THESIS
2022
1 online resource (xii, 40 pages) : illustrations (some color)
Abstract
The superior temporal and spectral resolution of ultrafast laser pulses is commonly adopted for
the ultrafast dynamic study of photo-induced states in complex materials referred to as pump-probe
spectroscopy. The corresponding probing method, time-domain spectroscopy, is a
formidable method for the measurement of complex optical response functions, including
dielectric constants ε̃(ω), conductivity σ̃(ω), and refractive index ñ(ω), without relying on
Kramers-Kronig analysis that is required by frequency-domain spectroscopy, such as
broadband Fourier-transform infrared spectroscopy (FTIR) and standard reflectance or
transmittance measurement. Although time-domain spectroscopy is an outstanding tool at mid-and
far-infrared wavelengths, the shorter wavelengths in near-infrared, visible, a...[
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The superior temporal and spectral resolution of ultrafast laser pulses is commonly adopted for
the ultrafast dynamic study of photo-induced states in complex materials referred to as pump-probe
spectroscopy. The corresponding probing method, time-domain spectroscopy, is a
formidable method for the measurement of complex optical response functions, including
dielectric constants ε̃(ω), conductivity σ̃(ω), and refractive index ñ(ω), without relying on
Kramers-Kronig analysis that is required by frequency-domain spectroscopy, such as
broadband Fourier-transform infrared spectroscopy (FTIR) and standard reflectance or
transmittance measurement. Although time-domain spectroscopy is an outstanding tool at mid-and
far-infrared wavelengths, the shorter wavelengths in near-infrared, visible, and ultraviolet
ranges are prohibited because of the compatibility of electro-optic (EO) sampling crystals in
the retrieval. The modern research of ultrafast optical study at the prohibitive range have been
obstructed to deliver dynamic measurements on reflectance, transmittance and absorbance only
in amplitude with limited information on phase. The complete measurement of an arbitrary
ultrashort pulse was complicated until the invention of the nonlinear technique, frequency-resolved
optical gating (FROG), by Rick Trebino and Daniel J. Kane. It is an improvement of
the autocorrelation to completely extract both amplitude and phase at the same time under the
nonlinear optical process without any initial information of pulses. We propose the usage of
second-harmonic generation (SHG) FROG among other types because of the simplicity and
high sensitivity for low-energy pulses. The asymmetric double-pulse scheme is introduced to activate the phase-locking mechanism, to solve the absolute zero-order phase ambiguity
existing in standard SHG FROG retrieval. The effectiveness of the method is examined by the
numerical simulation in both cases of equilibrium and non-equilibration states using two
semiconducting materials WS
2 or modulated-GaAs (redshifted 260 nm). The simulated results
can match with analytical models in visible probing range (550 ⎯ 750 nm).
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