Phase-matched enhancements of high-harmonic soft X-rays by adaptive wave-front control with a genetic algorithm

We demonstrate the enhancement of high-harmonic soft-X-ray generation by adaptive wave-front control of a 745-nm Ti:sapphire laser by use of a 59-channel membrane deformable mirror, combined with a genetic algorithm, for the first time to our knowledge. The harmonics ranging from 17 nm to 28 nm were enhanced by factors up to 13. The numerical calculations illustrate that the enhancements of the plateau harmonics are due to macroscopic phase-matching effect, whereas those of the cutoff harmonics are due to the increase in focal intensity. PACS 42.65.Ky     

Widely tunable ultraviolet sub-30-fs pulses from supercontinuum for transient spectroscopy

Focusing 800-nm pulses of 10–20 fs and ≤0.4 mJ into atmospheric-pressure argon gives rise to a supercontinuum extending down to 250 nm. We show that spectral cuts from this radiation can be shortened by a simple prism compressor down to 30 fs even near the UV cut-off. The resulting pulses have enough energy (several hundred nanojoules) to serve as a simple and rugged broadly tunable pump source in ultra-fast transient spectroscopy. Such an application is demonstrated for the first time, using pulses tuned over 280–320 nm to excite Cr(CO)₆; probing it by intense-field ionization at 800 nm, we determine the lifetime of initially populated states to be as short as 14 fs.PACS 42.65.Jx; 42.65.Ky; 42.65.Re; 42.72.Bj; 82.20.-w; 82.53.-k     

Solvent-dependent optical limiting behavior of lead nanowires stabilized by [60] fullerene derivative

The optical limiting performance of lead nanowires in chloroform, ethanol and dimethylformamide was measured with 532-nm, 8-ns-duration laser pulses. Experiments showed that the optical limiting is strong and solvent dependent. The origins and the solvent effect of the optical limiting were analyzed. It is proposed that the absorption-induced scattering is the main role responsible for the optical limiting behavior and the solvent effect. PACS 42.65.An; 42.65.Hw; 42.65.Jx     

Coherent control of high-order harmonics generated with intense femtosecond laser pulses

Experimental results and theoretical analysis on the coherent control of high-order harmonics with chirped femtosecond laser pulses are presented. The coherent control of high-order harmonic generation resulted in sharp harmonic spectra by compensating for induced harmonic chirp with the control of applied laser chirp and it was found to be crucial also in producing sharp and bright harmonics.Received: 18 November 2002, Published online: 8 July 2003PACS: 42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation - 32.80.-t Photon interactions with atoms - 42.65.Re Ultrafast processes; optical pulse generation and pulse compression     

Efficient high-order harmonic generation in a two-color laser field

We report efficient high-order harmonic generation achieved using a two-color laser field. By inserting a second-harmonic generation crystal in the path of a converging laser pulse, the overall efficiency for high-harmonic generation was enhanced by more than an order of magnitude. In addition, even-order harmonics were also generated. The 2(2n+1)th -order harmonics were very strong and, in particular, the 38th harmonic (21.6 nm), obtained with a single laser shot of 2 mJ, saturated the X-ray charge-coupled device (CCD) of an extreme-ultraviolet spectrometer even with an Al filter of 1.5 m thickness installed in front of the CCD. PACS 42.65.Ky; 32.80.-t; 42.65.Re     

A powerful diode-pumped laser source for micro-machining with ps pulses in the infrared, the visible and the ultraviolet

We report a ps diode-pumped Nd:YVO₄ laser system for micro-machining applications. The system consists of a passively mode-locked oscillator followed by a regenerative amplifier. It provides laser pulses at 1064 nm with a pulse duration of 10.2 ps, a repetition rate of 20 kHz and an average output power of 10.8 W. This average power corresponds to a pulse energy of 0.54 mJ. Second-harmonic generation in LBO and fourth-harmonic generation in BBO provide visible (532-nm) and ultraviolet (266-nm) radiation with pulse energies of 270 J and 75 J, respectively. Amplification in a diode-pumped single-pass Nd:YVO₄ amplifier increases the pulse energy of the fundamental 1064-nm laser pulses to 1 mJ. PACS 42.55.Xi; 42.60.Da; 42.65.Ky; 42.65.Re     

Two-photon-pumped amplified spontaneous emission and cavity lasing of an organic dye HEASPS in PHEMA polymer

Two-photon-absorption (TPA)-induced upconverted amplified spontaneous emission (ASE) and cavity lasing of a styrylpyridinium dye: trans-4-[p-(N-hydroxyethyl-N-ethylamino)styryl]-N-methylpyridinium p-toluene sulfonate (abbreviated as HEASPS) in poly(2-hydroxyethyl methacrylate) (abbreviated as PHEMA) polymer were observed under a picosecond pump condition. The spectral and temporal behaviors of TPA-induced fluorescence, ASE and cavity lasing were studied. The cavity lasing with a total oscillation time of more than 200 ps was achieved when pumped with 1064-nm, 50-ps laser irradiation. The population inversion could persist for three times longer than the duration of the pump pulse. The gain coefficients of the dye-doped polymer at various wavelengths were calculated based on cavity lasing spectra and one-photon fluorescence spectra. PACS 78.20.Ek; 42.65.Ky; 42.65.Re; 78.47.+p     

Temperature-dependent Sellmeier equation in the MIR for the extraordinary refractive index of 5% MgO doped congruent LiNbO<Subscript>3</Subscript>

We present a temperature-dependent Sellmeier equation for the extraordinary refractive index of 5 mol % MgO doped congruent lithium niobate. This equation is adapted for wavelengths in the range of 1.3–5 μm and temperatures between 40 °C and 200 °C. The calculation of the appropriate Sellmeier coefficients is based on the wavelengths of the signal and idler radiation measured for quasi-phase-matched optical parametric generators excited by 10-ns-long, 1064-nm pulses of a Q-switched Nd:YVO₄ laser. PACS 42.65.Ky; 42.70.Mp; 42.65.Yj     

Pump-depleting four-wave mixing in supercontinuum-generating microstructure fibers

Four-wave mixing processes are shown to provide a high efficiency of non-linear optical frequency conversion and spectral transformation of ultrashort pulses in supercontinuum-generating microstructure fibers. Pump-depleting conversion of 800-nm radiation to the spectral range around 500 nm is achieved by phase-matching the parametric four-wave mixing process for 80-fs Ti:sapphire-laser pulses. The ways to use microstructure fibers for generating frequency-tunable radiation through four-wave mixing with the maximum efficiency of pump-field frequency conversion are discussed. PACS 42.65.Wi; 42.81.Qb     

High brightness harmonic generation at 13 nm using self-guided and chirped femtosecond laser pulses

We have optimized the brightness of high-order harmonics from a long neon gas jet using self-guided and chirped laser pulses. The self-guided and chirped laser pulses effectively reduced the ionization effects in space and time, producing bright high-order harmonics with narrow bandwidth. The brightness of the 61st harmonic was about 10¹⁵ W/cm²/srad with a bandwidth of 0.7 Å. PACS 42.65.Ky; 42.65.Wi; 32.80.-t; 52.38.-r     

Harmonic generation by atoms in circularly polarized laser fields: far-off and near resonance regimes

The generation of harmonics by atoms interacting with two laser fields having coplanar circular polarizations and an integral frequency ratio is addressed through ab initio numerical simulations. A detailed characterization of a few specific harmonics is given. In particular, the two different cases where the total energy absorbed through photons is far off or close to the energy gap between different atomic states are investigated. It is found that the conversion efficiency in the harmonic generation is strongly dependent on the inner atomic structure and in certain specific cases it can be significantly enhanced within a small frequency range. PACS 42.50.Hz; 42.65.Ky     

Compression of harmonic pulses by using material dispersion

We utilize the dispersion property of an X-ray filter material for the generation of a single sub-50-as pulse from high-order harmonics. The attosecond pulse, formed by selecting the spectral range of high-order harmonic radiation, contains an intrinsic chirp corresponding to the quadratic phase variation during a half cycle of a laser pulse. We show that this chirp can be compensated by using the negative group-delay dispersion of a thin X-ray filter, compressing the attosecond pulse down to sub-50-as. PACS 42.65.Re; 42.50.Hz; 42.65.Ky     

Time-resolved measurements of high order harmonics confined by polarization gating

We investigate the temporal confinement of high order harmonic pulses generated by a femtosecond (fs) infrared (IR) pulse with a time varying polarization. We use a set of two birefringent quartz plates to modulate the IR polarization. It produces a short temporal gate of linear polarization where harmonics are efficiently generated during a small fraction of the IR pulse. By rotating one of the plates, the gate width can be continuously varied between 70 fs down to 7 fs. The XUV pulse duration is measured by cross-correlation with a probe IR pulse of 12 fs. When the gate width is decreased, a clear temporal confinement of the XUV emission is observed through the cross correlation signal. This experiment is the first direct experimental evidence in the temporal domain that the polarization gating technique can be used to significantly shorten the harmonic pulse duration. PACS 32.80.Wr; 42.65.Ky; 42.65.Re     

100-fs diode-pumped Yb:KGW mode-locked laser

We have developed a mode-locked diode-pumped Yb:KGW laser generating 100-fs pulses with an output power of 126 mW. The corresponding optical spectrum has a 13.4-nm FWHM bandwidth and is centered at 1037.4 nm. In the multiple-pulsing regime, bound states of solitons with rotating phase difference and separated by 917.5 fs were observed. We compare the performance of the Yb:KGW crystal to that of an Yb:KYW crystal with the same thickness and Yb concentration. PACS 42.55.Rz; 42.65.Re; 42.65.Tg     

Bismuth triborate (BiB3O6) optical parametric oscillators

We report the phase matching of parametric frequency conversion in the nonlinear material BiB₃O₆ (BiBO) and on an investigation of optical parametric oscillators (OPOs) of this new crystal. Based on the calculation of collinear type I and type II phase matching within the refractive-index planes, the most favorable directions for phase matching are identified for OPOs pumped by the fundamental or the harmonics of 1064-nm Nd-doped lasers. Based on these results, pulsed 532-nm-pumped ns OPOs are realized. The pump source is either a Q-switched high repetition rate (10 kHz) Nd:YVO₄ laser (with a pulse energy of 24 J) or a low repetition rate (10 Hz), high pulse energy (120 mJ) Nd:YAG laser system. The BiBO OPO pumped by the Nd:YVO₄ laser showed a very low threshold of 0.047 J/cm². At an average pump power of 2.4 W the total OPO output power was 630 mW. By changing the phase-matching angle within the yz plane from 0 to 11.6° the signal wavelength was tuned from 735 nm to 970 nm, while the spectral width changed from 0.2 nm to 1.4 nm. By pumping the OPO with the Nd:YAG laser, the OPO had a threshold of 0.12 J/cm², a steep slope (59%) and a high total efficiency (of up to 48%). Due to divergence broadening the spectral width changes from 8.5 nm at 800 nm to 70 nm near degeneracy. The properties of BiBO determined from the experimental results are compared with those of well-known nonlinear materials such as BBO, LBO and KTP. PACS 42.65.-k; 42.65.Yj; 42.70.-a; 42.70.Mp     

Scanning mirror on a vibrating membrane for dynamic optical trapping

A study of the variation of the spectral shape and the harmonic distribution of the high-order harmonics generated from silver plasma on the frequency chirp of the driving laser radiation (793 nm 48 fs) is reported. The results of the systematic study of the harmonic generation from the 21st order up to the 61st order (λ=13 nm) are presented. A tuning of the harmonic wavelength up to 0.8 nm can be accomplished by variation of the laser chirp. PACS 42.65.Ky; 42.79.Nv; 52.38.Mf     

Time-compensated grazing-incidence monochromator for extreme-ultraviolet and soft X-ray high-order harmonics

The use of a grating monochromator for the selection of one or more high-order laser harmonics produced by a femtosecond pulse interacting with a gas jet may alter the duration of the pulse itself. This is due to the differences in the optical paths of the rays caused by ordinary diffraction when a grating is used. The time stretching can be almost eliminated by using two gratings in time-compensated configurations. Unfortunately, the classical diffraction mounting has low efficiency, overall in the extreme-ultraviolet region. High broadband efficiency can be obtained by using the conical diffraction mounting. A time-compensated monochromator with toroidal gratings used in conical diffraction is here presented. It is shown that the time compensation is very effective in a broad spectral region, ranging from VUV to soft X-rays, with much higher efficiency than the classical diffraction mounting. PACS 42.65.Ky; 42.79.Dj; 42.65.Re; 41.85.Si     

Carrier concentration dependence of optical Kerr nonlinearity in indium tin oxide films

Optical Kerr nonlinearity (n₂) in n-type indium tin oxide (ITO) films coated on glass substrates has been measured using Z-scans with 200-fs laser pulses at wavelengths ranging from 720 to 780 nm. The magnitudes of the measured nonlinearity in the ITO films were found to be dependent on the carrier concentration with a maximum n₂-value of 4.1×10^-5 cm²/GW at 720-nm wavelength and an electron density of N_d=5.8×10²⁰ cm^-3. The Kerr nonlinearity was also observed to be varied with the laser wavelength. By employing a femtosecond time-resolved optical Kerr effect (OKE) technique, the relaxation time of OKE in the ITO films is determined to be ∼1 ps. These findings suggest that the Kerr nonlinearity in ITO can be tailored by controlling the carrier concentration, which should be highly desirable in optoelectronic devices for ultrafast all-optical switching. PACS 42.65.An; 42.65.Hw; 78.40.Fy     

Diode-laser-based sensor for ultraviolet absorption measurements of atomic mercury

A new sensor has been developed for measuring atomic mercury using absorption spectroscopy with 254-nm radiation generated from two sum-frequency-mixed diode lasers. Beams from a 375-nm external-cavity diode laser and a 784-nm distributed feedback diode laser are mixed in a beta-barium-borate crystal to generate approximately 4 nW of ultraviolet radiation. The development of the sensor is described along with extensive characterization experiments in a mercury vapor cell in the laboratory. An accuracy of ±6% in the absolute concentration of atomic mercury has been demonstrated by comparison with equilibrium vapor pressure calculations. The detection limit is approximately 0.1 parts per billion of atomic mercury in a meter path length for 300-K gas and a 10-s integration time. The insensitivity of the sensor to broadband attenuation is demonstrated. Measurements of collision-broadening coefficients for air, N₂, Ar, and CO₂ are reported, and implementation of wavelength-modulation spectroscopy with the sensor is demonstrated. Finally, results are presented from measurements with the sensor in situ in the exhaust stream of an actual coal-fired combustor. PACS 07.07.Df; 42.62.Fi; 42.65.Ky; 42.72.Bj     

Generation of high-order harmonics of femtosecond radiation from the surface of various media

The results of study of high-order harmonic generation from the surfaces of various materials by using femtosecond radiation with a variable chirp are presented. The analysis and optimization of radiation conversion into harmonics up to the fifteenth order (λ = 52.9 nm) are carried out. It is shown that the maximum conversion efficiency is realized for the surfaces of silver and glass. The wavelength of harmonics can be tuned over the 2-nm range by changing the chirp of the radiation converted.