A review of different integration platforms for high-Q Whispering Gallery Mode bulk resonators is presented, including SOI slotted photonics crystal waveguides, suspended Si photonic crystal membranes or suspended silica waveguides. While each of these approaches allows coupling to a specific monolithic resonator, including those made in low index materials, a novel architecture, based on metamaterial engineered silicon photonics waveguides, provides unique flexibility to couple a wide range of WGM microresonators, enabling the combination of high-performance resonators with complex Si photonic circuits.
Bulk whispering gallery mode optical resonators like spheres and disks provide a wide range of remarkable optical properties and ultra-high quality factors. In this invited presentation we will show our most recent results on the use of subwavelength metamaterial engineering to couple bulk resonators and integrated Si waveguides. We experimentally achieve up to 99% light coupling efficiency for microspheres and microdisks made of silica, lithium niobate, and calcium fluoride, with diameters between 300 µm and 3.6 mm. These results open promising prospects for the implementation of a new generation of devices combining high-performance bulk resonators and complex Si photonic circuits.
High quality factor bulk resonators made in different materials have demonstrated outstanding performance in key functionalities that are very challenging to achieve in planar photonics. However, they have made no significant technological impact mainly because of their stability and scalability limitations related to the way they are connected to the outside world using prisms or tapered fibers. Here, we show several demonstrations of efficient coupling of bulk resonators to integrated waveguides using different materials like lithium niobate or polymers. Preliminary results of a universal integrated coupler that can be implemented using silicon photonics are also presented.
This paper discusses ongoing research at Lawrence Livermore National Laboratory (LLNL) that investigates the effectiveness of spherical micro-resonators, coupled to a symmetrically tapered optical fiber, as a gas sensor. We will discuss silica-based microspheres and optimized tapered fiber coupling systems to detect greenhouse gases, i.e. CO2 in this context. The coupling setup is designed to be portable and amenable to different controlled environments, from constrained and controlled geometries to open and flexible enclosures. 3D-printed spherical resonator and tapered-fiber holders were made to satisfy different requirements. We produced microspheres for absorption spectroscopy of targeted gas and fabricated tapers by HF etching, using an HF-resistant fixture for safer handling and reduced waste. Detection within loose enclosures was performed as a preliminary study, where we observed spectral shift and broadening in the cavity resonances induced by the gaseous environments. Optically coupled vacuum-tight vessels have been designed and built to understand environmental effects.
An optical resonator like a fiber ring (FR) or a whispering gallery mode (WGM) resonator with two couplers along its loop is referred to be in the add-drop configuration, in analogy with the add-drop multiplexer in telecom networks. Both for practical applications as well as in several fundamental studies involving high-Q resonators, this configuration is of great interest and the assessment of the intrinsic properties of the resonator and of its interaction with the coupling systems is extremely important. We developed an original method able to fully characterize high-Q resonators in an add-drop configuration. The method is based on the study of the two cavity ringdown (CRD) signals, which are produced at the transmission and drop ports by wavelength sweeping a resonance in a time interval comparable with the photon cavity lifetime. All the resonator parameters can be assessed with a single set of simultaneous measurements. We implemented the model describing the two CRD profiles from which a best fit process of the measured profiles allows deducing the key parameters. We successfully validated the model with an experiment based on a FR resonator of known characteristics. Finally, we fully characterized a high-Q, home-made, MgF2 WGM disk resonator in the add-drop configuration, assessing its intrinsic and coupling parameters.
Whispering gallery mode resonators (WGMR) have attracted a great interest in the last decade. WGMR have been fabricated in different geometries, solid and hollow, spherical, toroidal, and bottled shaped. Hollow spherical WGMR or microbubble resonators (MBR) are the last arrived in the family of resonators. The approach used for their fabrication is based on surface tension driven plastic deformation on a pressurized capillary, similar to glassblowing. Using such technique we are able to fabricate large surface area and thin spherical shells with high quality factor (Q).
MBR are efficient phoxonic cavities that can sustain both optical photons and acoustic phonons. It has been demonstrated that MBR can be used to study Turing comb patterns (Kerr modulation) and Stimulated Brillouin Scattering (SBS). Radiation pressure is another mechanism that also leads to excitation of acoustic phonons with lower frequencies, in the range of hundreds of kHz to tens of MHz in the case of silica MBR. The frequency of such oscillations occurs very close to the mechanical eigenfrequencies of the cavity.
We have studied the temporal behavior of the cavity, the coexistence and the suppression of the oscillation while generating Turing comb patterns. The observed phenomenology can be explained by the geometrical characteristics of a MBR. MBRs are spheroidal WGM resonators with quite dense spectral characteristics. The total dispersion of MBR is anomalous and large, as expected for very large MBR. Thus, Kerr comb formation is allowed for all MBR used in this work.
The paper illustrates both review and original simulation results obtained via the modelling of different set-ups based on optical microresonators for applications in optical sensing, lasing and spectroscopy. Passive microbubbles and microspheres coupled via long period fiber gratings (LPGs) and tapered fibers are designed and/or constructed for sensing of biological fluids in the near infrared (NIR) wavelength range. Rare earth doped chalcogenide glass integrated microdisks are designed for active sensing in the medium infrared (MIR) wavelength range. A home-made numerical code modelling the optical coupling and the active behavior via rate equations of ion population is employed for a realistic design, by taking into account the most important active phenomena in rare earths, such as the absorption rates, the stimulated emission rates, the amplified spontaneous emission, the lifetime and branching ratios, the ion-ion energy transfers and the excited state absorption. Optical coupling is obtained by employing ridge waveguides, for micro-disks, and tapered fibers, for microspheres and microbubbles. Different dopant rare earths as Erbium (Er3+) and Praseodymium (Pr3+) are considered.
Surface tension induced whispering gallery mode (WGM) micro-resonators can be made in glass with very high quality factor Q. In fact, low losses amorphous glassy dielectrics can be easily shaped in high-surface-quality spheroids by thermal reflow. Since the pioneering works on fused silica microspheres showing several orders of magnitude higher Qs compared to previous findings, a large number of studies have been performed in the last years on glass based microresonators. Main results include frequency conversion through non-linear effects and micro-lasers, filtering and optical switching, RF photonics and sensing. Besides spheres, alternatives shapes like micro-bottles and micro-bubbles have been implemented to improve the resonator performances depending on the application. Other glasses rather than silica have been considered in order to enhance properties like transparency windows and non-linear effects. This presentation will review the main results we obtained on micro-laser sources in erbium doped microcavities, parametric conversion in silica microspheres, and stimulated Brillouin scattering in silica microbubbles. Potentials of coated silica microspheres implemented to add the functionalities of the coating material will be also presented.
The integration of the Whispering Gallery Modes (WGMs) resonators in a microfluidics platform represents an important feature towards the realization of a compact high performance label-free biosensor. These hollow resonant microstructures present the advantage to combine the WGM resonator properties with the intrinsic capability of integrated microfluidics. In this sense, optical microbubble resonators (OMBRs), intended as a hollow core spherical bulge realized in a glass microcapillary by a suitable fabrication process, with their high Q factors (< 107 in air) well satisfy this requirement. Their operation is based on the fact that, given a small enough wall thickness of the bubble, the WGM optical field extends on both sides of the wall, so that it is possible to couple light into the resonator from an outer waveguide, and at the same time to have interaction of the WGM field with the inner fluid and analyte.
The biosensing mechanism of these devices is based on the WGMs morphological dependence: any change on the OMBR inner surface, due to some chemical and/or biochemical binding, causes a shift of the resonance position and reduces the Q factor of the OMBR. By measuring these changes, important information about the sensing capability of the device can be obtained.
In order to develop an OMBR based biosensor and optimize its performance, a crucial step is represented by its chemical/biochemical functionalization. Here we present a novel technique able to guarantee that the chemical interaction occurs in the OMBR inner wall, leaving the other microfluidic parts completely inert from a biochemical point of view. The method is based on UV photoactivation, which allows to localize the biolayers only in correspondence of the OMBR inner wall. As a proof of concept, an immunoassay based on rabbit IgG/anti rabbit-IgG interaction was performed and. The anti rabbit-IgG antibody was labelled with Alexa Fluor 488 to verify, by a fluorescence characterization, the goodness of this procedure. Moreover, an anti mouse-IgG, labelled with the same fluorophore (Alexa Fluor 488) was used for specificity-tests of the IgG/anti-IgG interaction.
The immunoassay based on fluorescence was characterized using an optical microscope (Zeiss AXIO inverted fluorescence microscope) working at the wavelengths of 470 nm for excitation of Alexa Fluor 488.
The real time measurement of the resonance broadening after each functionalization step together with the high Q factor (< 105) measured after the IgG/anti-IgG interaction in water, gives a further proof for the method validity.
We report efficient generation of nonlinear phenomena related to third order optical non-linear susceptibility χ(3) interactions in resonant silica microspheres and microbubbles in the regime of normal dispersion. The interactions here reported are: Stimulated Raman Scattering (SRS), and four wave mixing processes comprising Stimulated Anti-stokes Raman Scattering (SARS) and comb generation. Unusually strong anti-Stokes components and extraordinarily symmetric spectra have been observed. Resonant SARS and SRS corresponding to different Raman bands were also observed. The lack of correlation between stimulated anti-stokes and stokes scattering spectra indicates that the signal has to be resonant with the cavity.
In order to optimize the performance of an optical microbubble resonator (OMBR) as biosensor, the chemical functionalization of its inner surface plays a key role. Here we report on a spatially selective photo – chemical procedure able to bind fluorescent biomolecules only in correspondence of the OMBR inner surface. This abruptly reduces the occurrence of an undesired specific biochemical bond event all along the microfluidic section of the device. The evidence of this method, which maintains high Q factor (> 105) for the OMBR in buffer solution, is proved by fluorescence microscopy and real time measurement of the resonance broadening.
A novel optical fiber coupler to whispering gallery mode (WGM) micro-resonators, which allows frequency selective addressing of different micro-resonators along the same fiber, is proposed. The coupling unit is based on a pair of identical long period fiber gratings (LPGs) and a thick adiabatic taper (>15 μm in waist) in between, where evanescent coupling from cladding modes to WGMs takes place. This robust unit can be replicated more times along the same fiber, simply cascading LPGs with different bands. Independent addressing of two different resonators along the same fiber is demonstrated.
We present linear and non linear optical applications of solid and hollow silica microresonators. Hollow microresonators or microbubble resonators combine the unique properties of whispering gallery mode resonators (WGMR) with the intrinsic capability of integrated microfluidics. Microbubbles were filled with water and aqueous solutions of ethanol in order to test the refractive index sensing capabilities of such resonators. We also tested the potential of microbubbles as nonlinear enhancement platform of both the filling material (Rhodamine 6G and Fluorescein) and the glass bubble.
Recently, optical micro-bubble resonators (OMBRs) have gained an increasing interest in many fields of photonics thanks to their particular properties. These hollow microstructures can be suitable for the realization of label – free optical biosensors by combining the whispering gallery mode (WGM) resonator properties with the intrinsic capability of integrated microfluidics. In fact, the WGMs are morphology-dependent modes: any change on the OMBR inner surface (due to chemical and/or biochemical binding) causes a shift of the resonance position and reduces the Q factor value of the cavity. By measuring this shift, it is possible to obtain information on the concentration of the analyte to be detected. A crucial step for the development of an OMBR-based biosensor is constituted by the functionalization of its inner surface. In this work we report on the development of a physical and chemical process able to guarantee a good homogeneity of the deposed bio-layer and, contemporary, to preserve a high quality factor Q of the cavity. The OMBR capability of working as bioassay was proved by different optical techniques, such as the real time measurement of the resonance broadening after each functionalization step and fluorescence microscopy.
We report the non linear fluorescence real-time detection of labeled IgG covalently bonded to the surface of a microspherical whispering gallery mode resonator (WGMR). The immunoreagents have been immobilized onto the surface of the WGMR sensor after being activated with an epoxy silane and an orienting layer. The developed immunosensor presents great potential as a robust sensing device for fast and early detection of immunoreactions. We also tested the potential of microbubbles as nonlinear enhancement platform. The dyes used in these studies are tetramethyl rhodamine isothiocyanate and Rhodamine 6G. All measurements were performed in a modified confocal microscope.
A novel method based on long period fiber gratings (LPGs) for coupling light to high-Q silica whispering gallery mode
(WGM) resonators is presented. An LPG couples the fundamental mode of a fiber to higher order LP cladding modes at
selected frequencies. At an adiabatically tapered section of the fiber following the LPG we demonstrated effective
coupling of these cladding modes to WGMs both in silica microspheres and microbubbles. The taper is about one order
of magnitude thicker than standard tapers used for the same purpose. Therefore this new method offers improved
robustness for practical applications.
Full exploitation of the unique properties of high quality factor micro-optical Whispering Gallery Mode (WGM) resonators requires a controllable and robust coupling of the light to the cavity, either for fundamental investigations or even more for practical applications. Fiber tapers are ideal phase-and-mode-matched couplers and are typically used for lab demonstrations in silica based micro-resonators or in low-index crystalline disks. Prism-based coupling basically adapts to any material and offers improved robustness and reliability for the implementation of devices based on larger resonators. We present the results of our studies on alternative methods based on integrated waveguides with specific reference to the coupling to lithium niobate disk resonators. We also demonstrate efficient coupling from fiber tapers to higher order azimuthal modes in coated microspheres and for third harmonic generation in silica microspheres. We finally propose a new method based on fiber gratings for improved robustness in biosensing applications.
High quality factor whispering gallery mode microresonators are ideally suited for nonlinear optical interactions. We demonstrate x(3)-based nonlinear interactions in silica microspheres, consisting in third harmonic generation and Raman assisted TSFG in the visible. A tunable, CW multicolour emission has been quantitatively measured controlling the cavity mode dispersion by choosing suitable sized microspheres and exciting the proper modes for efficient frequency conversion.
An ideal diagnostic device should be inexpensive, easy-to-use, rapid and reliable. Nanostructured porous silicon (PSi) satisfies these criterions including label-free optical detection and high throughput detection. Pore morphology (size, porosity) must be tailored for each specific application, and for immunosensing applications PSi morphology has been optimized for maximal pore infiltration of larger proteins as immuno gamma globlulin (IgG). Sensor degradation by high salt concentration induces a baseline drift. Different thermal oxidation procedures have been studied in order to obtain a stable sensor in the 3 hour incubation period of the immunoassay with negligible drift
We present results on the implementation of Whispering Gallery Modes (WGM) biosensors and on the demonstration of a new detection method for WGM based sensors. We first present a functionalization procedure based on the DNA-aptamer sequence immobilization on WGM resonators, able to recognize specifically thrombin protein. The protein binding was optically characterized in terms of specificity in buffer solution and in 10% diluted human serum. When performing the above measurements, we have used the typical detection scheme for WGM resonator based sensors, which relies on tracking the resonance shift − by scanning with a tunable laser − when a change of the refractive index in the region probed by the WGM takes place. In the second part of the presentation we propose a new sensing approach based instead on monitoring the position of the laser line of a fiber ring laser having a WGM microsphere in its loop. We demonstrate that the induced shift is the same for the ring laser line and for the microsphere resonance. The proposed method requires simpler and cheaper equipment and may also improve the sensor resolution because the ring laser line is narrower than the microsphere WGM resonance.
Microbubble resonators (MBRs) combine the unique properties of whispering gallery mode resonators with the intrinsic
capability of integrated microfluidics. Here an improved fabrication method of MBRs is presented, based on the heating
of a slightly pressurized capillary by a rotating arc discharge. Rotation of the electrodes ensures an homogeneous
distribution of the heat all over the capillary surface. The demonstrated MBRs have Q factors up to 107 at 773 nm.
Microbubbles were filled with water and aqueous solutions of ethanol in order to test the refractive index sensing
capabilities of such resonators, which also show a good temporal stability.
In order to fully exploit the unique properties of micro-optical resonators with whispering gallery modes (WGMs), both
for fundamental investigations as well as for practical applications, a critical point is an efficient, controllable, and robust
coupling of the light to the cavity WGMs. We present the results of our studies on phase-matched evanescent field
couplers, with particular reference to the coupling to high-index crystalline resonators like lithium niobate disks. We
focus on couplers based on different types of waveguide configurations and include demonstration of optical coupling to
high-Q lithium niobate resonators from integrated planar waveguides as well as from angle polished waveguides. These
systems are all in guided optics architectures. We also briefly present our recent achievements in the development of
microbubble resonators fabricated from silica capillaries. We show that, as high-Q hollow 3-D cavities with intrinsic
microfluidics, these resonators represent a promising biochemical sensing platform.
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