In this study we have examined structural and neurochemical aspects of retinal and optic nerve development in experimentally growth-retarded fetal guinea pigs following maternal unilateral artery ligation. Eye weight (n = 4) and total retinal area (n = 6) at 62 days gestation (term approximately 66 days) were both relatively spared when expressed as a percentage of body weight but in absolute terms were significantly reduced by 18% (P less than 0.001) and 13% (P less than 0.05) respectively when compared with age-matched controls. The numerical density of neurons in the ganglion cell layer was significantly higher at both 52 days (n = 4) and 62 days (n = 4) in growth-retarded fetuses compared with controls. However, there was no difference between the groups in the total number of neurons in this retinal layer at either age, since retinal areas are reduced in growth retardation. The area of neuronal somata in the ganglion and inner nuclear layers was significantly reduced in growth-retarded fetuses compared with controls. There was a concomitant reduction in the width of the cellular layers in the retina and also in the plexiform (synaptic) and photoreceptor layers. The growth of the outer segments of the photoreceptor layer was particularly affected in peripheral retina. The higher packing density of cells and the reduced growth of the plexiform layers suggests a reduction in the growth of the neuropile in growth-retarded fetuses compared with controls. The radial bundling of ganglion cell axons coursing across the retina to enter the optic nerve head was poorly defined in growth retardation. In addition myelination was delayed in the optic nerve with the numerical density of myelinated axons being significantly reduced (P less than 0.005) in growth-retarded fetuses compared with controls. There was a significant reduction (P less than 0.01) in the number of amacrine cells in the inner plexiform layer expressing Substance P-like immunoreactivity in growth-retarded fetuses compared with controls. Glutamate-like immunoreactivity was most intense in the five laminae of the inner plexiform layer and in the outer plexiform layer and less pronounced in photoreceptors, ganglion cells and their axons. There was no qualitative difference in glutamate immunoreactivity between control and growth-retarded fetuses in any of these structures. Thus we have shown that intrauterine growth retardation has specific effects on the development of the fetal guinea pig retina, reducing the growth of several types of neurons and their processes and affecting the expression of the neuropeptide substance-P in amacrine cells.

We report the design, growth, processing, and characterization of resonant cavity enhanced photodiodes for the midwave infrared at ∼3.72 μm on GaSb. Using AlAsSb/GaSb mirrors, AlAsSb barrier and spacer layers and a thin 96 nm InAsSb absorber, we observed dark current and detectivity behavior superior to common InAsSb nBn detectors in the literature, with peak specific detectivity values of 8×1010 and 1×1010 cm Hz1/2 W−1 measured at 250 K and 300 K, respectively. In the same temperature range, the linewidth of the detector response was <44 nm and the quality factor ∼80. The peak quantum efficiency was >60% where the enhancement due to the resonant cavity was ∼20x. We estimate that the devices can operate close to, or slightly above, the background-limited infrared performance limit imposed on broadband detectors for a 300 K scene.

No abstract

Resonant cavity‐enhanced photodiodes targeted within the spectral region of absorption by CH bonds are demonstrated. The 3.0 – 3.3   μ m region of the infrared spectrum contains many substances that are useful to measure spectroscopically. However, the measurement of individual substances requires a high spectral specificity, that is achieved by the resonant cavity photodiodes with spectral response widths of < 40   nm . Two material systems are investigated for detection at this wavelength range—an InAs absorber on an InAs substrate and an InAsSb absorber lattice‐matched to a GaSb substrate. The resonance wavelength of the InAs‐based device responds at ≈ 3.3   μ m , closely tuned to an absorption peak of methane to allow precise sensing of this gas. At 300   K a quantum efficiency of 52 % is achieved, with a specific detectivity of 2.5 × 10 10 cm Hz / W . The InAsSb‐based device is sensitive at ≈ 3.7   μ m , but the structure could be tuned to the methane absorption peak. Devices could be simply created to target other substances in the C−H absorption region by altering the layer thicknesses in the structure. Both structures can be used for spectrally specific gas sensing in this region of the infrared.

The design, fabrication, and characterization of a resonant cavity-enhanced photodetector (RCE PD) operating in the long-wavelength infrared regime are demonstrated. The incorporation of the low bandgap InAs/InAs0.70Sb0.30 type-II strained-layer superlattice into the absorber layer of the detector cavity, along with the high-reflectivity (Rm &gt; 0.9) AlAs0.08Sb0.92/GaSb distributed Bragg reflector pairs, results in resonant enhancement at 7.7–7.8 μm, which is a spectral region relevant in applications in sensing of chemical warfare agents and in medical biomarker diagnostics. These resonant wavelength peaks also display a high quality factor in the range of 76–86 and a small temperature coefficient of 0.52 nm K−1. An nBn architecture, where an Al0.71Ga0.29As0.08Sb0.92 layer acts as a barrier for majority electrons while minimizing the valence band offset with the absorber, is also incorporated into the cavity in order to improve the electrical properties of the detector. Spectral response measurements yield a peak external quantum efficiency of 14.6% and a peak responsivity of 0.91 A W−1 at 77 K and −0.8 V; meanwhile, a dark current density of 2.0 × 10−4 A cm−2 at 77 K results in a specific detectivity of 3.7 × 1010 cm Hz1/2 W−1, coming close to the theoretical background-limited D* of an ideal broadband photovoltaic detector with the superlattice composition as that of the RCE PD.