Micro-combs: A novel generation of optical sources

Detection principles of biological and chemical FET sensors.

A cavity optomechanical magnetometer is demonstrated. The magnetic-field-induced expansion of a magnetostrictive material is resonantly transduced onto the physical structure of a highly compliant optical microresonator and read out optically with ultrahigh sensitivity. A peak magnetic field sensitivity of $400\text{ }\text{ }\mathrm{nT}\text{ }{\mathrm{Hz}}^{\ensuremath{-}1/2}$ is achieved, with theoretical modeling predicting the possibility of sensitivities below $1\text{ }\text{ }\mathrm{pT}\text{ }{\mathrm{Hz}}^{\ensuremath{-}1/2}$. This chip-based magnetometer combines high sensitivity and large dynamic range with small size and room temperature operation.

Cavity optomechanical magnetometer.

Ultrasensitive nanoparticle detection holds great potential for early-stage diagnosis of human diseases and for environmental monitoring. In this work, we report for the first time, to our knowledge, single nanoparticle detection by monitoring the beat frequency of split-mode Raman lasers in high-Q optical microcavities. We first demonstrate this method by controllably transferring single 50-nm–radius nanoparticles to and from the cavity surface using a fiber taper. We then realize real-time detection of single nanoparticles in an aqueous environment, with a record low detection limit of 20 nm in radius, without using additional techniques for laser noise suppression. Because Raman scattering occurs in most materials under practically any pump wavelength, this Raman laser-based sensing method not only removes the need for doping the microcavity with a gain medium but also loosens the requirement of specific wavelength bands for the pump lasers, thus representing a significant step toward practical microlaser sensors.

https://www.pnas.org/content/pnas/111/41/14657.full.pdf

Single nanoparticle detection using split-mode microcavity Raman lasers

Optical Whispering Gallery Modes (WGMs) derive their name from a famous acoustic phenomenon of guiding a wave by a curved boundary observed nearly a century ago. This phenomenon has a rather general nature, equally applicable to sound and all other waves. It enables resonators of unique properties attractive both in science and engineering. Very high quality factors of optical WGM resonators persisting in a wide wavelength range spanning from radio frequencies to ultraviolet light, their small mode volume, and tunable in- and out- coupling make them exceptionally efficient for nonlinear optical applications. Nonlinear optics facilitates interaction of photons with each other and with other physical systems, and is of prime importance in quantum optics. In this paper we review numerous applications of WGM resonators in nonlinear and quantum optics. We outline the current areas of interest, summarize progress, highlight difficulties, and discuss possible future development trends in these areas.

https://iopscience.iop.org/article/10.1088/2040-8978/18/12/123002/pdf

Nonlinear and quantum optics with whispering gallery resonators

Label-free sensors based on electrical, mechanical and optical transduction methods have potential applications in numerous areas of society, ranging from healthcare to environmental monitoring. Initial research in the field focused on the development and optimization of various sensor platforms fabricated from a single material system, such as fiber-based optical sensors and silicon nanowire-based electrical sensors. However, more recent research efforts have explored designing sensors fabricated from multiple materials. For example, synthetic materials and/or biomaterials can also be added to the sensor to improve its response toward analytes of interest. By leveraging the properties of the different material systems, these hybrid sensing devices can have significantly improved performance over their single-material counterparts (better sensitivity, specificity, signal to noise, and/or detection limits). This review will briefly discuss some of the methods for creating these multi-material sensor platforms and the advances enabled by this design approach.

/pdf/hybrid-integrated-label-free-chemical-and-biological-sensors-2l41a5etof.pdf

Hybrid Integrated Label-Free Chemical and Biological Sensors

Over the past several decades, silica sol-gel materials have attracted significant interest from the optics community because of their extremely versatile synthesis method. Because silica sol gels are fabricated using liquid precursors, dopants can be directly and uniformly incorporated into the silica matrix. Additionally, judicious selection of the dopant material and sol-gel catalyst allows the refractive index of the final silica film to be tuned over a wide range. Tuning the refractive index of silica materials enables the direct integration of silica devices on a silicon substrate, benefiting applications in telecommunications and integrated optics. While previous materials characterizations studies have focused primarily on the near-IR, given the rapidly emerging field of biophotonics, it is equally important to understand how these materials behave at visible wavelengths. In the present work, thin silica sol-gel films are formed from either tetraethyl orthosilicate (TEOS) or methyltriethoxysilane (MTES) with titanium butoxide (Ti(OBu)4). We characterized the basic material properties using Fourier Transform Infrared Spectroscopy (FTIR) and ellipsometry. In addition, by spin-coating the sol gel films onto optical resonant cavities, we determined the thermo-optic coefficient and the transmission loss of the material at both visible and near-IR wavelengths. The addition of titanium allows the films’ refractive index, material loss, and thermo-optic coefficient to be tuned, making these films useful for integrated optics and sensing applications.

Characterization of thermo-optic coefficient and material loss of high refractive index silica sol-gel films in the visible and near-IR

Whispering gallery mode microcavities are ideally suited to form microlaser devices because the high circulating intensity within the cavity results in ultralow lasing thresholds. However, to achieve low-threshold Raman lasing in silica devices, it is necessary to have quality factors above 100 million. One approach to circumvent this restriction is to intercalate a sensitizer into the silica, which increases the Raman gain. In the present work, we demonstrate a Raman laser based on a titanium sensitized silica solgel coated toroidal microcavity. By tuning the concentration of the Ti, the Raman efficiency improves over 3× while maintaining sub-mW thresholds.

Titanium-enhanced Raman microcavity laser.

Waveguiding structures are one of the fundamental components of integrated photonic circuitry. Devices with low loss and a linear response across a wide wavelength range are especially desirable. In the present Letter, we have successfully developed and characterized low-loss silica waveguides integrated on a silicon substrate with a novel suspended cylinder geometry. The unique design creates a device that is effectively air clad, resulting in a large refractive index contrast for improved optical field confinement. The measured loss is constant from 658 to 1550 nm, and it is independent of the polarization of the input light and the input power.

Low-loss silica-on-silicon waveguides.

Optical microcavity sensors have demonstrated success in detecting analytes with high sensitivity. Typically, the sensor output is monitored using oscilloscopes or optical spectrum analyzers; however, these instruments can significantly limit the sensing performance. In the present work, we address this limitation by developing a neodymium-doped toroid microlaser and heterodyning it with a reference laser. The resulting beat signal is analyzed on an electrical spectrum analyzer. By performing comparative temperature sensing experiments, we demonstrate that the heterodyning improves the detection limit over 60-fold while also improving the temporal resolution and the signal to noise ratio up to 50-fold.

Ashley J. Maker

Papers

Hybrid Integrated Label-Free Chemical and Biological Sensors

Characterization of thermo-optic coefficient and material loss of high refractive index silica sol-gel films in the visible and near-IR

Titanium-enhanced Raman microcavity laser.

Low-loss silica-on-silicon waveguides.

Heterodyned toroidal microlaser sensor