Francisco Javier González

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.

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.

Response Increase of IR Antenna-Coupled Thermocouple Using Impedance Matching

The response of a bowtie antenna-coupled thermocouple operating at 10.6 μm is studied for varying lengths of a transmission line, which connects the antenna to the thermocouple and functions as an impedance-matching element. Peaks in the response are observed for several lengths of transmission line. Most notably, the response of a device with a transmission line length of 1.3 μm is increased 2.4 fold when compared to the device without the transmission line. The analytical response of a microwave circuit describing the detector is in agreement with the measurements, indicating that the increases in the response are caused by an improved impedance match between the antenna and thermocouple facilitated by the transmission line. This experiment demonstrates for the first time impedance matching principles applied to infrared antenna-coupled thermal detectors.

A Double-Blind, Randomized Clinical Trial of Niacinamide 4% versus Hydroquinone 4% in the Treatment of Melasma

Background. Multiple modalities have been used in the treatment of melasma with variable success. Niacinamide has anti-inflammatory properties and is able to decrease the transfer of melanosomes. Objective. To evaluate the therapeutic effect of topical niacinamide versus hydroquinone (HQ) in melasma patients. Patients and Methods. Twenty-seven melasma patients were randomized to receive for eight weeks 4% niacinamide cream on one side of the face, and 4% HQ cream on the other. Sunscreen was applied along the observation period. They were assessed by noninvasive techniques for the evaluation of skin color, as well as subjective scales and histological sections initially and after the treatment with niacinamide. Results. All patients showed pigment improvement with both treatments. Colorimetric measures did not show statistical differences between both sides. However, good to excellent improvement was observed with niacinamide in 44% of patients, compared to 55% with HQ. Niacinamide reduced importantly the mast cell infiltrate and showed improvement of solar elastosis in melasma skin. Side effects were present in 18% with niacinamide versus 29% with HQ. Conclusion. Niacinamide induces a decrease in pigmentation, inflammatory infiltrate, and solar elastosis. Niacinamide is a safe and effective therapeutic agent for this condition.

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.

High-Sensitivity Bolometers from Self-Oriented Single-Walled Carbon Nanotube Composites

In this work, films of horizontally aligned single-walled carbon nanotubes were thermally and electrically characterized in order to determine the bolometric performance. An average thermal time constant of τ = 420 μs along with a temperature coefficient of resistance of TCR = -2.94% K(-1) were obtained. The maximum voltage responsivity and detectivity obtained were R(V) =230 V/W and D* = 1.22 × 10(8) cm Hz(1/2)/W, respectively. These values are higher than the maximum voltage responsivity (150 V/W) and maximum temperature coefficient of resistance (1.0% K(-1)) previously reported for carbon nanotube films at room temperature. The maximum detectivity was obtained at a frequency of operation of 1.25 kHz.

Distribution of skin temperature in Mexican children

Background: Infrared thermography can be a valuable diagnostic tool in the evaluation and management of several pathologies; however, the temperature pattern of the healthy body should be known in order to perform a more precise clinical evaluation.

Seebeck nanoantennas for solar energy harvesting

We propose a mid-infrared device based on thermocouple optical antennas for light sensing and energy harvesting applications. We numerically demonstrate that antennas are able to generate low-power dc signals by beneficing of the thermoelectric properties of the metals that constitute them. We theoretically evaluate the optical-to-electrical conversion efficiency for harvesting applications and finally discuss strategies to increase its performance. Thermocouple optical antennas therefore open the route toward the design of photovoltaic devices.

Backscattering from particulate surfaces: experiment and theoretical modeling

The copolarized backscattered intensity from surfaces composed of metallic particles on conducting flat substrates is analyzed experimentally as a function of the incidence angle. The analysis is done for particle sizes smaller than, comparable to, and larger than the incident wavelength (0.633 μm) and for low particle surface densities. Numerical calculations based on the extinction theorem for a onedimensional surface model consisting of an infinitely long cylinder located on a flat substrate for the same optical constants used in the experiment are also presented for qualitative comparison with the experimental results. This serves to analyze the effect of particle aggregation. For the surfaces with particles smaller than the incident wavelength, conclusions are drawn concerning the possible relevance of this study in radar wave scattering from the sea surface.

Noninvasive determination of burn depth in children by digital infrared thermal imaging

Abstract. Digital infrared thermal imaging is used to assess noninvasively the severity of burn wounds in 13 pediatric patients. A delta-T (ΔT) parameter obtained by subtracting the temperature of a healthy contralateral region from the temperature of the burn wound is compared with the burn depth measured histopathologically. Thermal imaging results show that superficial dermal burns (IIa) show increased temperature compared with their contralateral healthy region, while deep dermal burns (IIb) show a lower temperature than their contralateral healthy region. This difference in temperature is statistically significant (p<0.0001) and provides a way of distinguishing deep dermal from superficial dermal burns. These results show that digital infrared thermal imaging could be used as a noninvasive procedure to assess burn wounds. An additional advantage of using thermal imaging, which can image a large skin surface area, is that it can be used to identify regions with different burn depths and estimate the size of the grafts needed for deep dermal burns.