T cell receptor signalling in gamma delta cell development: strength isn't everything

Published version can be found here: http://www.cell.com/trends/immunology/abstract/S1471-4906%2811%2900153-0

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

Quantum well saturable absorber mirror with electrical control of modulation depth

Liu X, Rafailov EU, Livshits D, Turchinovich D. Quantum well saturable absorber mirror with electrical control of modulation depth. Applied Physics Letters. 2010;97(5).We demonstrate a quantum well (QW) semiconductor saturable absorber mirror (SESAM) comprising low-temperature grown InGaAs/GaAs QWs incorporated into a p-i-n structure. By applying the reverse bias voltage in the range 0–2 V to the p-i-n structure, we were able to change the SESAM modulation depth in the range 2.5–0.5%, as measured by nonlinear reflectivity of 450 fs long laser pulses with 1065 nm central wavelength, in the pump fluence range 1.6–26.7 μJ/cm2. This electrical control of the modulation depth is achieved by controlling the small-signal loss of the SESAM via quantum-confined Stark effect in the QWs

Highly-stable monolithic femtosecond Yb-fiber laser system based on photonic crystal fibers

Liu X, Lægsgaard J, Turchinovich D. Highly-stable monolithic femtosecond Yb-fiber laser system based on photonic crystal fibers. Optics Express. 2010;18(15): 15475