Erik K. Duerr

Generation and detection of coherent terahertz waves using two photomixers

A general technique has been demonstrated at microwave and submillimeter-wave frequencies for photoconductive sampling in the frequency domain using photomixers and continuous-wave laser diodes. A microwave version in which two photomixers were coupled by a transmission line was developed to quantitatively test the concept from 0.05 to 26.5 GHz. A quasioptical version using antenna-coupled photomixers was demonstrated from 25 GHz to 2 THz. Such a system can outperform systems based on time-domain photoconductive sampling in frequency resolution, spectral brightness, system size, and cost.

InGaAsP/InP avalanche photodiodes for photon counting at 1.06 μm

Geiger-mode (photon-counting) operation at 1.06 μm has been demonstrated with InGaAsP/InP avalanche photodiodes operated at room temperature. A photon detection efficiency of 33% was measured on uncoated detectors, representing an internal avalanche probability of 60%. Under identical bias conditions a dark count rate as low as 1.7 MHz was measured at 290 K, consistent with a primary dark current of <0.3 pA. Dark count rates drop by approximately 50–200× by cooling the detectors to 210 K (−63 °C).

Arrays of InP-based Avalanche Photodiodes for Photon Counting

Arrays of InP-based avalanche photodiodes (APDs) with InGaAsP absorber regions have been fabricated and characterized in the Geiger mode for photon-counting applications. Measurements of APDs with InGaAsP absorbers optimized for 1.06 mum wavelength show dark count rates (DCRs) <20 kHz for room-temperature operation with photon detection efficiency (PDE) up to 50% and a reset or dead time of 1s. APDs with InGaAs absorbers optimized for 1.55 μm wavelength and 240 K temperature have DCRs <20 kHz, PDE up to 45%, and a reset time of ~6 mus. Arrays for both wavelengths have been fabricated and packaged with GaP microlenses (of 100 and 50 μm pitch) and CMOS readout integrated circuits (ROICs). Comparisons are made between ROICs that operate in the framed-readout mode as well as those that operate in continuous-readout mode.

Afterpulsing in Geiger-mode avalanche photodiodes for 1.06μm wavelength

We consider the phenomenon of afterpulsing in avalanche photodiodes (APDs) operating in gated and free-running Geiger mode. An operational model of afterpulsing and other noise characteristics of APDs predicts the noise behavior observed in the free-running mode. We also use gated-mode data to investigate possible sources of afterpulsing in these devices. For 30-μm-diam, 1.06-μm-wavelength InGaAsP∕InP APDs operated at 290K and 4V overbias, we obtained a dominant trap lifetime of τd=0.32μs, a trap energy of 0.11eV, and a baseline dark count rate 245kHz.

Arrays of III-V semiconductor Geiger-mode avalanche photodiodes

In this paper, InGaAsP/InP APDs is designed for detection of near infrared (1-1.5 /spl mu/m wavelength) light and GaN APDs designed for detection of ultraviolet (<365 nm wavelength) light. This paper will also describe ladar measurements which use arrays of G-M APDs matched with timing circuits to produce 3D images with near-infrared photons.

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.

Scaling of dark count rate with active area in 1.06μm photon-counting InGaAsP∕InP avalanche photodiodes

Reducing the active area of InGaAsP∕InP avalanche photodiodes operated in Geiger mode is investigated for reducing the dark count rate. The dark count rate in Geiger mode is found to scale linearly with the detector’s active area for mesa diameters of 10, 15, and 20μm. Scaling the mesa size from 20to10μm results in a reduction of the room-temperature dark count rate from 290to44kHz at ∼4.5V of overbias, while the photon detection efficiency is ∼45% for all device diameters. The trade-offs associated with shrinking the active area, including reduced optical coupling and higher on-state resistance, are discussed briefly.

Geiger-mode avalanche photodiodes for photon-counting communications

Arrays of photon-counting avalanche photodiodes enable laser-communications receivers with unprecedented sensitivity at 1.06-/spl mu/m wavelength. Arrays with 64 elements were fabricated in the InGaAsP/InP materials system and were bump-bonded to a custom CMOS integrated circuit. The integrated circuit uses a novel nonblocking architecture to continuously report both time-of-arrival for incoming photons as well as their spatial location on the array. Near room temperature, the best detectors have: 45% photon detection efficiency, 65-kHz dark count rate, and a 1.6-/spl mu/s reset time to avoid after-pulsing.

InP-based single-photon detector arrays with asynchronous readout integrated circuits

We have developed and demonstrated a high-duty-cycle asynchronous InGaAsP-based photon counting detector system with near-ideal Poisson response, room-temperature operation, and nanosecond timing resolution for near-infrared applications. The detector is based on an array of Geiger-mode avalanche photodiodes coupled to a custom integrated circuit that provides for lossless readout via an asynchronous, nongated architecture. We present results showing Poisson response for incident photon flux rates up to 10 million photons per second and multiple photons per 3-ns timing bin.

Micromachined room-temperature microbolometer for millimeter-wave detection and focal-plane imaging arrays

We have combined silicon micromachining technology with planar circuits to fabricated room-temperature niobium microbolometers for millimeter-wave detection. In this type of detector, a thin niobium film, with a dimension much smaller than the wavelength, is fabricated on a 1-micrometers thick Si3N4 membrane of square and cross geometries. The Nb film acts both as a radiation absorber and temperature sensor. Incident radiation is coupled into the microbolometer by a 0.37 (lambda) dipole antenna with a center frequency of 95 GHz and a 3-db bandwidth of 15%, which is impedance matched with the Nb film. The dipole antennas is placed inside a micromachined pyramidal cavity formed by anisotropically etched Si wafers. To increase the Gaussian beam coupling efficiency, a machined square or circular horn is placed in front of the micromachined section. Circular horns interface more easily with die-based manufacturing processes; therefore, we have developed simulation tools that allow us to model circular machined horns. We have fabricated both single element receivers and 3 X 3 focal-plane arrays using uncooled Nb microbolometers. An electrical NEP level of 8.3 X 10-11 W/(root)Hz has been achieved for a single- element receiver. This NEP level is better than that of the commercial room-temperature pyroelectric millimeter-wave detectors. The frequency response of the microbolometer has a ln(1/f) dependence with frequency, and the roll-off frequency is approximately 35 kHz.