| 1. |
- Gulbinas, V, et al.
(författare)
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Charge carrier photogeneration in conjugated polymer
- 2002
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Ingår i: Ultrafast phenomena in semiconductors 2001 / Material Science Forum. - 0255-5476. - 0878498907 - 9780878498901 ; 384-385, s. 279-286
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Konferensbidrag (refereegranskat)abstract
- The electric field-assisted charge carrier photogeneration in a Ladder-Type Methyl substituted Poly(Para-Phenylene) was investigated by studying the electromodulated ultrafast transient differential absorption spectra and kinetics. The field stimulated appearance of the polaron absorption and quenching of the singlet exciton absorption proves that dissociation of excitons into geminately bound electron-hole pairs occurs within the entire exciton lifetime and proceeds from the relaxed state. An influence of the optically created excitons and charge pairs on the Stark-shift of the absorption band was observed and interpreted as caused by the modification of the dielectric constant of the medium. The initial separation distance of the charge carriers and its evolution in time were determined from the Stark-shift dynamics. The initial separation distance was obtained being slightly larger than the distance between adjacent polymer chains.
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| 2. |
- Meng, Fanqi, et al.
(författare)
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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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Ingår i: 2015 40TH INTERNATIONAL CONFERENCE ON INFRARED, MILLIMETER AND TERAHERTZ WAVES (IRMMW-THZ). - : IEEE. - 9781479982721 - 9781479982714
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Konferensbidrag (refereegranskat)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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| 3. |
- Meng, Fanqi, et al.
(författare)
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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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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 91:7, s. 075201-
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Tidskriftsartikel (refereegranskat)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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| 4. |
- Soltani, Amin, et al.
(författare)
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Direct nanoscopic observation of plasma waves in the channel of a graphene field-effect transistor
- 2020
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Ingår i: Light: Science and Applications. - : Springer Science and Business Media LLC. - 2047-7538 .- 2095-5545. ; 9:1
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Plasma waves play an important role in many solid-state phenomena and devices. They also become significant in electronic device structures as the operation frequencies of these devices increase. A prominent example is field-effect transistors (FETs), that witness increased attention for application as rectifying detectors and mixers of electromagnetic waves at gigahertz and terahertz frequencies, where they exhibit very good sensitivity even high above the cut-off frequency defined by the carrier transit time. Transport theory predicts that the coupling of radiation at THz frequencies into the channel of an antenna-coupled FET leads to the development of a gated plasma wave, collectively involving the charge carriers of both the two-dimensional electron gas and the gate electrode. In this paper, we present the first direct visualization of these waves. Employing graphene FETs containing a buried gate electrode, we utilize near-field THz nanoscopy at room temperature to directly probe the envelope function of the electric field amplitude on the exposed graphene sheet and the neighboring antenna regions. Mapping of the field distribution documents that wave injection is unidirectional from the source side since the oscillating electrical potentials on the gate and drain are equalized by capacitive shunting. The plasma waves, excited at 2 THz, are overdamped, and their decay time lies in the range of 25–70 fs. Despite this short decay time, the decay length is rather long, i.e., 0.3-0.5 μm, because of the rather large propagation speed of the plasma waves, which is found to lie in the range of 3.5–7 × 10^6 m/s, in good agreement with theory. The propagation speed depends only weakly on the gate voltage swing and is consistent with the theoretically predicted 1414 power law.
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| 5. |
- Soltani, Amin, et al.
(författare)
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Unveiling the plasma wave in the channel of graphene field-effect transistor
- 2019
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Ingår i: International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz. - 2162-2027 .- 2162-2035. ; 2019-September
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Konferensbidrag (refereegranskat)abstract
- Coupling an electromagnetic wave at GHz to THz frequencies into the channel of a graphene field-effect transistor (GFET) provokes collective charge carrier oscillations of the two-dimensional electron gas (2DEG) known as plasma waves. Here, we report the very first experimental and direct mapping of the electric field distribution in a gated GFET at nanometer length scales using scattering-type scanning near-field microscopy (s-SNOM) at 2 THz. Based on the experimental results we deduce the plasma wave velocity for different gate bias voltages, which is in good agreement with the theoretical prediction.
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| 6. |
- Thomson, Mark D., et al.
(författare)
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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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Ingår i: 19th International Conference on Electron Dynamics in Semiconductors, Optoelectronics and Nanostructures (EDISON'19). - : Institute of Physics (IOP). ; , s. 012016-012019
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Konferensbidrag (refereegranskat)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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