Feb. 1, 2001
Quantum-cascade structure integrates into Si substrate; Quantum dot provides single-photon emitter; Liquid crystal exhibits wide nematic range ...
Quantum-cascade structure integrates into Si substrateResearchers from the Paul Scherrer Institute (Villigen, Switzerland), the University of Neuchatel (Neuchatel, Switzerland), and the Eidgenossische Technische Hochschule (ETH; Zurich, Switzerland) have built a quantum-cascade structure on a silicon (Si) chip. Although a poor light emitter, the structure keeps the electronic transitions within a single semiconductor band and is the first step toward integrating a quantum-cascade laser into Si. The device consists of a p-type Si/silicon-germanium (SiGe) structure of SiGe quantum wells grown on top of a high-resistivity Si substrate by means of molecular beam epitaxy (MBE). The researchers achieved 10-µm intersubband electroluminescence from the device with a center at 130 meV and a full width at half-maximum of 22 meV, up to a temperature of 180 K. Device lifetimes were comparable to those obtained in III-V cascade light-emitting diodes with vertical transitions. Polarization of the emitted light provided strong evidence that the luminescence originated from the intended quantum-well transitions between the heavy-hole states of the active well. Contact Gabriel Dehlinger at [email protected].Quantum dot provides single-photon emitterResearchers at the University of California Santa Barbara have devised a reliable single-photon turnstile device using single quantum dots. The indium-arsenide quantum dots were embedded in a high-Q-factor microcavity structure and were optically pumped by a 750-nm modelocked 250-fs Ti:sapphire laser. The quantum dots measured 40 to 50 nm in diameter and about 3 nm in height, and emitted 920- to 975-nm photons at a repetition rate of 82 MHz. Laser pump power was adjusted to ensure that two or more electron-hole pairs were captured by the quantum dot during each excitation pulse so that the probability of not having an electron-hole pair in the quantum dot would be negligible and single photons would be generated at the fundamental quantum-dot exciton transition. Single photon excitation was assured by keeping the total recombination time of the multi-exciton quantum-dot state (2.2 ns) longer than the recombination time of the free electron-hole pairs (100 to 200 ps). The researchers achieved a spectral resolution of 70 µeV, a spatial resolution of 1.7 µm, and a temporal resolution of 420 ps. This technique is expected to find application in quantum computing and cryptography. The researchers project that their system could operate as fast as 1 GHz and at temperatures up to 77 K. Contact Atac Imamoglu at [email protected].Liquid crystal exhibits wide nematic rangeNew types of liquid crystal have been synthesized by a team of researchers at HRL Laboratories (Malibu, CA), the Liquid Crystal Institute at Kent State University (Kent, OH), and Raytheon (Lexington, MA). Two series of alkenyl diphenyldiacetylene liquid crystals (PTTP, where P stands for a phenyl ring and T for a carbon-carbon triple bond) were shown to exhibit high birefringence, low viscosity, and wide nematic range—characteristics in demand for optical phased arrays using liquid crystal as an electro-optic medium.

The alkenyl PTTPs were synthesized with a double bond located between the second and third carbons from the phenyl ring that caused a dramatic nematic range broadening of 30 to 40°C when compared to dialkyl PTTPs. The researchers speculate that the double bond alters the side-chain flexibility, leading to denser molecular packing, and consequently enhancing the melting and clearing temperatures. Retaining a relatively small absorption coefficient at 1.06- and 1.55-µm wavelengths, these liquid crystals are particularly attractive for use in infrared laser-beam steering, pulse shaping, and network switching. Contact Shin-Tson Wu at [email protected].

Quaternary LEDs shine in all colorsMaking a multicolored light-emitting-diode (LED) array on a monolithic substrate could be one way to produce an inexpensive flat-panel display, especially if all the LEDs were made from the same material. Researchers at the City University of New York (New York, NY) are assembling the building blocks for such a display. They have created LEDs of varying colors, all based on zinc-cadmium-magnesium-selenide (ZnCdMgSe) quaternary materials with Zn, Cd, and Mg present in differing amounts. The LEDs are grown on an indium phosphide (InP) substrate, which lattice-matches to ZnCdMgSe.

In one experiment, the researchers grew red, yellow, and green LEDs on the same piece of InP using shadow-mask selective-area epitaxy. Striped and square quantum-well structures were grown with sizes between 15 and 60 µm. In a second experiment, the researchers formed harder-to-grow blue LEDs on another InP substrate. Because red, green, and blue lasing has been observed in other devices made from these materials, the researchers posit the plausibility of full-color integrated semiconductor laser displays. White-light illumination is another possible use of this technology. Contact Maria Tamargo at [email protected].

Low-energy femtosecond pulses write directional coupler in glassResearchers at Corning Inc. (Corning, NY) have fabricated a directional planar coupler in glass using a 25-fs, 80-MHz Ti:sapphire oscillator with pulse energies two to three orders of magnitude lower than used in prior research of this type. To reduce the order of the nonlinear process to two-photon order, the scientists exposed borosilicate glass to 400-nm, 2.8-nJ pulses produced by frequency doubling the 650-mW, 800-nm emission from the femtosecond oscillator. The 400-nm pulses were focused into a 10 x 15 x 20-mm3 sample by a 0.28-NA objective with a 20-mm focal length.

The single-mode coupler has a splitting ratio of 1.9 dB at 633 nm. It contains a straight waveguide, as well as a waveguide with 5-cm-radius arcs at its ends and a 9-mm-long straight sector in the middle; the latter feature is written parallel to the straight waveguide with a 3.5-µm center-to-center distance. Depending on the conditions of the writing process and the properties of the glass used, it is possible to fabricate waveguides with different refractive-index profiles. The writing technique can be used to create several planar structures in a single glass sample. Contact Alexander Streltsov at [email protected].

Efficiency of infrared upconversion gets a boostIn infrared (IR) upconversion, long-wavelength IR light is converted to the visible portion of the spectrum, where highly sensitive and low-noise radiation detectors are readily available. Well-established techniques using pulsed laser sources have been limited in the past by low conversion efficiencies. Scientists at the University of Rochester (Rochester, NY) and Texas A&M University (College Station, TX) have proposed a new technique that they claim will produce essentially 100% efficiency for IR upconversion on a steady-state basis while maintaining diffraction-limited imaging of the field. The technique is based on quantum coherence, in which a phase-coherent atomic ensemble ("phaseonium") renders a material system transparent to resonant laser radiation while retaining the large and desirable nonlinear optical properties associated with the resonant response of a material system.

Using a four-wave interaction in atomic sodium vapor, the team illustrated the technique for conversion of 100-µm radiation. The method is claimed to be applicable over a range spanning near-IR through submillimeter wavelengths. With these improvements, upconversion could prove useful for radiation detection down to terahertz frequencies. Contact Robert Boyd at [email protected].

Light at 157 nm writes fiber gratings fastMost commercial long-period fiber gratings (LPFGs) are fabricated by writing grating structures in hydrogen-rich optical fibers using ultraviolet lasers. Such gratings serve to tailor gain in erbium-doped fiber amplifiers, among other uses. Typical light sources for fabrication include excimer lasers emitting at 248 or 193 nm. Because grating exposure sensitivities are so low, high-intensity light must be used, potentially damaging the fiber. Researchers at Photonics Research Ontario and the University of Toronto (both Toronto, Ont., Canada) are now forming LPFGs using 157-nm light from a fluorine excimer laser. Light at such a wavelength creates gratings at an exposure dose of only 5 J/cm2—a level 250 times lower than the dose required for 248-nm radiation.

Photons from the fluorine laser are higher in energy than the bandgap of typical low-germanium-content glass, directly breaking the germanium-oxygen bonds in the glass to form a grating. Evidence exists for both color-center formation and bulk densification. This process is in contrast to the slower bleaching of defect states in glass, caused by longer-wavelength radiation. The researchers were able to write gratings in low-germanium (5%) optical fibers, but not in high-germanium (8%) fibers. Contact Kevin Chen at [email protected].

Conductive oxide film transmits 248-nm lightA deep-ultraviolet-transparent (DUV) conductive oxide film would be useful as transparent electrodes in ultraviolet optoelectronic devices, as well as for antistatic layers in phase-shift masks used for DUV photolithography. Conventional transparent conductive oxides such as indium tin oxide and zinc oxide do not transmit light below 300 nm. Researchers at the Japan Science and Technology Corporation (Kawasaki, Japan) and the Tokyo Institute of Technology (Yokohama, Japan) have developed a DUV-transmitting thin film consisting of beta gallium oxide (b-Ga2O3) that has an internal transmittance of 55% at a wavelength of 248 nm for a 100-nm-thick film. This makes the material the most ultraviolet-transparent conductive-oxide thin film to date.

The researchers created films on both glass and sapphire substrates; the films had a conductivity up to 1 S/cm (electron mobility: 0.44 cm2/V/s; carrier density: 1.4 x 1019 cm-3). The films were grown using pulsed laser deposition. High transparency and conductivity were achieved by tuning the growth condition to reductive by lowering oxygen pressure and raising the substrate temperature. Doping with tin ions likely enhanced conductivity. Contact Masahiro Orita at [email protected].

Output of 0.946-µm diode-pumped Q-switched Nd:YAG laser hits new highResearchers at NASA Langley Research Center (Langley Air Force Base, VA) have achieved energy output exceeding 75 mJ at 0.946 µm using a diode-pumped Q-switched Nd:YAG laser—performance they claim exceeds other lasers of this type by a factor of 20. According to scientists Norman Barnes, Theresa Axenson, and Don Reichle, a high-energy 0.946-µm Q-switched pulse is difficult to obtain in Nd:YAG, in part because Q-switched operation requires energy storage that implies a high gain before the Q-switch is opened. Reasonable gains on the 0.946-µm quasi-four-level transition are thus dwarfed by huge gains at the 1.064-m transition. This effect, combined with high energy storage, makes Q-switched operation subject to severe nonlinear loss processes such as amplified spontaneous emission.

The scientists used multiple gain modules in a single resonator, as well as special optics, to obtain the requisite energy storage required for a 75-mJ Q-switched pulse. The design builds on prior research by Axenson, who is with Science and Technology Corp. (Hampton, VA), and had previously proved that available Q-switched energy scales almost linearly with the number of gain modules. Between the gain modules, specially coated mirrors reflect the 0.946-µm radiation and discriminate against any 1.064-µm radiation. Optical-to-optical efficiency of the device exceeds 20% in normal mode and 9% in Q-switched mode. Contact Norman Barnes at [email protected].

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