Kai Zhao

InGaAs single photon avalanche detector with ultralow excess noise

An InGaAs single photon avalanche detector capable of sub-Geiger mode (Photomultiplier-tube-like) operation is reported. The device achieves a stable gain at around 106. The gain fluctuation is greatly suppressed through a self-quenching effect, thus an equivalent excess noise factor as low as 1.001 is achieved. In the photon counting experiment, the device is operated in the nongated mode under a dc bias. Because of its unique characteristics of self-quenching and self-recovery, no external quenching circuit is needed. The device shows a single photon response of around 30ns and a self-recovery time of about 300ns.

Self-quenching and self-recovering InGaAs∕InAlAs single photon avalanche detector

To prevent device damage through thermal runaway, conventional III–V single photon avalanche diodes (SPADs) operate in gated mode where the device is biased above breakdown only for a short gating period. Here a free-running In0.53Ga0.47As∕InAlAs SPAD with built-in negative feedback mechanism is reported. A physical model is also developed to formulate the avalanche process with negative feedback. Introducing negative feedback enables the device to possess self-quenching and self-recovering capabilities. Such devices have demonstrated free-running single photon detection at 1550nm wavelength with single photon detection efficiency of 11.5%, dark count rate of 3.3M∕s, and a self-recovery time of 60ns at 160K.

Self-Quenched InGaAs Single-Photon Detector

The requirement for external quenching circuits adds substantially to the complexity and processing difficulty for InGaAs single-photon detectors, particularly in array configurations. Using bandgap engineering, we have developed InGaAs SPADs with self-quenching and self-recovering capabilities. The quenching process occurs in less than 100 ps, determined by the gain buildup time and the magnitude of device overbias. On the other hand, the recovery time is determined by the carrier escape time over an energy barrier that is typically tens of meVs. The recovery time can range from 1 ns to > 100 ns from the design of device and material structures. The optimal recovery time is a function of dark count rate and afterpulsing rate. Our data show that a recovery time of around 10 ns is near the optimum in most operation conditions. The self-quenched SPADs also show great suppression in excess noise, yielding a very uniform intensity distribution of output response to single photons. This unique property favors resolving photon number in an array device. As in conventional InGaAs SPADs, the single-photon detection efficiency increases with the amount of overbias (bias above breakdown voltage) and so does the dark count rate. A detection efficiency of 13-16% is obtained while still keeping the dark count and afterpulsing rates low. To our knowledge, the self-quenched InGaAs SPAD is the only device in its class to be able to operate under DC bias without gating or external circuits. As a result, the device is particularly suitable for array structures often used in communications, sensing, and imaging.

Microfluidic-photonic-dielectrophoretic integrated circuits for biophotonic sensing

This work demonstrates microfluidic-photonic-dielectrophoretic integrated circuits fabricated using a novel technology involving micromolding, polymer bonding, and channel waveguides with capillary filling. This represents a new class of circuits particularly attractive to lab-on-a-chip and biomedical applications.

Engineering of quantum dot emission wavelength using conductive layer coating

Semiconductor quantum dot with a conducting half-shell was studied experimentally and theoretically. We sputtered a thin layer of gold on the semispherical surface of CdSe∕ZnS quantum dots. At room temperature, the emission wavelength for the half-metal-coated quantum dots was found to be redshifted by 10nm (38meV) from the wavelength of uncoated quantum dots, indicating the change of excitonic binding energy due to the gold cap layer. A theoretical model is presented to explain this effect. The results suggest that coating the quantum dots with a conducting shell can change the emission color of the quantum dots. The technique can significantly increase the number of quantum dot fluorescent labels for simultaneous observation of the activities of multiple biomolecules.

Ultra Low Noise InGaAs Single Photon Detector with Transient Carrier Buffer

Sub-Geiger mode single photon detection with ultra low excess noise is demonstrated on an InGaAs based avalanche detector with transient carrier buffer.

Fast and power-efficient infrared single-photon upconversion using hot-carrier luminescence

We analyze a new method for single-photon frequency upconversion. This technique uses a byproduct of the avalanche process - electroluminescence resulting from hot-carrier recombination - as a means of upconversion. Because the spectrum of the emitted photons peaks near the bandgap of the multiplying material and has a significant tail at higher energies, it is possible to generate secondary photons at significantly higher energies than the primary absorbed photon. The secondary photons can then be detected by a coupled CMOS silicon single-photon avalanche diode (SPAD), where the information can also be processes. This upconversion scheme does not require any electrical connections between the detecting device and the silicon SPAD, so glass-to-glass bonding can be used, resulting in inexpensive, high-density arrays of detectors. We calculate the internal and system upconversion efficiencies, and show that the proposed scheme is feasible and highly efficient for application such as quantum key distribution and near infrared low-light-level imaging.

External electroluminescence measurements of InGaAs∕InAlAs avalanche photodiodes

The external efficiency of electroluminescence resulting from hot-carrier recombination has been studied in an InGaAs∕InAlAs avalanche photodiode. An analytical model that quantifies this emission is presented. Experimental data suggest that the emission originates from an intrinsic layer above the multiplication region. This electroluminescence mechanism offers a novel way for frequency upconversion, where the upconverted frequency can be controlled with proper choice of device layers. Lastly, we report for the first time the optical absorption properties of In0.52Al0.48As.

Optical measurement of torsional spring modulus of a double-stranded DNA using half-coated nanoparticles

A technique of detecting the rolling and spinning of half-coated nanoparticles using interference ring patterns of the fluorescence has been applied to the measurement of torsional spring modulus of a double-stranded DNA. Using the unique ability to measure nanoparticle rotations in multiple degrees of freedom, we were able to determine the spinning of a nanoparticle tethered on the DNA and thereby the twisting of the DNA in real time. The detailed knowledge of the spinning as well as rolling behaviors of half-coated nanoparticles provides information about torsional elastic properties of the DNA under investigation.