The phenomenon of electromagnetically induced quantum coherence is demonstrated between three confined electron subband levels in a quantum well which are almost equally spaced in energy. Applying a strong coupling field, two-photon resonant with the 1-3 intersubband transition, produces a pronounced narrow transparency feature in the 1-2 absorption line. This result can be understood in terms of all three states being simultaneously driven into "phase-locked" quantum coherence by a single coupling field. We describe the effect theoretically with a density matrix method and an adapted linear response theory.

The refractive index of GaAs has been measured in the wavelength range from 0.97 to 17 μm, which covers nearly the entire transmission range of the material. Linear and quadratic temperature coefficients of the refractive index have been fitted to data measured between room temperature and 95 °C. In the midinfrared, the refractive index and temperature dependence are obtained from analysis of etalon fringes measured by Fourier-transform spectroscopy in undoped GaAs wafers. In the near infrared, the refractive index is deduced from the quasiphasematching (QPM) wavelengths of second-harmonic generation in orientation-patterned GaAs crystals. Two alternative empirical expressions are fitted to the data to give the refractive index as a function of wavelength and temperature. These dispersion relations agree with observed QPM conditions for midinfrared difference-frequency generation and second-harmonic generation. Predictions for various nonlinear optical interactions are presented, including tuning curves for optical parametric oscillators and amplifiers. Also, accurate values are predicted for QPM conditions in which extremely large parametric gain bandwidths can be obtained.

We report the wavelength dependencies of the two- and three-photon absorption coefficients of undoped GaAs in the spectral range 1.3-3.5 microm, as well as nonlinear refractive index n2 in the range 1.7-3.25 microm. The data were obtained by using the single-beam Z-scan method with 100-fs-long optical pulses. Anisotropy of the three-photon absorption coefficient was observed and found to be consistent with the crystal symmetry of GaAs.

We demonstrate an optical parametric oscillator (OPO) based on GaAs. The OPO utilizes an all-epitaxially-grown orientation-patterned GaAs crystal that is 0.5 mm thick, 5 mm wide, and 11 mm long, with a domain reversal period of 61.2 microm. Tuning either the near-IR pump wavelength between 1.8 and 2 microm or the temperature of the GaAs crystal allows the mid-IR output to be tuned between 2.28 and 9.14 microm, which is limited only by the spectral range of the OPO mirrors. The pump threshold of the singly resonant OPO is 16 microJ for the 6-ns pump pulses, and the photon conversion slope efficiency reaches 54%. We also show experimentally the possibility of pump-polarization-independent frequency conversion in GaAs.

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A ZnGeP>(2) (ZGP) optical parametric oscillator (OPO) with wide mid-IR tunability has been demonstrated. The singly resonant angle-tuned ZGP OPO was pumped by 100-ns erbium laser pulses at lambda =2.93mum and yielded output that was continuously tunable from 3.8 to 12.4 mum (type I phase matching) and from 4 to 10 mum (type II phase matching). An OPO pump threshold was less than 1 mJ in the whole 4-12 mum range of the output, and the quantum conversion efficiency reached 35%. An OPO linewidth was typically a few wave numbers; however, with a single intracavity etalon (uncoated Si plate) in a type II OPO it was narrowed to <0.5cm(-1). We demonstrate the sensitive detection of N(2)O gas with the narrow-linewidth OPO.

The effect of significant decrease of water absorptivity for the intense picosecond laser radiation at A = 2.79 and 2.94 pm being near the center of the OR stretching mode absorption band was discovered. In case of pure water a thermal mechanism dominated: A very fast temperature rise led to weakening of R-bonds and consequently to the absorption band shift towards higher frequencies. As a result, a considerable (up to 10 times) decrease in the optical density at the laser frequency was obtained. In the second case of ROO diluted in 0 2 0 the temperature effects were eliminated and a pure spectroscopic saturation of the v = 0 to v = 1 vibrational transition was displayed. The saturation intensity as high as Is = 2.5.10 11 W/cm 2 in this case gives the value of energy relaxation time of the OH excited state to be in the range 0.3 ps < TI < 0.6 ps. The width of the homogeneously broadened component of the fundamental OH band in HOO spectrum is evaluated to be :> 40 cm -I.

Received Month X, XXXX; revised Month X, XXXX; accepted Month X, XXXX; posted Month X, XXXX (Doc. ID XXXXX); published Month X, XXXX Broadband mid-IR output suitable for producing 1000-nm-wide frequency combs centered at 4.9 m was achieved in a degenerate subharmonic optical parametric oscillator (OPO) based on 500-m-long Brewster-angled orientation-patterned GaAs crystal. The OPO was synchronously pumped at 182 MHz repetition rate by 100-fs pulses from a Cr 2+ :ZnSe laser with the central wavelength of 2.45m and the average power of 100 mW. © 2010 Optical Society of America OCIS Codes: (190.4975) Parametric processes; (190.4410) Nonlinear optics, parametric processes Extending the spectral range of optical frequency combs to the mid-infrared (mid-IR) 'fingerprint' region where molecules have their tell-tale absorption features associated with ro-vibrational transitions is critical for numerous applications including frequency standards, precision spectroscopic measurements and trace molecular detection. With the aid of coherent Fourier transform spectroscopy using broadband frequency combs, one can access simultaneously a great variety of molecular signatures, accurately and with high sensitivity, spectral resolution and speed [1][2][3][4][5].Several methods were developed lately for extending broadband frequency combs (and more generally, supercontinuum sources) to the mid-IR. These include direct laser sources [6], sources based on supercontinuum generation in optical fibers driven by self-phase modulation [7][8][9] and engineered  (2) nonlinear-optical devices [10], optical rectification [11], difference-frequency generation [12][13][14], optical parametric oscillators [15,16] and amplifiers [17].This work extends the results of two earlier works on producing broadband frequency combs in synchronouslypumped subharmonic OPOs, based on periodically poled lithium niobate -with a 780-nm femtosecond (fs) Ti:Sapphire laser [18], and fs 1560-nm erbium-fiber laser [19] as a pump. In the latter case, a spectral span of 2.5 -3.8 m was achieved. Here we report on our first results on exploring even longer wavelength range -with a degenerate OPO based on orientation-patterned GaAs (OP-GaAs) crystal.OP-GaAs has excellent characteristics for mid-IR parametric frequency conversion. It has large secondorder nonlinear optical coefficient d14=94 pm/V [20] and good mid-IR transparency with a long-wavelength cutoff at 17 m. In addition it has smaller, as compared to lithium niobate, absolute value of group velocity dispersion at  > 3 m.The OPO was synchronously pumped (Fig.1) by a femtosecond Cr 2+ :ZnSe laser with the following parameters: central wavelength 2.45 m, repetition rate 182 MHz, pulse duration ~100 fs. The laser was mode locked using a semiconductor saturable absorber mirror (SESAM) [5,6] and used a sapphire plate for the 2-nd order dispersion compensation. Compared to [5,6], we used only a 5-mm sapphire plate for dispersion compensation and higher (6%) outcoupling, allowing to increase the output a...