C. G. Bethea

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.

Quantum cascade lasers: ultrahigh-speed operation, optical wireless communication, narrow linewidth, and far-infrared emission

Following an introduction to the history of the invention of the quantum cascade (QC) laser and of the band-structure engineering advances that have led to laser action over most of the mid-infrared (IR) and part of the far-IR spectrum, the paper provides a comprehensive review of recent developments that will likely enable important advances in areas such as optical communications, ultrahigh resolution spectroscopy and applications to ultrahigh sensitivity gas-sensing systems. We discuss the experimental observation of the remarkably different frequency response of QC lasers compared to diode lasers, i.e., the absence of relaxation oscillations, their high-speed digital modulation, and results on mid-IR optical wireless communication links, which demonstrate the possibility of reliably transmitting complex multimedia data streams. Ultrashort pulse generation by gain switching and active and passive modelocking is subsequently discussed. Recent data on the linewidth of free-running QC lasers (/spl sim/150 kHz) and their frequency stabilization down to 10 kHz are presented. Experiments on the relative frequency stability (/spl sim/5 Hz) of two QC lasers locked to optical cavities are discussed. Finally, developments in metallic waveguides with surface plasmon modes, which have enabled extension of the operating wavelength to the far IR are reported.

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.

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.

GaAs/AlGaAs multiquantum well infrared detector arrays using etched gratings

Efficient coupling of long‐wavelength infrared (LWIR) radiation to a two‐dimensional (2‐D) array of GaAs/AlGaAs multiple quantum well detectors is achieved by illumination through chemically etched diffraction gratings. Gratings were fabricated on the back surface of the GaAs substrate as well as selectively on the top contact of the detector mesas. Both top and bottom illumination schemes were employed. In all cases, high coupling efficiency (>90%) of the gratings was observed as measured by comparing the responsivity to that of an identical detector illuminated through an angle‐polished facet. The results demonstrate the feasibility of high‐sensitivity GaAs LWIR imagers.

Nonlinear Susceptibility of GaP; Relative Measurement and Use of Measured Values to Determine a Better Absolute Value

We have measured the ratio d36(GaP)/d11(SiO2) = 185 ± 10% at 1.318 μ. By using this value together with other values in the literature, we determine a better set of absolute values for the nonlinear susceptibility.