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.

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.

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.

Optical resonances in periodic surface arrays of metallic patches.

The transmission of light along the surface normal through an air-quartz-glass interface covered with a periodic array of thin, rectangular gold patches has been studied over the visible to infrared range. The various structures that are observed can be qualitatively understood as arising from standing-wave resonances set by the size and surroundings of the metal patches. A method-of-moments calculational scheme provides simulations in good quantitative agreement with the data. It is shown how the standing-wave picture provides a useful conceptual framework to understand and exploit such systems.

Modeling parameters for the spectral behavior of infrared frequency-selective surfaces.

Comparisons of experiment and theory are presented for transmission spectra over the range 2-15 mum of a set of frequency-selective surfaces consisting of arrays of simple dipole patches of aluminum on or in silicon. The arrays are fabricated by direct-write electron-beam lithography. Important parameters controlling the spectral shape are identified, such as dipole length, spacing, resistance, and dielectric surroundings. The separate influence of these variables is exhibited. Encouraging agreement between simple model calculations and the measurements is found.

Refractive-index and element-spacing effects on the spectral behavior of infrared frequency-selective surfaces.

Transmission and reflection characteristics of inductive-mesh frequency-selective surfaces were measured in the 4-12-microm range. Specific issues investigated include the effect of interelement spacing on the location and width of the resonance and the influence of superstrate and substrate refractive indices on the spectral response of the structure.

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) .

Spatial impulse response of lithographic infrared antennas.

We present measurements of the spatial response of infrared dipole and bow-tie lithographic antennas. Focused 10.6-microm radiation was scanned in two dimensions across the receiving area of each antenna. Deconvolution of the beam profile allowed the spatial response to be measured. The in-plane width of the antenna's spatial response extends approximately one dielectric wavelength beyond the metallic structure. Determination of an antenna's spatial response is important for several reasons. The power collected by the antenna can be calculated, if the collection area and the input irradiance (watts per square centimeter) are known. The actual power collected by the antenna is required for computation of responsivity and noise-equivalent power. In addition, the spatial response provides insight into the current-wave modes that propagate on an antenna and the nature of the fringe fields that exist in the adjacent dielectric.

Deconvolution method for two-dimensional spatial-response mapping of lithographic infrared antennas.

The spatial impulse response of antenna-coupled infrared detectors with dimensions comparable with the wavelength is obtained from a two-dimensional scan of a tightly focused CO(2)-laser beam. The method uses an experimental setup with submicrometer resolution and an iterative deconvolution algorithm. The measured spatial response is compared with numerically computed near-field distributions of a dipole antenna, with good agreement.

Tunable polarization response of a planar asymmetric-spiral infrared antenna.

We present measurements at 10.6 microm that demonstrate electronic tuning of the polarization response of asymmetric-spiral infrared antennas connected to Ni-NiO-Ni diodes. Continuous variation of the bias voltage applied to the diode results in a rotation of the principal axis of the polarization ellipse of the spiral antenna. A 90 degrees tuning range is measured for a bias voltage that varies from -160 to +160 mV .This effect is caused by a small asymmetry of the deposited diode contact or by a variation of the detector capacitance with the applied bias voltage.

Time-domain depolarization of waves retroreflected from dense colloidal media.

We report on depolarization measurements of femtosecond pulses retroreflected from dense suspensions of silica microspheres with solid loads increasing from 5% to 54%. Backscattered pulse shapes compare well with predictions of the diffusion theory for all volume fractions, and the inferred values of the transport mean free path agree with independent measurements of enhanced backscattering. The measured degree of polarization decays exponentially with temporal rates that scale with the solid load. It is newly found that, for all solid loads, depolarization sets in for path lengths longer than approximately five transport mean free paths.

Polarization response of asymmetric-spiral infrared antennas.

We present measurements on the polarization response of Ni-NiO-Ni diodes coupled to asymmetric spiral antennas. Our data are for the wavelength dependence of the orientation of the major axis of the polarization ellipse over the wavelength range 10.2-10.7 mum. The data are well fit by a two-wire antenna model. We find that the modes excited on the antenna are a combination of the balanced and unbalanced modes of a two-wire lossy transmission line.

Angular dependence of sampling modulation transfer function.

Sampling modulation transfer function (MTF) as defined in Park et al. [Appl. Opt. 23, 2527-2582 (1984)] as an x and y sampling can be generalized for image data not along x and y directions. For a given sampling lattice (such as in a laser printer, a scene projector, or a focal-plane array), we construct a two-dimensional sampling MTF based on the distance between nearest samples in each direction. Because the intersample distance depends on direction, the sampling MTF will be best in the directions of highest spatial sampling and poorer in the directions of sparse sampling. We compare hexagonal and rectangular lattices in terms of their equivalent spatial frequency bandwidth. We filter images as a demonstration of the angular-dependent two-dimensional sampling MTF.

Polarization asymmetry in waves backscattering from highly absorbant random media.

Within the range in which light penetration depth is approximately the same as or less than the diameter of the particles in the medium, particulate media with considerable absorption behave as two-dimensional, rough-surface structures. As penetration depth increases, a complicated transition between volume and surface effects is seen. For these media, low-order scattering sequences have small spatial extent, making observation of polarization characteristics difficult. We present an experimental technique to access the low-order scattered photons by artificially reinjecting them through total internal reflections. Using a dielectric layer in contact with the high-absorption medium, we are able to observe fourfold polarization asymmetry in backscattering from highly absorbant media. We discuss the origin of the polarization patterns in a ray-optics approximation and suggest possibilities for solving practical problems encountered in characterizing composites with appreciable absorption.

Dipole-on-dielectric model for infrared lithographic spiral antennas.

We present a dipole-on-dielectric model for lithographic antennas used for bolometer coupling in the infrared. The predicted antenna patterns show good agreement with measurements of Au-on-Si spiral antennas at 9.5-microm wavelength. Angle- and polarization-resolved measurements are proposed, which will further probe the behavior of these antenna structures and facilitate refinement of the analytical models.

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.