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Deep-UV LEDs become more powerful; Peak wavelength of quantum-well IR detector is voltage-tunable; Stimulated Raman emission and wavelength conversion observed in silicon; MORE...

Jan 1st, 2004
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Deep-UV LEDs become more powerful

Deep-UV light-emitting diodes (LEDs) are potentially useful for the detection of biological agents, purification of water, and decontamination of equipment. In addition, their development buttresses the technology behind more-ordinary near-UV LEDs. A group at the University of South Carolina (Columbia, SC) has been steadily pushing the power and short-wavelength edge of these devices (see Laser Focus World, February 2003, p. 13); now, researchers at Sandia National Laboratories have claimed their own device-power achievements.

The flip-chip continuous-wave LEDs are on sapphire substrates and are made of aluminum gallium nitride (AlGaN), with Al reaching up to 50% of the mix. One device emits at a wavelength of 290 nm and produces 1.3 mW of output power, while a second emits at 275 nm and produces 0.4 mW. A key step in achieving the high powers was getting high-quality material growth at such high aluminum percentages, considered to be very difficult. The Sandia team is part of the SUVOS (Semiconductor Ultraviolet Optical Sources) project headed by DARPA (the Defense Advanced Research Projects Agency; Arlington, VA). Contact Andy Allerman at aaaller@sandia.gov.


Peak wavelength of quantum-well IR detector is voltage-tunable

Just as wavelength-tunable lasers open up new applications, the same is potentially true for wavelength-tunable photodetectors, a version of which has been demonstrated by researchers at the Universität Linz (Linz, Austria), the Paul Scherrer Institut (Villigen, Switzerland), and the Fraunhofer-Institut für Angewandte Festkörperphysik (Freiburg, Germany). The device, a quantum-well IR photodetector (QWIP), has quantum-cascade injector structures and is tunable through a wavelength range of 3.2 to 5.2 µm at a temperature of 77 K by varying the bias voltage.

The tunability is a result of charge transfer from deep to shallow wells, and occurs when the ground states of adjacent QWs are aligned by an applied electric field. In the device, five silicon-germanium (Si1-xGex) QWs are spaced between Si layers; the whole structure is repeated ten times between thicker Si barriers. As voltage is varied from -7 to +8 V, the responsivity and peak wavelength change, with responsivity shifting from positive to negative, resulting in two detection bands. Peak detectivity magnitudes of 1 × 109 and 1.3 × 109 cm(Hz)0.5 W-1 at 5 and 3.2 µm, respectively are approximately 10 times less than for standard QWIPs. Contact Thomas Fromherz at thomas.fromherz@jku.at.


Stimulated Raman emission and wavelength conversion observed in silicon

Building upon a previous demonstration of spontaneous Raman light emission from silicon waveguides (see Laser Focus World, April 2003, p. 19), researchers at the University of California–Los Angeles have demonstrated optical gain using stimulated Raman emission as well as wavelength conversion in silicon. They observed amplification of the Stokes signal up to 0.25 dB at 1542.3 nm using a 1427-nm pump laser at a 1.6-W continuous-wave power in a silicon-on-insulator waveguide. Coherent anti-Stokes Raman scattering (CARS), a combination of Raman and four-wave mixing, enabled the wavelength conversion from 1542.3 nm to 1328.8 nm using the same pump laser.

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The CARS method couples two pump photons and one Stokes photon through a zone-center optical phonon to an anti-Stokes photon. The research team measured a maximum Stokes/anti-Stokes power-conversion efficiency of 1 × 10-5, and achieved a bandwidth, including the pump broadening effect, of 350 GHz. The researchers suggest use of multiple pumps suitably spaced in wavelength to increase the bandwidth. They also state that further waveguide optimization along with proper design of waveguide dispersion could lead to the significant increases in gain and conversion efficiency necessary to produce practical silicon-based integrated optics and optical-network components. Contact Bahram Jalali at jalali@ucla.edu.


Chain of metal nanospheres may create brilliant points of light

Clusters of metallic nanoparticles make possible the detection of single molecules—for example in giant Raman scattering, in which the scattering with respect to isolated molecules on colloidal fractal metal clusters can be enhanced by a factor of up to 1012. The Raman enhancement is proportional to the fourth power of the local field, meaning that the local field is enhanced relative to the excitation field by a factor of on the order of 103; in other words, the metal clusters are concentrating light.

Researchers at Georgia State University (Atlanta, GA) and Tel Aviv University (Tel Aviv, Israel) have taken this phenomenon one step further and proposed a nanolens made of a chain of several metal nanospheres of progressively decreasing size and separation (for example, radii of 45, 15, and 5 nm), or a similar but symmetric arrangement. When optically excited, each sphere-to-sphere proximal region produces a concentrated point of light, with the smallest two spheres bracketing the highest concentration, which can reach 2 × 103. If fabricated, such a nanolens could be used for single-molecule spectroscopy, nanomanipulation, or photonic-crystal-waveguide termination. Contact Kuiru Li at phykx1@panther.gsu.edu.


3.4-fs optical pulse approaches monocycle length

A monocycle has nothing to do with parades or circuses, but is instead a pulse that is a single 2p cycle long. To make an optical pulse shorter, its wavelength spectrum must be made larger; short femtosecond pulses have octave-sized bandwidths. Researchers at Hokkaido University (Sapporo, Japan) have produced a 3.4-fs optical pulse with a center wavelength of 655.4 nm, more-closely approaching a monocycle length (in this case, 2.2 fs) than any pulse yet created in the visible or near-IR region.

In the setup, a 30-fs, 790-nm pulse from a Ti:sapphire laser undergoes self-phase modulation in a single argon-filled hollow fiber, then passes through an active phase compensator with a liquid-crystal spatial-light modulator at its heart (the compensator transmits over a 300- to 1500-nm band). The resulting pulse, which spans a 495- to 1090-nm spectral band, is characterized by a modified SPIDER (spectral phase interferometry for direct electric-field reconstruction) technique. The 3.4-fs duration is close to the transform-limited (or theoretically shortest for that particular spectral band) pulse of 3.0 fs. Contact Keisaku Yamane at k-yamane@eng.houdai.ac.jp.


Radially polarized beam focuses to smallest spot

In another approach to concentrating light, researchers at the University Erlangen-Nuremberg (Erlangen, Germany) have determined what they say are the qualities of a focused free-space beam that produces the smallest spot size at focus, and have experimentally verified the results. Using a 632.8-µm beam and a 0.9-numerical-aperture objective, they showed that a radially polarized beam (in combination with an annular aperture) can produce spot sizes as small as 0.16λ2—smaller than the 0.26λ2 equivalent for a beam of linearly polarized light and 0.22λ2 for one of circularly polarized light.

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Their reasoning behind the discovery is especially interesting. When emitting photons, an atom (an oscillating dipole) produces a characteristic emission intensity versus direction, with all emitted light polarized parallel to the dipole moment. Because Maxwell's equations are symmetric with respect to time, it should be possible to reverse the emitted wavefront such that it converges to atomic dimensions. In terms of practical light beams, this ultimate result is not possible, but if a portion of a dipole's emission were to be collimated in the direction of the dipole, the resulting beam would be radially polarized—illustrating why such a beam focuses so compactly. Contact Gerd Leuchs at gerd.leuchs@physik.uni-erlangen.de.


Butterfly-wing-like structure produces same color over wide viewing angles

While most butterfly wings produce their color through thin-film interference, a few do so also through grating-style diffraction. The wings of the morpho butterfly produce an intense blue color viewable in this manner over a wide viewing angle—much wider than can be produced by a standard grating, which shifts colors as it is tipped. Researchers at Old Dominion University (Newport News, VA) and Alcoa Corporation (Richmond, VA and Alcoa Center, PA) have produced an artificial version of the morpho grating structure that barely shifts in color over a wide viewing angle.

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Although the morpho butterfly produces color through multilayer interference as well as diffraction, the researchers concentrated on duplicating the latter. Because the butterfly's wing contained many small gratings oriented in a random fashion, they used electron-beam lithography to fabricate a pattern of small hexagonal-close-packed gratings with three different orientations in photoresist on silicon. The gratings had equal lines and spaces and a linewidth of 0.22 µm. The resulting structure appears blue over viewing angles from 16° to 90°, and bluish-green for 0° to 16°. Similar structures may one day color consumer products. Contact Mool Gupta at mgupta@odu.edu.


Chiral fiber has polarization-selective stop band

A chiral optical fiber (one with left- or right-handed characteristics) can have unusual properties. Researchers at Chiral Photonics (Clifton, NJ) and Queens College, City University of New York (Flushing, NY) have fabricated microwave-transmitting chiral fibers with double-helical symmetry that show a polarization-selective photonic stop band only for circularly polarized light with the same handedness as the fiber structure. In the single-mode regime, a stop band exists at a wavelength (within the fiber) equal to the pitch of the helix.

Starting with a rod of high-impact polystyrene, two helical grooves were cut such that the fiber had a rectangular cross section of 13 × 8 mm at any point. The helix had 78 pitches with a pitch of 16 mm. A stop band was observed with a width of 0.01 of the center wavelength (about 20.5 mm) and a drop in transmission of about 28 dB. Cutting the fiber in half and rotating one part 85º caused a narrow pass band to appear within the stop band. Use in the microwave region facilitates easy fabrication; scaling to optical wavelengths is possible. Contact Victor Kopp at vickopp@chiraloptics.com.


Optical lattice sorts particles in flowing fluid

Taking the concept of optical manipulation to the ultimate degree, scientists at the University of St. Andrews (Fife, Scotland) and Illinois Wesleyan University (Bloomington, IL) have created an optical lattice that can sort large quantities of particles of different size, or of the same size but of different materials, without any reliance on fluorescence activation. The sorting efficiency of the lattice approaches 100%.

The lattice is created by passing a 1070-nm laser beam with 530 mW of optical power through a diffractive beamsplitter, which produces four beams that diverge from the central beam at 4.6° angles in a cross pattern. The amplitude and phase of each of the five resulting beams can be controlled independently. When simultaneously focused by a single lens, the beams interfere to produce a lattice greater than 40 µm in size. A stream of 2-µm silica and polystyrene spheres in a laminar flow of water (reaching or exceeding velocities of 35 µm/s) is separated, with a 45° angle of separation that allows the two types of spheres to be sent down separate collection tubes. Similarly, monodisperse (2-µm) protein microcapsules are separated from a polydisperse particle stream moving at 20 µm/s. Contact Michael MacDonald at mpm@st-and.ac.uk.

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