G. R. Allan

Generation of high-repetition-rate femtosecond pulses from 8 to 18 µm

We generated subpicosecond pulses from 8 to 18 mum by difference-frequency mixing in a 1-mm-thick AgGaSe(2) crystal, the 130- and 180-fs output pulses (1.45 < lambda < 1.85 mum) from an 84-MHz-repetition-rate optical parametric oscillator. Numerical simulations show that intrapulse and interpulse group velocity dispersion determine minimum pulse duration above and below 15 mum, respectively. By cross correlation (upconversion) of 10.5-mum pulses with 90-fs, 810-nm pulses in AgGaS(2), the pulse length was measured to be 310 fs in good agreement with simulations.

Influence of Amplified Spontaneous Emission on Energy Relaxation, Recombination and Ultrafast Expansion of Hot Carriers in Direct-Gap Semiconductors

The picosecond cooling and spatial expansion dynamics of high density ( > 1018 cm−3) photoexcited plasmas in direct gap semiconductors has commanded much attention over the past twenty years. However one aspect of the plasma evolution that has often been ignored is the role of amplified spontaneous emission (ASE). The common belief is that radiative recombination occurs on a nanosecond time scale and is therefore unimportant for plasma dynamics. Nonetheless, for a plasma which becomes degenerate as it cools, optical gain can occur and ASE can induce recombination and plasma reheating on a picosecond time scale, slowing down the cooling process. This can occur in density regimes where hot phonon effects1 have previously been surmised to be the major factor in determining a reduced cooling rate. In addition, ASE can lead to an effective ultrafast carrier diffusion2,3 through radiative transfer. Overall, for a plasma initially generated in a certain volume and with density above a certain critical value, we find that ASE can induce strong nonlocal behavior on a picosecond time scale whereby the density and temperature depend on position as well as excitation volume. To illustrate this behavior we present here results of simulations of plasma evolution in GaAs for a 1-D geometry, which is appropriate to many lasers or induced-grating experiments. Similar effects are expected in 2- and 3-D.

High-speed VGA CMOS image sensor

We present some design details and characterization results for a VGA CMOS image sensor designed for high sped inspection applications. The sensor has 16 analog outputs, which can each operate at 50 MHz data rate, and can capture images at 1600 frames per second. The image sensor has exposure control functionality, antiblooming capability and a on-rolling shutter architecture to implement snap-shot image capture mode. The pixel architecture incorporates 5 transistors on a 15.3 micron pitch with 50 percent fill factor.

Femtosecond Coherent Response of Spin-orbit Split-off Excitons in InP

From spectrally resolved femtosecond four-wave mixing (FWM) in InP at 5 K we observe excitons associated with the spin-orbit split-off gap at 1.52 eV. The FWM spectrum reveals Fano interactions of these excitons with continuum excitations when co-polarized pulses are used. For perpendicularly polarized pulses only weak excitonic features are observed and the spectrum is dominated by the continuum photon echo response.

Role of the Electron-Hole Interaction on Femtosecond Hole Relaxation in InP

Investigations of carrier relaxation in optically excited polar semiconductors are often frustrated by the inability to separate electron and hole relaxation processes. In general, photo-luminescence experiments measure the product of electron (f e ) and hole (f h ) occupation functions, whereas absorption saturation experiments measure their sum. Since holes relax much faster than electrons due to a stronger lattice-coupling and larger effective mass, e-h plasma dynamics are often dominated by electron dynamics. As a result, hole-relaxation mechanisms are less well understood than those involved in electron relaxation. Doped semiconductors have been used to provide initial conditions that isolate the minority-carrier dynamics. In particular, Zhou et al 1 observed hole relaxation in n-GaAs and n-InP by time-resolved luminescence. With excitation at 2.0 eV, the relaxation time was determined to be 400 fsec for n-InP and independent of doping density in the range 5 x 1017 to 2.5 x 1018 cm -3. The dominant hole-relaxation mechanism was identified as deformation-potential coupling to optical phonons. However, electrons are degenerate in these doped materials and, therefore, hole relaxation via electron-hole scattering is impeded2. We report on experiments performed in intrinsic InP with non-degenerate plasmas that are sensitive to hole relaxation via energy transfer from hot holes to low-energy electrons.

Nondegenerate two-photon absorption in CdS

A femtosecond continuum, with a bandwidth of 90 nm, has been used in a pump-probe geometry to investigate nondegenerate two-photon absorption (2 PA) from 660 nm to 570 nm, with pump wavelength (lambda) equals 620 nm, in hexagonal CdS at 300 K. A dependence upon probe wavelength and relative beam polarization is observed. The polarization anisotropy has allowed us to measure the relative magnitudes of particular (chi) (3) tensor elements. The nondegenerate 2 PA coefficient increases by a factor of about two from 660 nm to 570 nm. The anisotropy and dispersion are discussed in terms of a two-parabolic-band model.