Feb. 1, 2004
Growth of evanescent waves in negative-index material confirmed; Optical neural network learns phase values; Alkali-vapor lasers could be diode-pumped; MORE...

Growth of evanescent waves in negative-index material confirmed

Certain optical metamaterials with negative refractive index were first posited and then experimentally confirmed at microwave frequencies. If successfully fabricated in the optical region, such materials would lead to lenses that focus all Fourier components, including evanescent waves, resulting in an image having a resolution far below the diffraction limit. Another posited effect of the metamaterials is an exponential growth of evanescent waves within them, rather than the ordinary decay of all other materials.

A group at the University of California at Los Angeles has now experimentally confirmed the latter effect at optical frequencies. Rather than a metamaterial, they used a silver film, which, although strongly absorbing, has (like some other metals) a negative refractive index for some wavelengths. Depositing rough-surfaced films ranging from 20 to 90 nm thick on glass hemispheres, the researchers shone 514.5-nm argon-ion-laser light on the films, which, due to their roughness, converted some of the light to evanescent waves. By analyzing the transmitted light, the researchers found an evanescent-wave enhancement that was highest for the 50-nm film (thicker films absorbed too much light). Future lenses of engineered metamaterials should show the same effect without the absorption. Contact Xiang Zhang at [email protected].

Optical neural network learns phase values

Artificial neural networks are computing systems consisting of many simple interconnected processors that, based on data from the surrounding environment, adjust their interactions; that is, they "learn." Usually implemented electronically, neural networks excel at processing noisy data and are fault-tolerant. Now, researchers at the University of Tokyo are exploring the use of coherent all-optical neural networks that use phase to represent information.

In their setup, light from a laser diode (whose frequency can be varied over an 8-GHz range) is split into several beams, each of which has its path length adjusted independently by a liquid-crystal spatial light modulator. The light is subsequently interfered with a beam directly from the laser diode. A CCD camera and personal computer determine the output phase, which is an adaptive function of the laser diode's frequency. A given phase-versus-frequency curve was targeted at four discrete points; by 200 iterations, the all-optical neural network zeroed in on the proper solution. The researchers say that a more practical system could be constructed on a chip using photonic crystals along with small light modulators. Contact Sotaro Kawata at [email protected].

Alkali-vapor lasers could be diode-pumped

Researchers at Lawrence Livermore National Laboratory (Livermore, CA) have developed an optically pumped 795-nm-emitting resonance-transition rubidium (Rb) laser. It is potentially the first of what could become a new class of diode-pumped lasers—those using alkali vapors as gain media. Currently, the Rb laser is pumped by a continuous-wave Ti:sapphire laser emitting up to 500 mW of single-transverse-mode 780-nm light—qualities that laser diodes can achieve.

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The Rb laser's room-temperature vapor cell also contains helium and ethane buffer gases. The D2 transition line of Rb is 0.034 nm wide, which is overfilled by the Ti:sapphire pump laser's 0.1-nm linewidth. The Rb laser cavity includes a reflective angled thin-film polarizer; the laser's output is diffraction-limited with a purely S-polarized output. The laser emits up to approximately 30 mW with a slope efficiency of 54%. Other potential diode-pumped alkali lasers include cesium, pumped at 852 nm and emitting at 895 nm, and potassium, pumped at 766 nm and emitting at 770 nm. Diode-pumped versions of these lasers that operate at high power will be possible, say the researchers. Contact William Krupke at [email protected].

Bismuth manganite films have large optical nonlinearities

Researchers at Pennsylvania State University (University Park, PA) have tested a new candidate nonlinear optical material and deemed it good. The material, bismuth manganite (BiMnO3, or BMO), is ferroelectric. In the first optical tests of BMO, the researchers found large third-order nonlinearities and giant field-induced second-order nonlinearities.

Via pulsed-laser deposition, BMO films with thicknesses on the order of 100 nm were grown on strontium titanate substrates. A single-beam Z-scan technique using a modelocked Ti:sapphire laser was used to determine room-temperature nonlinear refractive index and absorption, which at 900 nm were -0.53 cm2/GW and -0.08 cm/kW, respectively. Electric-field enhancements of the 450-nm second-harmonic response reached three to four orders of magnitude; different experiments were done on a 100-nm BMO film, showing, for example, effective nonlinear coefficients of 40 or 115 pm/V at 300 or 473 K for applied fields of 707 or 177 V/mm, reaching enhancements of 1420 and 13,000, respectively (the latter applied field could not be boosted higher due to the larger dielectric losses at 473 K). As a result, BMO has become a potential material for optical devices such as modulators. Contact Alok Sharan at [email protected].

Entangled photons and local monochromator enable remote spectral measurement

Beyond their value in quantum-physics experiments, entangled photons have practical use in quantum cryptography and, potentially, imaging and lithography. Finding yet another real-world use for them, researchers at the University of Maryland (Baltimore, MD) have developed a remote-spectral-measurement technique that relies on entangled photons.

Two photons are created from a pump photon by spontaneous parametric down-conversion (SPDC); in the remote-measurement process, the wavelength of one photon is measured by a local scanning monochromator, thus determining the other (remote) photon's wavelength, which, through SPDC, can be made to be anywhere across the visible or IR. A remote detector simultaneously measures the arrival of the remote photon or lack thereof, but not its wavelength; the process is repeated many times while varying the photon's wavelength. Thus, the spectral transmission of an object at the remote location is done with only a simple, non-spectrally sensitive detector located there, with spectral resolution determined by the local monochromator. In a first test of the technique, the researchers spectrally characterized filters with bandpasses centered on 850, 886, and 916 nm. Potential remote-measurement locations include the Space Station. Contact Giuliano Scarcelli at [email protected].

Zinc-oxide waveguides produce amplified spontaneous emission

The wide bandgap of zinc oxide (ZnO) makes the material potentially suitable for UV emitters, including lasers. While optically pumped lasing has been observed in randomly oriented ZnO microcrystallite thin films (with the uneven microcrystal walls themselves serving as cavity mirrors), the optical output is not directional in this configuration and thus is not very useful. Creating a more-structured ZnO emitter would lead to a more functional device. Researchers at Nanyang Technological University (Singapore) have fabricated ZnO waveguides that achieve amplified spontaneous emission, pointing the way to directional ZnO lasers.

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A ZnO thin-film ridge waveguide structure is fabricated on a silicon substrate with a low-refractive-index silicon dioxide buffer layer serving as cladding between the waveguide and substrate. The waveguides are 100 nm high and 2 µm wide (sized to optimize the fundamental TE mode), and are separated by 500 µm. A frequency-tripled Nd:YAG laser emits 6-ns pulses of 355-nm light that are focused to a stripe 0.9 mm long and 0.8 µm wide to excite the waveguide structure, which has a net optical gain of 120 cm-1. The emitted 385-nm beam shows a high-quality single mode. Contact Siu-Fung Yu at [email protected].

Ultrafast organic thin film helps measure femtosecond-pulse timing

Scientists at the Communications Research Laboratory (Tokyo, Japan) and Fuji Xerox (Kanagawa, Japan) are using an organic thin film that acts over its entire area as an ultrafast optical switch to measure the arrival time of femtosecond laser pulses. The setup allows them to send a signal back to the laser to reduce pulse jitter.

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A 120-nm-thick film of squarylium dye J-aggregate was spin-coated onto a transparent substrate, forming an optical switch 5 × 5 cm in size. The film's nonlinear optical response is based on the Kerr effect and causes a reduction in transmittance that recovers to half its maximum in less than 100 fs; the film's center operating wavelength is approximately 800 nm. In operation, an angled 800-nm control pulse induces instantaneous dichroism in the film; a 760-nm input signal pulse can pass through a crossed polarizer (analyzer) only where it and the control pulse coincide. By tuning the timing of the two pulses, the timing of the signal pulse is evident by the lateral position at which it passes through the film. A 600-fs jitter was reduced to 120 fs by compensating the laser. Contact Makoto Naruse at [email protected].

Ultrafast white-light continua from filaments in water interfere

Given the proper conditions, a white-light continuum (WLC) can be generated by focusing pico- or femtosecond pulses into condensed matter. One property of a WLC produced in this manner is its coherence: each spectral component has the same coherence as the pump light and thus appears as if it were laser light. Scientists at Heriot-Watt University Edinburgh, Scotland, and Qinetiq (Malvern, England) have made this property plain to see by creating more than one WLC from the same laser and interfering their light to produce fringes.

Focusing the output of a Ti:sapphire laser (1-mJ, 12-fs pulses) into water using a cylindrical lens produced a stable one-dimensional array of white-light filaments. Reducing the pulse energy to 780 µJ caused all but two filaments to disappear. The two that remained were separated by 184 µm and had 1/e2 diameters of 25.8 and 19.3 µm for 700- and 600-nm wavelengths, respectively. Light from the pair interfered to produce fringes that were stable over greater than a 10-min time interval, easily observed at the 600- and 700-nm wavelengths with bandpass filters. Contact Ajoy Kar at [email protected].

Nonlinear lead silicate holey fiber produces Raman solitons

Small-core holey fibers, with their diminutive cross-sectional core area and large core/cladding refractive-index difference, and thus their tight confinement of light, are ideal for the production of nonlinear effects. Researchers at the University of Southampton (Southampton, England) are now fabricating holey fibers from SF57, a highly nonlinear lead silicate Schott glass that has a nonlinear refractive index of 4.1 × 10-19 m2/W at 1060 nm. A 0.37-m length of single-mode fiber with a 1.7-µm core diameter (effective mode area of 2.6 µm2) and loss of 9 dB/m has an effective nonlinear coefficient of 640 ± 60 W-1 km-1, enabling the production of Raman solitons at 1550 nm.

The structure of the fiber consists of a core held by three thin (approximately 250 nm thick) struts 5 µm in length to an outer tube. To make the fiber, a structured preform is drawn to 1.7 mm in diameter and inserted into a jacket of identical material; the assembly is then drawn to the final fiber diameter of 130 to 190 µm. Another fiber with a 2.2-µm core supports more than one mode and has a loss of only 2.6 dB/m. Contact Heike Ebendorff-Heidepriem at [email protected].


The hypothesis described in the Newsbreak "Light may hold ball lightning together" (December 2003, p. 11) was actually first published by a Russian group in 1982, translated and published in English in Sov. Phys.-Tech. Phys. 27, 905 (1982). The hypothesis is that ball lightning is formed by Rydberg atoms or molecules. The head scientist of this group was Prof. Edward Manykin at Kurchatov Institute, Moscow, Russia.

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