Generation of dual-wavelength pulses by frequency doubling with quasi-phase-matching gratings

We demonstrate generation of two synchronized picosecond pulses at different wavelengths near 778 nm by frequency doubling of a femtosecond pulse. We use nonlinear frequency filtering with quasi-phase-matching gratings, which allow us to obtain second-harmonic spectral intensities that are higher than the spectral intensities of the pump.     

Generation of subterawatt sub-10-fs blue pulses at 1-5 kHz by broadband frequency doubling

Blue pulses with durations of 8.5 fs are generated by broadband frequency doubling of subterawatt Ti:sapphire laser systems at 1-5-kHz repetition rates. The average powers are 1.6 W at 5 kHz and 1.9 W at 1 kHz. The peak power is 0.16 TW with a duration of 12 fs at 1 kHz.     

Tunable sub-10-fs ultraviolet pulses generated by achromatic frequency doubling

Tunable UV pulses shorter than 10 fs are generated by achromatic frequency doubling of a noncollinear optical parametric amplifier. With a suitable two-prism sequence we achieve first- and second-order achromatic phase matching and increase the natural bandwidth of the nonlinear crystal by a factor of 80. Extremely broad UV spectra with a Fourier limit of 2.9 fs are generated in a 360-microm-thick beta-barium borate crystal at a conversion efficiency of 20%. We compensate for the angular dispersion and the first-order chirp of the highly stable UV pulses with a second prism sequence and fully characterize the temporal pulse shape with zero-additional-phase spectral phase interferometry for direct electric-field reconstruction (ZAP-SPIDER). Pulses as short as 7 fs are generated by controlling the higher-order chirp with a deformable mirror.     

Generation of terawatt 10-fs blue pulses by compensation for pulse-front distortion in broadband frequency doubling

Pulse-front distortion as a result of broadband frequency doubling (BFD) has been compensated for over a 3-cm beam diameter by use of a telescope image relay system. As a result, distortion-free blue pulses with a peak power of 1.4 TW and a pulse width of 10 fs have been generated with a 10-Hz Ti:sapphire laser by use of this modified BFD configuration.     

Generation of intense sub-20-fs vacuum ultraviolet pulses compressed by material dispersion

We present our latest results on the generation of ultrashort vacuum UV (VUV) pulses by nonresonant four-wave mixing of chirped broadband pulses generated by filamentation of the fundamental of a Ti:sapphire laser with relatively narrowband pulses at the third harmonic. Positive chirp at the broadband idler yields negatively chirped VUV pulses necessary to compensate for material dispersion of a MgF(2) window in the VUV beam path. Pulse energies exceeding 400 nJ are available for time-resolved experiments. Pulse duration is measured by pump-probe ionization of Xe gas, providing the cross correlation between the fifth harmonic and the fundamental.     

Generation of broadly tunable sub-30-fs infrared pulses by four-wave optical parametric amplification

We report on the generation of sub-30-fs near-IR light pulses by means of broadband four-wave parametric amplification in fused silica. This is achieved by frequency downconversion of visible broadband pulses provided by a commercial blue-pumped beta-barium borate crystal-based noncollinear optical parametric amplifier. The proposed method produces the IR idler pulses with energy up to ～20 μJ and tunable in wavelength from 1 to 1.5 μm. The shortest pulse duration is 17.6 fs, measured at 1.2 μm.     

A novel quasi-phase-matching frequency doubling technique

A new quasi-phase-matching technique for efficient second-harmonic generation is reported. It is based on the spatial periodic modulation of the light intensity along the propagation direction, rather than the conventional spatial periodic modulation of the nonlinear optical coefficients. It can be realized by using a novel dual-channel waveguide frequency doubler structure for the desired light intensity distribution. This dual-channel waveguide device has major advantages including very small beam size, high light intensity within long nonlinear-waveguide interaction length, highly efficient second-harmonic generation, ease in fabrication of the nonlinear channel waveguides without any spatially periodic poling, and low waveguide propagation losses. The new quasi-phase-matching technique can also be applied to third-harmonic generation and other nonlinear optics processes.     

Ex vivo characterization of sub-10-fs pulses

An all-mirror dispersion-compensation setup is used to correct for quadratic and cubic phase distortions induced within a custom nonlinear optical microscope. Mouse tail tendon is used to characterize sub-10-fs pulses by interferometric autocorrelation. This is an ideal method for characterizing dispersion from the optical system, immersion medium, and wet biological sample. The generation of very short autocorrelations demonstrates the ability to compensate for phase distortions within the imaging system and efficient second-harmonic upconversion of the ultrashort pulse spectrum within collagen. Compensated autocorrelation traces are presented for biologically relevant objective lenses.     

Production of dense plasmas with sub-10-fs laser pulses

Close to solid state density plasmas with peak electron temperatures of about 190 eV have been generated with sub-10-fs laser pulses incident on solid targets. Extreme ultraviolet (XUV) spectroscopy is used to investigate the K shell emission from the plasma. In the spectra, a series limit for the H- and He-like resonance lines becomes evident which is explained by pressure ionization in the dense plasma. The spectra are consistent with computer simulations calculating the XUV emission and the expansion of the plasma.     

Efficient pulse compression and frequency conversion of phase-modulated laser pulses in engineered quasi-phase-matching gratings

The simultaneous generation of the 2nd and 3rd harmonics of a frequency-chirped pulse in a quadratically nonlinear medium encompassing a quasi-phase-matched grating with linearly varying inverse domain size has been numerically studied. The efficient pulse compression and energy conversion have been analyzed taking into account the group velocity mismatch and dispersion.     

Spectral analysis of high-order harmonics generated by 30-fs and sub-10-fs laser pulses

The spectral properties of harmonic radiation generated in neon by laser pulses with duration from 30 fs down to 5 fs have been investigated. In the 30-fs temporal regime we have achieved continuously tunable sub-20-nm radiation by changing the gas jet position relative to the laser beam waist. We show that the results of a one-dimensional propagation code partially including three-dimensional focusing effects are in good agreement with the experimental results. Finally, we have performed a spectral characterization of harmonic radiation in the sub-10-fs temporal regime.     

Characterization of sub-6-fs optical pulses with spectral phase interferometry for direct electric-field reconstruction

We demonstrate spectral phase interferometry for direct electric-field reconstruction (SPIDER) as a novel method to characterize sub-6-fs pulses with nanojoule pulse energy. SPIDER reconstructs pulse phase and amplitude from a measurement of only two optical spectra by use of a fast noniterative algorithm. SPIDER is well suited to the measurement of ultrabroadband pulses because it is quite insensitive to crystal phase-matching bandwidth and to unknown detector spectral responsivity. Moreover, it combines highly accurate pulse-shape measurement with the potential for online laser system diagnostics at video refresh rates.     

Phase-dependent loss due to nonadiabatic ionization by sub-10-fs pulses

For the first time to the author's knowledge, the effect of phase-dependent loss that can be used to stabilize the phase of a sub-10-fs laser pulse with respect to the envelope is shown. This loss is caused by the nonadiabatic response of an atom illuminated by a short pulse, which leads to the dependence of the tunneling ionization on the absolute phase of the pulse. The phase selectivity can be employed as a phase-dependent filter within the laser cavity with important applications in nonlinear optics in the single-cycle regime.     

Semiconductor saturable absorber mirrors supporting sub-10-fs pulses

Received: 18 March 1997/Revised version: 12 May 1997     

Generation of sub-4-fs pulses via compression of a white-light continuum using only chirped mirrors

Using noble gases as a nonlinear medium, it has become possible to compress energetic laser pulses into the sub-10-fs regime. Hollow fiber capillaries can serve to increase the effective interaction length of the pulses, and impressive white-light continua have been reported as a result. With demonstrated bandwidths exceeding the optical octave, this method holds the potential for generating single-cycle optical pulses. On the practical side, however, it becomes very difficult to compensate for dispersive effects and to fully exploit the enormous bandwidth. We will discuss chirped mirrors as one means for pulse compression. A further challenge lies on the characterization side. Utilizing advanced characterization schemes, we were able to demonstrate compression of a white-light continuum down to a pulse duration of 3.8 fs, which corresponds to only about 1.6 cycles at the carrier wavelength. These are the shortest pulses in the visible/near-infrared wavelength range that have ever been produced with a non-adaptive approach to dispersion compensation. Moreover, these are the shortest pulses generated using chirped mirrors, which compares favorably to previous results that were achieved with much more elaborate and lossier adaptive compression schemes. PACS 42.65.Re; 42.65.Wi     

High-efficiency blue generation by frequency doubling of femtosecond pulses in a thick nonlinear crystal

We have demonstrated highly efficient frequency doubling of femtosecond pulses in a thick, noncritically phase-matched KNbO(3) crystal under conditions of large group-velocity mismatch. At low power we observed a slope efficiency of ~300% nJ (-1) for harmonic conversion, and at higher powers we generated 170 mW of second-harmonic blue output for 300 mW of input light. Furthermore, we have shown that the focusing dependence for our conditions of large group-velocity mismatch is considerably different from that obtained for frequency doubling of continuous-wave light. We have also demonstrated that one can tune the spectral width of the generated blue light by varying the focusing conditions.     

Spectral features and modeling of high-order harmonics generated by sub-10-fs pulses

Harmonic radiation generated in a neon gas jet by sub-10-fs laser pulses was investigated both experimentally and theoretically. The spectral profile of the harmonics with respect to the order, their intensity and relative spectral shifts were measured as a function of the position of the gas jet. The results point out spectral features typical of the quasi-single-cycle excitation regime. A nonadiabatic three-dimensional numerical model was developed, which provides harmonic spectra in remarkable agreement with the experiments.     

Lateral patterning of nonlinear frequency conversion with transversely varying quasi-phase-matching gratings

We demonstrate control of the nonlinear conversion across a beam profile by using periodically poled lithium niobate with a laterally nonuniform quasi-phase-matching grating. As a representative experiment, generation of a flat-top second-harmonic beam is demonstrated.     

Generation of high average power, 7.5-fs blue pulses at 5 kHz by adaptive phase control

The spectral width of a 5-kHz Ti:sapphire laser system was broadened by spectral control in a regenerative amplifier consisting of broadband chirped mirrors. The dispersion over the wide spectral range was compensated by a deformable mirror along with a genetic algorithm, resulting in a pulse width of 15 fs. The pulse width is the shortest, to our knowledge, in chirped pulse amplification systems with a regenerative amplifier. The phase distortion of broadband frequency doubling in addition to the Ti:sapphire laser was compensated by using the self-diffraction intensity in sapphire as the feedback signal into the genetic algorithm, resulting in a pulse width of 7.5 fs. The average power of the second harmonic was 1 W with a fundamental input of 7 W.     

Generation of high-energy, sub-20-fs pulses in the deep ultraviolet by using spectral broadening during filamentation in argon

Generation of sub-20-fs UV pulses with more than 300 μJ energy at 268 nm is reported. First, the UV pulses are produced by successive second-harmonic and third-harmonic (TH) generation of 805 nm pulses of a 1 kHz Ti:sapphire laser amplifier. The spectral broadening of TH pulses is realized in a filament, generated in argon. The produced pulses are compressed in a simple double-pass prism-pair compressor. Starting from 100 fs pulses, we achieve a fivefold pulse shortening.