real-time image acquisitions via a reduced acquisition time, rather than the conventional raster scans that obtains an image by pixel-by-pixel acquisitions, a line-scan based systems utilizes an array detector with an 1 × n SBD array is preferable. In this study, we fabricated the InGaAs based SBD array detectors with broadband antennas of log-spiral and square-spiral patterns. To optimize leakage current and ideality factor, the dependence to the doping levels of ohmic and Schottky layers have been investigated. In addition, the dependence to the capacitance and resistance to anode size are also examined as well. As a consequence, the real-time THz imaging with our InGaAs SBD array detector have been successfully obtained.

The detection properties of a chalcopyrite zinc germanium diphosphide (ZnGeP₂, ZGP) electro-optic (EO) crystal, having thickness of 1080 μm and cut along the <012> plane, is studied in the terahertz (THz) frequency range. Outstanding phase matching is achieved between the optical probe pulse and the THz frequency components, leading to a large EO detection bandwidth. ZGP has the ability to measure frequencies that are 1.3 and 1.2 times greater than that of ZnTe for crystal thicknesses of 1080 and 500 μm, respectively. Furthermore, the ZGP crystal is able to detect frequency components that are ≥4.6 times larger than both ZnSe and GaP (for crystal thicknesses of 1080 μm) and ≥2.2 times larger than ZnSe and GaP (for crystal thicknesses of 500 μm).

We study the transmission of complementary THz split ring resonator (cSRR) arrays with THz time domain spectroscopy. Utilizing complementary THz metasurfaces and, varying the inter meta-atom separation, a regime of resonant coupling to surface plasmon polaritons (SPPs) is entered. The effective medium condition valid for the direct metasurface is not applicable anymore in the complementary case because of the high THz dielectric constant of gold. We observe a normalized strong coupling of 3.5% to the lattice SPP mode when tuned into resonance with the LC-mode of the cSRR at a frequency of 1.07 THz. The lattice constant is varied from 40 um to 160 um. Finite element simulations with CST MWS show the characteristic field distribution of the two modes and the intermixing of the LC-mode with the SPP-mode very clearly. Analytical modeling with a simple two oscillator model well describes the coupling. An effective relative permittivity of 11.6 for the coupled system was extracted. For broader linewidths, an apparent modulation of the effective Quality-factor can be observed which is of crucial importance for designing metasurfaces for applications. Measurements of the broader lambda/2 mode in orthogonal excitation direction reveal instead a Fano-like interaction. Rectangular array configuration reveal the excitation direction of the SPP modes being along the polarization of the exciting THz pulse. We demonstrate that the understanding of the SPP modes is fundamental for research and applications in which the metasurface has to be designed for special needs.

A cavity-free setup to generate short pulses at high repetition rate is introduced, which is based on four-wave mixing (FWM) in cascaded semiconductor optical amplifiers (SOAs). High repetition rate picosecond-pulse is important in optical communication and all-optic information processing systems. Cavity-free setup based on SOA means without ring cavity, which is stable and easy to be large-scale integrated. In this paper, we obtain picosecond-pulse around 10GHz repetition rate and the side-mode suppression ratio is 23 dB. Moreover, the repetition rate and center wavelength of optical pulse is tunable.

An ultra-compact Electro-Magnetic (EM) Wave Sensor working at 14GHz is designed and demonstrated experimentally. The sensor is based on electro-optics (EO) modulation and therefore has several important advantages over conventional electrical RF sensors including compact size and immunity to electromagnetic interference (EMI). The proposed sensor contains a set of bowtie antenna and a Mach-Zehnder interferometer (MZI) structure with one arm of slow-light enhanced EO polymer infiltrated one dimensional (1D) photonic crystal slotted waveguide and the other arm of silicon strip waveguide with tooth. To minimize the RC delay as well as the electrical connection between the two bowtie antenna, the innovative silicon tooth design are applied for both arms of the MZI respectively so that the device can be operated at 14Ghz. The bowtie antenna concentrates electrical field of the impinging wireless EM wave at its designed frequency of 14Ghz and applies it onto the EO polymer filled slot for modulating phase of the guided optical wave. By combining the effect of strong slow light effect of the slotted PCW, high field enhancement of the bowtie antenna, and also large EO coefficient of the EO polymer(r33=135pm/V), the device is only 4.6mmX4.8mm in size with active region of 300μm and has minimum detectable electromagnetic power density as low as 27 mW/m2.

First responders have the dangerous task of responding to emergency situations in firefighting scenarios involving homes and offices. The importance of this radar is its ability to see through walls and into adjacent areas to provide the first responder with information to assess the status of a building fire, its occupants, and to supplement his thermal camera which is obstructed by the wall. For the firefighter looking into an adjacent room containing unknown objects including humans, the challenge is to recognize what is in that room, the configuration of the room, and potential escape routes. We have just concluded a series of experiments to illustrate the performance of 77GHz radar in buildings. The experiments utilized the Delphi Automotive radar as the mm wave sensor and included display software developed by L. H. Kosowsky and Associates. The system has demonstrated the capability of seeing through walls consisting of sheetrock separated by two by four pieces of wood. It has demonstrated the ability to see into the adjacent room and to display the existence of persons and furniture Based on published data, the radar will perform well in a smoke, haze, and/or fog environment.

Recently, a wide interest has been gathered in using terahertz (THz) waves as the carrier waves for the next generation of broadband wireless communications. Upon this objective, the photonics technologies are very attractive for their usefulness in signal generations, modulations and detections with enhanced bandwidth and data rates, and the readiness in combining to the existing fiber-optic or wireless networks. In this paper, as a preliminary step toward the THz wireless communications, a THz wireless interconnection system with a broadband antenna-integrated uni-traveling-carrier photodiode (UTC-PD) and a Shottky-barrier diode (SBD) module will be presented. In our system, optical beating signals are generated and digitally modulated by the optical intensity modulator driven by a pulse pattern generator (PPG). As the receiver a SBD and an IF filter followed by a low-noise preamplifier and a limiting amplifier was used. With a 6-mA photocurrent of the UTC-PD which corresponds to the transmitter output power of about 30 μW at 280 GHz, an error-free (BER<10-9) transmission has been achieved at 2.5 Gbit/s which is limited by a limiting amplifier. With this system, a 1.485-Gbit/s video signal with a high-definition serial digital interface format was successfully transmitted over a wireless link.

The 5G era is nearly upon us, and poses several challenges for system designers; one important question is how the (soon to be standardized) mmWave bands of wireless mobile access can coexist harmoniously with optical links in fixed telecom networks. To this end, we present a Radio-over-Fiber (RoF) backhauling concept, interfaced to a 60-GHz indoor femto-cell via a field-installed optical fiber link. We successfully demonstrate generation of a RoF signal up to 1 Gb/s and transmit it optically over 43 km of deployed Single Mode Fiber (SMF), as well as investigate the performance of the 60-GHz access link as a function of distance. The optical link introduces negligible degradation, contrasting the effect of multipath fading in the 60-GHz wireless channel; the latter requires adaptive equalization using offline DSP. The proposed scheme is further validated by demonstration of a 60-GHz Remote Antenna Unit (RAU) concept, handling real traffic from commercial Gigabit Passive Optical Network (GPON) equipment. Proper RAU operation at 1.25 Gb/s is achieved, accommodating true data packets from a Media Converter emitting at 1310 nm through an in-building fiber link. System performance is confirmed through Bit Error Rate (BER) and Error Vector Magnitude (EVM) measurements. EVMs of ~11 and 19% are achieved with BPSK signals, for distances of 1 and 2 m respectively. As standardization of mmWave technologies moves from 5G testbeds to field-trial prototypes, successful demonstration of such 60-GHz wireless access scenarios over a telecom operator’s commercial fiber infrastructure is even more relevant.

A multimode horn differs from a single mode horn in that it has a larger sized waveguide feeding it. Multimode horns can therefore be utilized as high efficiency feeds for bolometric detectors, providing increased throughput and sensitivity over single mode feeds, while also ensuring good control of the beam pattern characteristics. Although a cavity mounted bolometer can be modelled as a perfect black body radiator (using reciprocity in order to calculate beam patterns), nevertheless, this is an approximation. In this paper we present how this approach can be improved to actually include the cavity coupled bolometer, now modelled as a thin absorbing film. Generally, this is a big challenge for finite element software, in that the structures are typically electrically large. However, the radiation pattern of multimode horns can be more efficiently simulated using mode matching, typically with smooth-walled waveguide modes as the basis and computing an overall scattering matrix for the horn-waveguide-cavity system. Another issue on the optical efficiency of the detectors is the presence of any free space gaps, through which power can escape. This is best dealt with treating the system as an absorber. Appropriate reflection and transmission matrices can be determined for the cavity using the natural eigenfields of the bolometer cavity system. We discuss how the approach can be applied to proposed terahertz systems, and also present results on how the approach was applied to improve beam pattern predictions on the sky for the multi-mode HFI 857GHz channel on Planck.

For their ability to be transmitted by materials opaque in the visible and IR ranges (clothes, plastic, …), for being non-ionizing, for providing sub-mm resolution imaging, for the specific signatures of numerous materials, Terahertz waves - ranging from 200 GHz to 10 THz - have been raising the interest of industrials for about fifteen years. This study focuses on the penetration of THz technologies into the industrial applications driving the THz market growth at short and long term: Non Destructive testing (NDT), Defense and Security, Biomedical. For 15 years, Terahertz technologies have been continuously tested on a wide variety of applications. Thanks to these ongoing feasibility studies, manufacturers and end-users gained a deeper knowledge about the abilities and the limitations of the different Terahertz systems (Time-Domain spectroscopy, Frequency-Domain spectroscopy, Time-Domain reflectometry, etc). The demand from end-users is more qualified and is segmented as follows: 1. Detection of objects and defects on large areas 2. Thickness measurement on large areas 3. Chemical and Structural characterization of small objects and defects on small areas (2D) or volumes (3D) Each of these 3 functions leads to a specific family of THz systems with distinct requirements in terms of performance and cost: 1. Detection: cheap and compact imaging systems. 2. Thickness measurement: cost-effective and high speed systems. 3. Characterization: high resolution, high reliability and real-time sensing systems. This article will present the existing and incoming THz systems and components addressing each function. Terahertz technologies are currently finding their place on the market, outside research and scientific applications. The objective of this article is to identify the industrial applications where THz techniques will be adopted and to provide market growth perspectives.

Pulsed terahertz systems are currently being deployed for online process control and quality control of multi-layered products for use in the building products and aerospace industries. While many laboratory applications of terahertz can allow waveforms to be acquired at rates of 1 – 40 Hz, online applications require measurement rates of in excess of 100Hz. The existing technologies of thickness measurement (nuclear, x-ray, or laser gauges) have rates between 100 and 1000 Hz. At these rates, the single waveform bandwidth must still remain at 2THz or above to allow thinner layers to be measured. In the applications where terahertz can provide unique capability (e.g. multi-layer thickness, delamination, density) long-term stability must be guaranteed within the tolerance required by the measurement. This can mean multi-day stability of less than a micron. The software that runs on these systems must be flexible enough to allow multiple product configurations, while maintaining the simplicity required by plant operators. The final requirement is to have systems that can withstand the environmental conditions of the measurement. This might mean qualification in explosive environments, or operation in hot, wet or dusty environments. All of these requirements can put restrictions on not only the voltage of electronic circuitry used, but also the wavelength and optical power used for the transmitter and receiver. The application of terahertz systems to online process control presents unique challenges that not only effect the physical design of the system, but can also effect the choices made on the terahertz technology itself.

In this contribution, we present a highly accurate approach for real-time thickness measurements of multilayered coatings using terahertz time domain spectroscopy in reflection geometry. The proposed approach combines the benefits of a model-based material parameters extraction method to calibrate the specimen under test, a generalized modeling method to simulate the terahertz radiation behavior within arbitrary thin films, and the robustness of a powerful evolutionary optimization algorithm to increase the sensitivity and the precision of the minimum thickness measurement limit. Furthermore, a novel self-calibration model is introduced, which takes into consideration the real industrial challenges such as the effect of wet-on-wet spray in the car painting process and the influence of the spraying conditions and the sintering process on ceramic thermal barrier coatings (TBCs) in aircraft industry. In addition, the developed approach enables for some applications the simultaneous determination of the complex refractive index and the coating thickness. Hence, a pre-calibration of the specimen under test is not required for such cases. Due to the high robustness of the self-calibration method and the genetic optimization algorithms, the approach has been successfully applied to resolve individual layer thicknesses within multi-layered coated samples down to less than 10 µm. The regression method can be applied in time-domain, frequency-domain or in both the time and frequency-domain simultaneously. The data evaluation uses general-purpose computing on graphics processing units and thanks to the developed highly parallelized algorithm lasts less than 300 ms. Thus, industrial requirements for fast thickness measurements with an “every-second-cycle” can be fulfilled.

Nyquist pulses, which are defined as responses of Nyquist filter, can be used in time-division multiplexing transmission which can simultaneously achieve ultrahigh data rate and spectral efficiency (SE). Generally, the methods for Nyquist pulse generation are based on optical Nyquist filters, nonlinear effects in fiber and phase-locked frequency comb. In this paper, we focus on the third method of phase-locked frequency comb. However, this method has a problem which the large duty cycle of generated Nyquist pulses limits their applications. To address this issue, we proposed a new setup in which one optical intensity modulator and an electrical arbitrary function generator (AFG) are employed. The various duty cycles of ideal Nyquist pulses are generated using one optical intensity modulator so that the phase-locking between the different RF signals is no need any more. And the ideal Nyquist pulses in microwave domain are generated successfully. The duty cycles ranging from 21% to 11% are obtained by programming the number of frequency comb lines in the RF signal which is generated by the AFG. The method has a potential to generate ideal Nyquist pulses in radio frequency domain if a high bandwidth AFG is used to replace the low bandwidth AFG used in this paper.

The distribution of analogue RF signals within a high performance radar system is challenging due to the limited space available and the high levels of performance required. This work investigates the gain, linearity and noise performance that can be achieved by an externally modulated direct detection link designed for operation up to 20 GHz using commercially available components. The aim was to assess the suitability of such links for use in future radar systems. Good correlation has been shown between modelled and measured results demonstrating that the performance should satisfy the linearity requirements for many radar applications.

Optical speckle in a multimode waveguide has been proposed to perform the function of a compressive sensing (CS) measurement matrix (MM) in a receiver for GHz-band radio frequency (RF) signals. Unlike other devices used for the CS MM, e.g. the digital micromirror device (DMD) used in the single pixel camera, the elements of the speckle MM are not known before use and must be measured and calibrated. In our system, the RF signal is modulated on a repetitively pulsed chirped wavelength laser source, generated from mode-locked laser pulses that have been dispersed in time or from an electrically addressed distributed Bragg reflector laser. Next, the optical beam with RF propagates through a multimode fiber or waveguide, which applies different weights in wavelength (or equivalently time) and space and performs the function of the CS MM. The output of the guide is directed to or imaged on a bank of photodiodes with integration time set to the pulse length of the chirp waveform. The output of each photodiode is digitized by an analog-to-digital converter (ADC), and the data from these ADCs are used to form the CS measurement vector. Accurate recovery of the RF signal from CS measurements depends critically on knowledge of the weights in the MM. Here we present results using a stable wavelength laser source to probe the guide.

Multi-core fiber (MCF) has been one of the main innovations in fiber optics in the last decade. Reported work on MCF has been focused on increasing the transmission capacity of optical communication links by exploiting space-division multiplexing. Additionally, MCF presents a strong potential in optical beamforming networks. The use of MCF can increase the compactness of the broadband antenna array controller. This is of utmost importance in platforms where size and weight are critical parameters such as communications satellites and airplanes. Here, an optical beamforming architecture that exploits the space-division capacity of MCF to implement compact optical beamforming networks is proposed, being a new application field for MCF. The experimental demonstration of this system using a 4-core MCF that controls a four-element antenna array is reported. An analysis of the impact of MCF on the performance of antenna arrays is presented. The analysis indicates that the main limitation comes from the relatively high insertion loss in the MCF fan-in and fan-out devices, which leads to angle dependent losses which can be mitigated by using fixed optical attenuators or a photonic lantern to reduce MCF insertion loss. The crosstalk requirements are also experimentally evaluated for the proposed MCF-based architecture. The potential signal impairment in the beamforming network is analytically evaluated, being of special importance when MCF with a large number of cores is considered. Finally, the optimization of the proposed MCF-based beamforming network is addressed targeting the scalability to large arrays.

In the past decade, there have been many studies on metamaterial based chemical and biological sensors due to their exotic resonance properties in microwave ranges. However, in spite of their non-destructive and highly sensitive properties, they have suffered from the use of bulky and expensive external measurement systems like a network analyzer for measuring resonance properties in the microwave regime. In this study, to increase accessibility of the metamaterial-based sensors, we propose a novel wireless chemical sensor system based on energy harvesting metamaterials at the microwave frequencies. The proposed metamaterial chemical sensor consists of a single split ring resonator and rectifier circuit to harvest the energy at the specific frequency, so that the chemical composition of the specific solution can be distinguished by the proposed metamaterial sensor by using the resonance property between the source antenna and the metamaterial which induces the variation in the energy harvesting rate of our sensor system. In our experimental setup, we used a 2.4 GHz Wi-Fi system as a source antenna. To verify the chemical sensitivity of the proposed sensor intuitively, we adopted a light emitting diode as an indicator of which luminescence is proportional to the energy harvesting rate determined by the ratio of ethanol and water in their binary mixture. With these results, it can be expected that our metamaterial-based wireless sensor can pave the way to the miniaturized wireless sensor systems and can be applied to not only for the chemical fluidic sensors but also for other dynamic environment sensing systems.

Conventional plasmonic materials are typically fabricated using a single homogenous metal and structured to obtain useful functionality. Alternatively, structures are occasionally made in which several homogenous materials are deposited using a layer-by-layer process, such as metal-dielectric-metal structures [1]. However additional control over the propagation properties of surface plasmon-polaritons should be possible if the metal conductivity could also be varied spatially. This is not straightforward using conventional microfabrication techniques. We demonstrate the ability to vary the conductivity spatially using a conventional inkjet printer, yielding either step-wise changes or continuous changes in the conductivity. We accomplish this using a commercially available inkjet printer, where one inkjet cartridge is filled with conductive silver ink and a second cartridge is filled with resistive carbon ink. By varying the fractional amounts of the two inks in each printed dot, we can spatially vary the conductivity. The silver ink has a DC conductivity that is only a factor of six lower than the bulk silver, while the carbon ink acts as a lossy dielectric at terahertz frequencies. Both inks sinter immediately after being printed on a treated PET transparency. We demonstrate the utility of this approach with both plasmonics and metamaterial applications, demonstrating the ability to control beam profiles, create new filter capabilities and hide images in THz metasurfaces.

Heterodyne terahertz spectrometers are highly in demand for space explorations and astrophysics studies. A conventional heterodyne terahertz spectrometer consists of a terahertz mixer that mixes a received terahertz signal with a local oscillator signal to generate an intermediate frequency signal in the radio frequency (RF) range, where it can be easily processed and detected by RF electronics. Schottky diode mixers, superconductor-insulator-superconductor (SIS) mixers and hot electron bolometer (HEB) mixers are the most commonly used mixers in conventional heterodyne terahertz spectrometers. While conventional heterodyne terahertz spectrometers offer high spectral resolution and high detection sensitivity levels at cryogenic temperatures, their dynamic range and bandwidth are limited by the low radiation power of existing terahertz local oscillators and narrow bandwidth of existing terahertz mixers. To address these limitations, we present a novel approach for heterodyne terahertz spectrometry based on plasmonic photomixing. The presented design replaces terahertz mixer and local oscillator of conventional heterodyne terahertz spectrometers with a plasmonic photomixer pumped by an optical local oscillator. The optical local oscillator consists of two wavelength-tunable continuous-wave optical sources with a terahertz frequency difference. As a result, the spectrometry bandwidth and dynamic range of the presented heterodyne spectrometer is not limited by radiation frequency and power restrictions of conventional terahertz sources. We demonstrate a proof-of-concept terahertz spectrometer with more than 90 dB dynamic range and 1 THz spectrometry bandwidth.

Advanced technology, such as sensing and communication equipment, has recently begun to combine optically sensitive nano-scale structures with customizable semiconductor material systems. Included within this broad field of study is the aptly named frequency-selective surface; which is unique in that it can be artificially designed to produce a specific electromagnetic or optical response. With the inherent utility of a frequency-selective surface, there has been an increased interest in the area of dynamic frequency-selective surfaces, which can be altered through optical or electrical tuning. This area has had exciting break throughs as tuning methods have evolved; however, these methods are typically energy intensive (optical tuning) or have met with limited success (electrical tuning). As such, this work investigates multiple structures and processes which implement semiconductor electrical biasing and/or optical tuning. Within this study are surfaces ranging from transmission meta-structures to metamaterial surface-waves and the associated coupling schemes. This work shows the utility of each design, while highlighting potential methods for optimizing dynamic meta-surfaces. As an added constraint, the structures were also designed to operate in unison with a state-of-the-art Ti:Sapphire Spitfire Ace and Spitfire Ace PA dual system (12 Watt) with pulse front matching THz generation and an EOS detection system. Additionally, the Ti:Sapphire laser system would provide the means for optical tunablity, while electrical tuning can be obtained through external power supplies.

The terahertz spectral regime, ranging from about 0.1–15 THz, is one of the least explored yet most technologically transformative spectral regions. One current challenge is to develop efficient and compact terahertz emitters/detectors with a broadband and gapless spectrum that can be tailored for various pump photon energies. A particularly essential and topical question is how to create nonlinear broadband terahertz devices using deeply subwavelength nanoscale meta-atom resonators. Here we demonstrate efficient single-cycle broadband THz generation, ranging from about 0.1–4 THz, in two model hybrid quantum nanostructures: (1) a thin layer of split-ring resonators (SRRs) with few tens of nanometers thickness by pumping at the telecommunications wavelength of 200 THz; (2) SRRs coupled to intersubband transitions in quantum wells pumping at 30 THz. We also reveal a giant sheet nonlinear susceptibility that far exceeds thin films and bulk non-centrosymmetric materials. Finally, I will also discuss their significances for THz enabled nonlinear spectroscopy and quantum phase discovery applications.

Mid-IR carbon dioxide (CO₂) gas sensing is critical for monitoring in respiratory care, and is finding increasing importance in surgical anaesthetics where nitrous oxide (N₂O) induced cross-talk is a major obstacle to accurate CO₂ monitoring. In this work, a novel, solid state mid-IR photonics based CO₂ gas sensor is described, and the role that 1- dimensional photonic crystals, often referred to as multilayer thin film optical coatings [1], play in boosting the sensor’s capability of gas discrimination is discussed. Filter performance in isolating CO₂ IR absorption is tested on an optical filter test bed and a theoretical gas sensor model is developed, with the inclusion of a modelled multilayer optical filter to analyse the efficacy of optical filtering on eliminating N₂O induced cross-talk for this particular gas sensor architecture. Future possible in-house optical filter fabrication techniques are discussed. As the actual gas sensor configuration is small, it would be challenging to manufacture a filter of the correct size; dismantling the sensor and mounting a new filter for different optical coating designs each time would prove to be laborious. For this reason, an optical filter testbed set-up is described and, using a commercial optical filter, it is demonstrated that cross-talk can be considerably reduced; cross-talk is minimal even for very high concentrations of N₂O, which are unlikely to be encountered in exhaled surgical anaesthetic patient breath profiles. A completely new and versatile system for breath emulation is described and the capability it has for producing realistic human exhaled CO₂ vs. time waveforms is shown. The cross-talk inducing effect that N₂O has on realistic emulated CO₂ vs. time waveforms as measured using the NDIR gas sensing technique is demonstrated and the effect that optical filtering will have on said cross-talk is discussed.

A non-invasive, unstaining tissue measurement method is expected to be an important tool for regenerative medical. As THz wave do not affect biological tissue because of their low energy, THz measurement method is expected to become a new modality of biomedical analysis as a noninvasive diagnosis. Due to the fact that biological tissues possess high level of hydration, it results in strong absorption at terahertz frequencies. A ridge waveguide LiNbO₃ based nonlinear terahertz generator was used to achieve high output power for THz time domain spectroscopy (THz-TDS). A ridge waveguide was designed for high efficiency emission from the LiNbO₃ crystal by the electro-optic Cherenkov effect. This THz-TDS system has realized six orders of dynamic range, and the bandwidth of the spectrum reaches 7 THz in the upper limit. We measured a reflected terahertz pulse shape at the interface between a plastic culture dish and biological tissues. By studying the gradient of phase spectroscopy of reflected THz pulse, we have been able to differentiate between human fibroblast tissue and cancer tissue. We demonstrate the application of terahertz time domain spectroscopy pulse in reflection geometry for the non-distractive measurement of biological tissues cultured on a plastic culture dish. These results demonstrate the potential of terahertz phase information for the study of biological tissues.

Metamaterials are subwavelength man-made plasmonic or dielectric structures designed to realize specific effects on the electromagnetic radiation interacting with the material. Here we aim to rotate the polarization of an incident terahertz beam using chiral metamaterials, while suppressing the circular dichroism which induces ellipticity of the output beam. An incident linearly polarized wave can be decomposed into left-circularly and right-circularly polarized waves, and the difference in propagation phase will result in rotation of the plane of polarization. Since chiral structures couple electric and magnetic fields, they are often implemented in complex geometries such as spirals or bi-layered plasmonic structures, which can achieve carefully balanced responses to the two fields. The important feature of the bi-layered plasmonic structure is the cross-coupling between the resonances of the two layers. It is precisely this coupling between the layers that induces currents in the structures that are mutually dependent producing chirality within the structure. By coupling a metallic structure to its complement, we are able to achieve strong transmission in the region of maximum polarization rotation, and relatively low ellipticity of the output state. Three different structures were fabricated for this work that will be referred to as: the plain crosses, crossed arrowheads, and crossed arcs, pictured in the figure below. The terahertz responses of the structures were compared using terahertz time-domain spectroscopy and numerical simulations using CST Microwave Studio software.

Metasurfaces represent the most promising class of metamaterials for real applications, whereby arbitrary wavefront and polarisation control can be achieved using just a single sub-wavelength layer. Therefore, allowing tunability over their capabilities is the next step to consolidate them as technology devices for light control. In our work we propose a new platform for creating tunable microwave devices based on gradient metasurfaces. Our study shows that the integration of a patterned elastic substrate in the design of functional metasurfaces is an effective approach to enable control over their electromagnetic properties. To demonstrate the new platform, we propose, design and experimentally realize a novel tuning mechanism that controls the focal length of an electromagnetic metasurface lens by exploiting the degree of freedom provided by the flexible substrate, which enables continuous elongation of the system. When such a metasurface is uniaxially stretched, the distance between embedded electromagnetic resonators increases, producing a change in the phase profile created by these resonators, and this leads to a change of the focal distance of the lens. Thus, the flexible metasurface displays a functionality that can be continuously controlled by unidirectional mechanical loading. We fully characterize the spherical-like aberration phenomenon which accompanies the tuning process. Finally, our study reveals that an equidistant separation between the resonators leads to reduced device performance of the operational metasurface and, therefore, the utilization of other degrees of freedom is mandatory if the efficiency needs to be preserved.

I propose a real-time demultiplexing method of terahertz-wave OFDM signal, which utilizes the optical technology. The received and amplified electrical OFDM signal is transferred into an optical OFDM signal through an optical modulator. Then, the optical OFDM signal is processed with an integrated-optic DFT circuit, which is mainly composed of a slab star coupler. This optical DFT circuit is compact, and can demultiplex up to a 10 × 10 Gbaud (100 Gbaud) optical OFDM signal. I report the operating principle of the method and some preliminary experimental results.

In this paper, a broadband, ultrathin metamaterial absorber (MA) using randomly distributed scatterers is presented. Each scattering element consists of two parallel strips. These elements can either be isolated or they may overlap with nearby elements. Three different randomly positioned structures are investigated for normal incident angle as well as oblique incident angles showing that these MAs can provide broadband absorption for all cases. The results presented here coincide with some previous works. Each structure obviously has different absorption spectrum and FWHM since the coupling between the randomly positioned scatterers is different in each case. The coupling between neighboring isolated and clustered scatterers form many resonating modes resulting in broadband absorption. The distribution of the electromagnetic fields are analyzed to obtain the physical behavior of the absorber. This shows that promising results can still be obtained for MAs when there is a significant tolerance distance between scatterers due to fabrication errors in micro and nanoscale metadevices.

Tailoring the geometry and arrangement of metasurface structures yields a complete control of the reflection/transmission amplitude, phase, and polarization states. The planar nature of the structures can be readily fabricated using existing technologies, which makes them more accessible particularly in the optical regime and potentially revolutionizes the design of integrated photonics. Although the ultrathin thickness in the wave propagation direction can greatly suppress the absorption, the impedance mismatching in many metasurfaces results in undesirable high insertion losses, and the performance of single-layer metasurface devices is yet unsatisfying in real world applications. In contrast, few-layer meta-surfaces can circumvent this impedance mismatching issue. Recently, we have developed a three-layer metasurface structure that is capable of rotating the incident linear polarization by 90° with a very high efficiency over a bandwidth of nearly two octaves. More importantly, the phase of the output light can be tuned over the entire 2π range with sub-wavelength resolution through simply tailoring the structure geometry of the basic building blocks. This creates an important opportunity in designing highly efficient optical devices for wavefront engineering, such as at lenses working in the microwave, terahertz, and infrared frequency ranges. Here we will present the design, fabrication, and characterization of high-performance terahertz metasurface lenses based on this concept.

One of the most peculiar characteristics of the insulator-to-metal transition (MIT) in vanadium dioxide (VO₂) material is its broadband response, manifested by drastic electrical and dielectric properties changes between the insulator and metallic states on a very large frequency spectrum. We are presenting the characterization of the MIT in VO₂ films over a wide range of the electromagnetic spectrum (75-110GHz, 0.1-1.4THz) and illustrate the materials’ capabilities for manipulating the electromagnetic radiation in the millimeter-waves and THz domains. We demonstrate the possibility of realizing tunable THz devices by introducing this phase transition material as localized patterns in the structure of THz planar metamaterials. We designed, simulated and fabricated tunable VO₂-based THz metamaterials devices which show significant variations in their THz transmission under the effect of thermal stimuli but also by applying an electrical voltage across the devices.

In genetic diagnostics, laboratory-based equipment generally uses analytical techniques requiring complicated and expensive fluorescent labelling of target DNA molecules. Intense research effort into, and commercial development of, Point-of-Care diagnostics and Personalized Healthcare are driving the development of simple, fast and cost-effective detection methods. One potential label-free DNA detection method uses Terahertz (THz) spectroscopy of the natural responses of DNA in metamaterial structures, which are engineered to have properties that are impossible to obtain in natural materials. This paper presents a study of the development of metamaterials based on asymmetric X-shaped resonator inclusions as a functional sensor for DNA. Gold X-shaped resonator structures with dimensions of 90/85 μm were demonstrated to produce trapped mode resonant frequency in the correct range for DNA detection. Realistic substrate materials in the form of 375 μm thick quartz were investigated, demonstrating that the non-transparent nature of the material resulted in the production of standing waves, affecting the system response, as well as requiring a reduction in scale of the resonator of 85%. As a result, the effect of introducing etched windows in the substrate material were investigated, demonstrating that increased window size significantly reduces the effect of the substrate on the system response. The device design showed a good selectivity when RNA samples were introduced to the model, demonstrating the potential for this design of device in the development of sensors capable of performing cheap and simple genetic analysis of DNA, giving label-free detection at high sensitivity.

This paper will discuss resonant tunnelling diode (RTD) sources being developed on a European project iBROW (ibrow.project.eu) to enable short-range multi-gigabit wireless links and microwave-photonic interfaces for seamless links to the optical fibre backbone network. The practically relevant output powers are at least 10 mW at 90 GHz, 5 mW at 160 GHz and 1 mW at 300 GHz and simulation and some experimental results show that these are feasible in RTD technology. To date, 75 - 315 GHz indium phosphide (InP) based RTD oscillators with relatively high output powers in the 0.5 – 1.1 mW range have been demonstrated on the project. They are realised in various circuit topologies including those that use a single RTD device, 2 RTD devices and up to 4 RTD devices for increasingly higher output power. The oscillators are realised using only photolithography by taking advantage of the large micron-sized but broadband RTD devices. The paper will also describe properties of RTD devices as photo-detectors which makes this a unified technology that can be integrated into both ends of a wireless link, namely consumer portable devices and fibre-optic supported base-stations (since integration with laser diodes is also possible).

The relative permittivity (real and imaginary component), absorption coefficient, and loss tangent of various cellulose nanocrystal (CNC) films, a dissolving pulp film, and a CNC powder are obtained by performing terahertz (THz) transmission spectroscopy experiments. The CNC films are constructed using different drying techniques (i.e. air-drying and freeze-drying) and are made from CNCs that have been extracted from various sources (i.e. hardwood, softwood, and dissolving pulp). Between frequencies of 0.2 and 1.5 THz, the real component of the permittivity is seen to range from 1.8-3.3 for the CNC films, suggesting that both the drying technique and CNC source material influence this dielectric property. Importantly, the CNC films are shown to exhibit relatively small THz absorption and loss tangent properties, such that CNC-based dielectric mirrors, waveguides, and transistors may be achieved.

The implementation of an integrated mm-wave transmitter (TX) and receiver (RX) in SiGe BiCMOS or CMOS technology offers a path towards a compact and low-cost system for gas spectroscopy. Previously, we have demonstrated TXs and RXs for spectroscopy at 238 -252 GHz and 495 - 497 GHz using external phase-locked loops (PLLs) with signal generators for the reference frequency ramps. Here, we present a more compact system by using two external fractional-N PLLs allowing frequency ramps for the TX and RX, and for TX with superimposed frequency shift keying (FSK) or reference frequency modulation realized by a direct digital synthesizer (DDS) or an arbitrary waveform generator. The 1.9 m folded gas absorption cell, the vacuum pumps, as well as the TX and RX are placed on a portable breadboard with dimensions of 75 cm x 45 cm. The system performance is evaluated by high-resolution absorption spectra of gaseous methanol at 13 Pa for 241 - 242 GHz. The 2f (second harmonic) content of the absorption spectrum of the methanol was obtained by detecting the IF power of RX using a diode power sensor connected to a lock-in amplifier. The reference frequency modulation reveals a higher SNR (signal-noise-ratio) of 98 within 32 s acquisition compared to 66 for FSK. The setup allows for jumping to preselected frequency regions according to the spectral signature thus reducing the acquisition time by up to one order of magnitude.

We will report our recent results using ultrathin NbN films (4-5 nm) for developing both conventional antenna-coupled hot-electron-bolometers (AC-HEBs) and of a novel type of phonon cooled HEB electrically-coupled to a metamaterial acting as a resonant absorber at THz frequencies (MM-HEB), optically-coupled through arrays of split ring resonators (metadevices). In a phonon-cooled HEB, being the active layer an ultrathin film of superconducting NbN, we have an ultrafast thermal direct detector that is also frequency selective thanks to the integration with the resonant metamaterial. We characterized both the AC-HEB and the MM-HEB by electro optical measurements using as a sources both the black body emission and terahertz quantum cascade lasers (THz-QCLs) and we compared their performances.

This paper presents a design and fabrication of 4 × 4 one-sided directional slot array antenna on indium phosphide (InP) substrate for 0.3 THz (300 GHz) wireless link. The antenna has top antenna metal layer and bottom floating metal layer. Polyimide dielectric layer is stacked between each metal layer. The antenna is placed on the deep etched InP substrate. By optimizing the length of the bottom floating metal layer, one-sided directional radiation can be realized. The branched coplanar wave guide (CPW) transmission line is connected to each antenna element with the same electrical length. The size of the 4 × 4 array antenna is 2,730 μm x 3,000 μm with uni-traveling-carrier photodiodes, DC bias and ground lines. Simulated realized gain in peak direction of the proposed antenna is 11.7 dBi. The transmission measurement is carried and measured received power.

An external modulation technique for the generation of optical linearly chirped frequency continuous waves is proposed and experimentally demonstrated. The output from a distributed feedback (DFB) laser is modulated by a single sideband (SSB) modulator which is driven by a linearly chirped electrical signal generated by an arbitrary waveform generator (AWG). A high precise linear optical frequency sweep with a tuning range of 2.5GHz is successfully achieved. Due to the good modulation quality of SSB and high stability of AWG, the chirp of the electrical signal can be perfectly modulated onto the optical signal. The linearity of the optical signal is almost consistent with the linearity of the electrical signal. The best linearity of 0.9996 and the lowest jitter rate of 4.76 % are obtained from the experimental measurements which are reported in this paper.

In order to effectively improve the coupling efficiency of terahertz (THz) detectors, we design a grating-coupled structure on the high-resistivity silicon substrate for 0.2 THz to 0.35 THz band to enhance the ability of coupling terahertz signals. We simulated the electric field distribution of the grating-coupled structure in surface and inside by using the finite difference time domain (FDTD) method. The electric field in the central area of the silicon surface can be enhanced more than 4 times compared with the non-structure silicon substrate. We also simulated the Fabry-Perot cavity in the frequency range from 0.2 THz to 0.35 THz, and the electric field in the central area of the silicon surface can be improved one time compared with the non-structure silicon substrate. In addition, the electric field distribution on the silicon surface can be changed by adjusting parameters of the grating-coupled structure. When the period of the grating is 560 μm, the width of the gold is 187 μm, and the thickness of the silicon substrate is 720 μm, a 4.7 times electric field could be achieved compared with the non-structure silicon substrate at 0.27 THz and around. So, the simulation result shows that the grating-coupled structure has an obvious advantage compared with the Fabry-Perot cavity at THz coupling efficiency.

A wafer fused GaP/GaAs waveguide was developed for THz QCLs to achieve high confinement factor benefiting from its lower refractive index in THz regime. The modal simulation of several waveguide structures using COMSOL showed an increase of confinement factor up to 2 as compared to regular waveguide; however it also resulted in high losses. Experimental results showed good electric characteristics but poor optical performance, which is mainly due to the degradation of crystal quality after high temperature process, confirmed by stress analysis and XRD. Therefore, a low temperature fusion process is necessary to fabricate GaP/GaAs THz waveguide.

A low-cost method to ink-jet print terahertz polarizers is presented. A liquid metal printer is used to deliver an eutectic Gallium and Indium alloy (EGaIn24.5) onto polyvinyl chloride film substrates to create flexible wire grid polarizers (10 x 20 mm) in the sub-THz band with a nominal pitch of 200 μm and 300 μm, respectively. A Terahertz Time Domain Spectroscopy (THz-TDS) setup has been used to characterize the polarizers. The experimental results have been compared with FIT (Finite Integration Technique) simulations (CST Studio) showing good agreement. The characterization of the polarizers shows an extinction ratio up to 14 dB in the 0.1-0.7 THz and low loss (<1 dB). A microscopic characterization of the polarizers shows a variance in the line spacing of about 9%. This fabrication method allows a quick and cost-effective approach for the development of sub-THz polarizers to be used in polarization-resolved spectroscopy, polarimetric quality monitoring sensors and the characterization of THz components.

A polarization insensitive metamaterial absorber is proposed consisting of a dielectric layer sandwiched between two tilted parallel metallic strips and a ground plane. First, a new analytical model is introduced to predict the resonance frequency for square, rectangular and wire geometries which shows less relative errors in comparison with previously proposed models. Then, ultra wide bandwidth absorption covering the entire x-band and Ku-band restricting a 10-dB absorption bandwidth is achieved with a relative material thickness λ₀/10 and a relative FWHM absorption bandwidth of 90% for normal incidence angle. The model also shows good absorption coefficients for oblique incident angles for both s- and p-polarizations. Three different resonance modes are observed, which led to such broadband absorption. Each resonance mode is investigated to determine its dependence on the scatterers and unit cell dimensions, which can help one to design a multiple band absorber. The electromagnetic field distributions of the scatterers are studied to explain the absorber mechanism. The results are compared to previous works showing remarkable improvement.

Photonic true-time delay (TTD) beamforming has been considered as a promising technique for future wide-band phased array antenna systems. A TTD beamforming system based on a linear chirped fiber grating (LCFG) requires a tunable multi-wavelength laser source, in which the wavelengths are tuned simultaneously with equally increased or decreased wavelength spacing. We present a novel approach of a TTD laser source employing a DQPSK modulator and a highly nonlinear fiber (HNLF). The wavelength tuning is achieved by tuning the frequency of the radio frequency signal generator that drives the DQPSK modulator. Up to 41 lasing wavelengths with 0.08 nm spacing, 29 lasing wavelengths with 0.06 nm spacing and 31 lasing wavelengths with 0.12 nm spacing in 3 dB bandwidth were obtained. In theory, the multi-wavelengths can be simultaneously tuned from 10 MHz to 20 GHz in our experiments based on the fact that the frequency range of the RF signal source is from 10 MHz to 20 GHz and the bandwidth of DQPSK modulator is up to 22 GHz. The tuning precision of the RF signal generator is up to 1Hz and phase noise is about -104dBc/Hz. Therefore, the wavelengths can be tuned precisely and stably.

We proposed and demonstrated a linearly frequency-swept multi-wavelength laser source for optical coherence tomography (OCT) eliminating the need of wavenumber space resampling in the postprocessing progress. The source consists of a multi-wavelength fiber laser source (MFS) and an optical sweeping loop. In this novel laser source, an equally spaced multi-wavelength laser is swept simultaneously by a certain step each time in the frequency domain in the optical sweeping loop. The sweeping step is determined by radio frequency (RF) signal which can be precisely controlled. Thus the sweeping behavior strictly maintains a linear relationship between time and frequency. We experimentally achieved linear time-frequency sweeping at a sweeping rate of 400 kHz with our laser source.

This work is focused on two promising concepts of microwave photonics- optoelectronic oscillators and optical frequency comb generators, and their use to generate a self-oscillating optical comb. In particular we develop a recirculating loop topology in which a Mach-Zehnder modulator is modulated recursively via a secondary loop that acts as a self-homodyne input.

Quantum well charge modulation in base region of light-emitting transistors and related electrical-to-optical responses are analyzed by constructing three-port small-signal equivalent circuit models under common-emitter, common-base, and common-collector configurations.