Oct. 1, 2001
SEMI to give keynote address at Marketplace Seminar; Liquid medium acts as optical diode; Photochromic polymer modulates phase; Bottom illumination doubles efficiency of RCE photodetector...

SEMI to give keynote address at Marketplace Seminar
With high-tech businesses still suffering an overcorrection in the computer and communications markets, many are wondering what the future holds for the semiconductor equipment industry. Dan Tracy, senior market analyst at Semiconductor Equipment and Materials International (Semi; Mountain View, CA), will shed some light on the subject in his keynote address, "Semiconductor Capital Equipment Trends and Outlook," at the 2002 Laser and Optoelectronics Marketplace Seminar in January.

Hosted by Laser Focus World and Strategies Unlimited (Mountain View, CA), the Marketplace Seminar provides business leaders, investors, and technology analysts with a close look at the laser and optoelectronics industries and where each market segment is headed. Presentations at the 2002 seminar will include Laser Focus World's annual review of global laser markets, including semiconductor lasers; a close look at the state of the industrial laser marketplace; and analyses of other key markets, including telecommunications and medical applications. Highlights include a review of market trends and product evolution of tunable laser diodes for optical communications by Tom Hausken, senior analyst of optical communications at Strategies Unlimited, and an overview of recent developments in the worldwide fiberoptic-communications infrastructure by Richard Mack, vice president and general manager of market research firm KMI (Newport, RI). The seminar will take place Monday, Jan. 21, 2002, during the Photonics West meeting in San Jose, CA. To register, contact the Registration Coordinator by phone at 603-891-9267, by fax at 603-891-9490, or e-mail: [email protected].

Liquid medium acts as optical diode
Scientists at the Raman Research Institute (Bangalore, India) have designed an optical device using fluorescent dye in a colloidal suspension that transmits certain wavelengths of light in one direction and prevents the transmission in the opposite direction, thus acting as an optical diode. The device consists of a 10-cm-long glass capillary 100 µm in diameter, filled with ethanol and a colloidal suspension of polystyrene microspheres (refractive index of 1.59; diameter of 0.21 µm). A drop of dye molecules inserted at one end of the capillary diffused to create a concentration gradient along the length of the capillary, which was then held horizontal and pumped by a line-focused, frequency-doubled pulsed Nd:YAG laser. Although the dye emitted in both directions from the two ends A and B of the capillary (A has a low dye concentration), a yellow wavelength peaking at 588 nm emerged from end A alone and a red wavelength of 608 nm from end B alone. Thus, the device acts as a diode, tunable by varying the density of microspheres, and may provide a means of making solid-state, exponentially graded gain media with many practical applications. Contact Sushil Mujumdar at [email protected].

Photochromic polymer modulates phase
A photochromic polymer is at the heart of an integrated optical interferometer, the phase shift of which can be optically controlled. Developed by researchers at Le Laboratoire d'Electromagnétisme Microondes et Optoélectronique (Grenoble, France) and Le Laboratoire d'Electronique de Technologie et d'Instrumentation (Gif-sur-Yvette, France), the interferometer is intended to demonstrate the optical switching capabilities of the polymer poly(methyl methacrylate) sensitized with Disperse Red 1.

The waveguide interferometer is clad on top with silica and contains a polymer-filled trench in the silica that constitutes the active zone. The two arms of the interferometer interfere at an output port, projecting fringes that show the phase shift with their movement. Light from a 1.064-nm-emitting Nd:YAG laser focused by a 25X microscope objective is coupled into the device. Light at 514.5 nm from an argon-ion laser illuminates the polymer over one inerferometer arm, causing the phase shift. The effect occurs over a write time of 2.5 s and with a write energy of 20 µJ. A photoinduced change of refractive index of approximately 0.01 causes a 4p phase shift, enough for applications such as optical switching. Contact Amparo Rodriguez at [email protected].

Bottom illumination doubles efficiency of RCE photodetector
Researchers at the Chinese Academy of Science (Beijing, China) have fabricated top-illumination and bottom-illumination silicon germanium/silicon (SiGe/Si) resonant-cavity enhanced (RCE) photodetectors on a separation-by-implanted-oxygen (SIMOX) wafer. The wafer had a top silicon layer thickness of 250 nm and a buried oxide thickness of 250 nm. Multiple quantum wells of SiGe/Si were grown on the SIMOX substrate by molecular beam epitaxy at 600°C, and a SiO2/Si distributed Bragg reflector was deposited by electron-beam evaporation to form a vertical microcavity with the buried oxide layer of the SIMOX wafer. The researchers achieved peak responsivities of 10.2 mA/W at 1285 nm in the top-illumination photodetector and 19 mA/W at 1305 nm in the bottom-illumination photodetector. The doubling of responsivity in the bottom-illumination photodetector was largely due to the ability to achieve mirror reflectivity on the order of 99%, as opposed to 80% in the top-reflectivity photodetector. The higher-efficiency microcavity in the bottom-illumination case also provided better wavelength selectivity—14-nm full-width at half maximum (FWHM) as opposed to 25-nm FWHM in the top-illumination case. External quantum efficiency increased by almost 3%. Contact Cheng Li at [email protected].

Holey fibers lead to 2R-regenerative all-optical switch
A team of researchers at the University of Southampton's Optoelectronics Research Centre (Southampton, England) has reported the fabrication of a highly nonlinear holey fiber made from pure silica with an effective area of approximately 2.8 µm2 at 1550 nm, which is believed to be the smallest effective area yet measured for a holey fiber at 1550 nm. To create such a fiber with high optical linearity and good polarization-preserving properties, a scale structure of the order of an optical wavelength with large air holes and a significant structural asymmetry of the core were used. The core diameter is approximately 1.5 µm, while the holes in the cladding range from 0.1 to 1 µm in diameter.

The team also demonstrated the use of their fiber within a simple optical switch that provides useful 2R-regenerative characteristics (amplitude restoration and pulse reshaping) based on accrued self-phase modulation and spectrally offset, narrowband optical filtering. Based on their results, the researchers believe that holey fiber technology can be extended to more-nonlinear compound glasses to ultimately enable meter-long nonlinear devices operating at approximately 100-mW switching powers. Contact Periklis Petropoulos at [email protected].

Autoparametric optical drive demonstrated for MEMS oscillators
The interaction between a laser light beam and a microelectromechanical system structure has been shown by researchers at Cornell University's Center for Materials Research (Ithaca, NY) to result in greatly enhanced performance and capabilities of the system. Self-generated vibration of a disk-shaped, single-crystal silicon micromechanical oscillator occurred when the power of a continuous-wave laser, focused on the periphery of the disk, exceeded a threshold value of 250 µW. The researchers attribute this to parametric amplification of mechanical vibration by laser light. With the laser power set just below the self-generation threshold, the mechanical quality factor Q for driven oscillations increased by an order of magnitude from 10,000 to 110,000. Self-generation and Q enhancement were provided by parametric amplification. Laser-heating-induced thermal stress changed the effective spring constant via the motion of the disk within the interference pattern of incident and reflected laser beams to provide a mechanism for parametric amplification and self-excitation. The researchers are now investigating whether a combination of a surface treatment and their optical parametric amplification technique can provide even higher Q for silicon oscillators. Contact Maxim Zalalutdinov at [email protected].

Deeply etched Bragg mirrors enable ultrashort 1550-nm lasers
Researchers at the University of Würzburg (Würzburg, Germany) and KTH Electrum (Kista, Sweden) have fabricated 1550-nm-emitting laser diodes with ultrashort 40-µm cavity lengths and threshold currents on the order of 12 mA. The experimental devices were fabricated in indium gallium arsenide phosphide/indium phosphide using metal-organic vapor-phase epitaxy. They consisted of a 500-nm lower cladding layer and a 40-nm separate-confinement heterostructure. The active region in the ridge-waveguide laser structures consisted of eight quantum wells, and the laser cavities were bounded by 400- to 600-nm-wide third- and fifth-order Bragg mirrors. In continuous-wave testing at room temperature, output powers of 0.6 mW were achieved with differential output efficiency on the order of 0.9 W/A. Output power in excess of 2 mW was achieved with a cavity length of 80 µm. Based on relative-intensity-noise measurements, the researchers estimated a thermally limited small-signal bandwidth for the devices on the order of 8.4 GHz. Contact Jürgen Hofmann at [email protected].

Ion implanting creates waveguides in polytetrafluoroethylene
Polytetrafluoroethylene (Teflon) has low loss at the visible and near-infrared wavelengths needed for optical waveguides. Its low refractive index of 1.30, however, has so far made it useful only as a cladding material. Researchers at the University of Duisberg (Duisberg, Germany) and Laval University (Quebec City, Que., Canada) have used ion implantation to create waveguides in Teflon by raising its refractive index within a region. For ion doses of 1015/cm2, a refractive index change of close to 0.1 was observed to a depth of 1 µm.

A film of Teflon was spun onto a glass substrate that was clamped onto a retroprism for leaky-mode spectroscopy. The reflectivity of 633-nm light striking the film was examined as a function of angle to study whether or not the layer could guide light. Next, a waveguide layer was implanted in the Teflon using helium ions and the experiment repeated, showing that a single mode could be guided in the layer, with coupling into the waveguide performed by tunneling through the unchanged layer. Through photochemical processes, the ion implanting created a 1.5-µm-deep depression in the Teflon as well as an index change. Contact Hilmar Franke at [email protected].

High-power ultraviolet LED grown on GaN
Using thick bulk gallium nitride (GaN) as a substrate, researchers at NTT Basic Research Laboratories (Kanagawa, Japan) have made progress toward the introduction of a compact and efficient ultraviolet light source from 200 to 360 nm similar to conventional longer-wavelength light-emitting diodes (LEDs). The device was grown by low-pressure metal organic vapor-phase epitaxy on a 500-µm-thick GaN substrate prepared by hydride vapor-phase epitaxy. A conductive GaN buffer layer was deposited on the GaN substrate to form the device structure, based on aluminum gallium nitride (AlGaN). Single quantum wells consisted of a 2.5-nm-thick Al0.04Ga0.96N well and Al0.1Ga0.9N barrier. The device was measured without coatings, mirrors, or moldings to have an output power of 0.55 mW at an injection current of 20 mA, increasing linearly to 3 mW at 100 mA. The peak wavelength was 352 nm, and estimated internal quantum efficiency was over 80%. With improvements in the nitride substrate and packaging technology to enhance extraction efficiency, the scientists claim such a compact and efficient ultraviolet-light source could replace mercury-arc lamps. Contact Toshio Nishida at [email protected].

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