Rugate filters are optical thin film interference structures with sinusoidal refractive index profiles. Two-wavelength reflection filters have been fabricated by codeposition of SiO(2) and TiO(2). Composition modulation was monitored and controlled using quartz crystal rate controllers. The resulting filters exhibited two well-defined stopbands. Microscopic examination revealed that the structure is glasslike without pronounced thin film microstructure.

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The microstructure and composition of the SiO2/TiO2 analog (codeposited) and digital (thin layer pairs) gradient-index films were examined by transmission electron microscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, scatterometry, and in situ ellipsometry. Both analog and digital structures were amorphous as-grown. The SiO2 was incompletely oxidized at some of the interfaces in the layered structures and in the codeposited films, indicated competition from the TiO2 for available oxygen during growth. Digital structures with thin (65 Å) layers remained well defined after annealing at 900 °C, but their order was completely destroyed by 1100 °C. Structures with thick (500 Å) layers remained intact up to 1100 °C, with the TiO2 crystallizing throughout the layer width. Both anatase and rutile TiO2 crystallites were present in the layered and codeposited films after a high-temperature anneal (T≳650 °C), while only the anatase phase was observed for 300 °C<T<650 °C. The optical scatter of the digital films increased with increasing annealing temperature and layer thickness. TiO2 precipitated out of the codeposited films after a high-temperature anneal; however, the scatter of these films at 0.633 μm was several orders of magnitude lower than that of the annealed layered films. Codeposited films consisting of greater than ∼60% TiO2 displayed form birefringence in the as-grown state.

Short wave infrared (SWIR) devices have been fabricated using Rockwell's double layer planar heterostructure (DLPH) architecture with arsenic-ion implanted junctions. Molecular beam epitaxially grown HgCdTe/CdZnTe multilayer structures allowed the thin, tailored device geometries (typical active layer thickness was -3.5 Mm and cap layer thickness was -0.4 Mm) to be grown. A planar-mesa geometry that preserved the passivation advantages of the DLPH structure with enhanced optical collection improved the performance. Test detectors showed Band 7 detectors performing near the radiative limit (-3-5X below theory). Band 5 detector performance was -4-50X lower than radiative limited performance, apparently due to Shockley-Hall-Read recombination. We have fabricated SWIR HgCdTe 256 x 12 x 2 arrays of 45 ~tm x 45 ~m detector on 45 pm x 60 ~m centers and with cutoff wavelength which allows coverage of the Landsat Band 5 (1.5-1.75 ~tm) and Landsat Band 7 (2.08-2.35 Mm) spectral regions. The hybridizable arrays have four subarrays, each having a different detector architecture. One of the Band 7 hybrids has demonstrated performance approaching the radiative theoretical limit for temperatures from 250 to 295K, consistent with test results. D* performance at 250K of the best subarray was high, with an operability of -99% at 101~ cm Hz'2/W at a few mV bias. We have observed 1/f noise below 8E-17 AHz -1/2 at 1 Hz. Also for Band 7 test structures, Ge thin film diffractive microlenses fabricated directly on the back side of the CdZnTe substrate showed the ability to increase the effective collection area of small (nominally <20 Mm pm) planar-mesa diodes to the microlens size of 48 ~tm. Using microlenses allows array performance to exceed 1-D theory up to a factor of 5.

An ultrahigh-Q, tunable optical filter with a FWHM bandwidth of 41 MHz is demonstrated. The filtering is produced by nondegenerate phase conjugation through four-wave mixing in atomic-sodium vapor. The filter is observed to have a maximum quantum efficiency of 4 X 10-3. However, degenerate phase-conjugation experiments in sodium suggest that a quantum efficiency of greater than unity can be attained on a cw basis.The frequency response of phase conjugation through nondegenerate four-wave mixing has been analyzed by several authors'-5 and has been shown to be capable of yielding a narrow-bandpass optical filter. The filter bandpass is a function of both phase-mismatch constraints and the frequency dependence of the coefficients coupling the waves in the nonlinear process. Optical filtering was recently demonstrated in CS 2 , a transparent medium, by using high-power pulsed lasers to provide the pump waves needed in the nonlinear process. 6 The filtering is dependent only on the phase-mismatch constraints in this type of medium. In this Letter we report the first known experimental demonstration of optical filtering by phase conjugation in a resonant system. A resonant system, sodium in this case, has the advantage of providing much larger nonlinear coupling constants than a transparent medium, enabling one to use low-power cw lasers to provide the pump waves. The frequency dependence of the coupling coefficients can result in a filter response that is much narrower than in a transparent medium. An ultrahigh-Q filter with a FWHM bandwidth of 41 MHz is demonstrated in our experiments.In this experiment, a phase-conjugate signal is generated by the coupling of two counterpropagating pump waves to a signal wave by an intensity-dependent modulation of the atomic population. Physically, the signal wave and one of the pump waves form an absorption grating that diffracts the second pump wave to create the phase-conjugate wave. The output of a passively stabilized Spectra-Physics cw ring dye laser is circularly polarized to provide optical isolation from the laser and is then retroreflected by a mirror to provide the counterpropagating pump waves. An actively stabilized Coherent Radiation cw ring dye laser generates the signal wave, which intersects the pump wave at an angle of 0.390. Sodium vapor acts as the nonlinear medium. A 2.44-cm-long cylindrical quartz cell of 2.2-cm diameter contains sodium metal in a side arm.The cell is heated to provide the sodium vapor. Cesium metal is also present in the cell and provides a background pressure of 10-2 Torr of cesium vapor. The cesium resonances are far from the sodium transition used and do not affect the nonlinear process. The signal wave is chopped mechanically, and the phase-conjugate signal is detected by a P-I-N photodiode that is connected to a lock-in amplifier to provide an excellent signal-to-noise ratio. Figure 1 shows the experimental setup. The pump beam has a FWHM diameter of 0.106 cm and a maximum power of 35.2 mW at the entrance to the cell. The signal beam ha...