Ultra-short laser-pulse-induced strain propagation in a Ge crystal is studied in the [111] and [100] directions using time-resolved X-ray diffraction (TXRD). The strain propagation velocity is derived by analysis of the TXRD signal from the strained crystal planes. Numerical integration of the Takagi–Taupin equations is performed using open source code, which provides a very simple approach to estimate the strain propagation velocity. The present method will be particularly useful for relatively broad spectral bandwidths and weak X-ray sources, where temporal oscillations in the diffracted X-ray intensity at the relevant phonon frequencies would not be visible. The two Bragg reflections of the Ge sample, viz. 111 and 400, give information on the propagation of strain for two different depths, as the X-ray extinction depths are different for these two reflections. The strain induced by femtosecond laser excitation has a propagation velocity comparable to the longitudinal acoustic velocity. The strain propagation velocity increases with increasing laser excitation fluence. This fluence dependence of the strain propagation velocity can be attributed to crystal heating by ambipolar carrier diffusion. Ge is a promising candidate for silicon-based optoelectronics, and this study will enhance the understanding of heat transport by carrier diffusion in Ge induced by ultra-fast laser pulses, which will assist in the design of optoelectronic devices.
An experimental study was performed to explore the effect of gas-density gradient in laser propagation direction on high-harmonic generation from neon-filled cells using a 50 fs annular and Gaussian Ti:sapphire laser beam. It was observed that, despite 20% lower power, the photon flux of the high-harmonics generated using an annular beam under optimum parameter conditions (∼2.5 × 1010 photons/sec for the 37th order in a 5 mm long cell) is on par with the maximum photon flux generated using a full Gaussian beam (∼2 × 1010 photons/sec for the 37th order in a 15 mm long cell). To elucidate the underlying mechanism for the experimental observation, a numerical simulation of the propagation of both the annular and Gaussian laser beams inside the cell was performed. The simulation was extended to estimate the high-harmonic intensity, after incorporating the effect of laser defocusing, the electron trajectory resolved phase-matching, and gas-density gradient. The dominant role of short electron trajectories was observed in the case of the annular beam, whereas, in the case of the Gaussian beam, a contribution of both short and long trajectories was found. Our analysis shows that, in neon-filled cells, the gas-density gradient present at the laser exit end of the cell plays a dominant role in achieving a high photon flux using an annular laser beam. Further, the annular beam not only provides a higher flux but also has lower divergence and higher coherence. This study will be useful in attosecond pulse metrology as well as in imaging applications viz coherent diffractive imaging.
The design and performance of an in-house developed double-solenoid magnetic bottle (MB) time-of-flight photoelectron spectrograph are presented. A combination of a strong permanent magnet (Sm2Co17) with a soft iron cone and a double-solenoid geometry is used to generate MB configuration. The first solenoid (length ∼150 mm) is placed inside the vacuum, and the second solenoid (length ∼1 m) is placed outside the vacuum. The double-solenoid geometry improves the effective conductance and reduces overall material outgassing. Due to this, an ultra-high vacuum (∼5 × 10−8 mbar) desirable for the working of the spectrograph was achieved using a small capacity (300 lps) turbo-molecular pump. An optimization of solenoid current generates a smooth magnetic field variation in MB, which keeps the adiabaticity parameter ∼0.6 at ∼25 eV photoelectron energy. The double-solenoid geometry also provides high collection efficiency as well as high energy resolution of the spectrograph. The experimentally measured energy resolution ( ΔE) of the spectrograph is better than ∼60 meV at ∼15 eV photoelectron energy. The collection efficiency is estimated to be ∼25% under optimum conditions as compared with ∼10−4 in field-free configuration. The calibrated MB spectrograph is used for the characterization of the attosecond pulse train using a cross-correlation “RABBITT” technique. The attosecond pulse train is generated from 15th to 25th odd high-harmonic orders, in argon filled cell. Attosecond pulses of average duration ∼260 as (FWHM) have been measured. The proposed MB electron spectrograph design provides a compact experimental setup for attosecond metrology and pump-probe studies with a relaxed requirement on vacuum pump capacity.
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