High-contrast infrared polymer photonic crystals fabricated by direct laser writing.

One-dimensional (1D) photonic crystals (PCs) were fabricated by three-dimensional (3D) direct laser writing using a single polymer to obtain reflectance values approaching that of a gold reference in the near-infrared (near-IR) spectral range. The PCs are composed of alternating compact and low-density polymer layers that provide the necessary periodic variation of the refractive index. The low-density polymer layers are composed of subwavelength-sized pillars which simultaneously serve as a scaffold while also providing refractive index contrast to the adjacent compact polymer layers. The Bruggemann effective medium theory and stratified-layer optical-model calculated reflectivity profiles were employed to optimize the PC's design to operate at a desired wavelength of 1.55 µm. After the fabrication, the PC's structure was compared to the nominal geometry using complementary scanning electron microscopy and optical microscopy micrographs identifying a true-to-form fabrication. The performance of the PCs was investigated experimentally using FTIR reflection and transmission measurements. A good agreement between stratified-layer optical-model calculated and measured data is observed. Therefore, we demonstrate the ease of predictive design and fabrication of highly efficient 1D PCs for the IR spectral range using 3D direct laser writing of a single polymer.

Broadband near-infrared antireflection coatings fabricated by three-dimensional direct laser writing.

Three-dimensional direct laser writing via two-photon polymerization is used to fabricate anti-reflective structured surfaces (ARSSs) composed of subwavelength conicoid features optimized to operate over a wide bandwidth in the near-infrared range from 3700 cm-1 to 6600 cm-1 (2.7-1.52 µm). Analytic Bruggemann effective medium calculations are used to predict nominal geometric parameters such as the fill factor of the constitutive conicoid features of the anti-reflective structured surfaces (ARSSs) presented here. The performance of the ARSSs was investigated experimentally using infrared reflection and transmission measurements. An enhancement of the transmittance by 1.35%-2.14% over a broadband spectral range from 3700 cm-1 to 6600 cm-1 (2.7-1.52 µm) was achieved. We further report on finite-element-based reflection and transmission data using three-dimensional (3D) model geometries for comparison. A good agreement between experimental results and the finite-element-based numerical analysis is observed once as-fabricated deviations from the nominal conicoid forms are included in the model. 3D direct laser writing is demonstrated here as an efficient method for the fabrication and optimization of ARSSs designed for the infrared spectral range.

Strong coupling between nanoscale metamaterials and phonons.

We use split ring resonators (SRRs) at optical frequencies to study strong coupling between planar metamaterials and phonon vibrations in nanometer-scale dielectric layers. A series of SRR metamaterials were fabricated on a semiconductor wafer with a thin intervening SiO(2) dielectric layer. The dimensions of the SRRs were varied to tune the fundamental metamaterial resonance across the infrared (IR) active phonon band of SiO(2) at 130 meV (31 THz). Strong anticrossing of these resonances was observed, indicative of strong coupling between metamaterial and phonon excitations. This coupling is very general and can occur with any electrically polarizable resonance including phonon vibrations in other thin film materials and semiconductor band-to-band transitions in the near to far IR. These effects may be exploited to reduce loss and to create unique spectral features that are not possible with metamaterials alone.

Experimental demonstration of tunable phase in a thermochromic infrared-reflectarray metamaterial.

For the first time, a tunable reflected phase reflectarray is demonstrated in the thermal infrared. This is done using thermochromic VO(2) square-patch elements in a reflectarray metamaterial configuration. A sixty degree change in reflected phase is measured using a Twyman-Green interferometer, and FTIR measurements show that the resonance reflection minima shifts from 9.2 to 11.2 mum as the sample is heated from 45 through 65 degrees C. These results are in agreement with finite-element method simulations using the optical properties of VO(2) which are measured by infrared ellipsometry.

Effect of thin silicon dioxide layers on resonant frequency in infrared metamaterials.

Infrared metamaterials fabricated on semiconductor substrates exhibit a high degree of sensitivity to very thin (as small as 2 nm) layers of low permittivity materials between the metallic elements and the underlying substrate. We have measured the resonant frequencies of split ring resonators and square loops fabricated on Si wafers with silicon dioxide thicknesses ranging from 0 to 10 nm. Resonance features blue shift with increasing silicon dioxide thickness. These effects are explained by the silicon dioxide layer forming a series capacitance to the fringing field across the elements. Resonance coupling to the Si-O vibrational absorption has been observed. Native oxide layers which are normally ignored in numerical simulations of metamaterials must be accounted for to produce accurate predictions.

Zernike-based matrix model of deformable mirrors: optimization of aperture size.

The actuator influence functions of a typical deformable mirror are expanded in a Zernike polynomial decomposition. This expansion is then extended to a matrix formalism that describes the modal operation of the mirror. The size of the aperture over which the Zernikes are defined affects the accuracy of the expansion. The optimum size of this aperture is found by minimizing the variance of the wave-front error.

Facet model for photon-flux transmission through rough dielectric interfaces.

When a photon flux is incident upon a rough interface that separates media with different refractive indices, the interface roughness influences the angular distribution of the transmitted flux. For the case of very rough surfaces with slopes of the order of unity, we find that a simple facet model is sufficient to describe the main features of the f lux-transmission behavior. We demonstrate the effect observed by Nieto-Vesperinas et al. [Opt. Lett. 15, 1261 (1990)] for plane-wave incidence, that the interface roughness tends to suppress the refractive-index contrast. In addition, for cases in which the incident flux is distributed in angle, we find that the direction of maximum transmitted flux can be predicted from the surface roughness.

Enhanced backscattering in a converging-beam configuration.

Enhanced backscattering (EBS) is investigated for a converging-beam geometry. We find that the functional form of the enhanced backscattering cone is preserved under a change of variables that involves the focal length of the lens and the lens-to-sample distance. In absolute terms, the effect of the lens is that the EBS enhancement cone is lowered in magnitude and narrowed in angle. The experimental data show good agreement with the theory presented, even for tightly focused beams, as long as the illuminated area is larger than the transport mean free path for the random medium.

Modal analysis of noise in signal-processing-in-the-element detectors.

Detector noise limits the performance of signal-processing-in-the-element detectors. For detectors to be optimized, an expression for the signal and noise must be found. The results of the eigenmode solution to the charge transport problem are used to derive the power spectral density of the noise in analytic form. This result is then coordinated with a similarly obtained modulation transfer function to yield a frequency-dependent signal-to-noise ratio (SNR). The SNR is used to reveal performance trends over several ranges of detector parameters. The most important result is that the contact boundary velocity strongly controls the SNR. The optimum SNR condition occurs when the contacts are not perfectly ohmic but exhibit a partially blocking behavior.

Modulation-transfer-function-enhanced readout for SPRITE detectors.

A new readout structure is investigated for signal-processing-in-the-element detectors that yields a modulation transfer function that is 3.5 dB better than those currently used. Experimental verification is performed in Si rather than HgCdTe, with similarity relations derived for the two semiconductors.

Modal analysis of transport processes in SPRITE detectors.

Carrier transport in signal-processing-in-the-element (SPRITE) detectors is an important phenomenon because it determines properties such as the responsivity and the modulation transfer function (MTF). The previous literature has presented approximate solutions to the transport problem that neglect boundary effects, which have long been thought to play a major role in SPRITE behavior. We present a new solution to the problem through the use of modal analysis. This method intrinsically includes boundary conditions and thus is more complete than the previous analysis. Furthermore we use this solution to derive expressions for the MTF. The effects of the boundary conditions on the MTF are studied to determine their optimum values.

Resolution-equivalent D* for SPRITE detectors.

It is desirable for design purposes to model a signal-processing-in-the-element (SPRITE) detector simply as a discrete-element detector with an integration-enhanced D*. We present a method for normalization of measured D* for SPRITE detectors to yield an equivalent-discrete D*. The multiplicative factor is the square root of the ratio of two noise-equivalent bandwidths: one is that of the SPRITE detector with no boost filter, and the other is that of the SPRITE detector with a boost filter that approximately compensates for carrier diffusion, yielding a spatial resolution that approaches that of a discrete detector the same size as the readout. This approach allows a resolution-equivalent D* comparison of SPRITE detectors with discrete-element detectors and facilitates such comparisons among SPRITE detectors. We find that, to obtain the D* of an equivalent-discrete detector, a measured SPRITE D* should typically be multiplied by a factor ranging from 0.85 to 0.57 for 8- to 12-µm SPRITE detectors and by a factor ranging from 0.50 to 0.23 for 3- to 5-µm SPRITE detectors.

Modulation Transfer Function Measurement Using Three- and Four-bar Targets.

Modulation-transfer-function (MTF) measurement often involves the use of three- and four-bar resolution targets. In the conversion of three- and four-bar image data to MTF, biased results can occur when we use series-expansion techniques appropriate for square-wave targets of infinite extent. For systems where the image data are digitally recorded, a convenient and accurate conversion of bar-target data to MTF can be performed using a Fourier-domain method.

On-axis and off-axis propagation of Gaussian beams in gradient index media.

The object-image formula and an expression for the lateral magnification are obtained for a gradient radial index medium used with Gaussian beams. Angular acceptance conditions for on-axis and off-axis incidence are discussed. Meridional and sagittal sections of a Gaussian beam are analyzed, and numerical methods are used to find the evolution of the beam. The eccentricity parameter of the elliptical spot and the astigmatism of the beam are obtained as a function of propagation distance.

Spatial filtering by a line-scanned nonrectangular detector: application to SPRITE readout MTF.

Any finite-sized photodetector has an effect on the spatial frequency content of the detected image. An expression for the modulation transfer function (MTF) of a nonrectangular detector in the along-scan direction is obtained. A comparison of our theoretical prediction is made with published experimental and numerical values for the MTF of a photosite having an exponentially tapered shape. Structures of this form are used as the readout region in SPRITE (signal processing in the element) detectors.

Modulation depth characteristics of a liquid crystal television spatial light modulator.

The performance of a commercially available liquid crystal TV display was characterized in terms of its modulation depth. Measurements of screen transmittance and modulation depth, as a function of signal level, showed that the primary limitations of the device as a spatial light modulator were due to the nature of the video scan format and the display drive electronics. The resolution of the device, as measured by the modulation transfer function, is limited more by the physical pixel spacing than by pixel crosstalk. The optical flatness of the screen was characterized interferometrically, both with and without polarizers, to show the improvement in wavefront quality obtained by replacing the original polarizers.

Modulation transfer function and number of equivalent elements for SPRITE detectors.

SPRITE (signal processing in the element) detectors are three-contact photoconductive structures made of HgCdTe, in which a time-delay-and-integration function is performed in the detector element itself without the need for external circuitry. Spatial frequency-dependent expressions are developed for the modulation transfer function and the number of equivalent elements N(eq) of the SPRITE. The development is based on a Green's function method, which accounts for carrier generation, recombination, and diffusion processes. The usual low-frequency approach of defining a square resolution element on the SPRITE is avoided. The resulting expressions are functions of spatial frequency and are also dependent on physical variables such as the length of the SPRITE element, carrier lifetime, carrier mobility, and operating voltage. These expressions are then applied to the design of a SPRITE element, optimized for operation over a particular range of spatial frequencies.

Parameters of spinning AM reticles.

A new method of obtaining amplitude modulation (AM) for determining target location with spinning reticles is presented. The method is based on the use of graded transmission capabilities. The AM spinning reticles previously presented were functions of three parameters: amplitude vs angle, amplitude vs radius, and phase. This paper presents these parameters along with their capabilities and limitations and shows that multiple parameters can be integrated into a single reticle. It is also shown that AM parameters can be combined with FM parameters in a single reticle. Also, a general equation is developed that relates the AM parameters to a reticle transmission equation.

Lithographic antennas at visible frequencies.

The response of antenna-coupled thin-film Ni-NiO-Ni diodes to 633-nm helium-neon laser radiation is investigated. Although these detectors and their integrated dipole antennas are optimized for the detection of mid-infrared radiation, a polarization dependence of the measured response to visible radiation is observed. The strongest signals are measured for the polarization parallel to the dipole antenna axis, which demonstrates antenna operation of the device in the visible in addition to the expected thermal and photoelectric effects. The connection structure of the diode also resonates and contributes to the polarization-dependent signal. The receiving area of the dipole antenna is approximately 2 microm(2) .

Effects of intensity thresholding on the power spectrum of laser speckle.

Spatial-frequency filtering of laser-speckle patterns has proved to be a useful tool in the measurement of the modulation transfer function for focal plane arrays. Intensity thresholding of the laser-speckle patterns offers nearly an order of magnitude savings in digital storage space. The effect of this thresholding on the spatial-frequency power spectral density of the speckle pattern is investigated. An optimum threshold level is found that minimizes distortion of the power spectrum for the classes of speckle data used for modulation transfer function testing.