Newsbreaks

Sept. 1, 2000
Photonic crystal achieves full bandgap in near-infrared; Dye-doped polymer optical-fiber laser reaches high efficiency; Multilayer structure boosts photodetector performance; Film of metal particles guides light ...

Photonic crystal achieves full bandgap in near-infrared

Scientists at Kyoto University (Kyoto, Japan) and the Ministry of International Trade and Industry (Ibaraki, Japan) have lithographically constructed a photonic crystal that has a full photonic bandgap at a 1.2-µm wavelength and is made from either gallium arsenide or indium phosphide, both semiconductors amenable to integration with light-emitting devices. The bandgap effect reaches more than 40 dB, or a reflection of 99.99%. If full bandgap is defined as more than 80% attenuation, then the bandgap covers the 1.3- to 1.55-µm telecommunications region.

The crystal is made in a "woodpile" configuration, in which layers of stripes are stacked in orthogonal directions. Every layer is shifted from its counterpart two layers below by half a period. The structure is assembled by bringing two wafers together in a wafer-fusion process, then removing one substrate; the process is repeated for up to eight layers. A stripe period of 0.7 µm necessitates a 30-nm alignment tolerance between layers to prevent a loss of the full bandgap. The researchers introduced a defect consisting of two removed orthogonal stripes, creating a waveguide with a 90° bend with a predicted transmission of greater than 95%.

Contact Susumu Noda at[email protected].

Dye-doped polymer optical-fiber laser reaches high efficiency

Solid-state dye lasers are far simpler than their liquid counterparts and often expose the researcher to less toxicity. Optical-fiber lasers also offer simplicity as a result of their waveguide makeup. Scientists at Keio University (Yokohama, Japan), Kanagawa Academy of Science and Technology (Kawasaki, Japan), and Polytechnic University (Brooklyn, NY) have combined both in a dye-doped polymer fiber laser. Although solid-state dye-doped fiber lasers have been made before, this version is the first to achieve high efficiency.

A graded-index optical-fiber preform made of poly(methyl methacrylate-co-2-hydroxyethyl methacrylate) and containing 0.1% rhodamine 6G dye by weight was drawn into a fiber with a 0.6-mm core diameter. When transversely pumped with 532-nm light in 1.5-mJ pulses from a frequency-doubled Q-switched Nd:YAG laser, the fiber emitted yellow light in 0.64 mJ pulses, for a slope efficiency of 43% (a comparable bulk solid-state dye laser reached only 7.7% slope efficiency). The experiments were performed at room temperature without external cooling. The output energy of the fiber laser dropped to half its original value after 110,000 pump-laser shots; the researchers ascribe such long life to the protection from oxygen provided by the fiber cladding. The group is investigating other laser-host media and pump-cavity configurations. Contact Yasuhiro Koike at[email protected].

Multilayer structure boosts photodetector performance

Researchers at Princeton University (Princeton, NJ) have used a multilayer stack structure to increase the bandwidth and quantum efficiency obtainable with organic photodetector materials to approximately 430 MHz and 75%, respectively. The active regions of their devices consisted of alternating layers of copper phthalocyanine (CuPc) and 3,4,9,10-perylenetertracarboxylic bis-benzimidazole grown on top of a transparent, 1500-Å-thick indium tin oxide anode atop a glass substrate. The CuPc layer was grown first atop the anode, and the entire layer-structure thickness was 320 Å. The number of layers, however, varied from two layers, each occupying half the thickness, to 64 layers that were 5 Å thick. The construction of photodetectors based on small-molecule multilayer organic thin films provided the high-performance results by facilitating the transport of photogenerated charge carriers to the electrodes prior to the occurrence of recombination or deep-charge trapping. The researchers expect devices based on this technology to find use in high-speed and large-area imaging applications in the visible and near-infrared spectral regions. Contact Stephen Forrest at [email protected].

Film of metal particles guides light

Brian Soller and Dennis Hall of the University of Rochester (Rochester, NY) have observed the propagation of guided light waves confined to the surface of a discontinuous film of silver (Ag) nanoparticles. The 457.9-nm light, which was p-polarized, was confined by a two-dimensional disordered array of particles with average diameter of 400 nm and average height of 150 nm. The finding has significance for the design of nanocomposite optical materials.

The mode propagates at frequencies close to the strong plasma-resonance absorption in the film and is least-lossy at frequencies where the nanoparticle layer has a large absorption. The mode is likely the result of coupling between incident light and the particle plasma resonances of the nanoparticle layer, say the researchers. The Ag particles themselves were formed on top of a 30-nm-thick layer of lithium fluoride (LiF) by evaporation in vacuum; surface irregularities in the LiF aided in the production of evenly sized Ag particles. No signs of surface waves were observed for s-polarized radiation. By comparing theoretical calculations with reflectivity data, the researchers determined that the TM2 mode is responsible for the excitation driving the surface-mode propagation. Contact Brian Soller at[email protected].

Thin twisted-nematic device achieves achromatic polarization rotation

Researchers at Pennsylvania State University (University Park, PA) have theoretically and experimentally demonstrated a simple device for achromatic polarization rotation based on the eigenmode of a twisted-nematic (TN) structure. The experimental device consisted of a thin (1.9-µm) TN liquid-crystal (LC) cell placed between two homogeneous LC waveplates. The TN cell had a specific twisted angle (phi) and was oriented parallel to a specific linear polarization angle (alpha) of incident radiation. The homogeneous LC cells were oriented such that the entrant and emergent polarizers were parallel to the entrant and emergent directors of the TN cell (alpha-45° and alpha+phi+45°, respectively). The purpose of the homogeneous LC cells was to transform the polarization state of all wavelengths between the linear polarization state and their corresponding eigenmodes. The theoretical explanation of the device was based on a Poincaré-sphere (PS) model representing the state of polarization as a curve on the PS. By combining the homogenous LC cell with the TN cells, the researchers achieved an emergent polarization rotation of 90° compared to incident polarization with almost 100% transmission over a wavelength range of 450 to 700 nm. Contact Zhizhong Zhuang at[email protected].

Vacuum-ultraviolet laser produces 10-µJ pulses at 52.9 nm

Helium is useful as a carrier gas for some experiments in photochemistry and photophysics (the study of laser-created nanoclusters, for example); these experiments benefit from laser sources capable of ionizing neutral atoms but falling short of the helium (He) photoionization threshold. One such source has been developed by researchers at Colorado State University (Fort Collins, CO). They have demonstrated a tabletop laser using neonlike chlorine as a gain medium that emits at 52.9 nm, just below the He ionization threshold. The laser emits 10-µJ pulses at 1 Hz, producing a beam with a divergence of 4 mrad.

The gain medium is produced by rapidly exciting a 3.2-mm inside-diameter capillary channel filled with preionized chlorine gas; a current pulse produces a plasma column 18.2 cm long. The fast current pulse compresses the column, causing collisional electron excitation and thus a population inversion. The optimum pressure for lasing was found to be 224 mTorr. The device produces pulses having a full-width-at-half-maximum duration of 1.46 ±0.25 ns and a peak power of 7 kW. The far-field pattern has a Gaussianlike shape. Contact Jorge Rocca at[email protected].

Gallium nitride avalanche photodiodes achieve UV photon counting

Researchers at Massachusetts Institute of Technology Lincoln Laboratory (Lexington, MA) have achieved ultraviolet photon counting with gallium nitride (GaN) avalanche photodiodes (APDs). The device structure, grown in a vertical chloride-transport hydride vapor-phase-epitaxy reactor on top of an unintentionally doped 10- to 15-µm-thick GaN layer, included a 0.6-µm-thick silicon-doped layer, an unintentionally doped layer, and a 0.2-µm-thick zinc-doped layer. A voltage source in series with a load resistor (50 to 200 kOmega) was used to temporarily bias the device above the breakdown voltage. The device underwent avalanche breakdown when the bias exceeded -86 V, while the load resistor reduced the load on the APD as its own internal resistance dropped during breakdown. Peak current values greater than 0.5 mA were measured for an effective gain on the order of 107. The maximum photon detection efficiency was 13% at 325 nm, room temperature, and less than 425-kHz dark-count rate. Contact Alexander McIntosh at[email protected].

Zinc selenide-based LED shows long life

When made into light-emitting diodes (LEDs), wide-gap II-VI semiconductors such as zinc selenide (ZnSe) emit light in the blue-green part of the spectrum. If it weren't for their short lifetimes, such devices could be competition for gallium nitride-based LEDs. Adding tellurium to the p layers of a ZnSe LED stabilizes the nitrogen receptor, lengthening lifetimes; also important is proper lattice spacings in the various layers of the device, leading to a tensile strain of the quantum well.

Researchers at the Universität Würzburg (Würzburg, Germany) have satisfied both these needs by building an LED that includes a layer of zinc magnesium selenide telluride, a zinc cadmium selenide quantum well, and an indium phosphide substrate. In contrast to conventional II-VI diodes made on a gallium arsenide substrate with a similar density of extended defects, the new LED shows no formation of dark-line defects, resulting in a lifetime increased by three orders of magnitude. Although the first such LED died catastrophically after 150 h when a contact failed, a reduction of the current density should increase device lifetime. Lifetime estimates for the first fabricated structure (which has only moderate quality) are at least several thousand hours. Contact Wolfgang Faschinger at[email protected].

OPO produces mid-infrared light at low pump power

Developing optical parametric oscillators (OPOs) that produce pulses at wavelengths beyond 5 µm has been difficult due to high idler absorption losses in the midinfrared. Although substantially boosting the power of the pump laser is one solution, this alternative rules out the use of small, convenient pulsed or continuous-wave lasers. Scientists at the University of St. Andrews (Fife, Scotland) have developed a low-power alternative.

Based on periodically poled lithium niobate synchronously pumped by a modelocked Ti:sapphire laser, their device uses a semimonolithic cavity design and hemispherical focusing to achieve operation at pump power thresholds of 17 mW and midinfrared idler powers of up to 64 mW in the 3.9- to 6-µm spectral range. Pulse-repetition rates reach to 322 MHz. The OPO produces a 280-mW signal output at 35% extraction efficiency, emitting transform-limited signal pulses of 0.4-ps duration and 1- to 1.14-µm wavelength. Minimizing the number of cavity components and coated surfaces—for example, coating the output mirror directly onto the nonlinear crystal—led to a low threshold and high stability. The maximum idler output was 64 mW at 4.3 µm, with output decreasing to 10 mW at 5.5 µm and 0.5 mW at 5.9 µm. Contact Majid Ebrahimzadeh at[email protected].

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