UV-curable adhesive microsphere whispering gallery mode resonators

We report the fabrication and optical characterization of spherical whispering gallery mode(WGM) resonators made from ultraviolet(UV)-curable adhesive. The fabricated microspheres have good sphericity and surface smoothness, and can directly adhere to the tip of half-tapered fibers for easy manipulation. WGMs are e?ciently excited in the microsphere using an evanescent field of the tapered silica optical fibers. Resonances with quality factors of 1.3×10 5 are observed. The dependence of wavelength shifts of WGM resonances on the input light powers shows that the resonant wavelength of the proposed microsphere resonators can be tuned thermo-optically.

Surface-enhanced Raman scattering on dielectric microspheres with whispering gallery mode resonance

Conventionally, metallic nanostructures are used for surface-enhanced Raman spectroscopy(SERS), but recently there has been increasing interest in the enhancement of Raman scattering from dielectric substrates due to their improved stability and biocompatibility compared with metallic substrates. Here, we report the observation of enhanced Raman scattering from rhodamine 6 G molecules coated on silica microspheres. We excite the whispering gallery modes(WGMs) supported in the microspheres with a tapered fiber coupler for efficient WGM excitation, and the Raman enhancement can be attributed to the WGM mechanism. Strong resonance enhancement in pump laser intensity and modified Raman emission from the Purcell effect in the microsphere resonator are observed from the experiment and compared with theoretical results. A total Raman enhancement factor of 1.4 × 10~4 is observed, with contribution mostly from the enhancement in pump laser intensity. Our results show that, with an efficient pumping scheme, dielectric microspheres are a viable alternative to metallic SERS substrates.

Impact of nanoparticle-induced scattering of an azimuthally propagating mode on the resonance of whispering gallery microcavities

Optical whispering gallery microcavities with high-quality factors have shown great potential toward achieveing ultrahigh-sensitivity sensing up to a single molecule or nanoparticle, which raises a huge demand on a deep theoretical insight into the crucial phenomena such as the mode shift, mode splitting, and mode broadening in sensing experiments. Here we propose an intuitive model to analyze these phenomena from the viewpoint of the nanoparticle-induced multiple scattering of the azimuthally propagating mode(APM). The model unveils explicit relations between these phenomena and the phase change and energy loss of the APM when scattered at the nanoparticle; the model also explains the observed polarization-dependent preservation of one resonance and the particle-dependent redshift or blueshift. The model indicates that the particle-induced coupling between the pair of unperturbed degenerate whispering gallery modes(WGMs) and the coupling between the WGMs and the free-space radiation modes, which are widely adopted in current theoretical formalisms, are realized via the reflection and scattering-induced free-space radiation of the APM, respectively, and additionally exhibits the contribution of cross coupling between the unperturbed WGMs and other different WGMs to forming the splittingresonant modes, especially for large particles.

All-silicon low-loss THz temporal differentiator based on microring waveguide resonator platform

Microring resonators have been widely used in passive optical devices such as wavelength division multiplexers,differentiators,and integrators.Research on terahertz(THz)components has been accelerated by these photonics technologies.Compact and integrated time-domain differentiators that enable low-loss,high-speed THz signal processing are necessary for THz applications.In this study,an on-chip THz temporal differentiator based on all-silicon photonic technology was developed.This device primarily consisted of a microring waveguide resonator and was packaged with standard waveguide compatibility.It performed time-domain differentiation on input signals at a frequency of 405.45 GHz with an insertion loss of 2.5 dB and a working bandwidth of 0.36 GHz.Various periodic waveforms could be handled by this differentiator.This device could work as an edge detector,which detected step-like edges in high-speed input signals through differential effects.This development holds significant promise for future THz data processing technologies and THz communication systems.

Photonic molecules with a tunable inter-cavity gap

Optical micro-resonators have broad applications.They are used,for example,to enhance light–matter interactions in optical sensors or as model systems for investigating fundamental physical mechanisms in cavity quantum electrodynamics.Coupling two or more micro-cavities is particularly interesting as it enlarges the design freedom and the field of application.In this context,achieving tunability of the coupling strength and hence the inter-cavity gap is of utmost importance for adjusting the properties of the coupled micro-resonator system.In this paper,we report on a novel coupling approach that allows highly precise tuning of the coupling gap of polymeric micro-resonators that are fabricated side by side on a common substrate.We structure goblet-shaped whispering-gallery-mode resonators on an elastic silicone-based polymer substrate by direct laser writing.The silicone substrate is mechanically stretched in order to exploit the lateral shrinkage to reduce the coupling gap.Incorporating a laser dye into the micro-resonators transforms the cavities into micro-lasers that can be pumped optically.We have investigated the lasing emission by micro-photoluminescence spectroscopy,focusing on the spatial localization of the modes.Our results demonstrate the formation of photonic molecules consisting of two or even three resonators,for which the coupling strengths and hence the lasing performance can be precisely tuned.Flexibility and tunability are key elements in future photonics,making our approach interesting for various photonic applications.For instance,as our coupling approach can also be extended to larger cavity arrays,it might serve as a platform for tunable coupled-resonator optical waveguide devices.