THz diffractive lenses have recently gathered a lot of attention as an effective alternative to conventional THz lenses which are bulky, thick, expensive and suffer from strong wavefront (geometric and chromatic) aberrations. It has already been shown that employing a direct binary search technique on the actual height profile of diffractive elements (DEs) can yield designs with better or comparable performance metrics with respect to kinoforms. Such search techniques are however, limited by their exponential time complexity in an unstructured design solution space. If the solution can be proven to exist, we can further perform a gradient descent optimization along with the binary search to overcome the time complexity required to arrive at the desired solution.
The semi-analytic and measured efficiency for all diffractive lenses under both narrowband and broadband focusing is > 80% based on our modified DBS employed design prediction. The modified DBS is observed to converge much faster for both 1D and 2D diffractive lens cases (>10X) with respect to conventional direct binary search based design prediction. For 2D diffractive lenses, the figure of merit is initially high and takes a longer time to converge to the desired solution, which can be understood from the fact that the number of “dielectric” pixels in a 2D lens is much greater than its 1D analogue. Furthermore, since the direct binary search is an iterative algorithm, it convergence depends a lot on the initial random pixel height profile, which is not the case in the modified DBS method.
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.
We analyze the terahertz properties of complex oxide hetero-structures with record-high carrier concentration approaching 1015 cm-2. Our results evidence a large room temperature terahertz conductivity, which corresponds to 3X to 6X larger mobility than what is extracted from electrical measurements. That is, in spite of a relatively lower mobility, when taking into account its ultra-large carrier concentration, the 2DEG in complex oxide hetero-structures can still attain a large terahertz conductivity, which is comparable with that in traditional high-mobility semiconductors or large-area CVVD graphene films. Moreover, we also discuss the perspectives off these hetero-structures for terahertz and high frequency electronic applications.
The phenomenon of anomalous transmission through subwavelength aperture arrays in metallic films (plasmonic
lattices) is thought to be mediated by surface plasmon polaritons (SPP) on the film surfaces. Using terahertz time-domain
spectroscopy we systematically studied the anomalous transmission spectrum through plasmonic lattices as a
function of the incidence angle, θ of the impinging beam. We observed splitting of the various transmission resonances
into two resonance branches when θ deviates from normal incidence that depends on the polarization direction of the
beam respect to the plane of incidence and θ. We show that the transmission resonance splitting is not related to
dispersion relation of different SPP branches, but rather is associated to the interference properties of the SPP waves on
the metal surface. The dependence of the split resonant frequencies vs. θ is fit with a theoretical formula that takes into
account the effective dielectric function of the plasmonic lattice vs. θ, which emphasizes the important role of the Fanotype
anti-resonances in the transmission spectrum. Finally, we introduced a simple way for making an efficient notch
filter with high Q factor exploiting the splitting of transmission resonance under rotation.
This report presents an overview of our study on the optical transmission and thermal light emission properties of sub-wavelength
hole arrays fabricated in a square lattice with 4 μm periodicity. The structures were fabricated in thin
aluminum (Al) films on silicon (Si) substrates using conventional photolithography. The spectra were obtained using a
Fourier transform infrared spectrometer with a port for an external cryostat configured for thermal emission
measurements. The perforated films showed extraordinary transmission bands in the mid-infrared spectral range, which
could be well explained as due to light coupling to surface plasmon-polaritons on the two film interfaces. We fitted the
transmission spectra and calculated the absorption spectra of these structures using a model for the dielectric response
that utilizes an effective plasma frequency determined by the individual holes, as well as several resonant modes
associated with the reciprocal vectors in the lattice structure factor. We found that the thermal emission spectrum from
the perforated films followed the transmission spectrum characteristics, rather than the obtained absorption spectrum; in
apparent contrast to Kirchhoff's law of radiation. We conclude that the perforated films behave as radiation filters,
where the thermal emission radiation is suppressed in the frequency range outside the transmission resonant bands in the
spectrum.
We observed resonantly enhanced (or anomalous transmission) terahertz transmission through two-dimensional (2D) periodic arrays of subwavelength apertures with various periodicities fabricated on metallic organic conducting polymer films of polypyrrole heavily doped with PF6 molecules [PPy(PF6)]. The anomalous transmission spectra are in good agreement with a model involving surface plasmon polariton excitations on the film surfaces. We also found that the resonantly enhanced transmission peaks are broader in the exotic metallic PPy(PF6) films compared to those formed in 2D aperture array in regular metallic films such as silver, indicating that the surface plasmon polaritons on the PPy(PF6) film surfaces have higher attenuation.
Terahertz (T-rays) spectroscopy has recently emerged as a powerful method to access a heretofore barely explored region of the electromagnetic spectrum where fundamental molecular resonances occur. Besides their importance for fundamental research, these resonances could be used as signatures in the identification of molecular species and as sensitive probes in a wide variety of molecular processes.
In this paper we consider the potential of THz spectroscopy in the application to relevant biomedical and homeland security problems such as the analysis of normal and diseased tissues and the detection of toxic biomolecules.
As examples, we present preliminary experimental data which suggest that THz spectroscopy: 1) can discriminate between cancerous and normal tissue, and 2) can reveal the presence of foreign substances hidden in an envelope and even allow their specific identification. This capability is of particular relevance as a straightforward homeland security tool for the detection of anthrax and other biotoxic molecules.
We discuss advances in the optoelectronic generation and detection of coherent terahertz radiation. The demonstrations utilize poled polymeric media, which exhibit characteristics that are extremely well suited for broadband THz applications. Using a 35 mm thick poled polymer film, we demonstrate electro-optic detection of freely propagating ultrashort baseband electromagnetic pulses with spectral sensitivity that extends from the far-infrared (l ~ 100 μm) to ~33 THz (l = 9 μm). Over a band of nearly 20 THz, a relatively flat frequency response is observed. We compare the performance of the poled polymer with that of established materials. We also present a novel waveguide geometry that allows for the phase-matched generation of broadband THz radiation in a polymer-based parallel plate metal waveguide via optical rectification. Both the optical pump beam and the generated THz radiation propagate in the fundamental mode of the waveguide. This allows for non-critical phase matching over a broad range of THz frequencies. We demonstrate guided wave interaction lengths of up to 3 mm.
A robust polymeric waveguide technology is proposed for affordable optoelectronic interconnects in massively parallel processing applications. We have developed high-performance organic polymeric materials that can be readily made into both multimode and single-mode optical waveguide structures of controlled numerical aperture and geometry. These materials are formed from highly-crosslinked acrylate monomers with specific linkages that determine properties such as flexibility, toughness, loss, and stability against yellowing. These monomers are intermiscible, providing for precise adjustment of the refractive index from 1.30 to 1.60. Waveguides are formed lithographically, with the liquid monomer mixture polymerizing upon illumination in the UV via either mask exposure or laser direct-writing. A wide range of rigid and flexible substrates can be used, including glass, quartz, oxidized silicon, glass-filled epoxy printed circuit board substrate, and flexible polyimide film. Our waveguides are low loss (0.02 dB/cm at 840 nm) as well as temperature resistant (over 65 years at 100 degree(s)C) and humidity resistant (no effect on unpackaged guides after 600 hours at 85 degree(s)C 85% RH), enabling use in a variety of demanding applications. We discuss the use of these materials on multi-chip modules, boards, and backplanes. Waveguiding structures measuring tens of inches in length can be produced on backplanes, and guides that are meters long can be laser-written on rolls of plastic. We also discuss the fabrication of symmetrically-clad flexible strips of waveguide arrays that are compatible with MT- type connectors.
The technical approach and progress achieved under the Polymer Optical Interconnect Technology (POINT) program are described in this paper. The POINT program is a collaborative effort among GE, Honeywell, AMP, AlliedSignal, Columbia University, and University of California at San Diego (UCSD), sponsored by DARPA/ETO, to develop affordable optoelectronic packaging and interconnect technologies for board and backplane applications. Specifically, progress is reported on (a) development of a plastic VCSEL array packaging technology using batch and planar fabrication, (b) demonstration of high-density optical interconnects for board and backplane applications using polymer waveguides to a length of 50 cm at an I/O density of 250 channels per inch, (c) development of low-loss optical polymer waveguides with loss less than 0.1 dB/cm at 850 nm, and (d) development of passively alignment processes for efficient coupling between a VCSEL array and polymer waveguides. Significant progress has also been made under the POINT program at Columbia University, in applying CAD tools to simulate multi-mode-guided wave systems and, at UCSD, to assist mechanical and thermal design in optoelectronic packaging. Because of space limitations, these results will be described elsewhere in future publications.
We report the electro-optic and thermal stability properties of a new class of novel thermostable fluorene-based cardo-type polymers. The electro-optic coefficients found for these materials are comparable to those obtained from many other reported side-chain polymers. Isothermal decay measurements for both the charge transfer absorption bands and for the electro-optic responses demonstrate that the thermal degradation of electro-optic response in these materials is dominated by chemical degradation and not by depoling, at least over a large range of elevated temperatures. One of these materials for example is characterized by a measured 1/e decay time of over 40 days at 190 degree(s)C with extrapolated lifetimes of over one year at 175 degree(s)C.
Practical applications of electro-optic polymers require thermally stable materials with high electro-optic coefficients and low absorption loss. In this report, we first review the properties of some azo-based electro-optic polymer materials and devices. We then describe a new class of electro-optic cardo polymers with glass transition temperatures greater than 200$DEGC.
We report loss measurements in polymer-bound Disperse Red I slab and photodelineated channel waveguides. Losses resulting from electronic charge-transfer and vibrational carbon- hydrogen stretch overtone absorptions, trans to cis isomerization, exposure to visible or ultraviolet (UV) light and changes in dye pendant group number density are investigated. A waveguide absorption spectrometer is described which can measured waveguide losses (alpha) ((lambda) ) from 600 - 1800 nm. Absorption losses are compared to the wavelength dependent electro-optic coefficient r33((lambda) ) and a figure-of-merit r33((lambda) )/(alpha) ((lambda) ) is determined for the material.
Polymeric materials suitable for the formation of both passive and electro-optically active waveguide devices have been produced. We have developed methods for delineating channel waveguides in thin polymer films via a photopatteming technique. A description of the performance characteristics of devices incorporating these materials will be presented. An analysis of the advantages and drawbacks of polymeric systems will also be included.
Organic polymeric materials offer great promise for the creation ofoptical guided-wave structures for use with silicon
or gallium arsenide semiconductor devices. We have developed a number of new polymeric materials for which the refractive
index may be photochemically controlled. These materials are ifiustrated by solid solutions of novel nitrone compounds in
polymer hosts such as PMMA. We have demonstrated the creation of planar guided-wave structures in these materials both
with direct laser writing and with traditional photolithographic techniques. We have also developed polymeric materials which
are electro-optically active and which provide for the photochemical delineation ofguided-wave structures. We have utilized
these materials to create electro-optic devices such as optical modulators.
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