Newsbreaks

March 1, 2001
Photonic crystal increases LED efficiency; Wavelength conversion uses SOA-based interferometric devices; Single-proton implantation tunes QWIP detection wavelength ...

Photonic crystal increases LED efficiency
Although semiconductor light-emitting diodes (LEDs) show potential for use as low-cost, long-life solid-state lighting sources for a variety of applications, they have been limited by low light-extraction efficiency—most of the light emitted has been lost to guided modes within the high dielectric material. Scientists at the Research Laboratory of Electronics and the Center for Materials Science and Engineering at Massachusetts Institute of Technology (Cambridge, MA) may have resolved the problem by using a two-dimensional photonic crystal to enhance light extraction in the vertical direction as much as sixfold. The crystal also couples pump light at normal incidence into the structure, which strengthens optical excitation. Scientists etched the photonic crystal, which consists of a triangular lattice of holes, into the upper cladding layer of an asymmetric LED structure that emits at 980 nm. The structure includes an indium-gallium-phosphide/indium-gallium-arsenide (InGaP/InGaAs) active region on top of a low-dielectric spacer layer and a distributed Bragg reflector. The asymmetric active region consists of 32 nm of InGaP beneath an 8-nm InGaAs quantum well. The upper InGaP has a thickness of either 95 or 158 nm. Contact Alexei Erchak at [email protected].

Wavelength conversion uses SOA-based interferometric devices
Researchers at the Technical University of Denmark (Lyngby) have demonstrated a novel scheme for all-optical wavelength conversion in semiconductor-optical-amplifier (SOA)-based interferometric devices. David Wolfson and colleagues investigated the scheme experimentally at 10 Gbit/s and found simultaneously high modulation bandwidth and excellent transmission properties. They accomplished this by injecting an additional clock signal into the lower interferometer arm, with the signal synchronized to the data signal at a repetition rate corresponding to the bit rate.

When the clock and data signals are identical, a logical "1" is present in the data signal. The continuous-wave light experiences equal phase conditions in the interferometer arms, resulting in a constant output power. When a "0" results in the data signal, the interferometer is unbalanced, causing a pulse at the output, entirely determined by the clock signal. The resulting eye diagram shows narrower peaks when compared to the eye diagram of the conventional scheme, indicating an improvement in speed performance. Bit-error rates are also greatly improved for transmission through 25 km of fiber. The scheme is applicable for use with both a Mach-Zehnder and a Michelson device. Contact David Wolfson at [email protected].

Single-proton implantation tunes QWIP detection wavelength
Researchers at the Australian National University (Canberra, Australia) and the Chinese Academy of Science (Shanghai, People's Republic of China) have used high-energy proton implantation to tune the detection wavelengths of quantum-well infrared photodetectors (QWIPs).

A single high-energy proton implantation at 0.9 MeV provided homogeneous quantum-well intermixing across a structure of 50 quantum wells, each consisting of 4.5 nm of silicon-doped gallium arsenide (GaAs) with 50-nm undoped aluminum GaAs barriers. Grown by molecular beam epitaxy on a semi-insulating GaAs substrate, the quantum wells were sandwiched between 2-µm-top and 1.3-µm-bottom n+ GaAs contact layers grown on a 0.5-µm aluminum arsenide layer.

The implantation procedure, performed at room temperature, consisted of masking with the QWIP material to provide a reference portion without implantation and implantation portions with doses of 1, 2, 3, and 4 x 1016 cm-2. Peak detection wavelength increased with dosage from 6.8 µm unimplanted to 7, 7.3, 7.6, and 8.6 µm through the dosage range. Similarly, spectral line width changed from a full width at half maximum of 13.8% of peak wavelength unimplanted to 14.3%, 12.8%, 17.7%, and 20.2%. Contact Hoe Tan at [email protected].

Asymmetric diffraction magnifies x-ray image
Based on refraction effects in the object to be studied, x-ray phase-contrast imaging can potentially become a powerful tool for materials science, especially when done in real time. One drawback of the technique is the lack of a means to highly magnify an x-ray image in two dimensions. Researchers at NEC Corp. (Tsukuba, Japan) and the Himeji Institute of Technology (Ako, Japan) have come up with a way to magnify x-rays based on extremely asymmetric Bragg diffractions off two silicon crystals. The technique produces images of submicron resolution.

A monochromatic beam with a 0.0826-nm wavelength strikes the first crystal at a glancing angle near the critical angle and is diffracted into a high order, increasing the beam width by a factor of 294 in one dimension. Another diffraction off the second crystal magnifies the beam by an equal amount in the other dimension. Setting the incidence angle to near the critical angle not only increases the magnification, it also minimizes penetration depth into the crystal. A comparison of images of 0.7-µm gold lines to a simulation showed that the phase-contrast profile of small, thin samples is strongly affected by the sample shape. Contact Kenji Kobayashi at [email protected].

Diode laser provides mode-hop-free output
One problem with conventional high-brightness 635-nm laser diodes is a tendency to high levels of mode-hop noise. While a laser that emits in a single mode at fairly constant temperature can offer noise levels lower than 0.05% root mean square, temperature changes or aging of the diode increase the likelihood that the cavity will want to support a different mode, which causes hopping and higher noise. To reduce this jumping from mode to mode, engineers at Coherent Auburn Division (Auburn, CA) have developed a technique to force the laser to remain always in a multilongitudinal mode. As a result, several wavelengths very close together will always be present in the laser cavity, and there are essentially several modes of lower intensity. With temperature changes, these modes move like a caterpillar across the wavelength spectrum without allowing abrupt changes. As a result, the laser diode—which was demonstrated at Photonics West this January in San Jose, CA—operates as if there are no mode-hops. Contact Michael Cook at [email protected].

Nitric-oxide tagging reveals air flow
In molecular-tagging velocimetry of a gas, a pulsed laser tags a spatially defined distribution of molecules in the gas, which is then affected in shape by the gas flow. The modified distribution of the tagged molecules is probed by laser-induced fluorescence of the molecules, resulting in a picture of the flow velocity distribution. For air, it is desirable to tag the air itself rather than an introduced seeding gas, and also to have the tagging scheme be long-lived and easy to visualize.

Researchers at the University of Nijmegen (Nijmegen, The Netherlands) have developed a method of tagging air based on the creation of nitric oxide (NO) by a pulse from a mildly focused argon-fluoride excimer laser. After a time delay, NO fluorescence induced by a single shot of a read laser reveals the modified spatial distribution, which is captured by an imaging monochromator and camera. Mean flow velocities below 1 cm/s can be measured. In one experimental example, the complex vortex flow from an air valve was characterized. Diffusion of the NO molecules limits the velocity resolution after a few hundred microseconds has elapsed. Contact Nico Dam at [email protected].

Silicon AR structures are made using porous alumina membrane
A subwavelength structured (SWS) surface, which is a surface-relief grating having a period smaller than the light wavelength, can behave as an antireflection (AR) surface. Researchers at Tohoku University (Sendai, Japan) have fabricated SWS surfaces consisting of hexagonal gratings on crystal silicon substrates with higher periods and aspect ratios than other existing two-dimensional AR structures. An ordered anodic porous alumina membrane was used as a lithographic mask for fast-atom-beam etching to generate a 100-nm-period AR structure on the substrate.

The antireflection structure consists of a deep hexagonal grating with an aspect ratio of 12. Over a wavelength range of 370 to 800 nm, the structure lowers the reflectivity of a silicon surface from 40% to less than 1.6%. The experimental findings are a good match to theoretical calculations based on rigorous coupled-wave analysis. Since the porous alumina membrane is easily widened, researchers claim the process will be useful for the fabrication of AR surfaces over a large area of a silicon substrate. Contact Yoshiaki Kanamori at [email protected].

Near-field microscopy sees nanoscale refractive-index variations
Researchers at the University of Tokyo (Tokyo, Japan) have used near-field scanning optical microscopy (NSOM) to perform refractive-index patterning on a nanoscale level in a transparent polymer film without significantly affecting the transparency or surface morphology of the sample. The polymer used in the study, 3-phenyl-2, 5-norbornadiene-2-carboxylic acid doped in poly(methyl methacrylate), was spin-coated in toluene solution into 3-µm-thick film samples on glass cover slips. The refractive-index patterning was performed using an illumination-mode NSOM apparatus with shear-force tip-sample distance regulation, and a tapered probe constructed of a chemically etched, aluminum-coated, ultraviolet-transparent optical fiber.

Light at 325 nm from a helium-cadmium laser was coupled to the optical-fiber probe and scanned over the surface at a power density on the order of 4 W/cm2 at the sample surface for a 10-min duration at a scan speed of 2 µm/s. After photoisomerization, the refractive index of the sample had decreased by about 0.006, as measured by light of a 442-nm wavelength from the same laser. The technique is expected to prove useful for investigating electromagnetic interactions between small structures and has potential applications in data storage, according to the researchers. Contact Satoshi Takahashi at [email protected].

Undersampling results in full object recovery
Traditionally, full recovery of an object from its discretely sampled diffraction pattern requires that the optical signal be sampled at a certain minimum rate, the so-called Nyquist rate. Calculations can then completely reconstruct the object (which must be band-limited to satisfy the Nyquist criterion). Now, Levent Onural of Bilkent University (Bilkent, Turkey) claims that an object can be fully recovered when its diffraction pattern is undersampled, even if the undersampling is severe.

The form of the convolution kernel that describes Fresnel diffraction allows another, alternative, full-reconstruction procedure of the object from the samples of its diffraction pattern when the object is space-limited. Starting from the condition of space limitation is interesting, says Onural, since even when the well-known Nyquist criterion is not satisfied (for example, as a result of severe aliasing), a complete object can be reconstructed. To confirm his calculations, Onural carried out experiments on inline holograms that provided supporting experimental data. The technique could result in computational savings for image-processing applications involving diffraction. Contact Levent Onural at [email protected] or [email protected].

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