News breaks

Scientists have previously shown that highly localized plasmons can occur in chains of metallic nanospheres, striplike metal films, metal nanorods, and in narrow gaps between two metal interfaces.

Mar 1st, 2005
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Metal V-grooves guide channel plasmon-polaritons

Scientists have previously shown that highly localized plasmons can occur in chains of metallic nanospheres, striplike metal films, metal nanorods, and in narrow gaps between two metal interfaces. But recently, researchers in the Applied Optics Program at the Queensland University of Technology (Brisbane, Australia) have shown that a type of highly localized plasmon, a channel plasmon-polariton (CPP), can be guided in metallic V-grooves, demonstrating superior features for subwavelength guiding when compared to other techniques.

Previous analysis of CPPs in a metal V-groove showed that the number of modes increases with increasing localization of the wave. However, the Queensland team was able to demonstrate that all higher CPP modes could be effectively removed from the V-groove simply by adjusting its depth. By reducing the depth of the V-groove to approximately the penetration depth of the fundamental mode of the CPP, higher modes are not supported by the structure and single-mode operation is possible. Single-mode subwavelength guiding of CPPs is necessary for interconnectors in integrated nano-optics. Contact David Pile at d.pile@qut.edu.au.

Rotary-disk laser can reach high powers

The first commercial rotary-disk laser was introduced by Sparkle Optics (Rolling Hills Estates, CA) at Photonics West 2005 (San Jose, CA), in paper 5707-32 of the “Solid State Lasers XIV: Technology and Devices” conference within the LASE symposium. It is the only solid-state laser in which the gain medium is rotated between two cooling plates to dissipate waste heat. The rotary-disk modules are pumped with fiber-coupled laser diodes and are classified by the type of laser-gain media such as neodymium:yttrium aluminum garnet (Nd:YAG) or ytterbium:yttrium aluminum garnet (Yb:YAG) along with the pump-power-handling capability (0 to 1500 W).

Experiments using a multimode fiber-coupled diode laser at 808 nm with 95 W of incident power to pump a rotating Nd:YAG disk laser produced a single-mode output with power of 30.8 W and 32.4% optical efficiency. The beam-pointing jitter was measured to be less than 59 µrad peak to peak. The rotary disk laser interfaces with most commercial fiber-coupled pump-laser diodes through a standard fiberoptic connector and has the potential for generating higher powers than other bulk solid-state lasers and higher-energy pulses than fiber lasers. Contact Santanu Basu at basu@sparkleoptics.com.

Projection display uses eVCSEL source

Scientists at Principia Lightworks (Woodland Hills, CA) and the Lebedev Physical Institute (Moscow, Russia) are developing a rear-projection television display using an electron-beam-pumped vertical-cavity surface-emitting laser, or eVCSEL. The eVCSEL concept, presented in the OPTO 2005 symposium at Photonics West 2005 (Jan. 22-27; San Jose, CA), represents a low-cost alternative to ultra-high-pressure lamps and proposed displays using laser diodes and diode-pumped solid-state lasers.

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The gain layer for eVCSELs consists of quantum wells of different materials to obtain red (640 nm), green (540 nm), and blue (460 nm) light (their far-field output is shown here). These eVCSELs can be placed in a single cathode-ray tube and scanned either sequentially or simultaneously in a three-beam system, with each eVCSEL capable of an output power of up to 5 W in each primary color. By using a low-cost optical arrangement for eVCSEL illumination, rear-projection television displays are possible with screen diagonals exceeding 80-inches, power requirements comparable to conventional white-light sources, device lifetimes in excess of 10,000 hours, and packaging that can fit within the footprint of today’s thinner televisions. Contact Michael Tiberi at mdtiberi@principia-lightworks.com.

Waveguide Ti:sapphire laser has high slope efficiency

Ti:sapphire lasers are broadly tunable, but have a small peak emission cross section. As a consequence, they require high pump-power densities for efficient continuous-wave (CW) emission. While conventional Ti:sapphire lasers are based on bulk laser crystals, a channel-waveguide version-which allows good overlap of the pump and laser modes-could be more efficient. Researchers at the Optoelectronics Research Centre at the University of Southampton (Southampton, England) have fabricated a laser of this type that has a slope efficiency of 5.3%.

The waveguides are fabricated on sapphire substrates using pulsed-laser deposition followed by photolithographic patterning and ion-beam etching. An experimental device contains multiple rib waveguides 3.5 µm in depth and ranging from 10 to 24 µm in width, clad with a 5-µm-thick sapphire layer. The end faces are cut, polished, and optically coated. The pump source is an argon-ion laser operating on all visible lines. For a 10-µm rib, the pump-power threshold for CW operation was 315 mW; for 1 W of pump power, the laser emitted 27 mW of 792.5-nm light in the fundamental transverse mode (vertical M2 of 1.2; horizontal M2 of 1.3). Contact Christos Grivas at chg@soton.ac.uk.

Optical roll-angle sensor is stable, sensitive, and compact

In angular terms, when one object rolls with respect to another, it is rotating around the axis that connects the two objects. Neither tip nor tilt, this angular rotation is the hardest rotation to measure optically. One approach is to have a polarized light beam emanating from one object; a Faraday rotator on the second object modulates the polarization which, when monitored, produces relatively precise information on the roll angle. But such a setup can be complex and bulky.

Researchers at Tsinghua University (Beijing, China) have developed a compact, sensitive roll-angle sensor based on a magnetic-garnet single-crystal Faraday rotator a few microns thick. A pigtailed light source passes through a collimator and polarizer; at the other end, an electrical coil provides the crystal’s field (the crystal works in a deep saturation state for stability). The largest component (the crystal-coil unit) is 12 × 12 × 5 mm in size and the saturation field is 70 to 120 Oe. The device has an angular range of ±30° and a resolution of 0.5 min, or 0.0084°, determined by the 1-mV sensitivity of the lock-in amplifier. Contact Shiguang Li at lishiguang@tsinghua.org.cn.

Lower limits on HC-PCF transmission determined

Relying on a combination of theory, experiment, and long experience, as well as a tip from Eli Yablonovich of the Massachusetts Institute of Technology (Cambridge, MA), researchers at the University of Bath and BlazePhotonics (both of Bath, England) have determined the ultimate low-loss limit of hollow-core photonic-crystal fibers (HC-PCFs). With air cores, such fibers mostly avoid the problem regular solid-core fibers have, which is that of glass absorption (resulting in a minimum optical attenuation for solid-core glass fibers of 0.15 dB/km).

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Key to the determination was the identification of surface-capillary waves (unavoidable surface roughness appearing during fiber fabrication, resulting from the surface tension of molten glass) as a source of loss; Yablonovich provided this clue. The researchers measured surface roughness inside a HC-PCF with an atomic-force microscope, performed angular-resolved scattering measurements on a HC-PCF having an attenuation of 1.7 dB/km at 1565 nm (cross section is shown; horizontal bands within the core are imaging artifacts), and measured attenuation spectra. Analysis shows an attenuation lower limit of 0.1 dB/km is credible (the researchers have experimentally achieved down to 1.2 dB/km at 1620 nm); a more speculative approach assuming theoretical near-perfection gives a probably unachievable lower limit of 0.03 dB/km. Contact Tim Birks at pystab@bath.ac.uk.

Mid-IR two-photon detector is resonantly enhanced

Dynamic characterization of mid-IR laser sources-for example, quantum-cascade lasers and optical parametric oscillators-using nonlinear laser spectroscopy is made difficult by the technique’s lack of detection sensitivity. Now, researchers at the Fraunhofer-Institut für Angewandte Festkörperphysik (Freiburg, Germany) and the National Research Council (Ottawa, Ont., Canada) have developed a sensitive two-photon mid-IR detector based on resonantly enhanced nonlinear absorption that handles pulses in the femtosecond regime.

Two experimental devices were fabricated, each with 20 gallium arsenide quantum wells; the devices were optimized for wavelengths of 7.9 and 10.2 µm, respectively. The resonantly enhanced nonlinear absorption between subbands in the quantum wells is six orders of magnitude higher than in that of semiconductors and three orders of magnitude higher than in nonresonant devices. The power density for the onset of quadratic detection is a low 0.1 W/cm2 at 70 K and can be further lowered by cooling to lower temperatures. Autocorrelation-trace measurements of the response to 160-fs, 10.3-µm laser pulses determined the intersubband relaxation time (460 fs) and dephasing time (100 fs) for one of the devices. Contact Harald Schneider at harald.schneider@iaf.fhg.de.

Negative-luminescent device promises quick temperature changes

In some semiconductors, negative luminescence (NL) can be produced by suppressing the concentration of electrons and holes below their thermal-equilibrium values. The result of NL is a device that becomes a net absorber of the surrounding radiation; in other words, it cools down. One potential application is as a temperature-calibration source for IR sensors; in this role, a NL device can switch to different temperatures at megahertz frequencies.

Researchers at QinetiQ (Malvern, England) have developed a room-temperature long-wavelength mercury cadmium telluride-on-silicon NL device with a cutoff wavelength at 7.2 µm; silicon offers good compatibility with external circuits. Square devices ranging from 40 to 400 µm in size were fabricated; their response versus wavelength was calibrated with the help of a blackbody at 500 K. The peak quantum efficiency was 45% (an antireflection coating would boost this to 70%). Blackbody emittance was integrated across the 2- to 10-µm wavelength range; the calculated apparent temperature of the NL device was derived, and proved to be 10 K below room temperature, approaching the radiative limit with a mere 0.3-V reverse bias. Contact Mary Haigh at mkhaigh@qinetiq.com.

Metal wires guide terahertz waves

With applications in sensing, imaging, and spectroscopy, terahertz radiation has potential in many markets. Waveguiding in this spectral region intermediate between microwave and visible radiation remains a challenge, however; neither metal waveguides for microwaves nor dielectric fibers for visible and near-IR radiation can be used to guide terahertz waves over long distances, as a result of the high loss from finite conductivity of metals and the high absorption coefficient of dielectric materials in the terahertz range. Now, scientists at Rice University (Houston, TX) have shown that a cylindrical stainless-steel wire with a diameter of 0.9 mm can guide terahertz waves, producing a mode similar to the TEM mode of a conventional coaxial waveguide.

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In experiments that studied the propagation of terahertz waves focused onto the metal waveguide, the average attenuation coefficient was less than 0.03 cm-1, lower than that of any terahertz waveguide reported to date. The scientists have successfully fabricated an endoscope to deliver the terahertz radiation. Contact Daniel Mittleman at daniel@rice.edu.

CORRECTION

In “Surface-plasmon resonance enhances random lasing” (January 2005 Newsbreaks, p. 13) the researcher’s name and e-mail address were misspelled. The researcher is Abdulhakem Elezzabi and his e-mail is elezzabi@ece.ualberta.ca.

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