337 research outputs found
Exciton and electron-hole plasma formation dynamics in ZnO
Euan Hendry, M. Koeberg, and M. Bonn, Physical Review B, Vol. 76, article 045214 (2007). "Copyright © 2007 by the American Physical Society."We employ optical pump-THz probe measurements to study the formation of excitons and electron-hole plasmas following photogeneration of a hot electron-hole gas in the direct gap semiconductor zinc oxide. Below the Mott density, we directly observe the evolution of the hot electron-hole plasma into an insulating exciton gas in the 10 to 100 ps following photoexcitation. The temperature dependence of this process reveals that the rate determining step for exciton formation involves acoustic phonon emission. Above the Mott density, the density of the hot electron-hole plasma initially decreases very rapidly (~1.5 ps) through Auger annihilation until a stable plasma is formed close to the Mott density. In contrast to exciton formation, Auger annihilation is found to be independent of lattice temperature, occurring while the plasma is still hot
Extracting the sample response function from experimental two-dimensional terahertz-infrared-visible spectra
Terahertz molecular motions are often probed by high-frequency molecular
oscillators in different types of non-linear vibrational spectroscopy. Recently
developed two-dimensional terahertz-infrared-visible spectroscopy allows direct
measuring of this coupling and, thus, obtaining site-specific terahertz
vibrational spectrum. However, these data are affected by the intensity and
phase of the employed laser pulses. In this work, we develop a method of
extracting sample response - representing solely physical properties of a
material - from experimental spectra. Using dimethyl sulfoxide (DMSO) as a
model molecule to verify this method, we measure the coupling between C-H
stretch vibration of its methyl groups and terahertz intramolecular twist and
wagging modes
Reduction of carrier mobility in semiconductors caused by charge-charge interactions
Euan Hendry, M. Koeberg, J. Pijpers, and M. Bonn, Physical Review B, Vol. 75, article 233202 (2007). "Copyright © 2007 by the American Physical Society."We investigate the effect of charge-charge interactions on carrier mobility in titanium dioxide (TiO2) and silicon (Si) using terahertz spectroscopy. Charge scattering times and plasma frequencies are directly determined as a function of charge density. In Si, a linear increase in scattering rate for densities exceeding 1021 m−3 is attributed to electron-hole scattering. In contrast, in TiO2, charge-charge interactions are suppressed due to dielectric screening, highlighting the vastly different dielectric properties for these two materials
Detection of deep-subwavelength dielectric layers at terahertz frequencies using semiconductor plasmonic resonators
Plasmonic bowtie antennas made of doped silicon can operate as plasmonic
resonators at terahertz (THz) frequencies and provide large field enhancement
close to their gap. We demonstrate both experimentally and theoretically that
the field confinement close to the surface of the antenna enables the detection
of ultrathin (100 nm) inorganic films, about 3750 times thinner than the free
space wavelength. Based on model calculations, we conclude that the detection
sensitivity and its variation with the thickness of the deposited layer are
related to both the decay of the local THz field profile around the antenna and
the local field enhancement in the gap of the bowtie antenna. This large field
enhancement has the potential to improve the detection limits of plasmon-based
biological and chemical sensors
Retrieving the susceptibility from time-resolved terahertz experiments
Copyright © 2007 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Journal of Chemical Physics 127 (2007) and may be found at http://link.aip.org/link/?JCPSA6/127/094308/1We present an analytical expression for the observed signal in time- and phase-resolved pump-probe studies, with particular emphasis on terahertz time-domain spectroscopy. Maxwell's equations are solved for the response of damped, harmonic oscillators to a driving probe field in the perturbative regime. Our analytical expressions agree with the one previously reported in the literature [Nemec et al., J. Chem. Phys. 122, 104503 (2005)] in the Drude limit; however, they differ in the case of a vibrational resonanc
High-frequency dielectric relaxation of liquid crystals: THz time-domain spectroscopy of liquid crystal colloids
Copyright © 2006 Optical Society of America. This paper was published in Optics Express and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-23-11433 . Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.Terahertz time-domain spectroscopy has been used to study the dielectric relaxation of pure 4’–n-pentyl–4–cyanobiphenyl (5CB) liquid
crystal (LC) and its mixtures with 10 µm SiO2 particles in the frequency
range 0.2–2 THz. For the pure sample, we find that spatial inhomogeneities
consisting of oriented domains, comparable in size to our probe area (~1
mm2), cause a large scatter in the measured dielectric function, due to varying
contributions from the ordinary and extraordinary components. In the
LC/particle mixtures, ordering of the LC at the surface of the SiO2 particles
results in a break-up of these domains, giving rise to a spatially much more
homogeneous dielectric response. The inferred dielectric function can be
interpreted using effective medium theory and the Debye relaxation model.
We observe this stabilizing effect for interparticle distances < ~30 µm,
setting a lower limit for the size of oriented domains in the bulk LC
Density-dependent electron scattering in photoexcited GaAs in strongly diffusive regime
Mics Z, D'Angio A, Jensen SA, Bonn M, Turchinovich D. Density-dependent electron scattering in photoexcited GaAs in strongly diffusive regime. Applied Physics Letters. 2013;102(23).In a series of systematic optical pump–terahertz probe experiments, we study the density-dependent electron scattering rate in photoexcited GaAs in the regime of strong carrier diffusion. The terahertz frequency-resolved transient sheet conductivity spectra are perfectly described by the Drude model, directly yielding the electron scattering rates. A diffusion model is applied to determine the spatial extent of the photoexcited electron-hole gas at each moment after photoexcitation, yielding the time-dependent electron density, and hence the density-dependent electron scattering time. We find that the electron scattering time decreases from 320 to 60 fs, as the electron density changes from 1015 to 1019 cm−3
Both Inter- and Intramolecular Coupling of O-H Groups Determine the Vibrational Response of the Water/Air Interface
Vibrational coupling is relevant not only for dissipation of excess energy
after chemical reactions but also for elucidating molecular structure and
dynamics. It is particularly important for OH stretch vibrational spectra of
water, for which it is known that in bulk both intra- and intermolecular
coupling alter the intensity and line shape of the spectra. In contrast with
bulk, the unified picture of the inter/intra-molecular coupling of OH groups at
the water-air interface has been lacking. Here, combining sum-frequency
generation experiments and simulation for isotopically diluted water and
alcohols, we unveil effects of inter- and intramolecular coupling on the
vibrational spectra of interfacial water. Our results show that both inter- and
intramolecular coupling contribute to the OH stretch vibrational response of
the neat H2O surface, with intramolecular coupling generating a double-peak
feature, while the intermolecular coupling induces a significant red shift in
the OH stretch response
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