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.     

Picosecond pulses from mode-locked dye lasers

Picosecond pulses have been generated in laser pumped (15 psec) and flashlamp pumped (6 psec) mode-locked dye lasers. Two-photon fluorescence and streak camera measurements are described.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Subnanosecond multiphonon relaxation of divalent copper impurities in LiTaO3

The temperature dependence of the lifetime of the excited ²T₂ state of the Cu²⁺ ion in a LiTaO₃ crystalline host has been measured with picosecond optical pulses. Its value ranges from 450 psec at 22°K to 10 psec at 423°K. Consistent with this result is the observation of weak fluorescence at 1.75 μm having a low temperature quantum efficiency of ～ 10^?6. The experimental results are in reasonably good agreement with recent theoretical models based on strong coupling of the Cu²⁺ impurities to the host lattice.     

Direct observation of the lifetime of a polariton mode in gallium phosphide

A polariton mode not far from the fundamental TO frequency in GaP is excited using the stimulated Raman effect. The degree of lattice excitation is directly observed as a function of time by a picosecond probe pulse. A relaxation time of 5.5 psec was measured in agreement with infrared data.     

Positron annihilation in n- and p-type, B- and P-doped silicon crystals of 100 and 111 orientations

Positron annihilation spectra are compared for oriented and doped silicon crystals. A critical three-component deconvolution analysis of the lifetime spectra resulted in a previously undetected lifetime of 60 psec and 25% relative intensity. The sensitivity of the lifetimes to the details of analysis is discussed.     

Suppression of the spectral narrowing effect in lasers mode-locked by saturable absorbers

A gain profile with a very flat top is produced by incorporating a Fabry-Pérot interferometer in the cavity of a mode-locked Nd-glass laser. With proper adjustment of the interferometer parameters, the spectral narrowing effect is significantly reduced, and bandwidth-limited pulses as short as 1.5 psec have been observed.