May 1, 2001
Superlattice quantum-cascade lasers get more power; Stoichiometric LiNbO3 and LiTaO3 show better nonlinear performance; Solid-state Raman amplifier optically induces pulse delay...

Superlattice quantum-cascade lasers get more power
Physicists at the Universita e Politecnico di Bari (Bari, Italy) report achieving the highest peak power ever for a quantum-cascade (QC) laser, using a novel injector design that allows the tunneling of electrons into high-energy states of the excited miniband. The QC laser was designed with superlattice active regions of 25 stages, grown by molecular-beam epitaxy on an indium phosphide (InP) substrate and embedded between two gallium indium arsenide layers n-doped to 5 x 1016 cm-3. These layers, together with the active region, form the waveguide core and enhance the refractive-index contrast with respect to the cladding layers. Index guiding on the lower side is provided by the InP substrate and on the upper side by two aluminum indium arsenide layers. At a wavelength of 8.4 µm and a temperature of 80 K, peak power of 2.2 W per facet was measured. With 25 active stages, this corresponds to a record power of 88 mW/stage. A slope efficiency of 160 mW/A over a current range six times larger than the laser threshold was observed. Contact Gaetano Scamarcio at [email protected].

Stoichiometric LiNbO3 and LiTaO3 show better nonlinear performance
Researchers at the National Institute for Research in Inorganic Materials (NIRIM; Tsukuba, Japan) have grown 3-in.-diameter boules of single-crystal stoichiometric lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) and have commercialized the supply via a spin-off company called Oxide Corp. Stoichiometric crystals have qualities superior to their conventional, congruently grown counterparts. For example, ordinary congruently grown crystals have a high coercive electric field resulting from stoichiometric impurities, limiting aperture size for optical applications—a limit that does not apply to stoichiometric crystals.

Congruently grown crystals are typically lithium-deficient, while stoichiometric materials maintain the nearly ideal one-to-one lithium-to-niobium ratio, reducing the defect density by two orders of magnitude. This high quality in turn leads to larger nonlinear and electro-optic coefficients, improved transparency in the ultraviolet, increased resistance to optical damage, and—most importantly for nonlinear and parametric processes—a factor of between 5 and 10 reduction in the poling fields required. The new crystals should find widespread use in telecommunications, optical storage, frequency doubling, and parametric conversion. Contact Yasunori Furukawa at or Kenji Kitamura at [email protected].

Solid-state Raman amplifier optically induces pulse delay
Brown University (Providence, RI) researchers investigating the group-velocity change of a probe beam in a solid-state Raman amplifier (as a function of pump intensity) have demonstrated that time delays longer than the probe-pulse duration can be achieved at room temperature in a solid-state barium nitrate [Ba(NO3)2] crystal. Using a Raman-shifted modelocked and Q-switched Nd:YAG laser operating at a 10-Hz repetition rate, 1.197-µm probe pulses with 90-ps duration were generated and propagated through a 5-cm-long Ba(NO3)2 crystal synchronously pumped by 7-ns-long, 1.06-µm pulses. The time delay of the pulse peak was measured with varying pump intensity and compared with theoretical calculation up to the point where amplified spontaneous Raman emission becomes dominant. The maximum delay was found to be 105 ps, which shows that the delay depends on the Raman gain in linear fashion until the amplified spontaneous emission becomes significant. These results indicate that significant lossless delays can be obtained across the near-infrared to the ultraviolet using a compact solid-state approach. Contact Kijoon Lee at [email protected].

ZnMgS photodetectors show promise for sunburn evaluation
Existing aluminum gallium nitride Schottky-barrier photodiodes can be used to estimate ultraviolet (UV) radiation doses to human skin, but the lack of lattice-matched substrates in such detectors can lead to the formation of cracks and density dislocations that limit sensitivity. Physicists at the Hong Kong University of Science and Technology (Kowloon, Hong Kong) have reported construction of zinc magnesium sulfide (ZnMgS) photodiodes that offer high performance and improved visible-wavelength rejection. Molecular-beam epitaxy was used to grow Zn1-xMgxS alloy thin-films on gallium phosphide substrates. In situ studies of reflection high-energy electron diffraction show that stable alloys can be grown for x composition of Mg less than 30%. Critical thickness for the thin-film growth was found to be sensitively dependent on the Mg composition, ranging from 500 nm for x slightly larger than 30% to about 10 nm for pure MgS. Abrupt long-wavelength cut-offs covering 325, 305, 295, and 270 nm were achieved for devices with Mg composition of 16%, 44%, 57%, and 75%, respectively. The device's response curve closely matches that of the accepted standard erythemal action spectrum for human-skin sensitivity to UV light. Contact Phillip Sou at [email protected].

Semiconductor nanocrystal films form ultrafast holograms
Scientists at the Los Alamos National Laboratory (Los Alamos, NM) have demonstrated the use of solid-state films of close-packed cadmium selenide nanocrystal solids as the active material for generating dynamic holographic gratings. The gratings were formed using two 400-nm, 100-ps pump pulses that interfered at the sample plane to form an intensity grating. This, in turn, formed a population grating in the nanocrystal sample, which modulated its absorption via such mechanisms as state filling and the carrier-induced Stark effect. The real part of the refractive index was modified through Kramers-Kronig transformation, and the dynamic grating formed in the samples was probed at the Bragg angle with tunable 100-fs pulses generated by an optical parametric amplifier. A high diffraction efficiency of up to 0.5% for films of approximately 0.5 µm was calculated as the ratio of the intensities of the diffracted and incident beams. The spectral maximum of diffraction efficiency is located in the vicinity of the lowest (1S) absorption maximum (the nanocrystal band edge) and is tunable over a wide range by varying the size of the nanocrystals used in fabricating the solid. Contact Victor Klimov at [email protected].

1400-nm-region pump laser reaches output of 1 W
Single-mode diode lasers emitting in the 1400-nm range are used to pump erbium-doped fiber amplifiers (EDFAs) and Raman amplifiers. Higher pump power leads to higher channel count for EDFAs and longer transmission distances without optical regeneration for both types of amplifiers. Researchers at Princeton Lightwave Inc. (Cranbury, NJ) have developed ridge-waveguide lasers emitting between 1400 and 1480 nm at output powers of up to 1 W continuous-wave that, when coupled into a single-mode fiber, result in a fiber output of up to 710 mW.

The narrow-stripe laser structure contains a strained indium gallium arsenide phosphide quantum-well active region. Accurate channel-etch depth, and thus reliable single-mode operation, is maintained during fabrication by including a grown-in etch-stop layer in the laser structure. Cavity lengths ranged from 2 to 3.5 mm; a laser with a 2.4-mm cavity emitted 1420-nm light at an efficiency of 42 mW/A. The researchers have begun lifetime testing of the devices, beginning with a 70°C burn-in at 1.5 A, then moving to extended testing. Future efforts will include lifetime testing at 2.5 A, or about 470 mW of output power at 20°C. Contact Dimitri Garbuzov at [email protected].

Free-electron laser pulses manipulate semiconductor quantum bits
Researchers at the University of California (Santa Barbara, CA) and the University of Glasgow (Glasgow, Scotland) have used intense pulses of terahertz radiation to manipulate quantum bits, the fundamental elements of proposed quantum-information processors, in a sample of gallium arsenide semiconductor material. The sample was exposed to terahertz radiation provided by the free-electron laser (FEL) at the University of California. The 2- to 6-µs, 2.52-THz FEL pulses were chopped by optically activated semiconductor switches into pulses with durations varying from a few to 50 ps. The pulses subsequently induced coherent, damped Rabi oscillations in the sample, which was placed in a 3.5-Tesla magnetic field to tune the sample response to the stimulating FEL frequency. During the Rabi oscillations, the states of electrons bound to donor impurities oscillated between hydrogen-atom-like 1s and 2p states, providing both the pair of states and rapid manipulation that characterize quantum bits. The researchers expect their work to provide an accessible model of semiconductor quantum-bit manipulation. Contact Mark Sherwin at [email protected].

NIST develops excimer dose calibration
The National Institute of Standards and Technology (NIST; Boulder, CO) has developed the capability to accurately measure pulse-energy density of deep-ultraviolet radiation produced by excimer lasers; this new capability is being used to provide dose (energy density) measurement services. Richard Jones, of NIST's Optoelectronics Division, and Holger Labs, a guest researcher, offer absolute responsivity calibrations of laser dose meters at the 193-nm argon fluoride (ArF) excimer-laser wavelength. Additional excimer-laser wavelengths will be added in the near future.

The dose measurements are performed using a beamsplitter-based calibration system in which a spatially uniform beam from an ArF excimer laser is generated using a beam homogenizer. The beam propagation properties, including uniformity and homogeneity, are fully characterized with a beam-profile measurement system based on a pyroelectric camera array. This uniform beam is then used to irradiate a NIST-calibrated aperture placed immediately in front of the test detector. Measurement traceability for these calibrations comes from an electrically calibrated, primary standard calorimeter developed by Chris Cromer and Marla Dowell, also of the Optoelectronics Division. Contact Richard Jones at [email protected].

X-ray diffraction provides nonthermal measurements in real time
Researchers from the École Polytechnique (Paliseau, France), the Niels Bohr Institute (Copenhagen, Denmark), the Royal Veterinary and Agricultural University (Frederiksberg C, Denmark), and Freidrich Schiller University (Jena, Germany) have used ultrafast, time-resolved x-ray diffraction to measure nonthermal melting in a semiconductor material. In typical thermal melting, atomic motion tends to be diverse, random and relatively slow compared to ultrafast, laser-induced melting in semiconductors, in which the kinetic energy for melting comes from the orderly and rapid movement of 10% or more of the material's valence electrons, which have been optically stimulated into the conduction band. The researchers used x-ray diffraction instead of optical spectroscopy to study the phenomenon because the former enables direct probing of the changing atomic structure.

The experiment consisted of exciting an indium antimony (InSb) semiconductor sample with 120-fs pulses from an 800-nm-emitting laser, and probing the response using 0.713-nm x-ray radiation. The probe signal was obtained by focusing a 23-mJ, 120-fs, 800-nm laser pulse onto a silicon target. The researchers achieved unambiguous measurements of this nonthermal melting process in real time and found a loss of long-range order inside the InSb crystal up to 90 nm, with time constants as short as 350 fs. The ability to make these types of measurements is also expected to enable picosecond resolution of vibrational relaxation in solids and biological macromolecules, as well as subpicosecond atomic motion. Contact Antoine Rousse at [email protected].

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