Doppler broadening

About: Doppler broadening is a research topic. Over the lifetime, 5509 publications have been published within this topic receiving 92552 citations.

Theory of THz generation by Optical Rectification using Tilted-Pulse-Fronts

Abstract: A model for THz generation by optical rectification using tilted-pulse-fronts is developed. It simultaneously accounts for (i) the spatio-temporal distortions of the optical pump pulse, (ii) the nonlinear coupled interaction of THz and optical radiation in two spatial dimensions (2-D), (iii) self-phase modulation and (iv) stimulated Raman scattering. The model is validated by quantitative agreement with experiments and analytic calculations. We show that the optical pump beam is significantly broadened in the transverse-momentum (kx) domain as a consequence of the spectral broadening caused by THz generation. In the presence of this large frequency and transverse-momentum (or angular) spread, group velocity dispersion causes a spatio-temporal break-up of the optical pump pulse which inhibits further THz generation. The implications of these effects on energy scaling and optimization of optical-to-THz conversion efficiency are discussed. This suggests the use of optical pump pulses with elliptical beam profiles for large optical pump energies. It is seen that optimization of the setup is highly dependent on optical pump conditions. Trade-offs of optimizing the optical-to-THz conversion efficiency on the spatial and spectral properties of THz radiation is discussed to guide the development of such sources.

Energy shift and broadening of the spectra of electrons backscattered elastically from solid surfaces

Abstract: In the present work our experimental results on the energy shifts and energy widths (full width of half-maximum) of the quasi-elastic peaks (1-5 keV) obtained using a high-energy-resolution electron spectrometer and different (C, Si, Ni and Au) surfaces are compared with those calculated by assuming single elastic scattering on free atoms having a Maxwell-Boltzmann thermal velocity distribution. There is a good agreement in the case of the energy shifts as well as for the energy broadenings obtained using higher atomic number polycrystalline samples (Ni, Au). In the case of Si, however, the measured energy broadening is systematically larger by 15-20% than the calculated broadening for the whole primary beam energy range. Compared with the calculated values, considerably larger broadenings (by 30-60%, depending on the primary beam energy) were observed for carbonic samples. The contribution of the multiple elastic scattering to the yield of the electrons backscattered elastically, and the effect of the multiple scattering on the energy shifts and Doppler broadenings, have been determined using Monte Carlo simulations. Our results show that multiple scattering causes only small changes in energy shifts and energy broadenings of elastic peaks in the case of the samples and primary electron energy region studied.

Effects of velocity changing collisions on line shapes of HF in Ar

Abstract: The generalized Hess method (GHM) gives a line shape expression which is formally equivalent to the Rautian–Sobel’man hard collision model of Dicke narrowing, but differs radically in the definition of one of the relaxation terms. The relaxation term leading to pressure broadening is the same, but the term leading to Dicke narrowing and ultimately to Doppler line shapes at zero density differs in certain important respects: (1) in GHM it is a weighted sum of the pressure broadening coefficient and an optical diffusion coefficient and (2) there is no sharp distinction between ‘‘velocity changing’’ and ‘‘phase changing’’ collisions. The Dicke narrowing term should thus be understood as including both collision types irretrievably intermixed, with GHM providing a prescription for both relaxation terms. Applied to HF v=0→1, j→j±1 absorption spectra in a bath of Ar and using an accurate interaction potential obtained from spectra of the van der Waals complex and essentially exact close coupling scattering S ma...

Wideband nonlinear spectral broadening in ultra-short ultra - silicon rich nitride waveguides.

Abstract: CMOS-compatible nonlinear optics platforms with high Kerr nonlinearity facilitate the generation of broadband spectra based on self-phase modulation. Our ultra – silicon rich nitride (USRN) platform is designed to have a large nonlinear refractive index and low nonlinear losses at 1.55 μm for the facilitation of wideband spectral broadening. We investigate the ultrafast spectral characteristics of USRN waveguides with 1-mm-length, which have high nonlinear parameters (γ ∼ 550 W−1/m) and anomalous dispersion at 1.55 μm wavelength of input light. USRN add-drop ring resonators broaden output spectra by a factor of 2 compared with the bandwidth of input fs laser with the highest quality factors of 11000 and 15000. Two – fold self phase modulation induced spectral broadening is observed using waveguides only 430 μm in length, whereas a quadrupling of the output bandwidth is observed with USRN waveguides with a 1-mm-length. A broadening factor of around 3 per 1 mm length is achieved in the USRN waveguides, a value which is comparatively larger than many other CMOS-compatible platforms.