Fiber optic-based chemical sensors are created by coating fiber Bragg gratings (FBG) with the glassy polymer cellulose
acetate (CA). CA is a polymeric matrix capable of localizing or concentrating chemical constituents within its structure.
Some typical properties of CA include good rigidity (high modulus) and high transparency. With CA acting as a sensor
element, immersion of the gratings in various chemical solutions causes the polymer to expand and mechanically strain
the glass fiber. This elongation of the fiber sections containing the grating causes a corresponding change in the
periodicity of the grating that subsequently results in a change in the Bragg-reflected wavelengths. A high-resolution
tunable fiber ring laser interrogator is used to obtain room-temperature reflectance spectrograms from two fiber gratings
at two different wavelengths - 1540nm and 1550nm. The graphical representation from this device enables the display
of spectral shape, and not merely shifts in FBG central wavelength, thereby allowing for more comprehensive analysis of
how different physical conditions cause the reflectance profile to move and alter overall form. Wavelength shifts on the
order of 1 to 80 pm in the FBG transition edges and changes in spectral shape are observed in both sensors upon
immersion in a diverse selection of chemical analytes.
A promising new fiber optic sensor is under development that combines fiber Bragg gratings coated with polymer materials for sensitive and rapid detection and identification of chemical and/or biological agents. Volumetric expansion of the polymer coating transfers characteristic strain to the Bragg grating, modifying directly its grating period rather than sensing through a change of effective guide index of refraction. The optical interrogation of the sensor element utilizes a sensitive transmission spectroscopy technique with a balanced receiver that minimizes polarization and laser intensity noise problems. A compact, rugged, all-solid-state laser at 1550 nm is being adapted for rapid tuning between discrete preset locked wavelengths. To accompany use of this laser at these few discrete wavelengths, a sampled (superstructure) fiber Bragg grating is being designed using coupled mode theory. Hence, the need for a continuously tunable laser, often with moving mechanical optical elements, and its attendant reference etalon will be avoided entirely. This process exploits a novel vernier effect between the discrete laser wavelengths and the sampled grating responses to create 'signatures' for an artificial neural network. Therefore, the total spectral response pattern of strain can constitute a unique fingerprint used to identify and quantify chemical agents or biomarkers. The sensor is intended for applications requiring multi-functionality, sensitivity, speed, mobility and remote operability in vibrational, electromagnetic, and explosive environments.
In this work, we investigate the DC and high-frequency performance of heterostructure InGaAs/InAlAs metal-semiconductor-metal (MSM) photodetectors fabricated using e-beam nanolithography. The device finger electrodes are approximately 250 nm in width with gaps of 100 nm, and are arranged interdigitally in a circular geometry with ~4.5 μm outer diameter. The InGaAs absorption layer is 200 nm thick. High-bandgap material layers are utilized to form carrier transport barriers and to enhance Schottky barrier heights at the contacts. Also, a buried Bragg reflector (resonant cavity enhanced structure) serves to increase the photodetector responsivity given the very thin absorption layer thickness. Current-voltage measurements of seven devices were taken at room temperature under dark and illuminated conditions. Additionally, the frequency performance was evaluated using a 1550-nm diode laser with an integrated 50-GHz electroabsorption modulator and a microwave network analyzer. A fairly large range of dark currents from 100 pA-1 μA is observed. External responsivities for these small devices also vary widely between 0.05-0.24 A/W. Under high-frequency intensity modulation, the devices exhibit an RC-limited and/or diffusion-limited performance, with usable bandwidths to 15 GHz. Physical origins of the device and parasitic capacitances are discussed as well as optical diffraction effects, both of which are thought to contribute to the limited high-frequency performance.
Efforts to exploit reduced dimensionality systems in semiconductor devices are presently driven by the continuing need to improve speed performance, transport efficiency, device density, and power management. In this work, we investigate the performance of novel GaAs/AlGaAs and InGaAs/InAlAs heterostructures for high-speed photodetector devices. First, a modulation-doped AlGaAs/GaAs device, suitable for monolithic integration with planar HEMT and FET devices, produces a built-in electric field that aids in the high-speed collection of photogenerated carriers. Surface Schottky electrodes on this structure form a planar interdigitated metal-semiconductor-metal (MSM) device for use at 850-nm wavelength. A second structure, an InGaAs/InAlAs quantum-well MSM photodetector for use at 1550-nm wavelength, utilizes recessed electrodes to contact directly the two-dimensional (2D) transport channel. Unfortunately, rather low Schottky barrier heights on undoped InGaAs lead to excessive dark currents when metal contacts are deposited directly on this material. To remedy this situation, we propose to form barrier-enhancement regions between the optically active 2D-quantum well and the lateral 3D-metal contacts by means of ion-implantation-induced quantum-well intermixing. Results indicate a reduction in dark current of nearly three orders of magnitude. Additionally, the high-speed performance appears not to be adversely affected under normal operating conditions by the potentially deleterious effects of carrier emission and accumulation at these heterojunction interfaces. The Fourier transform of a simulated transient current response to a light impulse indicates an electrical 3-dB bandwidth in excess of 50 GHz in a device with a recessed electrode gap of 1 μm.
The Applied Electrophysics Laboratory at the University of Virginia has developed a web based device simulation environment to accurately model and simulate terahertz and gigahertz electronic devices. This environment allows us to provide the device simulation codes developed over the years to the larger research community. In this paper, we describe the simulation environment and the particular device simulations available.
The role of integrated device/circuit simulation is critical to understanding the gigahertz-photonic operation of photomixing circuits containing metal-semiconductor-metal (MSM) devices. This work presents an efficient convolution- based time-domain approach to circuit simulation that incorporates an advanced numerical MSM device model. Complete millimeter-wave circuit simulation requires consideration of both the dynamic, high-frequency behavior of the electron and hole charge carriers in the large-signal device, and the frequency-dependent, distributed nature of the embedding circuit. The modeled device is an MBE-grown GaAs MSM photodetector with trench electrodes. Device and circuit performance is assessed by calculating the optical responsivity and bandwidth. Simulations with the device alone demonstrate the effects of a new current density boundary condition, as well as the effects of using low- growth- versus conventional-growth-temperature GaAs MSM's. Global simulations illustrate the effect that the embedding circuit has on bandwidth. Both types of simulations aid in the co-design of device and circuit, with applications to millimeter-wave generation in phased-array antennas and optoelectronic-based communication systems.16
We describe a unique and powerful global time-domain simulation technique for terahertz diodes such as GaAs Schottky diode mixers, GaAs Schottky diode frequency multipliers, and InP Transferred Electron Oscillators (TEOs). 1D, finite difference, drift-diffusion nonlinear device simulation codes have been linked with a convolution- based circuit analysis. These simulators allow designers to observe both the transient and steady state time domain behavior of the nonlinear circuits. Since physical device simulators have been used, the spatial and temporal behavior of the electrons and electric field within the device under large signal drive can be observed. This gives great insight into the internal device physics at high frequencies. The mixer code allows for the direct and fully self-consistent calculation of the conversion loss and noise temperature; the TEO code allows for fully autonomous calculation of oscillator start-up and frequency selection. Simulation results for 2.5 THz GaAs Schottky mixers and 140 GHz InP TEOs are given.
Planar metal-semiconductor-metal (MSM) devices fabricated on gallium arsenide (GaAs) are promising candidates for use as photodetectors in coherent optical communications and millimeter-wave phased-array applications. Their primary features are broad bandwidth, large responsivity, high power-handling capability, and compatibility with monolithic optoelectronic integrated circuits. We have characterized the performance of an interdigitated GaAs MSM photodetector grown by molecular beam epitaxy at 350 degree(s)C using a fast sampling technique in the time domain. A key factor for undoped GaAs material grown at this temperature is the optimal combination of both low dark current and high photocurrent. Experimental measurements are made of the temporal response of the MSM detector to optical impulses generated by a mode-locked titanium-sapphire (Ti:Al<SUB>2</SUB>O<SUB>3</SUB>) laser. Speed and responsivity are characterized over a range of optical powers and DC bias voltages. Results demonstrate that this device can switch up to 69% of the applied DC bias voltage under high optical pulsed power. Results also indicate responsivities exceeding 80 mV/pJ and bandwidths approaching 20 GHz. This high-efficiency, broad-bandwidth photodetector may find critical applications in the optical production of millimeter-wave signals by frequency conversion (mixing) and harmonic generation.