Fabio Alves

Strong terahertz absorption using SiO2/Al based metamaterial structures

Metamaterial absorbers with nearly 100% absorption in the terahertz (THz) spectral band have been designed and fabricated using a periodic array of aluminum (Al) squares and an Al ground plane separated by a thin silicon dioxide (SiO2) dielectric film. The entire structure is less than 1.6 mm thick making it suitable for the fabrication of microbolometers or bi-material sensors for THz imaging. Films with different dielectric layer thicknesses exhibited resonant absorption at 4.1, 4.2, and 4.5 THz with strengths of 98%, 95%, and 88%, respectively. The measured absorption spectra are in good agreement with simulations using finite element modeling.

Three-band quantum well infrared photodetector using interband and intersubband transitions

This paper presents the design, fabrication, and characterization of a quantum well infrared photodetector capable of detecting near infrared (NIR), midwavelength infrared (MWIR), and long wavelength infrared (LWIR) simultaneously. The NIR detection was achieved using interband transition while MWIR and LWIR were based on intersubband transition in the conduction band. The quantum well structure was modeled by solving self-consistently the Schrodinger and Poisson equations with the help of the shooting method. Intersubband absorption in the sample was measured for the MWIR and LWIR using Fourier transform infrared spectroscopy, and the measured peak positions were found at 5.3 and 8.7 μm, respectively, which are within 5% of the theoretical values, indicating the good accuracy of the self-consistent model. The photodetectors were fabricated using a standard photolithography process with exposed middle contacts to allow separate bias and readout of signals from the three wavelength bands. The background li...

Bio-Inspired Miniature Direction Finding Acoustic Sensor

A narrowband MEMS direction finding sensor has been developed based on the mechanically coupled ears of the Ormia Ochracea fly. The sensor consists of two wings coupled at the middle and attached to a substrate using two legs. The sensor operates at its bending resonance frequency and has cosine directional characteristics similar to that of a pressure gradient microphone. Thus, the directional response of the sensor is symmetric about the normal axis making the determination of the direction ambiguous. To overcome this shortcoming two sensors were assembled with a canted angle similar to that employed in radar bearing locators. The outputs of two sensors were processed together allowing direction finding with no requirement of knowing the incident sound pressure level. At the bending resonant frequency of the sensors (1.69 kHz) an output voltage of about 25 V/Pa was measured. The angle uncertainty of the bearing of sound ranged from less than 0.3° close to the normal axis (0°) to 3.4° at the limits of coverage (±60°) based on the 30° canted angle used. These findings indicate the great potential to use dual MEMS direction finding sensor assemblies to locate sound sources with high accuracy.

Estimation of radiant intensity and average emissivity of Magnesium/Teflon/Viton (MTV) flares

This paper presents a comparison between measurements of the spectral radiant intensity of Magnesium/Teflon®/Viton® (MTV) flares and theoretical estimation using available mathematical models in the range of 0.6 μm up to 11.5 μm. It is used an indigenous system capable of hold the flare pellets in the same position during the burning time and reproduce the airflow after an actual air launching at different velocities. Two mathematical models are studied and adapted to predict MVT flare radiant intensity and average emissivity from visible to long wavelength infrared. The results indicate that the adapted models can be use to estimate the MTV flare parameters in different situations with good accuracy.

Widely Separate Spectral Sensitivity Quantum Well Infrared Photodetector Using Interband and Intersubband Transitions

Recent commercial and military infrared sensors have demanded multispectral capabilities, high sensitivity and high selectivity, usually found in quantum well infrared photodetectors (QWIPs). This paper presents the design and characterization of a three-band QWIP capable to detect simultaneously near infrared (NIR), mid-wavelength infrared (MWIR), and long-wavelength infrared (LWIR), using interband and intersubband transitions. Separate readouts provide the flexibility to optimize each band detection by allowing the application of different bias voltages. The quantum well structure was designed using a computational tool developed to solve self-consistently the Schrodinger-Poisson equation with the help of the shooting method. The detector comprises of three different stacks of uncoupled (wide barriers) quantum wells that combine AlGaAs, GaAs, and InGaAs, separated by contact layers, grown by molecular beam epitaxy (MBE) on a GaAs substrate. The spectral responses in all three bands were measured using a standard photocurrent spectroscopy setup with light coupling via a 45 facet. The measured photoresponse showed peaks at 0.84, 5.0, and 8.5 wavelengths with approximately 0.8, 0.03, and 0.12 A/W peak responsivities for NIR, MWIR, and LWIR bands, respectively. A good agreement between the measured and simulated figures of merit shows the possibility to improve and tailor the detector for several applications with low computational effort. Finally, this work has demonstrated the possibility of detection of widely separated wavelength bands using interband and intersubband transitions in quantum wells.

Tunable NIR quantum well infrared photodetector using interband transitions

This paper presents the design and characterization of a near infrared (NIR) tunable quantum well infrared photodetector (QWIP). The detection was achieved using interband electron transitions between quantized energy levels for holes (light and heavy) in the valence band and quantized energy levels for electrons in the conduction band. The quantum wells are made asymmetric (step wells) to allow transitions between energy levels with different parity quantum numbers. The structure is modeled by solving self-consistently the Schrodinger and Poisson equations with the help of the shooting method. The photocurrent of the fabricated GaAs/InGaAs photodetector is measured at the temperature of 10 K and the observed response lies between 825 and 940 nm. When the bias is 0.5 V, a narrow response centered in 840 nm is achieved. Applying 4.5 V the peak response moves to 930 nm. The results demonstrate the possibility of tunable detection in the NIR band with great versatility.

Acousto-optic spectrum analysis of RF pulsed signals having Bartlett and Blackman envelope waveforms

The application of the acousto optic spectral analysis technique to RF pulsed signals having either Bartlett or Blackman envelope waveforms is investigated based on a generalized formulation which yields their spectra as they evolves through the Bragg cell. It is shown that, even with uniform light beam, power spectrum with side lobe supression as high as 58 dB can be attained. The detected temporal responses from a real-time photodetector array are analyzed as a function of the pulse envelope waveforms and the relative photodetector to spectra centroid location. The appearance of flattop and multiple humps are discussed.

Comparison of experimental three-band IR detection of buried objects and multiphysics simulations

A buried-object detection system composed of a LWIR, a MWIR and a SWIR camera, along with a set of ground and ambient temperature sensors was constructed and tested. The objects were buried in a 1.2x1x0.3 m3 sandbox and surface temperature (using LWIR and MWIR cameras) and reflection (using SWIR camera) were recoded throughout the day. Two objects (aluminum and Teflon) with volume of about 2.5x10-4 m3 , were placed at varying depths during the measurements. Ground temperature sensors buried at three different depths measured the vertical temperature profile within the sandbox, while the weather station recorded the ambient temperature and solar radiation intensity. Images from the three cameras were simultaneously acquired in five-minute intervals throughout many days. An algorithm to postprocess and combine the images was developed in order to maximize the probability of detection by identifying thermal anomalies (temperature contrast) resulting from the presence of the buried object in an otherwise homogeneous medium. A simplified detection metric based on contrast differences was established to allow the evaluation of the image processing method. Finite element simulations were performed, reproducing the experiment conditions and, when possible, incorporated with data coming from actual measurements. Comparisons between experiment and simulation results were performed and the simulation parameters were adjusted until images generated from both methods are matched, aiming at obtaining insights of the buried material properties. Preliminary results show a great potential for detection of shallowburied objects such as land mines and IEDs and possible identification using finite element generated maps fitting measured surface maps.

Understanding of self-terminating pulse generation using silicon controlled rectifier and RC load

Recently a silicon controlled rectifier (SCR)-based circuit that generates self-terminating voltage pulses was employed for the detection of light and ionizing radiation in pulse mode. The circuit consisted of a SCR connected in series with a RC load and DC bias. In this paper, we report the investigation of the physics underlying the pulsing mechanism of the SCR-based. It was found that during the switching of SCR, the voltage across the capacitor increased beyond that of the DC bias, thus generating a reverse current in the circuit, which helped to turn the SCR off. The pulsing was found to be sustainable only for a specific range of RC values depending on the SCR’s intrinsic turn-on/off times. The findings of this work will help to design optimum SCR based circuits for pulse mode detection of light and ionizing radiation without external amplification circuitry.

High sensitive MEMS directional sound sensor with comb finger capacitor electronic readout

The conventional directional sound sensing systems employ an array of spatially separated microphones to achieve directional sensing by monitoring the arrival times and amplitudes of different microphones. However, there are insects such as Ormia ochracea fly that can determine the direction of sound using its miniature-hearing organ much smaller than the wavelength of sound it detects. The fly’s eardrums that are coupled mechanically with separation merely by about 1 mm have remarkable sensitivity to the direction of sound. Our MEMS based sensor, which consists of two 1 mm2 wings connected in the middle, similar to the fly’s hearing system, was designed and fabricated using silicon on insulator (SOI) substrate. The vibration of the wings in response to incident sound at the bending resonance was measured using a laser vibrometer found to be about 1 μm/Pa. For measuring sensor response electronically, comb finger capacitors were integrated on to the wins and the measured output using a MS3110 capacitive to voltage converter was found to be about 25 V/Pa. The fabricated sensors showed cos2q directional response similar to a pressure gradient microphone. The directional response of the sensor was measured down to about 30 dB.