Carrier concentration profiles of two-dimensional electron gases are investigated in wurtzite, Ga-face AlxGa1−xN/GaN/AlxGa1−xN and N-face GaN/AlxGa1−xN/GaN heterostructures used for the fabrication of field effect transistors. Analysis of the measured electron distributions in heterostructures with AlGaN barrier layers of different Al concentrations (0.15<x<0.5) and thickness between 20 and 65 nm demonstrate the important role of spontaneous and piezoelectric polarization on the carrier confinement at GaN/AlGaN and AlGaN/GaN interfaces. Characterization of the electrical properties of nominally undoped transistor structures reveals the presence of high sheet carrier concentrations, increasing from 6×1012 to 2×1013 cm−2 in the GaN channel with increasing Al-concentration from x=0.15 to 0.31. The observed high sheet carrier concentrations and strong confinement at specific interfaces of the N- and Ga-face pseudomorphic grown heterostructures can be explained as a consequence of interface charges induced by ...

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Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures

We review the field of femtosecond pulse shaping, in which Fourier synthesis methods are used to generate nearly arbitrarily shaped ultrafast optical wave forms according to user specification. An emphasis is placed on programmable pulse shaping methods based on the use of spatial light modulators. After outlining the fundamental principles of pulse shaping, we then present a detailed discussion of pulse shaping using several different types of spatial light modulators. Finally, new research directions in pulse shaping, and applications of pulse shaping to optical communications, biomedical optical imaging, high power laser amplifiers, quantum control, and laser-electron beam interactions are reviewed.

Femtosecond pulse shaping using spatial light modulators

We examine the current performance and future demands of interconnects to and on silicon chips. We compare electrical and optical interconnects and project the requirements for optoelectronic and optical devices if optics is to solve the major problems of interconnects for future high-performance silicon chips. Optics has potential benefits in interconnect density, energy, and timing. The necessity of low interconnect energy imposes low limits especially on the energy of the optical output devices, with a ~ 10 fJ/bit device energy target emerging. Some optical modulators and radical laser approaches may meet this requirement. Low (e.g., a few femtofarads or less) photodetector capacitance is important. Very compact wavelength splitters are essential for connecting the information to fibers. Dense waveguides are necessary on-chip or on boards for guided wave optical approaches, especially if very high clock rates or dense wavelength-division multiplexing (WDM) is to be avoided. Free-space optics potentially can handle the necessary bandwidths even without fast clocks or WDM. With such technology, however, optics may enable the continued scaling of interconnect capacity required by future chips.

Device Requirements for Optical Interconnects to Silicon Chips

Intracavity semiconductor saturable absorber mirrors (SESAM's) offer unique and exciting possibilities for passively pulsed solid-state laser systems, extending from Q-switched pulses in the nanosecond and picosecond regime to mode-locked pulses from 10's of picoseconds to sub-10 fs. This paper reviews the design requirements of SESAM's for stable pulse generation in both the mode-locked and Q-switched regime. The combination of device structure and material parameters for SESAM's provide sufficient design freedom to choose key parameters such as recovery time, saturation intensity, and saturation fluence, in a compact structure with low insertion loss. We have been able to demonstrate, for example, passive modelocking (with no Q-switching) using an intracavity saturable absorber in solid-state lasers with long upper state lifetimes (e.g., 1-/spl mu/m neodymium transitions), Kerr lens modelocking assisted with pulsewidths as short as 6.5 fs from a Ti:sapphire laser-the shortest pulses ever produced directly out of a laser without any external pulse compression, and passive Q-switching with pulses as short as 56 ps-the shortest pulses ever produced directly from a Q-switched solid-state laser. Diode-pumping of such lasers is leading to practical, real-world ultrafast sources, and we will review results on diode-pumped Cr:LiSAF, Nd:glass, Yb:YAG, Nd:YAG, Nd:YLF, Nd:LSB, and Nd:YVO/sub 4/.

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Semiconductor saturable absorber mirrors (SESAM's) for femtosecond to nanosecond pulse generation in solid-state lasers

We present what is to our knowledge the first imaging system based on optoelectronic terahertz time-domain spectroscopy Terahertz time-domain waveforms are downconverted from the terahertz to the kilohertz frequency range, and the waveform for each pixel is frequency analyzed in real time with a digital signal processor to extract compositional information at that point We demonstrate applications to package inspection and chemical content mapping in biological objects

Imaging with terahertz waves

Nowadays, smart composite materials embed miniaturized sensors for structural health monitoring (SHM) in order to mitigate the risk of failure due to an overload or to unwanted inhomogeneity resulting from the fabrication process. Optical fiber sensors, and more particularly fiber Bragg grating (FBG) sensors, outperform traditional sensor technologies, as they are lightweight, small in size and offer convenient multiplexing capabilities with remote operation. They have thus been extensively associated to composite materials to study their behavior for further SHM purposes. This paper reviews the main challenges arising from the use of FBGs in composite materials. The focus will be made on issues related to temperature-strain discrimination, demodulation of the amplitude spectrum during and after the curing process as well as connection between the embedded optical fibers and the surroundings. The main strategies developed in each of these three topics will be summarized and compared, demonstrating the large progress that has been made in this field in the past few years.

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Fiber Bragg grating sensors toward structural health monitoring in composite materials: challenges and solutions.

We demonstrate integration of GaAs-AlGaAs multiple quantum well modulators to silicon CMOS circuitry via flip-chip solder-bonding followed by substrate removal. We obtain 95% device yield for 32/spl times/32 arrays of devices with 15 micron solder pads. We show operation of a simple circuit composed of a modulator and a CMOS transistor. >

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GaAs MQW modulators integrated with silicon CMOS

We present a micromechanical modulator for fiber-in-the-loop applications with projected optical bandwidths from 1.3 to 1.55 /spl mu/m and data rates of several Mbits/sec. The device behaves as a damped oscillator. We have made a device with a ringing frequency of 1.1 MHz and a damping time constant of 1 /spl mu/s. We indicate that with an appropriate linear filter the device could operate digitally with data rates of 1 Mbit/sec. >

Silicon modulator based on mechanically-active anti-reflection layer with 1 mbit/sec capability for fiber-in-the-loop applications

A direct-view display comprising an array of micro-mechanical modulators is disclosed, he modulator used to form the display comprises a suspended, vertically moving membrane and a substrate. The device functions based on optical interference effects between the membrane and the substrate which cause the modulator to either substantially reflect or absorb an optical signal. The interference effects are a function of the size of the air gap between the membrane and substrate, which varies as the membrane moves. The membrane moves in response to a data signal, representative of an image, delivered to the modulator. The display generates an image based on the pattern of light and dark sections of the display corresponding to the reflectivity of each modulator at a given point in time.

Direct view display based on a micromechanical modulation

An apparatus for modulating an optical signal and a method for fabricating such an apparatus, are disclosed. The apparatus, which may be formed on a semiconductor wafer or chip, consists of a membrane that is supported over a substrate by flexible support arms. An air gap is defined between the membrane and substrate. The membrane thickness and refractive index are chosen so that the reflectivities of the membrane and substrate are not 180° out of phase. Under the action of bias, the membrane moves vertically from a first position to a second position relative to the substrate, changing the air gap. The reflectivity of the modulator changes as the air gap changes. Membrane thickness and the air gap are suitably selected to achieve zero overall modulator reflectivity in one of the two membrane positions. Equations define acceptable values for the thickness of the membrane thickness and the air gap. The membrane and air gap are formed by various etching and photolithographic methods.

Keith W. Goossen

Papers

Fiber Bragg grating sensors toward structural health monitoring in composite materials: challenges and solutions.

GaAs MQW modulators integrated with silicon CMOS

Silicon modulator based on mechanically-active anti-reflection layer with 1 mbit/sec capability for fiber-in-the-loop applications

Direct view display based on a micromechanical modulation

Phase-mismatched fabry-perot cavity micromechanical modulator