Thermal properties of a photonic resonator, determined by both intrinsic properties of materials and the geometry and structure of the resonator, play important roles in various applications including radiation detection, biosensing, and microlaser. In this work, we propose and demonstrate a method to measure the thermal relaxation time and thermal conductance of an optical microresonator. The method utilizes the optothermal effect of two nearby optical modes in the transmission spectrum of the same resonator to extract the thermal properties of the resonator. We show that the thermal relaxation time, as well as thermal conductance, can be tailored by changing the geometric parameters of the resonator. Furthermore, we provide an analytical model that can be used to estimate the thermal relaxation time of a microtoroid resonator given its geometric parameters. The experimental results agree well with the analytical predictions. Our method can be exploited to characterize and optimize the thermal properties of other types of optical microresonators.
Whispering-Gallery-Mode (WGM) resonators are emerging as an excellent platform to study optical phenomena resulting from enhanced light-matter interactions due to their superior capability to confine photons for extended periods of time. The monolithic fabrication process to achieve ultra-high-Q WGM resonators without the need to align multiple optical components, as needed in traditional design of resonators based on precise arrangement of mirrors, is especially attractive. Here we explain how to process a layer of thin film doped with optical gain medium, which is prepared by wet chemical synthesis, into WGM structures on silicon wafer to achieve arrays of ultra-low threshold on-chip microlasers. We can adjust the dopant species and concentration easily by tailoring the chemical compositions in the precursor solution. Lasing in different spectral windows from visible to infrared was observed in the experiments. In particular, we investigated nanoparticle sensing applications of the on-chip WGM microlasers by taking advantages of the narrow linewidths and the splitting of lasing modes arising from their interactions with nano-scale structures. It has been found that a nanoparticle as small as ten nanometers in radius could split a lasing mode in a WGM resonator into two spectrally separated lasing lines. Subsequently, when these lasing lines are photo-mixed at a photodetector a heterodyne beat note is generated which can be processed to signal the detection of individual nanoparticles. We have demonstrated detection of virions, dielectric and metallic nanoparticles by monitoring the changes in this self-heterodyning beat note of the split lasing modes. The built-in self-heterodyne method achieved in this monolithic WGM microlaser provides an ultrasensitive scheme for detecting and measuring nanoparticles at single particle resolution, with a theoretical detection limit of one nanometer.
Whispering-Gallery-Mode (WGM) microresonators have shown great promise for ultra-sensitive and label-free chemical
and biological sensing. The linewidth of a resonant mode determines the smallest resolvable changes in the WGM
spectrum, which, in turn, affects the detection limit. The fundamental limit is set by the linewidth of the resonant mode
due to material absorption induced photon loss. We report a real-time detection method with single nanoparticle
resolution that surpasses the detection limit of most passive micro/nano photonic resonant devices. This is achieved by
using an on-chip WGM microcavity laser as the sensing element, whose linewidth is much narrower than its passive
counterpart due to optical gain in the resonant lasing mode. In this microlaser based sensing platform, the first binding
nanoparticle induces splitting of the lasing line, and the subsequent particles alter the amount of splitting, which can be
monitored by measuring the beat frequency of the split modes. We demonstrate detection of polystyrene and gold
nanoparticles as small as 15 nm and 10 nm in radius, respectively, and Influenza A virions. The built-in self-heterodyne
interferometric method achieved in the monolithic microlaser provides a self-referencing scheme with extraordinary
sensitivity, and paves the way for detection and spectroscopy of nano-scale objects using micro/nano lasers.
Ultra-sensitive and label-free chemical and biological sensing devices are of great importance to biomedical research,
clinical diagnostics, environmental monitoring, and homeland security applications. Optical sensors based on ultra-highquality
Whispering-Gallery-Mode (WGM) micro-resonators, in which light-matter interactions are significantly
enhanced, have shown great promise in achieving compact sensors with high sensitivity and reliability. However,
traditional sensing mechanisms based on monitoring the frequency shift of a single resonance faces challenges since the
resonant frequency is sensitive not only to the sensing targets but also to many types of disturbances in the environment,
such as temperature variation and mechanical instability of the system. The analysis of the signals is also affected by the
positions of sensing targets on the resonator. Thus, it is difficult to distinguish signals coming from different sources,
which introduces 'false positive' detection. We report a novel self-reference sensing mechanism based on mode splitting,
a phenomenon in which a high-quality optical mode in a WGM resonator splits into two modes due to intra-cavity
Rayleigh scattering. In particular, we demonstrated that the two split modes that can be induced by a single nanoparticle
reside in the same resonator and serve as a reference to each other. As a result, a self-reference sensing scheme is
formed. This allows us to develop a position-independent sensing scheme to accurately estimate the sizes of
nanoparticles. So far we have achieved position-independent detecting and sizing of single nanoparticles down to 20 nm
in radius with a single-shot measurement using an on-chip high-quality WGM microtoroid resonator.
Optical microcavities with high quality factors (Q factor) and small mode volumes have shown their potentials in various
sensing applications. Here we experimentally demonstrate the real-time detection of single nanoparticles down to 30 nm
in radius, using an ultra-high-Q microtoroid on a silicon chip. Mode splitting phenomenon of WGMs caused by their
strong interactions with a single nanoparticle is utilized as the sensing signal. Frequency and linewidth information of
the split modes is used to accurately derive the size of the particle detected. Theoretical calculations and finite element
simulations are in good agreement with the experimental results. The mode splitting technique provides a self-reference
scheme that is more immune to noise than the techniques based on the detection of changes of a single mode.
Whispering gallery mode (WGM) optical microcavities trap light in micro-scale volumes by continuous total internal
reflection which leads to enhancement of light intensity within a confined region and longer photon lifetime.
Consequently, light-matter interaction is enhanced making the WGM resonator an extremely sensitive platform for the
detection of perturbations in and around the resonator. Here, we report mode-splitting in monolithic ultra-high-Q WGM
microcavities for real-time and in-situ detection of single nanoparticles. We investigate experimentally and theoretically
particle detection and sizing at single nanoparticle resolution using the mode-splitting technique. Theoretical calculations
are in good agreement with the experimental results. The mode-splitting effect provides a <i>'self-reference sensing'</i>
technique that can overcome the limitations of current resonator-based sensors and in the meantime keep the advantages
offered by resonant structures for high-performance sensing.