| 1. |
- Meng, Fanqi, et al.
(author)
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Relativistic Doppler Frequency Up-conversion and Probing the Initial Relaxation of a Non-Equilibrium Electron-Hole Plasma in Silicon
- 2015
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In: 2015 40TH INTERNATIONAL CONFERENCE ON INFRARED, MILLIMETER AND TERAHERTZ WAVES (IRMMW-THZ). - : IEEE. - 9781479982721 - 9781479982714
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Conference paper (peer-reviewed)abstract
- we demonstrate experimentally the relativistic Doppler frequency up-conversion of the THz pulses from the counter-propagating ionized plasma front in silicon. The observed frequency up-conversion can be well modeled by the 1D FDTD simulations if significant short scattering time (well below 10 fs) in the plasma is assumed. To further elucidate the scattering rate in the electro-hole plasma, we performed pump probe experiment employing ultra-broadband (150 THz) THz-Mid-Infrared pulse. The results show the scattering time decreases from similar to 200 fs down to similar to 20 fs when the carrier density increases up to 10(19)-cm(-3), and then saturates for higher densities. Such scattering time dependence on plasma carrier density can be very well fitted by the Drude model for thermalized electron-holes, and the saturation behavior is attributed to electron-hole phase-space restriction as the plasma becomes degenerate. The resultant much shorter scattering time measured with non-thermalized plasma is in good accordance with the Doppler experiment, which demonstrates Doppler geometry an effective method for probing non-equilibrium plasma dynamics.
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| 2. |
- Meng, Fanqi, et al.
(author)
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Ultrafast dynamic conductivity and scattering rate saturation of photoexcited charge carriers in silicon investigated with a midinfrared continuum probe
- 2015
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In: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 91:7, s. 075201-
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Journal article (peer-reviewed)abstract
- We employ ultrabroadband terahertz-midinfrared probe pulses to characterize the optical response of photoinduced charge-carrier plasmas in high-resistivity silicon in a reflection geometry, over a wide range of excitation densities (10(15)-10(19) cm(-3)) at room temperature. In contrast to conventional terahertz spectroscopy studies, this enables one to directly cover the frequency range encompassing the resultant plasma frequencies. The intensity reflection spectra of the thermalized plasma, measured using sum-frequency (up-conversion) detection of the probe pulses, can be modeled well by a standard Drude model with a density-dependent momentum scattering time of similar to 200 fs at low densities, reaching similar to 20 fs for densities of similar to 10(19) cm(-3), where the increase of the scattering rate saturates. This behavior can be reproduced well with theoretical results based on the generalized Drude approach for the electron-hole scattering rate, where the saturation occurs due to phase-space restrictions as the plasma becomes degenerate. We also study the initial subpicosecond temporal development of the Drude response and discuss the observed rise in the scattering time in terms of initial charge-carrier relaxation, as well as the optical response of the photoexcited sample as predicted by finite-difference time-domain simulations.
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| 3. |
- Thomson, Mark D., et al.
(author)
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Relativistic Doppler reflection as a probe for the initial relaxation of a non-equilibrium electron-hole plasma in silicon
- 2015
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In: 19th International Conference on Electron Dynamics in Semiconductors, Optoelectronics and Nanostructures (EDISON'19). - : Institute of Physics (IOP). ; , s. 012016-012019
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Conference paper (peer-reviewed)abstract
- This paper reviews the status of investigations of the relativistic Doppler reflectionof a broadband terahertz pulse at a counter-propagating plasma front of photo-excited chargecarriers in undoped silicon. When a THz pulse with 20-THz bandwidth impinges onto amoving plasma front with a carrier density in the range of 1019 per cm3, one observes a spectralup-shift, which is, however, much less pronounced than expected from simulations assuming a Drude plasma characterized by a single carrier relaxation time τ of the order of 15-100 fs.Qualitative agreement between simulations and experiments can be achieved if τ is chosen tobe less than 5 fs. In order to explore carrier relaxation in more detail, optical-pump/THz-probeexperiments in the conventional co-propagation geometry were performed. If the pump-probedelay is long enough for monitoring of the equilibrium value of the scattering time, τ rangesfrom 200 fs at low carrier density to 20 fs in the 1019-cm-3 density range. For small (subpicosecond)pump-probe delay, the data reveal a significantly faster scattering, which slowsdown during energy relaxation of the charge carriers.
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