Steven Huang

Highly sensitive detection of nanoparticles with a self-referenced and self-heterodyned whispering-gallery Raman microlaser

Recently optical whispering-gallery-mode resonators (WGMRs) have emerged as promising platforms to achieve label-free detection of nanoscale objects and to reach single molecule sensitivity. The ultimate detection performance of WGMRs are limited by energy dissipation in the material they are fabricated from. Up to date, to improve detection limit, either rare-earth ions are doped into the WGMR to compensate losses or plasmonic resonances are exploited for their superior field confinement. Here, we demonstrate, for the first time, enhanced detection of single-nanoparticle induced mode-splitting in a silica WGMR via Raman-gain assisted loss-compensation and WGM Raman lasing. Notably, we detected and counted individual dielectric nanoparticles down to a record low radius of 10 nm by monitoring a beatnote signal generated when split Raman lasing lines are heterodyne-mixed at a photodetector. This dopant-free scheme retains the inherited biocompatibility of silica, and could find widespread use for sensing in biological media. It also opens the possibility of using intrinsic Raman or parametric gain in other systems, where dissipation hinders the progress of the field and limits applications.Significance To date, loss compensation in optical microresonators has been done using rare-earth ions, which requires additional processing steps and costs and raises biocompatibility concerns. An alternative to integrating rare-earth ions for loss compensation is the use of intrinsic gain mechanisms such as Raman and parametric gain present in the materials from which resonators are fabricated. Here, we report the first implementation to our knowledge of Raman gain-induced loss compensation in silica whispering-gallery-mode (WGM) resonators for improved detection and the first demonstration to our knowledge of mode splitting in a WGM Raman microlaser for detecting and counting single nanoparticles down to 10 nm. This intrinsically self-referenced, self-heterodyned, and biocompatible scheme has enabled achieving record-high polarizability sensitivity (down to 3.82 × 10−6 μm3) without using plasmonic effects, passive or active stabilization, or frequency locking. Optical whispering-gallery-mode resonators (WGMRs) have emerged as promising platforms for label-free detection of nano-objects. The ultimate sensitivity of WGMRs is determined by the strength of the light–matter interaction quantified by quality factor/mode volume, Q/V, and the resolution is determined by Q. To date, to improve sensitivity and precision of detection either WGMRs have been doped with rare-earth ions to compensate losses and increase Q or plasmonic resonances have been exploited for their superior field confinement and lower V. Here, we demonstrate, for the first time to our knowledge, enhanced detection of single-nanoparticle-induced mode splitting in a silica WGMR via Raman gain-assisted loss compensation and WGM Raman microlaser. In particular, the use of the Raman microlaser provides a dopant-free, self-referenced, and self-heterodyned scheme with a detection limit ultimately determined by the thermorefractive noise. Notably, we detected and counted individual nanoparticles with polarizabilities down to 3.82 × 10−6 μm3 by monitoring a heterodyne beatnote signal. This level of sensitivity is achieved without exploiting plasmonic effects, external references, or active stabilization and frequency locking. Single nanoparticles are detected one at a time; however, their characterization by size or polarizability requires ensemble measurements and statistical averaging. This dopant-free scheme retains the inherited biocompatibility of silica and could find widespread use for sensing in biological media. The Raman laser and operation band of the sensor can be tailored for the specific sensing environment and the properties of the targeted materials by changing the pump laser wavelength. This scheme also opens the possibility of using intrinsic Raman or parametric gain for loss compensation in other systems where dissipation hinders progress and limits applications.

Inverted-wedge silica resonators for controlled and stable coupling.

Inverted-wedge silica resonators with Q ≥ 106 were fabricated using conventional semiconductor processing. Robust wide-range control of coupling was demonstrated by horizontally moving a fiber taper on the top surface of the resonator.

6.2 133Mpixel 60fps CMOS image sensor with 32-column shared high-speed column-parallel SAR ADCs

To realize next-generation highly realistic sensation broadcasting systems, the research and development of 8K ultrahigh-definition television (UHDTV) systems have been promoted. To realize 8K video cameras, 33Mpixel sensors [1-2] and a full-resolution camera system that uses three 33Mpixel sensors [3] have been reported. However, the weight of the camera with three sensors is over 40kg because the camera requires a large-format color-separation prism. To reduce the size of the camera, single-chip imaging is a promising approach, and a compact single-chip 8K camera that weighs only 2kg has already been developed using a color 33Mpixel CMOS image sensor [4]. However, a conventional single-chip camera has a lower image quality than a full-resolution camera because the total pixel count of the single-sensor camera is only one-third of that of the three-sensor camera, and pixel interpolation is required to configure a full-resolution image. In this paper, a 133Mpixel sensor that can be operated at 60fps to realize a full-resolution 8K single-chip camera is described. To achieve both high speed and suitable ADC resolution, 32-column multiplexing analog readout circuitry and 14b high-speed redundant successive approximation register (SAR) ADCs [5] are adopted. As a result, a full-size image with a data rate of 128.71Gb/s at 60fps has been captured.

Experimental color video capturing equipment with three 33-megapixel CMOS image sensors

We have been developing an ultra high definition television (UHDTV) system with a 7,680 horizontal by 4,320 vertical pixel resolution and a 60 Hz frame rate. This system, which is called Super Hi-vision (SHV), is expected to serve the next generation of broadcasting services. We have just completed the worlds first imaging equipment that is capable of capturing video at a full SHV resolution. In designing this equipment, we decided to develop three new devices, taking into account the camera performance and the ease of implementation. First, we developed a 33-megapixel CMOS image sensor. Its pixel size of 3.8 &mgr;m sq. retained the dynamic range of the sensor above 60 dB even with a 3-transistor pixel structure. Second, a fixed focal length lens was developed to create an adequate MTF right up to the limiting resolution of the sensor. Third, we developed a signal-processing device capable of handling 72 Gbps signals and cascading boards to expand the process. SHV images with a modulation of 20% at the Nyquist frequency were obtained by using these three key technologies.

Whispering gallery mode resonator sensor for in situ measurements of hydrogel gelation

Whispering gallery mode (WGM) resonators are compact and ultrasensitive devices, which enable label-free sensing at the single-molecule level. Despite their high sensitivity, WGM resonators have not been thoroughly investigated for use in dynamic biochemical processes including molecular diffusion and polymerization. In this work, the first report of using WGM sensors to continuously monitor a chemical reaction (i.e. gelation) in situ in a hydrogel is described. Specifically, we monitor and quantify the gelation dynamics of polyacrylamide hydrogels using WGM resonators and compare the results to an established measurement method based on rheology. Rheology measures changes in viscoelasticity, while WGM resonators measure changes in refractive index. Different gelation conditions were studied by varying the total monomer concentration and crosslinker concentration of the hydrogel precursor solution, and the resulting similarities and differences in the signal from the WGM resonator and rheology are elucidated. This work demonstrates that WGM alone or in combination with rheology can be used to investigate the gelation dynamics of hydrogels to provide insights into their gelation mechanisms.

Protein-based flexible whispering gallery mode resonators

The idea of creating photonics tools for sensing, imaging and material characterization has long been pursued and many achievements have been made. Approaching the level of solutions provided by nature however is hindered by routine choice of materials. To this end recent years have witnessed a great effort to engineer mechanically flexible photonic devices using polymer substrates. On the other hand, biodegradability and biocompatibility still remains to be incorporated. Hence biomimetics holds the key to overcome the limitations of traditional materials in photonics design. Natural proteins such as sucker ring teeth (SRT) and silk for instance have remarkable mechanical and optical properties that exceed the endeavors of most synthetic and natural polymers. Here we demonstrate for the first time, toroidal whispering gallery mode resonators (WGMR) fabricated entirely from protein structures such as SRT of Loligo vulgaris (European squid) and silk from Bombyx mori. We provide here complete optical and material characterization of proteinaceous WGMRs, revealing high quality factors in microscale and enhancement of Raman signatures by a microcavity. We also present a most simple application of a WGMR as a natural protein add-drop filter, made of SRT protein. Our work shows that with protein-based materials, optical, mechanical and thermal properties can be devised at the molecular level and it lays the groundwork for future eco-friendly, flexible photonics device design.

Surface enhanced Raman scattering in whispering gallery mode microresonators

In this talk I will discuss surface enhanced Raman scattering in silica microsphere resonators based on whispering gallery mode resonance. Recently silica microspheres have attracted attention as a novel substrate for surface enhanced Raman scattering. Whispering gallery mode resonance has been identified as a major enhancement mechanism, along with other effects such as photonic nanojets. In most of the previous experiments, however, free space pumping of the microsphere has been used, which has low efficiency in coupling to the whispering gallery modes. In our approach, we use a tapered fiber coupler for a highly efficient coupling to the whispering gallery modes. Coupling to the microresonator is monitored using a tunable laser. We observe both pump enhancement and Purcell enhancement in the microresonator. Since the linewidth of the whispering gallery modes is much smaller than that of the Raman peaks, sharp peaks corresponding to the whispering gallery modes are overlaid on top of the Raman spectrum of the bulk material. To demonstrate the system’s potential for Raman analysis, I will present the whispering gallery mode surface enhanced Raman spectrum of rhodamine 6G thin film coated on a microsphere resonator.

On-chip whispering-gallery-mode microlasers and their applications for nanoparticle sensing

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.