Spatial beam shaping of ultrashort laser pulses: theory and experiment

We studied the spatial intensity profile of an ultrashort laser pulse passing through a laser beam shaping system, which uses diffractive optical elements to reshape a Gaussian beam profile into a flat-topped distribution. Both dispersion and nonlinear self-phase modulation are included in the theoretical model. Our calculation shows that this system works well for ultrashort pulses (approximately 100 fs) when the pulse peak intensity is less than 5 x 10(11) W/cm2. Experimental results are presented for 136 fs pulses at 800 nm wavelength from a Ti:sapphire laser with a 6 nJ pulse energy. We also studied the effects of lateral misalignment, beam-size deviation, and defocusing on the energy fluence profile.     

Generation of a train of ultrashort pulses from a compact birefringent crystal array

A linear array of n calcite crystals is shown to allow the generation of a high contrast (>10:1) train of 2(n) high energy (>100 microJ) pulses from a single ultrafast laser pulse. Advantage is taken of the pulse-splitting properties of a single birefringent crystal, where an incident laser pulse can be split into two pulses with orthogonal polarizations and equal intensity, separated temporally in proportion to the thickness of the crystal traversed and the difference in refractive indices of the two optic axes. In the work presented here an array of seven calcite crystals of sequentially doubled thickness is used to produce a train of 128 pulses, each of femtosecond duration. Readily versatile properties such as the number of pulses in the train and variable mark-space ratio are realized from such a setup.     

Spatial and temporal properties of ultrashort pulses propagating in free space

Abstract

By means of the Rayleigh diffraction integral and analytic signal complex representation, the propagation equation of ultrashort pulses in free space is derived, which permits us to study spatial and temporal properties of ultrashort pulses during propagation in free space. It is shown that the pulse deformation and broadening, spectrum redshifting, narrowing and distortion can take place with increasing diffraction angle, and the pulse form changes with propagation distance in the near field, but is preserved in the far field. A comparison with previous work is also made.     

Spectral superresolution with ultrashort optical pulses

A superresolution technique for the measurement of transmission, reflection, and absorption spectra is proposed. An ultrashort laser pulse is propagated in a dispersive element and then periodically phase modulated. The temporal modulation is transformed into periodic spectral modulation, for which the number of harmonics, 2M+1, is determined by the modulation index. The modulated pulse is transmitted through (reflected from) the sample to be tested and measured by a spectrometer. By performing 2M+1 measurements for 2M+1 delays between the dispersed pulse and modulation signal, one can restore the spectral response of the sample with superresolution after simple processing. We numerically demonstrate the measurement of the transmission spectrum of an ultranarrow optical filter with a minimum feature of 0.43 pm by an optical spectrum analyzer with a 10 pm resolution. A twentyfold enhancement of the resolution is achieved in the presence of noise with a level of 0.1%. The advantage of the system is its full reconfigurability.     

Phase mask for spatial and temporal control of ultrashort light pulses focused by lenses

The field amplitude associated with ultrashort light pulses was analyzed by using the phase-space formalism of the Wigner distribution function (WDF). The diffraction integral was properly modified to take into account the dispersion effects (up to second order). A two-dimensional WDF associated with a reduced pupil function was derived, from which the on-axis irradiance was obtained for varying times. A two-dimensional and rotationally symmetric quartic-phase mask to control the temporal stretching of femtosecond light pulses passing through optical systems was proposed and analyzed. A Gaussian spatial and temporal pulse passing through a single lens with and without the phase mask was investigated.     

Controlling semiconductor nanoparticle size distributions with tailored ultrashort pulses

The laser generation of size-controlled semiconductor nanoparticle formation under gas phase conditions is investigated. It is shown that the size distribution can be changed if picosecond pulse sequences of tailored ultra short laser pulses (<200?fs) are employed. By delivering the laser energy in small packages, a temporal energy flux control at the target surface is achieved, which results in the control of the thermodynamic pathway the material takes. The concept is tested with silicon and germanium, both materials with a predictable response to double pulse sequences, which allows deduction of the materials' response to complicated pulse sequences. An automatic, adaptive learning algorithm was employed to demonstrate a future strategy that enables the definition of more complex optimization targets such as particle size on materials less predictable than semiconductors.     

Absorption of ultrashort optical pulses in water

We investigate the linear propagation of 800 and 1530 nm ultrashort optical pulses in water. For all pulse repetition rates studied, we observe pure exponential decay down to a transmission of 2.5 x 10(-5). We further demonstrate that previous observations of nonmonoexponential decay and pulse splitup in broadband pulses are consistent with Beer's law in the purely linear regime.     

A review of non-linear terahertz spectroscopy with ultrashort tabletop-laser pulses

Over the past decade, breakthroughs in the generation and control of ultrafast high-field terahertz (THz) radiation have led to new spectroscopic methodologies for the study of light-matter interactions in the strong-field limit. In this review, we will outline recent experimental demonstrations of non-linear THz material responses in materials ranging from molecular gases, to liquids, to varieties of solids – including semiconductors, nanocarbon, and correlated electron materials. New insights into how strong THz fields interact with matter will be discussed in which a THz field can act as either a non-resonant electric field or a broad bandwidth pulse driving specific resonances within it. As an emerging field, non-linear THz spectroscopy shows promise for elucidating dynamic problems associated with next generation electronics and optoelectronics, as well as for demonstrating control over collective material degrees of freedom.     

Electron transitions and radiation of atoms interacting with ultrashort electromagnetic pulses

Excitation and ionization of an atom interacting with a short electromagnetic pulse is studied within the framework of the sudden perturbation approximation. The excitation and ionization probabilities and the spectra and cross sections of reradiation of the pulse by the atom are calculated. It is suggested that the process of reradiation of ultrashort electromagnetic pulses by multielectron atoms possesses a coherent character.     

On the second order characterization of ultrashort pulses

Abstract

The spectral properties of almost-Gaussian functions are considered and applied to the characterization of the second-order approximation in the expansion of the coefficients of almost-perfect optical pulses. Specifically, adding small amounts of odd-order Hermite-Gaussians to a Gaussian induces a second-order increase in the time-bandwidth product, while the increase in the time-bandwidth product from adding even-order Hermite-Gaussians is higher-order and hence smaller. We indicate the class of small perturbations of Gaussian functions which change neither the temporal profile of the intensity nor the intensity of the spectral profile.     

Simple method for accurate characterization of birefringent crystals

We present a simple method to determine the cutting angle and thickness of birefringent crystals. Our method is based on chromatic polarization interferometry and allows for accuracies of typically 0.1 degrees in the cutting angle and 0.5% in the thickness.     

Efficient femtosecond optical parametric generator with a birefringent delay compensator

A novel method based on a birefringent crystal is presented to compensate for the group-velocity mismatch between a femtosecond pulsed pump and the signal or idler in an optical parametric generator (OPG). With a thin calcite plate inserted between two beta-barium borate crystals for synchronization, an efficient four-pass OPG pumped by a frequency-doubled ~200-fs Ti:sapphire laser is obtained. The conversion efficiency is almost doubled throughout the whole tunable region compared with a conventional OPG pumped at the same power density, indicating that the total conversion efficiency of femtosecond optical parametric amplifier system can be raised significantly by use of this new four-pass OPG to generate the seed.     

Efficient type I second-harmonic generation of subpicosecond laser pulses with a series of alternating nonlinear and delay crystals

We propose a novel configuration of efficient type I second-harmonic generation (SHG) with ultrashort laser pulses by group-velocity compensation. The configuration is composed of a type I SHG crystal and a series of alternating time-delay and type I SHG crystals. The numerical calculations show that the conversion efficiency can be increased to almost 100% by using crystal pairs in series, and the duration of the second-harmonic pulse is almost the same as that of the fundamental pulse.     

超短激光脉冲腔外光谱展宽与压缩技术

超短激光脉冲是超快光学和强场激光物理研究领域的重要驱动光源,通常可以通过脉冲后压缩方法获得。本文详细介绍了超短脉冲后压缩的发展现状、原理和相关技术,包括块状固体材料压缩、薄片组压缩、多通腔压缩、中空波导压缩以及光子晶体光纤压缩等。并通过对现有的脉冲后压缩技术进行总结,为未来的研究与发展方向提供了理论指导。     

Propagation of ultrashort RF pulses in a polluted atmosphere

The effect of absorption and dispersion on the spectral characteristics and the shape of ultrashort RF pulses propagating in a polluted atmosphere is studied theoretically. The frequency band occupied by the pulses essentially covers the rotational spectrum of gas molecules encountered in a real atmosphere.     

Frequency doubling of ultrashort laser pulses in biological tissues

Theoretical and experimental studies of second-harmonic generation (SHG) in biological tissues was performed by use of ultrashort laser pulses (<1 ps). A simplified one-dimensional model for the generation and the propagation of frequency-doubled light inside tissue was developed. This model was tested in vitro against measurements of pig and chicken tissue and human tooth. The experimental results indicate that the intensity of SHG varies significantly among tissue types and between test sites in individual tissue. Possibilities of using this nonlinear tissue property in imaging and diagnostics are discussed.     

Generation of high-power ultrashort optical pulses by semiconductor lasers

Fiber-coupled semiconductor lasers have been studied when pumped by high-power short electrical pulses of 5 ns width and leading front duration below 1 ns. In this pumping regime, it is possible to ensure significant sharpening of output pulses, the duration of which decreases below 80 ps for a single-mode laser and below 120 ps for a broad aperture multimode laser at an output peak optical power as high as 1.5 and 27 W, respectively.