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In this work, we discuss and experimentally investigate the whispering gallery modes (WGMs) in a 500 μm radius silicon sphere. We begin by reviewing the basics of WGM resonators, followed by simulations and experimental results obtained with the aforementioned silicon spherical WGM resonator. The recorded WGM signatures in the transmission and scattering spectra excited, in the near-infrared (near-IR) region, agree well with the simulations. Thermo-optic tuning of these WGMs is achieved by introducing a pump laser in the violet-blue region. Red-shifts in wavelength are observed, which increase with the increase in the pump power. A shift of 0.63 nm is observed at a pump power of 6.32 mW. We also study switching by analysing the transient response of the WGM spectra. The measured WGMs exhibit high quality factors ($Q\sim 10^5$Q∼105), which along with the aforesaid tunability, demonstrates the potential of spherical silicon resonator as a platform for photonic applications, e.g. sensing and communication. In summary, this work contributes towards the understanding of the fundamental physics of WGMs and provides insights into silicon WGM resonators. [ABSTRACT FROM AUTHOR]

A novel micron-range displacement sensor based on a whispering-gallery mode (WGM) microcapillary resonator filled with a nematic liquid crystal (LC) and a magnetic nanoparticle- coated fiber half-taper is proposed and experimentally demonstrated. In the proposed device, the tip of a fiber half-taper coated with a thin layer of magnetic nanoparticles (MNPs) moves inside the LC-filled microcapillary resonator along its axis. The input end of the fiber half-taper is connected to a pump laser source and due to the thermo-optic effect within the MNPs, the fiber tip acts as point heat source increasing the temperature of the LC material in its vicinity. An increase in the LC temperature leads to a decrease in its effective refractive index, which in turn causes spectral shift of the WGM resonances monitored in the transmission spectrum of the coupling fiber. The spectral shift of the WGMs is proportional to the displacement of the MNP-coated tip with respect to the microcapillary's light coupling point. The sensor's operation is simulated considering heat transfer in the microcapillary filled with a LC material having a negative thermo-optic coefficient. The simulations are in a good agreement with the WGMs spectral shift observed experimentally. A sensitivity to displacement of 15.44 pm/µm and a response time of 260 ms were demonstrated for the proposed sensor. The device also shows good reversibility and repeatability of response. The proposed micro-displacement sensor has potential applications in micro-manufacturing, precision measurement and medical instruments. [ABSTRACT FROM AUTHOR]

Tunable high-refractive-index nanostructures are highly desired for realizing photonic devices with a compact footprint. By harnessing the large thermo-optic effect in silicon, we show reversible and wide thermal tuning of both the far- and near-fields of Mie resonances in isolated silicon nanospheres in the visible range. We perform in situ heating in a transmission electron microscope and electron energy-loss spectroscopy to show that the Mie resonances exhibit large spectral shifts upon heating. We leverage the spectral shifts to demonstrate near-field tuning between different Mie resonances. By combining electron energy-loss spectroscopy with energy-dispersive X-ray analysis, we show a reversible and stable operation of single silicon nanospheres up to a temperature of 1073 K. Our results demonstrate that thermal actuation offers dynamic near-field tuning of Mie resonances, which may open up applications in tunable nonlinear optics, Raman scattering, and light emission.

Millimeter-wave band opens new opportunities for ultra-high data rates and dense connectivity in forthcoming 5G and 6G wireless cellular networks. However, the major challenge is the significant propagation loss experienced by millimeter-wave. The beneficial solution to overcome this is to exploit beamforming with multiple antennas. In this paper, we propose a silicon-on-insulator waveguide technology based 1 × 2 integrated photonic beamformer for 28 GHz signal. The true time delay line is realized by utilizing micro-ring resonator. Ring resonator of 30 µm radius designed using single mode rib waveguide is considered. Firstly, the dimensions of waveguide to maintain single mode operation are estimated. Next, mathematical analysis and design of the beamforming structure are presented. True time delay of 12.46 ps is achieved, corresponding to a phase difference of 0.708π and beam pointing angle of 45.06˚. Finally, thermo-optic tuning is performed to incorporate continuous tunability feature into the delay device. By adjusting the power coupling coefficient of coupler, the designed beamformer can provide continuous tuning range of 0˚ to 60˚. [ABSTRACT FROM AUTHOR]

A novel micron-range displacement sensor based on a whispering-gallery mode (WGM) microcapillary resonator filled with a nematic liquid crystal (LC) and a magnetic nanoparticle- coated fiber half-taper is proposed and experimentally demonstrated. In the proposed device, the tip of a fiber half-taper coated with a thin layer of magnetic nanoparticles (MNPs) moves inside the LC-filled microcapillary resonator along its axis. The input end of the fiber half-taper is connected to a pump laser source and due to the thermo-optic effect within the MNPs, the fiber tip acts as point heat source increasing the temperature of the LC material in its vicinity. An increase in the LC temperature leads to a decrease in its effective refractive index, which in turn causes spectral shift of the WGM resonances monitored in the transmission spectrum of the coupling fiber. The spectral shift of the WGMs is proportional to the displacement of the MNP-coated tip with respect to the microcapillary’s light coupling point. The sensor’s operation is simulated considering heat transfer in the microcapillary filled with a LC material having a negative thermo-optic coefficient. The simulations are in a good agreement with the WGMs spectral shift observed experimentally. A sensitivity to displacement of 15.44 pm/µm and a response time of 260 ms were demonstrated for the proposed sensor. The device also shows good reversibility and repeatability of response. The proposed micro-displacement sensor has potential applications in micro-manufacturing, precision measurement and medical instruments.

We demonstrate continuous tuning of a second-order silicon microring filter over the full 7.8-nm FSR of the resonators using the thermo-optic effect. Strongly correlated linear relationships between the heater powers and the filter center wavelength were obtained even in the presence of thermal crosstalks in the resonant frequencies and coupling coefficients, which allowed the filter to be tuned to any arbitrary wavelength with an accuracy of ±27 pm without the need for further optimization. We also demonstrate the use of integrated on-chip thermistors for precise tracking of the local temperatures of the microring resonators, thereby providing an indirect mechanism for monitoring the center wavelength and spectral characteristics of the filter.

We report a 4th-order Vernier cascaded microring (VCMR) filter on Silicon-on-Insulator that can be continuously tuned over an effective free spectral range of 32 nm using the thermo-optic effect. The filter consists of two cascaded double-microring filter stages with free spectral ranges of 7.8 nm and 6.5 nm. The cascaded architecture allows the two stages to be thermally isolated and individually calibrated, which enables accurate tuning of the VCMR filter without further spectral optimization at each set wavelength. We achieved a wavelength tuning accuracy of ±15 pm over 32 nm and a 3-dB bandwidth variation of ±6.5%, which represent the most accurate tuning performance reported for silicon Vernier microring filters to date. In addition, integrated on-chip thermistors allow for precise tracking of the local temperatures of the microrings, thereby providing a mechanism for monitoring the filter performance. [ABSTRACT FROM AUTHOR]

We demonstrate continuous tuning of a second-order silicon microring filter over the full 7.8-nm FSR of the resonators using the thermo-optic effect. Strongly correlated linear relationships between the heater powers and the filter center wavelength were obtained even in the presence of thermal crosstalks in the resonant frequencies and coupling coefficients, which allowed the filter to be tuned to any arbitrary wavelength with an accuracy of ±27 pm without the need for further optimization. We also demonstrate the use of integrated on-chip thermistors for precise tracking of the local temperatures of the microring resonators, thereby providing an indirect mechanism for monitoring the center wavelength and spectral characteristics of the filter. [ABSTRACT FROM AUTHOR]

We investigated both theoretically and experimentally, the thermo-optic tuning of whispering-gallery modes (WGMs) in a microcapillary resonator filled with nematic liquid-crystal (LC). The tuning of WGMs was realized due to the photo-thermal effect of magnetic nanoparticles (MNPs) on the surface of a fiber half-taper connected to a pump laser source, resulting in temperature-induced refractive index (RI) changes of the LC material. Based on perturbation theory, we analyzed the influence the RI and the wall thickness on the sensitivity of the proposed tuning scheme. Furthermore, we experimentally demonstrated that increasing the thickness of the MNPs coating on the fiber taper surface leads to a stronger photo-thermal effect and to a larger RI change of the LC material within the microcapillary core. Thermo-optic tuning of WGM resonances with a sensitivity of 256.63 ± 5.67 pm/mW to the laser pump power and tuning range of 10.43 nm has been achieved. The developed thermo-optic tuning scheme has many potential applications as tunable devices, optical filters and sensors. • A thicker MNPs layer raises the sensitivity of thermo-optic tuning to pump power. • Capillary with thinner walls offers a better tuning performance. • A higher refractive index material in the capillary can increase sensitivity. [ABSTRACT FROM AUTHOR]

Ultracompact thermooptically tuned photonic crystal waveguide (PCW) silicon-on-insulator Mach-Zehnder interferometers (MZIs) have been proposed and fabricated. A novel thermal design was employed to improve the device switching performance. Both steady-state and transient thermal analyses were performed to evaluate the thermal performance of the thermooptic MZIs. A switching time less than 20 mus has been experimentally achieved, which clearly demonstrated the speed advantage using the new heating approach. The active length of the PCW-based MZIs was 80 mum, nearly one order of magnitude shorter than the conventional silicon waveguide-based MZIs. A maximum modulation depth of 84% for a switching power of 78 mW was obtained at a wavelength of 1548 nm [ABSTRACT FROM PUBLISHER]