Tunable light sources based on high harmonics generation for time-resolved VUV spectroscopy

The project of set-up for time resolved femtosecond VUV spectroscopy, which is based on a high harmonic generation source and a VUV monochromator is presented. We discuss the generation conditions of quasi-continuous VUV spectrum via HHG in Ar using the intense chirped IR fundamental pulse and the relatively weak second harmonic of IR fundamental. The desired narrow VUV spectral range is selected by time compensated double monochromator with concave spherical diffraction gratings. PACS 42.65.Ky; 42.79.Dj; 78.47.+p; 39.30.+w     

Femtosecond laser pulses measured with a photodiode – FROG revisited

We demonstrate complete characterization of a femtosecond laser pulse electric field amplitude and phase using a FROG setup with a thick second harmonic crystal and a photodiode replacing a spectrometer. The scheme we propose is intended for high-repetition-rate pulse trains. It takes advantage of the photodiode high dynamic range and can be implemented in a compact size. PACS 42.65.Ky; 42.65.Re     

Coherent control multiphoton processes in semiconductor saturable Bragg reflector with freezing phase algorithm

A freezing phase concept has been proposed for adaptive coherent control with a femtosecond pulse shaper. We applied the scheme to investigate multiphoton processes in InAs quantum dot saturable Bragg reflector (SBR). The optical transition of InAs quantum dots can be revealed in the spectral phase sensitivity plot of second harmonic signal. We also achieved a three-time increase in image contrast on regions with photoluminescent wavelength differing only 18 nm by using coherent control nonlinear optical microscopy. Our results suggest the new freezing phase scheme to be useful for various investigations which require fast and reliable complete-field characterization and coherent control on one setup. PACS 42.65.Re; 42.50.Md; 78.67.Hc     

Generation of attosecond pulses with ellipticity-modulated fundamental

We have studied experimentally and theoretically high-order harmonic generation using a laser field with a time-dependent ellipticity. We show that the harmonic emission can be confined into a narrow temporal window, in which the fundamental polarization is quasi-linear. This allows a single attosecond pulse (200 as) with a fundamental field obtained from 10 fs pulse to be generated. PACS 42.65.Ky; 42.65.Re; 32.80.Wr     

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     

All-optical stabilization of carrier-envelope phase by use of difference frequency generation with seeded amplification of colored conical emission

We stabilized the carrier-envelope phase of pulses emitted by a femtosecond regenerative amplifier through difference frequency generation between pump and seeded amplification of colored conical emission. Seeded amplification of colored conical emission was induced by modulational instability in the second harmonic generation with a supercontinuum injected and amplified. As a consequence, it inherited the origin phase of the pump pulse. After difference frequency with the pump pulses, the generated tunable idler pulses were carrier-envelope phase stabilized, which was verified with a simple and robust spectral interference setup. PACS 42.65.Re; 42.65.Yj; 42.25.Kb     

Generation of shaped ultraviolet pulses at the third harmonic of titanium-sapphire femtosecond laser radiation

We experimentally demonstrate a method to generate shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 267 nm, the third harmonic of conventional titanium-sapphire femtosecond laser systems. Employing a 128-pixel liquid-crystal spatial light modulator, we impose variable spectral phase modulations upon the near-infrared laser pulses. By this, complex laser pulses can be shaped whose overall spectrum is still conserved. Our experiments show that it is possible to easily transfer these pulses into the ultraviolet at 267 nm via sum-frequency mixing in nonlinear crystals and to predictably generate multistructured ultraviolet femtosecond laser pulses. We analyze the temporal and spectral composition of these pulses after frequency conversion into the ultraviolet using difference-frequency cross-correlation and XFROG (cross-correlation frequency-resolved optical gating) techniques with an unmodulated fundamental laser pulse. The method can be employed to facilitate adaptive quantum control experiments in the ultraviolet wavelength regime, where the major absorption bands of many organic molecular systems are located. PACS 42.65.Re; 42.72.Bj; 32.80.Qk     

Femtosecond-resolution measurement of soft-X-ray pulse duration using ultra-fast population increase of singly charged ions induced by optical-field ionization

We demonstrate sub-100-fs resolution of a cross-correlation method for measuring the duration of soft-X-ray pulses. The method uses the ultra-fast increase in a singly charged ion population induced by optical-field ionization as a soft-X-ray -absorption switch. We measured the pulse duration of the 51st harmonic of a Ti:sapphire laser pulse using Kr gas as a soft-X-ray absorption medium and found it to be 60 fs assuming that the harmonic envelope is equal to a squared secant hyperbolic. This confirmed that our method achieves a shorter temporal resolution than the 100-fs pulse duration of the ionizing laser pulse. The temporal resolution obtained in this way is expected to be from one-third to one-half the duration of the ionizing laser pulse, according to our calculation of the time-evolving population of the Kr⁺ ions. The experimental demonstration and calculation show that methods based on optical-field-induced ionization are promising for femtosecond temporal characterization of an ultra-short pulse in the soft-X-ray region. PACS 42.50.Hz; 42.65.Ky; 32.80.Rm; 06.60.Jn     

Characterization of a broadband pulse for phase controlled multiphoton microscopy by single beam SPIDER

We present what we believe to be a new version of spectral phase interferometry for direct electric field reconstruction (SPIDER) using only a single-phase and polarization controlled laser beam. Two narrow pulses and one broadband pulse are selected out of an ultrafast laser pulse by a polarization and phase control technique to generate second harmonic generation (SHG) signals, which are equivalent to a spectral shear interferogram in the conventional SPIDER method. The spectral phase of the broadband laser pulse is extracted analytically with double quadrature spectral interferometry (DQSI). An arbitrary spectral phase can be retrieved with great precision and compensated in situ at the sample position of a microscope. This new method requires no separate reference beam and is suitable for nonlinear optical microscopy with a phase controlled laser pulse.     

Generation of tunable 7-fs ultraviolet pulses: achromatic phase matching and chirp management

Ultraviolet pulses with a duration of 7 fs are efficiently generated by frequency doubling the output of a noncollinear optical parametric amplifier. The ultraviolet pulses are tunable between 275 to 335 nm. The acceptance bandwidth of the doubling crystal is increased by a factor of 80 through high-order achromatic phase matching. The chirp of the visible pulses and the dispersion introduced along the beam path are compensated partially before and partially after the doubling crystal. For the design of the dispersion management, we investigate the second harmonic generation of pulses with mixed orders of chirp and explicitly discuss the transfer of the spectral phase in frequency doubling. A simple analytic theory is derived that correctly describes frequently observed spectral narrowing effects. We find that chirped SHG avoids spectral narrowing and allows for precompression of dispersion encountered in the ultraviolet beam path. We apply chirped SHG to generate 18.7 fs ultraviolet pulses in an extremely simple setup. PACS 42.65.Re; 42.65.Ky; 42.65.Yj     

Comparison of the filamentation and the hollow-core fiber characteristics for pulse compression into the few-cycle regime

The gas-filled hollow-core fiber compression and the optical filamentation technique are compared experimentally in a parameter regime suitable for intense few-cycle pulse generation. In particular, pointing stability, spectral properties, and spatial chirp are investigated. It is found that in the case of filamentation, the critical parameter for pointing stability is gas pressure inside the generation cell whereas for the hollow-core fiber it is alignment that plays this role. The hollow-core fiber technique yields spectra that are better suited for chirped-mirror pulse compression whereas filamentation offers higher throughput and prospects for easy-to-implement self-compression. We present spectral phase interferometry for direct electric-field reconstruction (SPIDER) measurements that directly show the transition in the spectral phase of the output continua into the self-compression regime as the gas pressure is increased. PACS 42.65.Re; 42.65.Jx; 42.65.Tg     

Single-shot multiple-delay crossed-beam spectral interferometry for measuring extremely complex pulses

We demonstrate a single-shot measurement technique based on spectral interferometry (SI) for measuring the complete intensity and phase vs. time of extremely complex ultrashort laser pulses. Ordinarily, such a method would require an extremely-high-resolution spectrometer, but, by temporally interleaving many SI measurements, each using a different reference-pulse delay, our method overcomes this need. It involves introducing a transverse time delay into the reference pulse by tilting its pulse front transversely to the spectrometer dispersion plane. The tilted reference pulse then gates the unknown pulse by interfering with it at the image plane of a low-resolution imaging spectrometer, yielding an effective increase in the delay range and spectral resolution—by a factor of 30 in our proof-of-principle implementation. Our device achieved a temporal resolution of ~ 130 fs and a temporal range of 120 ps. This simple device has the potential to measure even longer and more complex pulses.     

Use of nonlinear properties of intracavity type II second harmonic generation for image processing

We discuss the use of intracavity type-II second harmonic generation for all-optical processing of images. Injecting an image in a pump beam linearly polarized and a homogeneous field in the orthogonally polarized pump, it is possible to magnify the contrast and to recognize the contour of any part of the image whose intensity is above a tunable reference level. This can be done using ideal planar cavities where all the fields are resonant as well as in the more realistic situation where only the fundamental fields are resonant and using cavities with spherical mirrors. PACS 42.30.-d; 42.65.Ky; 42.65.Pc     

Single-atom effects in high-order harmonic generation: role of carrier-envelope phase in the few-optical-cycle regime

Recently the role of the carrier-envelope phase of few-optical-cycle light pulses on high-order harmonic generation has been experimentally observed. By using a three-dimensional nonadiabatic propagation model, we show that the observed carrier-envelope phase dependence of the spectral characteristics of the harmonic radiation is essentially a single-atom effect. Nonadiabatic single-atom response has been calculated in the framework of the saddle-point approximation. PACS 42.65.Ky; 42.65.Re; 42.50.Hz     

采用光栅角色散的宽带二倍频及聚焦特性分析

针对带宽为20 nm左右、中心波长为1 053 nm的线性啁啾宽带激光,分析了采用光栅角色散方式宽带二倍频的原理,讨论了光栅角色散性能及光栅加工误差引入的基频光位相扰动对二倍频光转换效率及聚焦特性的影响.研究结果表明:采用光栅角色散方式的宽带二倍频光的转换效率、脉冲宽度和带宽均明显增大;在0～22 nm基频光带宽范围内,二倍频光均可保持80％左右的转换效率;对于入射基频光带宽为22 nm的情况,光栅角色散率在30～80 μrad/nm范围内变化时,宽带二倍频转换效率均可保持在70％以上;基频光有位相扰动时,二倍频光带宽对二倍频聚焦光斑的主瓣影响不大,但对旁瓣有一定的匀滑作用,采用光栅角色散方式宽带二倍频的聚焦光斑旁瓣的匀滑效果更为明显.     

Soft-x-ray wavelength shift induced by ionization effects in a capillary

Coherent soft x rays are produced by high-harmonic generation in a capillary filled with Ar gas. We demonstrate that the tuning of the harmonic wavelengths with intensity and chirp arises from changes in the Ar ionization level. Control over the tuning can be achieved either by changing the average intensity of the laser pulse or by varying the quadratic spectral phase of the laser pulse. We observe an ionization-dependent blueshift of the fundamental wavelength that is directly imprinted on the harmonic wavelengths. The harmonic tuning is shown to depend on nonlinear spectral shifts of the fundamental laser pulse that are due to the plasma created by ionization, rather than directly on any chirp imposed on the fundamental wavelength.     

Frequency doubling of multi-terawatt femtosecond pulses

We present an experimental and theoretical study of the influence of the spatial beam quality (fluence and phase distributions) on the second-harmonic generation in KDP crystals pumped by 180-fs pulses at 790 nm. Conversion efficiency and beam focusability are investigated experimentally and theoretically by the numerical analysis of the second harmonic, considering effects due to the cubic nonlinearity, beam diffraction, group-velocity walk-off, and dispersion of the pulses. It was found that the uniform intensity and phase distributions of the fundamental beam are essential to obtain a high focal intensity of the second-harmonic beam. PACS 42.65.Ky; 42.65.Re     

Optimization of a femtosecond pulse self-compression region along a filament in air

We have identified the pulse self-compression region in a filament produced by 55 fs, 4 mJ, 805 nm radiation propagating in air without geometrical focusing. In our experiment the pulse self-compression region is attained by the propagation distance, where the shortest wavelength in the supercontinuum blue wing reaches a minimum, and the growing conversion efficiency to white light has a large gradient. Numerical tracking of the pulse along the filament shows a single-peak 9 fs pulse with a flat spectral phase at the optimum compression distance. PACS 42.65.Jx; 42.65.Re; 42.65.Ky     

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.     

Four-wave mixing with non-resonant pump and resonant probe for OH detection in flames

We report a new long-term interferometric stabilization scheme for a delay line to be used for pump–probe spectroscopy on the attosecond time scale. A separate interferometer with a He–Ne laser beam is used to stabilize the delay line with respect to a slow drift in the optical delay induced mainly by ambient temperature changes in the laboratory. The power of the stabilization scheme is demonstrated with the characterization of an attosecond pulse train in the extreme ultraviolet formed through high order harmonic generation of Ti:sapphire laser radiation. We use the RABBITT technique for the attosecond pulse-train characterization in a reaction microscope employed here for photoelectron–photoion-coincidence spectroscopy. PACS 42.65.Ky; 42.65.Re; 07.60.Ly; 07.81.+a; 32.80.Fb