Showing papers by "Elliott R. Brown published in 2005"

Ultrafast photoresponse at 1.55 μm in InGaAs with embedded semimetallic ErAs nanoparticles

Abstract: We have grown epitaxial metal/semiconductor superlattice materials by molecular beam epitaxy that exhibit subpicosecond photocarrier lifetimes at 1.55 μm. The superlattice samples consist of layers of semimetallic ErAs nanoparticles embedded in a semiconducting In0.53Ga0.47As matrix. Time-resolved optical measurements are performed using a fiber-based transmission pump-probe technique with an erbium-doped-fiber mode-locked laser. Photocarrier lifetimes decrease with increasing ErAs deposition and decreasing spacing between the ErAs layers. Further reduction in the lifetime is achieved by selective beryllium doping of the superlattice; measured lifetimes ⩽0.3ps were achieved in optimized structures.

Ultrasound crack detection in a simulated human tooth.

Abstract: Objective: Currently, diagnosis of cracked teeth generally depends upon the overall clinical assessment, or on exclusion of other clinical possibilities, not primarily on the direct identification of cracks themselves. Owing to its short wavelength in hard tissues and associated high resolution, ultrasound has the potential to allow detection of cracks within tooth structure. However, ultrasound detection of dental cracks has not previously been achieved. The purpose was to determine if an ultrasound imaging system was capable of imaging cracks in simulated tooth structure. Methods: A complete ultrasound system including a novel transducer made of PLZT-98, a novel gallium-indium alloy coupling agent, and customized electronic and digital signal processing (DSP) algorithms was developed for the specific application of optimizing crack detection within teeth. A simulated tooth with a known and uniform internal structure and acoustic properties similar to those of natural enamel and dentin was designed to mo...

Tunable all epitaxial semimetal-semiconductor schottky diode system : ErAs on InAlGaAs

Abstract: We report the growth, fabrication, and electrical properties of a fully epitaxial semimetal-semiconductor Schottky diode materials system: ErAs on InAlGaAs. The coherent, thermodynamically stable, lattice-matched interface eliminates oxide and interfacial third phases, making the Schottky barrier height and associated electrical parameters dependent on the fundamental molecular bonding (e.g., interface dipoles) rather than unintentional extrinsic effects. ErAs:InAlGaAs diodes thus have highly tunable properties; by adjusting composition, doping, and interface type, the Schottky barrier height is tunable from ∼100meVto∼620meV, short circuit responsivity is tunable from 0.5to19A∕W, differential resistance is tunable from 103to1010Ωμm2, and capacitance is tunable from 0.4to2fF∕μm2. Rectification of a rf source has been demonstrated up to 20GHz. ErAs:InAlGaAs is thus a very promising materials system for design of an upper mm-wave, zero-bias, square-law detector with a significantly reduced noise floor.

Semimetal-semiconductor rectifiers for sensitive room-temperature microwave detectors

Abstract: In contrast to traditional metal-semiconductor (i.e., Schottky) junctions, ErAs:InAlGaAs heterojunctions have recently been shown to provide highly “engineerable” electrical rectification characteristics through the tuning of the Schottky barrier height and differential resistance, while maintaining the very low specific capacitance for which Schottky diodes are famous. We demonstrate that these new rectifiers can operate effectively at zero bias and can be extremely sensitive at microwave frequencies. We report a tangential sensitivity of −63dBm at 3.1 GHz in an 8.0 KHz bandwidth, corresponding to a noise-equivalent power (NEP) of 8.9×10−13W∕Hz1∕2. We attribute this noise performance to the high rf-to-dc current responsivity (∼8A∕W), zero bias operation, and noise source reduction. The new diodes are found to be ∼19dB more sensitive than the best available Hewlett–Packard zero-bias diodes at comparable frequencies.

Sensitivity measurement and analysis of an ErAs:GaAs coherent photomixing transceiver

Abstract: A frequency domain transceiver is implemented with a pair of 780-nm-based ErAs:GaAs photomixers, one serving as the transmitter and the other as the receiver. The transceiver is all solid state, operated at room temperature, and has a fast response. The receiver detects the amplitude of the incoming electric field coherently and the sensitivity is limited by 1∕f mechanisms which are excited by a photovoltage that is always present in the receiver photomixer. This letter reports the first measurements and calculations of the signal-to-noise (S/N) ratio in the transceiver. Measurement yields a differential terahertz power-to-current sensitivity of 4mA∕W and noise equivalent power of 0.1pW∕Hz. The maximum S/N ratio was 58dB∕rtHz around 86GHz which is 17dB lower than our theoretical estimate. The discrepancy is attributed to radiative coupling losses and nonoptimal laser spatial overlap.

State-of-the-art in 1.55 µm ultrafast InGaAs photoconductors, and the use of signal-processing techniques to extract the photocarrier lifetime

Abstract: A summary of ultrafast In0.53Ga0.47As photoconductors at λ = 1.55 µm is provided, with emphasis placed on recent ion-implanted and ErAs-nanoparticulate materials that have displayed response times <1 ps. Signal-processing techniques are then developed to deconvolve the photocarrier lifetime from pump–probe photo-transmission data. These techniques are formulated in a way that allows the estimation of photocarrier lifetime values less than the pump–probe laser pulse width, and are applied to the fastest ErAs:In0.53Ga0.47As material measured to date with a photocarrier lifetime of ≈0.3 ps.

Evaluation of gallium-indium alloy as an acoustic couplant for high-impedance, high-frequency applications

Abstract: Materials are investigated for coupling ultrasound between solid piezoelectric transducers and solid targets having acoustic impedances ranging between 10–20 MRayl. The majority of available acoustic couplants have impedances below 5 MRayl, leading to large surface reflections and acoustic losses. The desired couplant is preferably liquid or semiliquid with good conformability and low attenuation. This study examines the acoustic properties of various couplants, including gallium–indium alloys. Transmission tests were conducted at 19 MHz, concluding that these alloys are most appropriate for the stated application, with low attenuation, high acoustic impedance (17.4 MRayl), and a measured compressional velocity of 2740 m/s.

An ultra-compact low loss 30-GHz micromachined coaxial filter

Abstract: In this paper, we present a novel low-loss (1.5 dB) 3rd order Chebyshev filter with a center frequency of 29.5 GHz and a 3 dB bandwidth of 1.4 GHz. The filter fits within a compact 5.1 mm /spl times/ 3.3 mm footprint. The filter was designed with a fully shielded integrated coax structure and fabricated using EFAB/spl trade/ technology, a selective electrochemical metal deposition process. We have demonstrated minimal performance effects from two 180 degree bends in the filter structure.

First MMW characterization of ErAs/InAlGaAs/InP semimetal-semiconductor-Schottky diode (S3) detectors for passive millimeter-wave and infrared imaging

Abstract: We present the first mm-wave characterization of Semimetal Semiconductor Schottky (S3) diodes for direct detector applications from 94 GHz to 30 THz. The S3 devices use molecular-beam epitaxy growth of binary compounds that are closely lattice-matched and crystallographically perfect across the heterointerface to reduce 1/f and burst noise while maintaining ultra-high-frequency performance. The S3 diodes are fabricated from an InAlGaAs/InP based material system with both the Schottky layer and contact layer having n and n+ doping levels. The semimetal Schottky contact is ErAs which is grown in-situ during the MBE growth. By varying the InAlAs percentage content in the epitaxial layer structure, the diode dc I-V characteristics and its zero bias responsivity are optimized. Diode s-parameter data from dc-100 GHz is used to determine the diode responsivity as a function of frequency and diode capacitance and resistance. These measurements then allow the device intrinsic and extrinsic equivalent-circuit elements to be optimized for direct detection from 94 GHz to ~30 THz.

Retrodirective noise-correlating radar at X band: first demonstration of small target detection

Abstract: This paper reports the realization of a four-channel retrodirective noise correlating radar operating in X band around 10 GHz. It is designed for rapid detection of small targets at short range. Detection times less than 50 ns are demonstrated for a 30-caliber bullet, and the autotracking capability of the RNC radar is demonstrated for the first time with the bullet in motion.

Terahertz signature characterization of bio-simulants

Abstract: Collaboration with the University of Virginia (UVa) and the University of California, Santa Barbara (UCSB) has resulted in the collection of signature data in the THz region of the spectrum for ovalbumin, Bacillus Subtilis (BG) and RNA from MS2 phage. Two independent experimental measurement systems were used to characterize the bio-simulants. Prior to our efforts, only a limited signature database existed. The goal was to evaluate a larger ensemble of biological agent simulants (BG, MS2 and ovalbumin) by measuring their THz absorption spectra. UCSB used a photomixer spectrometer and UVa a Fourier Transform spectrometer to measure absorption spectra. Each group used different sample preparation techniques and made multiple measurements to provide reliable statistics. Data processing culminated in applying proprietary algorithms to develop detection filters for each simulant. Through a covariance matrix approach, the detection filters extract signatures over regions with strong absorption and ignore regions with large signature variation (noise). The discrimination capability of these filters was also tested. The probability of detection and false alarm for each simulant was analyzed by each simulant specific filter. We analyzed a limited set of Bacillus thuringiensis (BT) data (a near neighbor to BG) and were capable of discriminating between BT and BG. The signal processing and filter construction demonstrates signature specificity and filter discrimination capabilities.

A novel I-RTD based optically-pulsed hybrid device for generating THz oscillations

Abstract: The double-barrier AlGaSb/InAs/AlGaSb heterostructure with staggered bandgap alignment can admit significant interband tunneling current in addition to the conduction band electron transport. The resulting positive hole-charge accumulation in the right valence-band (VB) well will electrostatically modify the spatial potential profile across the device structure, thereby effectively altering the conduction of conduction-band electron transport. A sequentially triggered optical discharging process can be used to annihilate, or substantially reduce, the trapped holes that are generated from the interband tunneling process. Hence, an artificially induced electro-optic interaction can be used to return the device to its initial state and to produce a two-cycle oscillation process - i.e., one with a interband-induced charging transient followed by a optically-induced discharging transient to the initial state. These charging-discharging cycles obtained from this hybrid type of interband resonant-tunneling-diode (I-RTD) device constitute steady-state oscillatory behavior at very high frequency and produce alternating-current (ac) power as long as very short (i.e., sub-picosecond) and intense far-infrared laser pulses are presented to the diode. Initial studies of non-optimized structures and designs predict impressive figures of merit for oscillation frequencies (e.g., - 300-600 GHz) and substantial output powers (e.g., ∼ 10 mW) for very modest device areas (i.e., 100 μm 2 ). This paper will present physics-based I-RTD diode simulation results to precisely describe transport dynamics and transient electric current for both charging (initiated by Zener tunneling) and discharging (artificially induced by photons flux) processes. A basic electro-optical design concept and modeling approach for the analysis and synthesis of non-linear hybrid I-RTD circuits will also be presented. The main objectives of this paper are: (1) to perform a detailed assessment of the ac output power and efficiency of an optically-triggered (OT) I-RTD hybrid oscillator in the frequency range approximately 300 to 600 GHz, and (2) to prescribe the general requirements for realizing a diode-laser pair upon a single solid-state platform in the future. Therefore, guidelines for a practical engineering implementation and performance estimates for an OT-I-RTD hybrid oscillator design will be presented.

Bright MM-Wave and THz Luminescence by Down-Conversion of Near-IR Amplified-Spontaneous-Emission

Abstract: We report the measurement of exceptionally bright, incoherent radiation in the THz region by frequency down-conversion of amplified spontaneous emission around 775-nm wavelength. The down-conversion technique is optical mixing in an interdigital photoconductive capacitor made from ultra fast ErAs:GaAs. The brightness temperature into a single spatial mode is approximately 1.1×105 K, making the new radiation at least 70 times more intense than common incandescent sources in the THz region.

Semimetal-Semiconductor Junctions for Low Noise Zero-Bias Rectifiers

Abstract: ErAs and InAlGaAs heterojunctions promise to act as low noise Schottky diode. In this paper, we present responsivity measurements and 2–8 GHz tangential sensitivity measurements on a room- temperature zero-bias rectifier intended for use in THz imaging arrays. This novel interface promises to allow for a room-temperature zero-bias rectifier that significantly simplifies detector design, mitigates current induced flicker and burst noise, and eliminates bias circuitry and related noise, potentially leading to a square-law detector with a noise floor of ~10−12 W/Hz1/2. The ability to tune barrier height, responsivity, capacitance and differential resistance is also investigated.