Javier Alda

Optical antennas for nano-photonic applications

Antenna-coupled optical detectors, also named optical antennas, are being developed and proposed as alternative detection devices for the millimetre, infrared, and visible spectra. Optical and infrared antennas represent a class of optical components that couple electromagnetic radiation in the visible and infrared wavelengths in the same way as radioelectric antennas do at the corresponding wavelengths. The size of optical antennas is in the range of the detected wavelength and they involve fabrication techniques with nanoscale spatial resolution. Optical antennas have already proved and potential advantages in the detection of light showing polarization dependence, tuneability, and rapid time response. They also can be considered as point detectors and directionally sensitive elements. So far, these detectors have been thoroughly tested in the mid-infrared with some positive results in the visible. The measurement and characterization of optical antennas requires the use of an experimental set-up with nanometric resolution. On the other hand, a computation simulation of the interaction between the material structures and the incoming electromagnetic radiation is needed to explore alternative designs of practical devices.

Laser and Gaussian Beam Propagation and Transformation

Optical engineers and researchers working on optics deal with laser beams and optical systems as usual tools in their specific areas. The knowledge of the special characteristics of the propagation of laser beams through optical systems has to be one of the keystones of their actual work, and the clear definition of their characteristic parameters has an important impact in the success of the applications of laser sources. In this article, we will provide some basic hints about the characterization and transformation of laser beams that also deserve special attention in basic and specific text books (e.g., see Refs. [7–13]). The Gaussian beam case is treated in the first place because of its simplicity. Besides, it allows to introduce some characteristic parameters whose definition and meaning will be extended along the following sections to treat any kind of laser beam. In between, we will show how the beam is transformed by linear optical systems. These systems are described by using the tools of matrix optics. In the following, we will assume that laser beams have transversal dimensions small enough to consider them as paraxial beams. What it means is that the angular spectrum of the amplitude distribution is located around the axis of propagation, allowing a parabolic approximation for the spherical wavefront of the laser beam. In the paraxial approach, the component of the electric field along the optical axis is neglected. The characterization of laser beams within the nonparaxial regime can be done, but it is beyond the scope of this presentation. We will take the amplitudes of the beams as scalar quantities. This means that the polarization effects are not considered, and the beam is assumed to be complete and homogeneously polarized. A proper description of the polarization dependences needs an extension of the formalism that is not included here. Pulsed laser beams also need a special adaptation of the fundamental description presented here.

Antenna-coupled infrared detectors for imaging applications

Infrared focal plane arrays (IRFPAs) are a critical component in advanced infrared imaging systems. IRFPAs are made up of two parts, a detector array and a readout integrated circuit (ROIC) multiplexer. Current ROIC technology has typical pitch sizes of 20/spl times/20 to 50/spl times/50 /spl mu/m/sup 2/. In order to make antenna-coupled detectors suited for infrared imaging systems, two-dimensional (2-D) arrays have been fabricated that cover a whole pixel area with the penalty of increasing the noise figure of the detector and, therefore, reducing its performance. By coupling a Fresnel zone plate lens to a single element antenna-coupled detector, infrared radiation can be collected over a typical pixel area and still keep low-noise levels. A Fresnel zone plate lens coupled to a single-element square-spiral-coupled infrared detector has been fabricated and its performance compared to single element antenna-coupled detectors and 2-D arrays of antenna coupled detectors. Measurements made at 10.6 /spl mu/m showed a two-order-of-magnitude increase in SNR and a /spl sim/3/spl times/ increase in D/sup */ as compared to 2-D arrays of antenna-coupled detectors.

Complex beam parameter and ABCD law for non-Gaussian and nonspherical light beams

We define the width, divergence, and curvature radius for non-Gaussian and nonspherical light beams. A complex beam parameter is also defined as a function of the three previous ones. We then prove that the ABCD law remains valid for transforming the new complex beam parameter when a non-Gaussian and nonspherical, orthogonal, or cylindrical symmetric laser beam passes through a real ABCD optical system. The product of the minimum width multiplied by the divergence of the beam is invariant under ABCD transformations. Some examples are given.

Principal-component characterization of noise for infrared images

Principal-component decomposition is applied to the analysis of noise for infrared images. It provides a set of eigenimages, the principal components, that represents spatial patterns associated with different types of noise. We provide a method to classify the principal components into processes that explain a given amount of the variance of the images under analysis. Each process can reconstruct the set of data, thus allowing a calculation of the weight of the given process in the total noise. The method is successfully applied to an actual set of infrared images. The extension of the method to images in the visible spectrum is possible and would provide similar results.

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) .

Analytical expression for the complex radius of curvature tensor Q for generalized gaussian beams

Abstract An expression for the complex radius of curvature tensor describing every kind of gaussian beam is proposed. This expression can be used in the tensor ABCD law. The circular, orthogonal astigmatic and non orthogonal astigmatic beams are expressed by means of this tensor.

Conversion efficiency of broad-band rectennas for solar energy harvesting applications

Optical antennas have been proposed as an alternative option for solar energy harvesting. In this work the power conversion efficiency of broadband antennas, log-periodic, square-spiral, and archimedian-spiral antennas, coupled to Metal-Insulator-Metal and Esaki rectifying diodes has been obtained from both theoretical and numerical simulation perspectives. The results show efficiencies in the order of 10(-6) to 10(-9) for these rectifying mechanisms, which is very low for practical solar energy harvesting applications. This is mainly caused by the poor performance of diodes at the given frequencies and also due to the antenna-diode impedance mismatch. If only losses due to antenna-diode impedance mismatch are considered an efficiency of about 10(-3) would be obtained. In order to make optical antennas useful for solar energy harvesting new rectification devices or a different harvesting mechanism should be used.

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.

Infrared antennas coupled to lithographic Fresnel zone plate lenses

Several designs for Fresnel zone plate lenses (FZPLs) to be used in conjunction with antenna-coupled infrared detectors have been fabricated and tested. The designs comprise square and circular FZPLs with different numbers of Fresnel zones working in transmissive or reflective modes designed to focus infrared energy on a square-spiral antenna connected to a microbolometer. A 163x maximum increase in response was obtained from a 15-zone circular FZPL in the transmissive mode. Sensor measurements of normalized detectivity D* resulted in a 2.67x increase with FZPLs compared with measurements made of square-spiral antennas without FZPLs. The experimental results are discussed and compared with values obtained from theoretical calculations.