K. Alexander McIntosh

Readout circuitry for continuous high-rate photon detection with arrays of InP Geiger-mode avalanche photodiodes

An asynchronous readout integrated circuit (ROIC) has been developed for hybridization to a 32x32 array of single-photon sensitive avalanche photodiodes (APDs). The asynchronous ROIC is capable of simultaneous detection and readout of photon times of arrival, with no array blind time. Each pixel in the array is independently operated by a finite state machine that actively quenches an APD upon a photon detection event, and re-biases the device into Geiger mode after a programmable hold-off time. While an individual APD is in hold-off mode, other elements in the array are biased and available to detect photons. This approach enables high pixel refresh frequency (PRF), making the device suitable for applications including optical communications and frequency-agile ladar. A built-in electronic shutter that de-biases the whole array allows the detector to operate in a gated mode or allows for detection to be temporarily disabled. On-chip data reduction reduces the high bandwidth requirements of simultaneous detection and readout. Additional features include programmable single-pixel disable, region of interest processing, and programmable output data rates. State-based on-chip clock gating reduces overall power draw. ROIC operation has been demonstrated with hybridized InP APDs sensitive to 1.06-μm and 1.55-μm wavelength, and fully packaged focal plane arrays (FPAs) have been assembled and characterized.

Large-Format Geiger-Mode Avalanche Photodiode Arrays and Readout Circuits

Over the past 20 years, we have developed arrays of custom-fabricated silicon and InP Geiger-mode avalanche photodiode arrays, CMOS readout circuits to digitally count or time stamp single-photon detection events, and techniques to integrate these two components to make back-illuminated solid-state image sensors for lidar, optical communications, and passive imaging. Starting with 4 × 4 arrays, we have recently demonstrated 256 × 256 arrays, and are working to scale to megapixel-class imagers. In this paper, we review this progress and discuss key technical challenges to scaling to large format.

Demonstration of terahertz water-vapor spectroscopy with a photomixer transceiver

Measurements are reported that demonstrate the first use of a photomixer-transceiver system for terahertz rotational spectroscopy of airborne molecules. The photomixer transmitter and receiver were coupled to free space with twin-slot antennas that were optimized for operation in the 1.1 - 1.7 THz range. Both atmospheric-pressure and low- pressure conditions were investigated.

Strong intersubband absorption by photogenerated carriers in quantum wells

Intersubband absorption spectra in the 10-micrometers region are measured between 4.2 and 290 K in four n-type GaAs/Al0.28Ga0.72As multiple-quantum-well samples. The carriers responsible for the absorption are generated by a cross-gap pump laser operating at 0.75 micrometers . The absorption strength per unit pump power is found to depend strongly on the background electron sheet density (sigma) B, and is greatest by far in a sample having (sigma) B approximately equals 4 X 1010 cm-2. By measuring the speed of response, the cause of the strong photoabsorption is found to be a long photoelectron lifetime. The sample with the strongest photoabsorption is used to make an efficient CO2-laser modulator.