K. T. Tsen

Time-resolved Raman studies of the decay of the longitudinal optical phonons in wurtzite GaN

Decay of the longitudinal-optical (LO) phonons in wurtzite GaN has been studied by subpicosecond time-resolved Raman spectroscopy. Our experimental results show that among the various possible decay channels, the LO phonons in wurtzite GaN decay primarily into a large wave-vector TO and a large wave-vector LA or TA phonon. These experimental results are consistent with the recent theoretical calculations of the phonon dispersion curves for wurtzite GaN.Decay of the longitudinal-optical (LO) phonons in wurtzite GaN has been studied by subpicosecond time-resolved Raman spectroscopy. Our experimental results show that among the various possible decay channels, the LO phonons in wurtzite GaN decay primarily into a large wave-vector TO and a large wave-vector LA or TA phonon. These experimental results are consistent with the recent theoretical calculations of the phonon dispersion curves for wurtzite GaN.

Subpicosecond time-resolved Raman studies of LO phonons in GaN: Dependence on photoexcited carrier density

Subpicosecond time-resolved Raman spectroscopy has been used to measure the lifetime of the LO phonon mode in GaN for photoexcited electron-hole pair density ranging from 1016to2×1019cm−3. The lifetime has been found to decrease from 2.5ps, at low density, to 0.35ps, at the highest density. The experimental findings should help resolve the recent controversy over the lifetime of LO phonon mode in GaN.

Observation of large electron drift velocities in InN by ultrafast Raman spectroscopy

Electron transport in an InN film grown on GaN has been studied by transient Raman spectroscopy at T=300K. Our experimental results demonstrate that under the subpicosecond laser excitation and probing, electron drift velocity of carriers in the Γ valley can exceed its steady-state value by as much as 40%. Electron velocities have been found to cut off at around 2×108cm∕s, significantly larger than those observed for other III-V semiconductors, such as GaAs and InP. These experimental results have been compared with ensemble Monte Carlo simulations and good agreement has been obtained.Electron transport in an InN film grown on GaN has been studied by transient Raman spectroscopy at T=300K. Our experimental results demonstrate that under the subpicosecond laser excitation and probing, electron drift velocity of carriers in the Γ valley can exceed its steady-state value by as much as 40%. Electron velocities have been found to cut off at around 2×108cm∕s, significantly larger than those observed for other III-V semiconductors, such as GaAs and InP. These experimental results have been compared with ensemble Monte Carlo simulations and good agreement has been obtained.

Direct measurements of electron-longitudinal optical phonon scattering rates in wurtzite GaN

Electron-longitudinal optical phonon scattering rates in wurtzite GaN have been directly measured by subpicosecond time-resolved Raman spectroscopy. We find that the total electron-longitudinal optical phonon scattering rate in GaN is about one order of magnitude larger than that in GaAs. We attribute this enormous increase in the electron-longitudinal optical phonon scattering rate to the much larger ionicity in GaN.

Nonequilibrium electron distributions and phonon dynamics in wurtzite GaN

We report experimental measurements of nonequilibrium electron distributions as well as phonon dynamics in wurtzite GaN by using subpicosecond time‐resolved Raman spectroscopy. Our experimental results have demonstrated that for electron densities n≥5×1017 cm−3, the nonequilibrium electron distributions in wurtzite GaN can be very well described by Fermi–Dirac distribution functions with the temperature of electrons substantially higher than that of the lattice. The population relaxation time of longitudinal optical phonons was directly measured to be τ≂5±1 ps at T=25 K.

Ultrafast phenomena in semiconductors

1. Coherent Dynamics of Photoexcited Semiconductor Supperlattices with Applied Homogenous Electric Fields.- 2. Ultrafast Nonequilibrium Dynamics of Intersubband Excitations in Quasi-two-dimensional Semiconductors.- 3. Bloch Oscillations in Semiconductors: Principles and Applications.- 4. Electron Velocity Overshoot, Electron Ballistic Transport, and Non-equilibrium Phonon Dynamics in Nanostructure Semiconductors.- 5. Coherent Control of Photocurrents in Semiconductors.- 6. Ensemble Monte Carlo Simulations of Ultrafast Phenomena in Semiconductors.- 7. Theory of Coherent Phonon Oscillations in Bulk GaAs.- 8. Coherent Spectroscopy on Quantum Wires.- 9. The Vectorial Dynamics of Coherent Emission from Excitons.

Inactivation of viruses by coherent excitations with a low power visible femtosecond laser.

BackgroundResonant microwave absorption has been proposed in the literature to excite the vibrational states of microorganisms in an attempt to destroy them. But it is extremely difficult to transfer microwave excitation energy to the vibrational energy of microorganisms due to severe absorption of water in this spectral range. We demonstrate for the first time that, by using a visible femtosecond laser, it is effective to inactivate viruses such as bacteriophage M13 through impulsive stimulated Raman scattering.Results and discussionBy using a very low power (as low as 0.5 nj/pulse) visible femtosecond laser having a wavelength of 425 nm and a pulse width of 100 fs, we show that M13 phages were inactivated when the laser power density was greater than or equal to 50 MW/cm2. The inactivation of M13 phages was determined by plaque counts and had been found to depend on the pulse width as well as power density of the excitation laser.ConclusionOur experimental findings lay down the foundation for an innovative new strategy of using a very low power visible femtosecond laser to selectively inactivate viruses and other microorganisms while leaving sensitive materials unharmed by manipulating and controlling with the femtosecond laser system.

Inactivation of viruses by laser-driven coherent excitations via impulsive stimulated Raman scattering process

The inactivation of viruses such as M13 bacteriophages subject to excitations by a very low power visible femtosecond laser has been studied. Our experimental results show that for a visible femtosecond laser having lambda = 425 nm and a pulse width of 100 fs, the M13 bacteriophages are inactivated when the laser power density is greater than or equal to 49 MW/cm(2). The medium lethal laser power density (LD(50)) is 51.94+/-0.14 MW/cm(2). The functionality of M13 bacteriophages has been shown to be critically dependent on the pulse width as well as power density of the excitation laser. Our work demonstrates that by using a very low power visible femtosecond laser, it is plausible to inactivate viruses such as the M13 bacteriophages through impulsive stimulated Raman scattering process. These experimental findings suggest a novel avenue of selectively inactivating microorganisms while leaving the sensitive materials unharmed by manipulating and controlling with femtosecond laser systems.

Photonic approach to the selective inactivation of viruses with a near-infrared subpicosecond fiber laser

We report a photonic approach for selective inactivation of viruses with a near-infrared subpicosecond laser. We demonstrate that this method can selectively inactivate viral particles ranging from nonpathogenic viruses such as the M13 bacteriophage and the tobacco mosaic virus to pathogenic viruses such as the human papillomavirus and the human immunodeficiency virus (HIV). At the same time, sensitive materials such as human Jurkat T cells, human red blood cells, and mouse dendritic cells remain unharmed. The laser technology targets the global mechanical properties of the viral protein shell, making it relatively insensitive to the local genetic mutation in the target viruses. As a result, the approach can inactivate both the wild and mutated strains of viruses. This intriguing advantage is particularly important in the treatment of diseases involving rapidly mutating viral species such as HIV. Our photonic approach could be used for the disinfection of viral pathogens in blood products and for the treatment of blood-borne viral diseases in the clinic.

Prospects for a novel ultrashort pulsed laser technology for pathogen inactivation

The threat of emerging pathogens and microbial drug resistance has spurred tremendous efforts to develop new and more effective antimicrobial strategies. Recently, a novel ultrashort pulsed (USP) laser technology has been developed that enables efficient and chemical-free inactivation of a wide spectrum of viral and bacterial pathogens. Such a technology circumvents the need to introduce potentially toxic chemicals and could permit safe and environmentally friendly pathogen reduction, with a multitude of possible applications including the sterilization of pharmaceuticals and blood products, and the generation of attenuated or inactivated vaccines.