134 research outputs found
Self-phase modulation of a single-cycle terahertz pulse by nonlinear free-carrier response in a semiconductor
We demonstrate the self-phase modulation (SPM) of a single-cycle THz pulse in
a semiconductor, using bulk n-GaAs as a model system. The SPM arises from the
heating of free electrons in the electric field of the THz pulse, leading to an
ultrafast reduction of the plasma frequency, and hence to a strong modification
of the THz-range dielectric function of the material. THz SPM is observed
directly in the time domain. In the frequency domain it corresponds to a strong
frequency-dependent refractive index nonlinearity of n-GaAs, found to be both
positive and negative within the broad THz pulse spectrum, with the
zero-crossing point defined by the electron momentum relaxation rate. We also
observed the nonlinear spectral broadening and compression of the THz pulse.Comment: 5 pages, 6 figure
Semiconductor saturable absorbers for ultrafast THz signals
We demonstrate saturable absorber behavior of n-type semiconductors GaAs, GaP
and Ge in THz frequency range at room temperature using nonlinear THz
spectroscopy. The saturation mechanism is based on a decrease in electron
conductivity of semiconductors at high electron momentum states, due to
conduction band nonparabolicity and scattering into satellite valleys in strong
THz fields. Saturable absorber parameters, such as linear and non-saturable
transmission, and saturation fluence, are extracted by fits to a classic
saturable absorber model. Further, we observe THz pulse shortening, and an
increase of the group refractive index of the samples at higher THz pulse peak
fields.Comment: Submitted to Appl. Phys. Lett
Highly-stable monolithic femtosecond Yb-fiber laser system based on photonic crystal fibers
Liu X, LĂŚgsgaard J, Turchinovich D. Highly-stable monolithic femtosecond Yb-fiber laser system based on photonic crystal fibers. Optics Express. 2010;18(15): 15475
Role of dynamical screening in excitation kinetics of biased quantum wells: Nonlinear absorption and ultrabroadband terahertz emission
Turchinovich D, Monozon BS, Jepsen PU. Role of dynamical screening in excitation kinetics of biased quantum wells: Nonlinear absorption and ultrabroadband terahertz emission. Journal of Applied Physics. 2006;99(1).In this work we describe the ultrafast excitation kinetics of a biased quantum well, arising from the optically induced dynamical screening of a bias electric field. The initial bias electric field inside the quantum well is screened by the optically excited polarized electron-hole pairs. This leads to a dynamical modification of the properties of the system within an excitation pulse duration. We calculate the excitation kinetics of a biased quantum well and the dependency of resulting electronic and optical properties on the excitation pulse fluence, quantum well width, and initial bias field strength. Our calculations, in particular, predict the strongly nonlinear dependency of the effective optical absorption coefficient on the excitation pulse fluence, and ultrabroadband terahertz emission. Our theoretical model is free of fitting parameters. Calculations performed for internally biased InGaNâGaN quantum wells are in good agreement with our experimental observations [Turchinovich et al., Phys. Rev. Bâ68, 241307(R) (2003)], as well as in perfect compliance with qualitative considerations
Compression of fiber supercontinuum pulses to the Fourier-limit in a high-numerical-aperture focus
A multiphoton intrapulse interference phase scan (MIIPS) adaptively and automatically compensates the combined phase distortion from a fiber supercontinuum source, a spatial light modulator pulse shaper, and a high-NA microscope objective, allowing Fourier-transform-limited compression of the supercontinuum pulses at the focus of the objective. A second-harmonic-generation-based method is employed to independently validate the transform-limited compression. The compressed pulses at the focus of the objective have a tunable duration of 10.8â38.9 fs (FWHM), a central wavelength of ~1020 nm, an average power of 18â70 mW, and a repetition rate of 76 MHz, permitting the application of this source to nonlinear optical microscopy and coherently controlled microspectroscopy
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