B. F. Levine

Quantum‐well infrared photodetectors

The extensive literature on quantum‐well infrared photodetectors (QWIPs) is reviewed. A detailed discussion is given on the device physics of the intersubband absorption and hot‐carrier transport processes for individual detectors, as well as the high performance which has been achieved for large staring arrays. QWIPs having widely different structures, materials, and spectral responses are covered, as is the optimization of the quantum‐well parameters for maximum performance.

New 10 μm infrared detector using intersubband absorption in resonant tunneling GaAlAs superlattices

We demonstrate a novel 10.8 μm superlattice infrared detector based on doped quantum wells of GaAs/AlGaAs. Intersubband resonance radiation excites an electron from the ground state into the first excited state, where it rapidly tunnels out producing a photocurrent. We achieve a narrow bandwidth (10%) photosensitivity with a responsivity of 0.52 A/W and an estimated speed of 30 ps.

An organic crystal with an exceptionally large optical second‐harmonic coefficient: 2‐methyl‐4‐nitroaniline

We have grown and measured the optical second‐harmonic coefficient dijk of the new nonlinear crystal 2‐methyl‐4‐nitroaniline (MNA). We find that the dijk are very large with d12 being 5.8 times larger than d31 of LiNbO3 giving a birefringence phase‐matching figure of merit d2/n3 which is 45 times larger than LiNbO3. The other coefficient d11 is 40 times larger than LiNbO3, giving a huge figure of merit which is 2000 larger than LiNbO3.

High sensitivity low dark current 10 μm GaAs quantum well infrared photodetectors

By increasing the quantum well barrier width, we have dramatically reduced the tunneling dark current by an order of magnitude and thereby significantly increased the blackbody detectivity D*BB. For a GaAs quantum well infrared detector having a cutoff wavelength of λc=10.7 μm, we have achieved D*BB =1.0×1010 cm (Hz)1/2/W at T=68 K, a temperature which is readily achievable with a cryogenic cooler.

Strong 8.2 μm infrared intersubband absorption in doped GaAs/AlAs quantum well waveguides

We have measured the infrared intersubband absorption at 8.2 μm in doped GaAs/AlAs quantum well superlattices. Waveguide geometry experiments demonstrate strong absorption with 95% of the incident infrared energy being absorbed.

Molecular hyperpolarizabilities determined from conjugated and nonconjugated organic liquids

The magnitude and sign (positive) of the molecular hyperpolarizability γ has been determined for several mixtures of nitrobenzene and benzene. This has allowed the testing of the adequacy of Onsagers local field factors, which are found to be valid. By comparing γ of nitrobenzene with that of nitromethane, we find that the delocalized conjugated electrons are an order of magnitude more effective in contributing to γ than are the localized nonconjugated electrons.

Photoexcited escape probability, optical gain, and noise in quantum well infrared photodetectors

We present a detailed and thorough study of a wide variety of quantum well infrared photodetectors (QWIPs), which were chosen to have large differences in their optical and transport properties. Both n‐ and p‐doped QWIPs, as well as intersubband transitions based on photoexcitation from bound‐to‐bound, bound‐to‐quasicontinuum, and bound‐to‐continuum quantum well states were investigated. The measurements and theoretical analysis included optical absorption, responsivity, dark current, current noise, optical gain, hot carrier mean free path, net quantum efficiency, quantum well escape probability, quantum well escape time, as well as detectivity. These results allow a better understanding of the optical and transport physics and thus a better optimization of the QWIP performance.

High‐detectivity D*=1.0×1010 cm √H̄z̄/W GaAs/AlGaAs multiquantum well λ=8.3 μm infrared detector

We report the first high‐detectivity (D*=1.0×1010 cm (Hz)1/2/W), high‐responsivity (Rv =30 000 V/W) GaAs/AlxGa1−xAs multiquantum well detector, sensitive in the long‐wavelength infrared band at λ=8.3 μm (operating at a temperature of T= 77 K). Because of the mature GaAs growth and processing technologies as well as the potential for monolithic integration with high‐speed GaAs field‐effect transistors, large focal plane arrays of these detectors should be possible.

Multiple quantum well 10 μm GaAs/AlxGa1−xAs infrared detector with improved responsivity

We have achieved a high responsivity, R=1.9 A/W, 10 μm infrared detector using intersubband absorption in GaAs/AlxGa1−xAs quantum well superlattices. The photocurrent is produced by intersubband absorption followed by efficient photoexcited tunneling. This responsivity is nearly four times higher than our previous results and has been obtained by using thicker and higher AlxGa1−xAs superlattice barriers thereby reducing the dark current and allowing the detector to be operated at higher biases.

Normal incidence hole intersubband absorption long wavelength GaAs/AlxGa1−xAs quantum well infrared photodetectors

The first long wavelength quantum well infrared photodetector based on valence band intersubband absorption holes is demonstrated. A normal incidence quantum efficiency of η=28% and detectivity of D*λ=3.1×1010 cm √Hz/W at T=77 K, for a cutoff wavelength λc=7.9 μm, have been achieved.