Newsbreaks

April 1, 2001
QC laser becomes free-space communications link; Laser prints active indium-oxide optical microstructures; Extra layers make ZnTe quantum-well structure lase in green...

QC laser becomes free-space communications link
Quantum cascade (QC) lasers provide light at tailor-made wavelengths in the mid-infrared between 3.5 and 20 µm. As a result of the unipolar intersubband nature of the lasing mechanism, QC lasers should have very good direct-current modulation properties, with theoretical predictions showing an upper limit of several hundred gigahertz. The combination of favorable modulation characteristics and a wavelength that can be positioned within a window of atmospheric transparency make the QC laser a potential candidate for high-speed free-space optical transmission. Researchers at Lucent Technologies Bell Laboratories (Murray Hill, NJ) and the Stevens Institute of Technology (Hoboken, NJ) have taken a step toward demonstrating this potential.

A QC laser emitting 7.347-µm light at an average power of 10 mW was modulated at speeds between 100 MHz and 2 GHz. Light from the laser was collected at a distance of 1.5 m away by a mercury-cadmium-telluride detector. Data collected by the researchers show a signal-to-noise ratio of 35 dB at 2 GHz, allowing them to estimate a 5-GHz cutoff frequency. Next, a 10-MHz audio and video signal centered at 66 MHz was transmitted over a 70-m distance (limited by laboratory size) with no noticeable loss in quality. More-advanced modulation schemes are under investigation. Contact Rainer Martini at [email protected].

Laser prints active indium-oxide optical microstructures
Indium-oxide (InOx) diffractive optical microstructures can be activated with light. Researchers at the Hellas Institute of Electronic Structure and Laser (Heraklion, Greece) are fabricating nonstoichiometric InOx surface-relief gratings using reactive pulsed-laser deposition. In the process, InOx thin-film material is selectively transferred onto a glass substrate by ultrafast-laser microprinting via a demagnified photomask to form the structure. The microprinting laser—a hybrid distributed-feedback dye laser and krypton-fluoride excimer laser—delivers 500-fs pulses; the InOx film is selectively ablated onto a receiver substrate placed 2 to 5 µm from the source substrate. A step-and-repeat process forms the structure.

The resulting transmissive grating has a 10-µm period, a size of 2 x 2 mm, and an initial diffraction efficiency of 5%. A normal-incidence 325-nm helium cadmium laser activates the grating within 4 s, which then diffracts a collinear 633-nm beam from a helium neon laser at higher efficiency. Switching the activation light off causes the InOx to revert to its original state in about 4 s. The change in diffraction efficiency reaches approximately 10%. The maximum efficiency change exhibited a strong temperature dependency, achieving its maximum of 10% at 40°C, dropping to zero at 30°C, and dropping further to -8% at 20°C. Optimizing the grating height will greatly boost efficiency. The researchers have also activated microprinted InOx gratings electrically. Contact Nikos Vainos at [email protected].

Extra layers make ZnTe quantum-well structure lase in green
Although robust and long-lived blue-output semiconductor lasers exist in the form of gallium-nitride-based laser diodes, no practical green equivalent has ever been developed despite much research. Scientists at Tohoku University (Sendai, Japan) are working with a new material they believe has potential for pure-green light-emitting devices. Quantum-well (QW) structures based on zinc telluride (ZnTe), a wide-gap semiconductor, are fabricated that contain some layers of ZnTe made into ternary and quaternary compounds containing cadmium, magnesium, and selenium. In some of these devices, the researchers have achieved room-temperature optically pumped lasing.

The alloy composition and thickness of the QW layers are monitored during growth by tracking high-energy electron diffraction intensity oscillations. The QW structure contains 9-nm layers of ZnTe inserted between the QW and cladding layers to reduce interface fluctuations such as interdiffusion, thickness variations, and variations in composition. When optically pumped at 355 nm, the QW structure shows bright intrinsic emission even under weak excitation. When pumping is increased, lasing occurs at 552 nm. The high threshold power of 215 kW/cm2 will be reduced by optimizing the laser structure, say the researchers. Contact Jamaica Chang at [email protected].

Polymeric modulator has zero state with no electrical bias
A digital optical modulator based on an asymmetric Y-branch waveguide has been fabricated by Sang-Yung Shin and colleagues at the Advanced Institute of Science and Technology (Taejon, Korea). The modulator requires no precise control of the bias and the drive voltage because of its digital response, which makes it desirable for an optical modulator array. The narrow arm of the branch is used for output, while the wide arm is used for monitoring the output. The operating point is initially shifted to the off-state, using the asymmetry in the branch to provide an initial zero-state with no electrical bias, unlike interferometric modulators. The core layer is made from an electro-optic polymer using the so-called Disperse Red 1 dye side-chain attached to a poly(methylmethacrylate) backbone. The optimum output waveguide-pattern width of the modulator was determined to be 5.8 µm. The high extinction ratio was obtained with a low voltage; an extinction ratio of 25 dB was demonstrated for a drive voltage of 20 V at an electro-optic coefficient of 5 pm/V at 1.3 µm. Contact Sang-Yung Shin at [email protected].

Epitaxial process produces dual lasers for DVD pickups
Two-wavelength integrated laser diodes (TWINLDs) with aluminum-free active areas have been fabricated by researchers at the National Chiao Tung University (Taiwan, Republic of China). Such devices reduce the number of optical components in a digital-versatile-disk (DVD) optical pickup head, as well as the dimension of the head, assembly time, and overall cost. The team used a controlled regrowth technique carried out in a low-pressure metalorganic chemical-vapor-deposition system containing a multiwafer rotating disk (7 x 2-in. wafers). A gallium arsenide substrate having a surface 10° off in orientation from the (100) lattice plane was used to avoid the ordering phase of aluminum gallium indium phosphide during the epitaxial growth. The TWINLD emits red light at 650 nm and infrared at 780 nm with good performance. The distance between the two emission spots of the monolithic TWINLD can be defined exactly by photolithography. The adoption of etching stop layers allowed the device to maintain high performance. This monolithic single-chip approach may be applicable to next-generation DVD systems using a 400-nm blue laser diode. Contact Shing Wang at [email protected].

Ferroelectric barium titanite adds tunability to photonic crystal
Ferroelectrics constitute a special group of materials with high dielectric constants and tunable dielectric properties that vary with external conditions, such as temperature, electric field, and pressure. Introducing such materials into a photonic crystal could thus help modulate its bandgap. In line with this, researchers at Nanyang Technological University (Singapore), working with colleagues at Tsinghua University (Beijing, China), have synthesized a silicon-dioxide colloid crystal infilled with barium titanite (BaTiO3). The process combined a self-assembly method with a sol-gel technique. In the vicinity of the ferroelectric phase-transition point of BaTiO3 (100 to 150°C), the photonic bandgap of the resulting assembly exhibited a strong temperature dependence. At the Curie point, they identified a 20-nm redshift in the bandgap. This is also where optical transmittance is at a minimum. According to the scientists, the tuning of the bandgap could be used not only for simple on-off switching, but also in devices requiring more localized control of light propagation. Contact Ji Zhou at [email protected].

Long monolithic-cavity VCSELs provide high single-mode output power
Scientists at the University of Ulm (Ulm, Germany) have fabricated selectively oxidized vertical-cavity surface-emitting lasers (VCSELs) operating near 980 nm that have effective cavity lengths of up to 9.2 µm and an integrated n-gallium-arsenide spacer up to 8 µm long. Using a device with a 6.5-µm aperture, they achieved a maximum single-mode output power of 5.4 mW with a side-mode suppression ratio exceeding 30 dB. The VCSEL thus combines high single-mode power with a full-width-at-half-maximum far-field angle below 9° and a differential series resistance below 85 W. Future research efforts of the Ulm team will explore the practical limits to cavity length and single-mode output power. The group also plans to transfer the technology to 850-nm VCSELs using aluminum-gallium-arsenide spacer layers. Contact Karl Ebeling at [email protected].

Organic solar cell achieves threefold efficiency improvement
The search for inexpensive photovoltaic power seems to point to solar-cell materials that can be made in sheets suitable for mass manufacture; lower optical-to-electrical efficiencies of such materials (as compared to crystalline silicon cells) would be more than compensated by the low cost of fabrication. Although amorphous silicon on a substrate is the most well-known sheet-based solar cell, organic materials also can be used as photovoltaic devices. Now, researchers at Johannes Kepler University and Quantum Solar Energy Linz (both of Linz, Austria) and the University of Groningen (Groningen, The Netherlands) have developed an organic solar cell that has a 2.5% efficiency, three times higher than previously reported values for such a device (although still lower than that for amorphous silicon).

The material used is a conjugated polymer/methanofullerene blend. The researchers discovered that the solvent used to cast the organic cells has a large effect on efficiency. By casting with fluorobenzene as opposed to toluene, they were able to make solar cells that had a surface with much more uniform fullerene concentration. The short-circuit current density, fill factor, and charge-carrier mobility all improved with the change in solvent. Monochromatic power conversion efficiency at 488 nm reached 9.5%. Contact Cristoph Brabec at [email protected].

Crystal generates red, green, and blue laser light simultaneously
Researchers at Universidad Miguel Hernández (Alicante, Spain) and Universidad Autónoma de Madrid (Madrid, Spain) have simultaneously generated red, green, and blue continuous-wave (CW) laser radiation using a neodymium-doped aperiodically-poled lithium niobate crystal. The crystal used in the experiment was 3-mm long with a cross-sectional area of 4 x 4 mm. It was doped with a 0.7 mol % concentration of neodymium ions and had a chirped ferroelectric domain distribution. The crystal was placed in a 9.8-cm-long, quasi-hemispherical linear laser cavity bounded by a near-flat dichroic input mirror and a 10-cm radius-of-curvature output coupler. The experimental laser was pumped with the focused CW 744-nm output of a Ti:sapphire laser. The input mirror was highly transmissive for the pump signal but highly reflective for the 1084- and 1372-nm fundamental infrared wavelengths of the experimental laser. Red light at 686 nm and green light at 542 nm were obtained by self-frequency-doubling the fundamental laser lines at 1084 and 1372 nm. Blue light at 441 nm was obtained by self-sum-frequency mixing of the pump wavelength and the 1084-nm laser line. Contact Juan Capmany at [email protected].

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