Picosecond x-ray chronoscopy

An X-ray/VUV streak camera with a time resolution of ≈20 psec is described. X-ray pulses, from a laser plasma, down to 22 psec half width, separated by 42 psec, are clearly resolved. Photoelectron time-dispersion effects and the absolute sensitivity of the camera are discussed.     

Direct linear measurement of ultrashort light pulses with a picosecond streak camera

The durations of ultra-short (4–7 psec) laser pulses have been directly measured with an image tube streak camera designed for time resolution in the picosecond region. The camera was calibrated with simultaneously recorded two photon fluorescence pulse profiles and it is shown that a time resolution limit of ≈1 psec should be possible with this system.     

Direct measurement of pulse broadening in the second harmonic of mode-locked Nd: glass laser

Direct measurement of pulse broadening in the second harmonic of a mode-locked Nd: glass laser has been made using a high-speed streak camera. Recorded pulsewidths varied from 3.8 psec to greater than 15 psec.     

Direct observation of the relaxation time of the symmetric molecular CO2-3 vibration of calcite

The relaxation of the internal A_1g-mode of the CO^2-₃ group (? = 1086 cm^-1) is investigated with picosecond pulses. The measured decay times are 4.4 psec and 8.7 psec at 295 K and 90 K, respectively. Our values are in agreement with spontaneous Raman data. Possible relaxation processes are discussed.     

10 μm detection system with 40 picosecond resolution

A 10 μm detection system with a temporal resolution of ? 40 psec has been used to investigate the output of a multi-atmosphere CO₂ laser. Mode-locking, by means of p-type germanium, has permitted the observation of pulses as short as ? 75 psec.     

Pulse propagation through multimode fibres

To evaluate their communication potential, measurements have been made on multimode glass fibres. Detector limited pulses of 180 psec risetime from a Ga-As laser were transmitted through various lengths of fibre. After a 120 m length the risetime was still not greater than 450 psec. Measurements with a mode locked He-Ne laser, done on shorter lengths of fibre, agreed with the above results.     

Picosecond time-resolved luminescence of hydrogenated amorphous silicon-carbon

Photoluminescence decay curves and time-resolved luminescence spectra of amorphous Si_0.15C_0.85 : H films are measure. The decay curves are nearly exponential with a time-constant of about 200 psec at room temperature. The peak of the emission spectrum is shifted from 440 nm immediately after the excitation to 490 nm at 400 psec after the excitation.     

Photoisomer generation and absorption relaxation in the mode-locking dye 3,3'-diethyloxadicarbocyanine iodide

A quantum efficiency of ≈ 0.01 for the generation of the photoisomer of DODCI has been obtained. The establishment of equilibrium concentrations for the two forms of the dye in mode-locked rhodamine dye lasers has been confirmed. The measured saturable absorber recovery times of ≈ 200–250 psec are shown to be consistent with an excited state lifetime of ≈ 300 psec, in agreement with fluorescence measurements.     

Dynamics of photoinduced electron transfer in model multiporphyrin ensembles

It is shown by the methods of picosecond fluorescent and femtosecond absorptive spectroscopy that in the nanosize self-assembled complexes based on a Zn-octaethylporphyrin dimer and electron acceptors of different nature (quinone, anthraquinone, pyromellitimide) deactivation of the energy of electron excitation is caused by the nonradiative S-S-energy transfer (for times ≤10 psec) and photoinduced charge transfer (within the range of 300 fsec-700 psec). In tetrads an electron is transferred from a porphyrin extra-ligand to quinone for distances r_DA≈18–21 Å by the “superexchange” mechanism. In triads, the charge transfer from a dimer to a fluorinated porphyrin extra-ligand is efficient at 120-77 K.     

Polymethine IR laser dyes for passive mode-locking of ruby lasers

Several polymethine IR laser dyes, in addition to the well known DDI and cryptocyanine, are found to be suitable for mode-locking of ruby lasers. While with DDI pulses as short as 12 psec are obtained these new mode-lockers generate pulses in the 40 to 100 psec range, presumably due to their longer recovery time of the ground-state absorption.     

Production of single subnanosecond multi-line 10.6 μm laser pulses by fast optical gating

An electro-optic shutter, activated by a pulse generator switched with a laser triggered spark gap having a risetime of 120 psec, has been used to produce optical pulses of variable duration, as short as 600 psec, from the output of a uv photo-preionized CO₂ laser, which simultaneously oscillates on several rotational transitions.     

Measurements of exciton absorption with pico-second light pulses produced by a self-phase modulation technique

Time-resolved absorption spectra of the A exciton in CdSe are measured by using pico-second white-light pulses produced by a self-phase modulation technique. The absorption peak of the n = 1 A exciton shifts to the high-energy side due to the high-density effect. The magnitude of the shift reaches a maximum at 20 psec after the pulse excitation, which is considered to indicate that the time constant for the formation of the n = 1 excitons by band-to-band excitation is about 20 psec.     

Time-resolved spectra of spontaneous luminescence from high density electron-hole plasma in CdS

Pico-second time-resolved spectra of the spontaneous luminescence from high density electron-hole plasma in CdS are measured at 4.2 K suppressing the stimulation effect. It is found that after the pulse excitation hot carriers are cooled rapidly for the first 100 psec, and that therafter up to the 400 psec delay time the shape of spectra hardly changes but the spectral width depends on excitation density. Although it seems as if some kind of state like electron-hole liquid is formed, one cannot easily regard that state as the condensed electron-hole drop state.     

SPEAR3 Short-Pulse,Low-α Operation

The SPEAR3 storage ring at SLAC National Accelerator Laboratory, Stanford Synchrotron Radiation Lightsource, is delivering beam in a short-pulse, low-α operational mode. In order to deliver short photon pulses, the stored current in SPEAR3 must be reduced. In low-α mode, we presently deliver 100 mA total current with bunch lengths of 15 psec FWHM, compared to our standard operations with 500 mA and 46 psec FWHM bunch lengths. It is possible to deliver still shorter pulses at lower current. The low-α beam is stored in four bunch trains, interleaved with up to four “camshaft” timing bunches for pump-probe experiments. This simultaneously provides short pulses for timing experiments while ensuring 100 mA for experiments at other beamlines.     

Positron annihilation in boron-implanted n-type silicon

The ion damaged effect and subsequent isothermal annealing in boron-implanted Si was studied by positron annihilation lifetime measurements. The mean positron lifetime in preimplanted n-type Si is 243 psec. The variation of mean lifetime is detectable when the implanted boron dose is greater than 1.0x10¹⁵/cm². The saturated mean positron lifetime (247 psec) occurs when the implantation dose reaches 2.5x10¹⁵/cm². The mean electron density of the positron sensitive defects is estimated to be about 85% less than that in the perfect parts of the crystal. Isothermal annealing was held in every 5-minute step at 1000°C. In the first step, the positron lifetime in the implanted sample increases slightly and then decreases completely to its initial state in the 3rd step. Sheet resistance of the sample monitored by 4-point probe method has been found closely related to the positron lifetime.     

Measurement of the cascade time of ∑ in liquid helium

In a study of ∑⁻π⁺ production by stopped K⁻ in liquid helium, (19 ± 6)% of stopped ∑⁻ were observed to decay. This corresponds to a “cascade time” of 28 ± 8 psec.     

Suppression of fast-ions from a laser-produced plasma

Fast-ion emission occurs when a target is irradiated with single 25 psec pulses from a Nd: YAG/glass laser but is suppressed when the same target is irradiated with a mode-locked pulse train at the same intensity. Possible explanations for this effect are given.     

Picosecond study of color center translocation in LiTaO3

By the use of dual-beam picosecond absorption spectroscopy technique, we observed color center dislocation in LiTaO₃ single crystal. The time constant for the translocation was found to be 100 ± 40 psec, and independent of excitation laser power.     

Nonlinear optical properties of Cu x S and CuInS2 semiconductor nanoparticles in sol-gel glass

Quartz glasses containing ultradisperse particles (with a mean radius of 5 ± 3 nm) of semiconductor compounds Cu_xS and CuInS₂ were synthesized using the sol-gel technology. The line and differential absorption spectra of such glasses were investigated. In the range 600–700 nm, the samples demonstrate absorption bands that are assigned to the lowest-lying energy transition in quantum-size particles. Excitation by laser pulses with a wavelength of 540 nm causes the effect of induced absorption in the range of 500–950 nm. The induced absorption in the samples containing Cu_xS nanoparticles undergoes relaxation which has a characteristic time of ∼500 psec, and in the samples with CuInS₂ nanoparticles it has a two-exponential form with τ₁ ≈ 30 psec and τ₂>1 nsec. Translated from Zhurnal Prikladnoi Spektroskopii, vol. 67, No. 4, pp. 507–511, July–August, 2000.     

Time resolved spectroscopy of picosecond laser pulses with the aid of optical Kerr effect photography

An extension of the transverse gating, focal plane, optical Kerr effect photographic technique has permitted the direct observation of a frequency sweep within single ≈ 10 psec duration 0.53 μ wavelength pulses derived from a mode-locked Nd: glass laser.