Femtosecond Resolution Research Articles

Ultrafast relaxation of photoexcited carriers in quantum wells and superlattices

The dynamics of photoexcited carriers in GaAs quantum wells and superlattices has been measured with femtosecond resolution using luminescence as well as pump-probe experiments. The electron capture mechanism shows well defined resonances with a time as short as 500 fs at room temperature for a well thickness of 60 AA. This gives strong evidence for the importance of the quantum mechanical, LO phonon-assisted capture mechanism predicted by theoretical calculations. In type-II superlattices, we show that the transfer of electrons from the GaAs layers to the AlAs layer is mediated by zone edge phonons (both LO and TA) in a way similar to intervalley scattering. The observed times range from 140 fs up to 30 ps depending on the thickness of the GaAs layer.

Ultrafast dynamics of intersubband relaxation in GaAs quantum wells: hot carrier and phonon populations effects

The dynamics of intersubband relaxation in GaAs quantum wells and the role of hot carriers and the phonon distributions have been investigated using two different optical techniques with femtosecond resolution: 1) time-resolved photoluminescence and 2) pump and probe experiments. The (2→1) intersubband relaxation times have been measured as functions of well widths (100Å < L well < 220Å), under different experimental conditions (15K < T lattice < 300K, and 1×10 10 cm -2 < excitation densities < 1×10 12 cm -2). The electron intersubband relaxation time is deduced from the decay time of the n=2 well luminescence (or differential transmission) intensity. For thin wells (<150Å), a fast intersubband (2→1) relaxation time ≤ 3 ps has been measured. For thicker wells, the measured decay times are found to be critically dependent on the excitation conditions (vary from 5 ps to 40 ps). The well width dependence of the intersubband relaxation time does not show the strong dependence (2 orders of magnitude) predicted theoretically for electron-LO phonon scattering. Our results show that the hot phonon populations and the slow carrier cooling rate limit the observation of subpicosecond relaxation time. For thick well widths, our results also suggest that hot carriers effects play an important role in the intersubband relaxation mechanisms.

Correlation method for femtosecond optical pulses

A correlation method for accurate measurement of ultrafast optical pulses of femtosecond resolution is presented. It uses a lock-in amplifier and a variable mixing method of reference frequencies in a Michelson interferometer. The read-out system consists of an X-Y plotter or a computer monitor. This technique does not require high-speed photodetectors but is sufficiently sensitive to plot femtosecond pulses at low optical power level (about 10 mu W average power). The measurement of 320 fs FWHM light pulses from an actively mode-locked semiconductor laser and a contrast ratio of 80 were demonstrated. The resolution of this technique is believed to be a few femtoseconds.

Ultrafast optical evidence for resonant electron capture in quantum wells.

The dynamics of carrier capture in GaAs quantum wells have been measured with femtosecond resolution using pump and probe experiments under resonant excitation conditions. The electron capture mechanism at room temperature shows a resonance with a time as short as 650 fs for a well thickness of 58 \AA{}. This gives strong evidence for the importance of the quantum-mechanical LO-phonon-assisted capture mechanism predicted by theoretical calculations.

Electro-optic sampling using an aluminum gallium arsenide probe

We demonstrate a new electro-optic sampling probe with femtosecond resolution. The probe is a 7-μm-thick film of 80% AlGaAs used in both total internal reflecting and free-standing geometries. A thin-film probe eliminates many of the problems associated with the use of bulk crystals as electro-optic sensors.

Femtosecond pump and probe of low band gap thiophene-based polymers

Dynamics of the photoinduced absorption with femtosecond resolution have been performed on newly syntetized low band gap thiophene-based polymers in solution. Comparison between compounds with different substituents has been made. The results are consistent with creation and recombination of self-trapped excitons.

Carrier capture in quantum wells

We have studied the mechanisms of capture in quantum wells by time resolved luminescence with femtosecond resolution. The measured decay times of the barrier luminescence are always shorter than 3 ps and the well luminescence rise time is of the order of 4 ps at 100 K. These coupled results indicate that electron and hole capture is always very fast. We carry out a quantum mechanical calculation which takes into account LO phonon scattering and impurity scattering as well as hot carrier effects. The short capture times in all cases are shown to be explained by the competing influence of these different scattering processes.

Femtosecond Resolution of Soft Mode Dynamics in Structural Phase Transitions

The microscopic pathway along which ions or molecules in a crystal move during a structural phase transition can often be described in terms of a collective vibrational mode of the lattice. In many cases, this mode, called a "soft" phonon mode because of its characteristically low frequency near the phase transition temperature, is difficult to characterize through conventional frequency-domain spectroscopies such as light or neutron scattering. A femtosecond time-domain analog of light-scattering spectroscopy called impulsive stimulated Raman scattering (ISRS) has been used to examine the soft modes of two perovskite ferroelectric crystals. The low-frequency lattice dynamics of KNbO(3) and BaTiO(3) are clarified in a manner that permits critical evaluation of microscopic models for their ferroelectric transitions. The results illustrate the advantages of ISRS over conventional Raman spectroscopy of low-frequency, heavily damped soft modes.

Ultrafast carrier dynamics in III-V semiconductors grown by molecular-beam epitaxy at very low substrate temperatures

Ultrashort carrier lifetimes were experimentally observed in a variety of As-based III-V crystalline semiconductor layers grown by molecular beam epitaxy over a range of very low substrate temperatures. Time-resolved optical absorption, reflection, and photoconductive switching measurements with femtosecond resolution have been used to directly measure the ultrafast carrier dynamics in these materials. GaAs and In/sub 0.52/Al/sub 0.48/As grown at temperatures of 200 and 150 degrees C, respectively, exhibited subpicosecond carrier lifetimes, while lattice-mismatched In/sub x/Ga/sub 1-x/As on GaAs displayed a 7-ps carrier lifetime in material grown at 200 degrees C. A distinct decrease in carrier lifetime with decreasing growth temperature is documented for all three material systems. For epilayers annealed at high-temperature, resistivities are high, and thus these materials were found to be ideally suited for a number of ultrafast optoelectronic applications. A number of applications are reviewed and discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Subpicosecond fluorescence dynamics of dye molecules

Time- and frequency-resolved fluorescence of dye molecules with femtosecond resolution gives new information on the intramolecular and intermolecular dynamics of large molecules in solution. We analyze data that were obtained recently by the upconversion technique together with new results: Access to vibrational coherence effects and to vibrational redistribution, thermalization, and cooling is illustrated for fluorescent dyes. The ultrafast fluorescence decay that accompanys internal conversion of a triphenylmethane dye is also analyzed.

Ultrafast chi /sup (3)/-related processes in semiconductor doped glasses

Experimental results are described for exploiting the fast and large nonlinear susceptibility of semiconductor-doped glasses (SDGs). Self-diffraction experiments were performed using incoherent broadband lasers with femtosecond time resolution yielding new information on the time response of the SDG nonlinearity. The use of backward nondegenerate wave-mixing to demonstrate yellow-to-green frequency conversion with femtosecond resolution is reported. In addition, results are presented which demonstrate the use of the SDG as a picosecond all optical gate at a 100-MHz repetition rate. >

Femtosecond dynamics of the n=2 image-potential state on Ag(100).

We report the first time-resolved studies of higher-order image-potential states on Ag(100). Two-photon photoemission measurements are performed with femtosecond resolution using pump and continuum probe techniques. We measure the lifetime of the n=2 image state on Ag(100) to be \ensuremath{\sim}180 fs, substantially longer than that of the n=1 state. These results have implications for theoretical models used for predicting the lifetimes of these states.

Perspectives on the use of very long undulators in high-energy storage rings (invited)

A few electron storage rings exist (and possibly could be available in the future), with energy &amp;gt;10 GeV and with straight sections of the order of 100 m on which very long undulators could be installed. The exceptionally low emittance that these machines can reach when run at relatively low energies would allow highly degenerate radiation to be produced in the soft and medium x-ray range, with applications to biological imaging near the C and N K edges, holography, and nonlinear optics down to a few angstroms, pumping of soft x-ray ion lasers, picosecond or femtosecond resolution spectroscopy, etc.

Terahertz fluorescence quantum beats in a dye solution

Time-resolved fluorescence of a dye molecule in liquid solution at room temperature, monitored with femtosecond resolution, reveals small-amplitude quantum beats with subpicosecond periods and picosecond damping. Specifically, the observation of damped ripples on the electronic fluorescence of a large dye molecule oxazine 1 (MW=324) in different liquid solvents (methanol, water) manifesting terahertz vibrational quantum beats is reported.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Femtosecond studies of image-potential dynamics in metals.

We report the first time-resolved studies of image-potential states on Ag(100). Femtosecond ultraviolet-pump and visible-probe techniques are combined with two-photon photoemission spectroscopu to measure transient photoemission spectra with femtosecond resolution. The lifetime of the $n=1$ image-potential state on Ag(100) is observed to be several tens of femtoseconds.

Femtosecond time-resolved measurements of weak localization of light

Time-resolved backscattering of coherent light from a disordered medium under weak localization conditions has been observed for the first time with femtosecond resolution using a light-gating technique based on second-harmonic generation. The time dependence of the observed enhanced backscattering is consistent with simple diffusion theory.

Femtosecond streak tube

A new design concept of a streak tube is conceived to obtain femtosecond temporal resolution. It is based on four factors as follows. First, in order to decrease the photoelectron transit time spread, one must investigate how high electric field is practically usable near the photocathode. It is found that the value can be increased up to ∼6 kV/mm and this is used as a design value. Second, as the other method to decrease the photoelectron transit time spread, an electromagnetic focusing method must be adopted because there is no region similar to the focusing section of an electrostatic focusing type where the electric potential is low. Third, the focusing magnetic field should be located in a limited region to obtain a very high sweep speed of ∼2×108 m/s. And fourth, in the operation, a special readjustment of the focusing magnetic field should be performed to compensate the photoelectron beam defocusing caused by the very rapid transient deflection field. Based on the above, a new streak tube has been designed, manufactured, and tested. The limiting temporal resolution of ∼400 fs has been predicted theoretically and the value less than ∼500 fs has been experimentally produced.

Femtosecond study of the recovery dynamics of malachite green in solution

The ground state recovery process of malachite green in water and ethylene glycol has been studied in the time domain with ∼50 femtosecond resolution using the equal-pulse correlation technique. The observed recovery law is consistent with a single exponental decay with time constant t21=0.93+/−0.05 ps for malachite green in water and t21=4.46+/−0.05 ps for malachite green in ethylene glycol.

Fentosecond Time Resolved Surface Structural Dynamics of Optically Excited Silicon

ABSTRACTLaser processing of materials has raised a number of interesting issues relating to phase transitions and the structure of optically excited semiconductors. In this paper we describe the dynamics of structural changes that take place on a silicon surface following excitation with an intense optical pulse. Second harmonic generation from the silicon surface is used as a tool to measure crystalline order with 90 femtosecond resolution. The three-fold rotational symmetry of the silicon &lt;111&gt; surface is observed to become rotationally isotropic within a picosecond after excitation consistent with a transition from the crystalline to the liquid molten state.

Picosecond Dynamics of Photoexcited Gap States in Polyacetylene

The time dependence of photoinduced optical absorption in the prototype one-dimensional semiconductor, polyacetylene, is investigated with femtosecond resolution. It is found that the induced absorption appears in less than 150 fsec. The dynamics of the absorption decay are measured and compared with a model based on geminate recombination on a one-dimensional chain.