In recent years whispering gallery mode (WGM) resonators have attracted interest due to their various potential passive (filters, resonators, sensors) and active (lasers, four-wave mixing) applications. By choosing an appropriate material with very low absorption, and fabricating a very smooth surface, WGM resonators can reach ultra-high quality (Q) factors. Additionally, the surface of the WGM resonator can be functionalized with nanoparticles or nanomaterial layers, which can enhance optical properties. Recently, we have been interested in the functionalization of the WGM resonator surface for active applications. WGM resonators are suitable for nonlinear optical interactions due to their ultra-high Q factors, significantly lowering necessary pumping power. WGM resonators can be used to generate optical frequency combs (OFCs), which have many applications in optical clocks, spectroscopy, and communications. After coating WGM resonator with quantum dots, besides the OFC generation, we have observed the third harmonic generation. Functionalization with erbium ions leads to the observation of lasing.
We have created a highly sensitive humidity sensor that is based on glycerol droplet. Glycerol is hygroscopic and has a specific glycerol-water ratio for every relative humidity (RH) %. It is cheap and environmentally friendly. We use the droplet as a whispering gallery mode (WGM) microresonator. WGMs are known for their high Q factors, which lead to high sensitivity and precision. As RH changes, the resonant wavelength shifts due to a change in the droplet’s radius and refractive index. We have successfully created an experimental set-up and original data analysis method that allow us to follow the resonant wavelength shift in real-time. Results show that the sensor has an average sensitivity of 2.85 nm/% RH in the 50–70 % RH range, it is stable and has a long lifetime. To further investigate the properties of the glycerol droplet sensor, we tested its selectivity and tried two coupling methods (free-space and tapered fiber). We decided to test the sensor’s response to two different gases – ethanol and acetone. Results show that glycerol is highly selective and does not absorb ethanol/acetone molecules, meaning that it can be used for trustworthy humidity measurements.
The whispering gallery modes (WGM) micro resonators are based on elliptical objects, which can be made from optically transparent materials, The geometry of the object enables optical wave circulating inside the ellipse using total internal reflection. If there is a monochromatic light source with constant intensity to the ellipse, constructive interference may be observed. Poly methyl methacrylate acrylic (PMMA) WGM micro resonators are commercially available with typical optical quality factor of 103-104. These could limit problems with WGM micro resonator expensive manufacturing. Thanks to advances in high resolution image processing, read-outs using spectroscopy (single photo detector) could be replaced with image processing. Image processing (4.5μm/px) allows to split elliptical WGM micro resonator in regions and analyze separate sectors of the ellipse, which can used as a representation of surface irregularity interaction with higher order special mode groups. In the present work new type of image processing for micro-resonators were developed, to analyze intensity distribution in separate regions for the PMMA WGM micro-resonators (40-70 μm). Resonators were coupled using a tapered fiber and fixed wavelength VCSEL laser (760nm). Temperature was change from 20-80 0C which affected the PMMA refractive index (α), for 760nm dn/dT = -1.32 10-4 (0C-1) and thermal expansion (β) dR/dT = 2.60 10-4(0C-1). Combining the following physical changes, total changes (α+ β), WGM PMMA micro-resonator mode mapping was obtained. The following work offers new type of intensity processing methods for measuring applications using PMMA WGM micro resonators.
Whispering gallery mode resonators (WGMRs) are small axial symmetrical structures from transparent material, that can exceptionally well confine light within, thus making them ideal for studying light-matter interactions and using them as sensors. Various WGMR designs can be simulated using COMSOL Multiphysics. Sometimes an extra layer is coated on the surface of the resonator for achieving desirable effects. The extra layer changes quality factor of the resonator and ads extra modes for some frequencies. Different methods and studies are used for the exploration of this topic such as changing the thickness of the coating and using random functions to describe the roughness of the surface, which in micro and nanoscale makes a difference.
Optical frequency combs (OFC) using different kinds of whispering-gallery-mode (WGM) microresonators have already shown different applications and especially their applications in fiber optical communication systems as replacements of laser-arrays. For this application the free spectral range (FSR) of 200 GHz or less is desirable. Besides the fabrication material for microspheres, the resonator radius can be modified to change the FSR. In this paper use of silica microspheres for OFC represents an inexpensive alternative over the other microcombs: microring, microdisk, and microtoroid. We experimentally present a microsphere fabrication process from a different kind of silica (SiO2) fibers by use of the hydrogen-oxygen melting technique. We experimentally review the OFC generation process the main microresonator parameters as FSR, Q-factor and evaluate the resulting WGM resonator generated OFC comb light source for further applications. An OFC was excited inside a 166 μm silica microsphere WGM resonators using a 1548 nm laser light. The obtained broadband OFC spanned from 1400-1700 nm with FSR of (397 ± 10) GHz.
Whispering gallery mode resonators (WGMRs) are very interesting for sensing because a resonance shift could be caused by any perturbation of the surrounding environment. Additionally such a resonator is coated with nanomaterials to tailor and enhance the sensitivity for a specific purpose. WGMR were fabricated using standard telecommunication fiber and a hydrogen flame, characterized using the scan method to obtain the quality factors and then coated with gold nanoparticles (Au NPs) using dip coating method and characterized again for comparison. Au NPs were chosen because their positive impact on microresonator sensitivity has been mentioned before and the surface can later be functionalized. The deposited layer was investigated and new properties no over-coupling due to Van der Waals forces and suppression of higher order modes were observed after coating the resonator. To observe the localized surface plasmon resonance a glucose sensor test was performed using the WGMRs coated with Au NPs and glucose oxidase. Comparing the results with control measurements, the resonance shifted more for samples with Au NPs.
Whispering Gallery Mode (WGM) resonators are very sensitive to nanoparticles attaching to the surface. We simulate this process using COMSOL Wave Optics module. Our spherical WGM resonators are produced by melting a tip of an optical fiber and we measure optical Q factors in the 105 range. Molecular oxygen lines of the air in the 760 nm region are used as reference markers when looking for the shifts of the WGM resonance lines. We demonstrate WGM microresonator surface coating with a layer of ZnO nanorods as well as with polystyrene microspheres. Coatings produce increased contact surface. Additional layer of antigens/antibodies will be coated to make high-specificity biosensors.
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