A colliding-pulse mode-locked ring laser with a four stage Nd:YAG-pumped dye amplifier generates femtosecond pulses at 630 nm with a repetition rate of 10 Hz. Frequency doubling to 315 nm provides photolysis pulses, while continuum generation offers probe wavelengths for transient spectroscopy throughout the visible. Since "ordinary" hemoglobin and myoglobin have now been characterized on all time scales, it is appropriate to turn attention toward the comparison of proteins from different species, which vary in specific ways, and to model compounds which incorporate specific features whose function one wishes to understand. In this presentation, the prospects for such studies are outlined and early results are reported.

Recent picosecond transient absorption studies' show that the yield of geminate rebinding is sensitive to protein structure. Picosecond transient Raman studies 2 reveal that over the time course (T = 200 ps) of the geminate rebinding, the effective tertiary structure about the heme is not relaxing. The results of this study also imply strong coupling among the elements of tertiary structure that control both the frequency of iron-proximal histidine stretching motion and the ligand binding properties of the heme. These findings and conclusions are used to further develop a working model in which reactivity and structure about the heme are controlled by an effective tertiary structure associated with what Karplus and co-workers3 call an "allosteric core" comprised of the heme, the proximal histidine (F-8), a portion of the F-helix and the a1-B2 interface.

Picosecond infrared transient bleaching experiments have been performed to measure the population lifetime (T1) of vibrationally excited (v=1) functional groups chemisorbed on high surface area colloidal silicas (SiO2). The experimental method and results for vibrational modes of -OH, -OD, -NH2 and -OCH3 coordinated to surface silicon atoms and for the -BOH surface species are presented. Lifetimes for these groups at both vacuum and liquid interfaces indicate that the adsorbate degrees of freedom, chemical coordination and nearby substrate modes play an important role in damping vibrational energy. It is also surmised that the vibrational lifetime, especially that for the hydroxyl group (T1=150 ps), is related to the chemical reactivity of adsorbates on surfaces.

Picosecond transient absorption spectra and kinetics of room temperature iodine solutions following 532 nm or 683 nm excitation are presented. The results indicate that a significant amount of recombination occurs through theA'(3112u) state. Studies at high concentrations show that the A' state is quenched by ground state iodine by way of an I4 or I3 intermediate. The geminate recombination and relaxation of Br2 in CC14 solution is also reported. The similarities and differences of the Br2 and I2 results are interpreted in terms of the respective potential curves.

Picosecond and subpicosecond spectroscopic studies have revealed evidence for an isomerization intermediate between cis and trans in the photoinduced isomerism of both stilbene and biindanyledene ("stiff" stilbene). In stiff stilbene, a transient absorption at 351 nm displays time evolution and viscosity dependence consistent with absorption by a twisted intermediate ("phantom" state) with a lOps lifetime. An analagous bottleneck state with a life-time of 4ps is also consistent with the ground state recovery dynamics of t-stilbene following excitation of c-stilbene when monitored with 0.1ps resolution.

We describe a simple system for the generation of tunable picosecond pulses in the millijoule range. The system consists of an active/passive mode-locked Nd:YAG pump laser which emits a single pulse, and the dye laser part which employs a single mode short cavity dye laser and two double pumped amplifiers.

The output of a continuous wave (cw) mode-locked Nd:YAG laser has been compressed and then frequency doubled to provide a new pumping source for a synchronously pumped dye laser. Using Rhodamine 6G, tunable subpicosecond pulses have been generated in both synchronously pumped and synchronously pumped cavity dumped configurations.

We report on the design of a synchronously pumped dye laser using the hybrid mode-locking technique to generate subpicosecond pulses. A detailed description of the optical cavity is given. Pumped by the second harmonic pulse train from a CW mode-locked Nd: YAG laser, the dye laser has produced pulses as short as .6 psec (FNTA of the autocorrelation trace) in the R6G and styryl 9 frequency range. The efficiency of the DODC and IR 140 dyes used as saturable absorbers is discussed. The problems associated with the design of a dye laser synchronously pumped by the second harmonic train of a mode-locked, Q-switched Nd: YAG laser are examined. We report on a dye laser which has produced as much as 50 μJ in a single pulse.

The synchronous pumping of a cw Rhodamine 6G dye laser with the ultrashort pulses from the fiber-optically compressed second harmonic of a Nd:YAG laser has resulted in the generation of dye laser pulses of duration less than 300 fsec, tunable over a range of 400 A. With continuous temporal tuning of the duration of the pump pulses from 460 fsec to their uncompressed duration of 34.4 psec, we made the first investigation of the dynamics of synchronous modelocking as a function of pump pulsewidth. We describe a method for the generation of tunable femtosecond dye laser pulses that should be applicable to the large class of visible and near infrared dyes that can be pumped by a frequency doubled Nd:YAG laser.

We have developed a new dye laser and c.w.-pumped regenerative amplifier system capable of generating white light continuum at a repetition rate higher than 1 KHz.

It is shown that the sech shape assumption does not apply generally for the pulses produced by the passively-mode-locked ring dye laser. Therefore, pulse duration estimates based on intensity autocorrelations alone are not reliable, as demonstrated by accurate measurements of the pulse amplitude and phase modulation. Precise control of the phase modulation guarantees an optimum resolution in measurements of ultrafast events.

The dynamics of energy redistribution in multiphoton ionization dissociation (MPID) of 2,4-hexadiyne have been studied with a two-color picosecond laser mass spectrometric technique. Two picosecond lasers, (25 ps fwhm) the 266 nm and 532 nm outputs of a Nd:YAG laser are used as the two pump sources in this experiment. By monitoring the parent and daughter ions as a function of a variable delay between the two pump lasers, dynamic information about energy redistribution in the electronic states of the molecule and the parent ion responsible for the formation of the different daughter ions may be obtained.

We measured picosecond transient Raman spectra of Si trans-stilbene in hexane and Si diphenylbutadiene in THF. We observed 18 vibrational modes in the trans-stilbene spectra and 9 modes in the diphenylbutadiene spectra. The band positions and band shapes of the transient Raman spectra suggest a distribution of molecular conformers in Si having different single and double bond strengths. The band intensities in the trans-stilbene spectra suggest a redistribution of electron density in Sn that is concentrated in the olefinic region and in the ortho and meta region of the phenyl rings. We also measured the low wavenumber Stokes and anti-Stokes spectra in trans-stilbene.

We have recently demonstrated a passive, picosecond, Si optical-limiting switch for 1 micron radiation. Since this device functions below, at, and above the Si melting threshold, its operating characteristics are determined by a large number of mechanisms including nonlinear transmission, self-diffraction, nonlinear reflection, phase transitions, and resolidification morphologies. Here, we review measurements in which we apply a wide variety of picosecond spectroscopic techniques in order to characterize these optically-induced phenomena.

Following recent developments in ultrashort pulse laser technology, there are now many examples of applications to transient spectroscopy of semiconductors where distinctly new physical information has been obtained at a fundamental microscopic level. Our work has focused on selected aspects of exciton kinetics in the so-called 'semimagnetic' semiconduc-tors, sepcifically Cdl_xMnxSe and Cdi_xMnxTe 1. These II-VI materials, while possessing many of the conventional features of their parent compound semiconductors, have additional spin-spin interactions which lead to a number of interesting and useful properties2. Excitons, in turn, control many of the light emitting properties, especially at low temperatures. The emphasis below, however, is to show through a specific example that the experimental techniques in question can yield real time kinetic information of excitonic energy relaxation processes with considerable detail. In the next section we review some of the technical aspects of a two-laser excite-probe approach, termed here as photomodulation spectroscopy. In Section III we show an example of exciton energy relaxation and subsequent trapping by alloy potential fluctuations in the mixed crystal Cd1-xMnxTe. Some concluding remarks follow in Section IV.

Semiconductor lasers are potentially devices of great importance for optical transmission as well as short pulse generation for various sampling, characteration and dispersion measurements. Since semiconductor lasers are currently driven devices, it is relatively easy to modulate the optical output and to generate short pulses, on the order of lOps long, by directly modulating the injection current into the laser. This paper will present some recent developments in injection lasers which are capable of being analog or digitally mod-ulated at rates up to lOGHz, as well as generating short optical pulses at repetition rates from several hundred megahertz to tens of gigahertz.

Recent developments of streak cameras at Hamamatsu are reviewed. Excellent performance characteristics have been obtained for individual items, e.g. time resolution of 400 femto-second, dynamic ranae of 106 : 1, direct observation of 1.3 μm diode laser, and so on. Combination of all of them in a particular streak camera is the subject for a future study.

Two particular requirements in the area of picosecond optical detection are discussed. On the one hand, there is a need for high efficiency, fast response detectors in the near ultraviolet. On the other hand, there is a need for ultrafast (<10 ps) response direct detectors, especially at wavelengths beyond 1.1 Tim. In both cases, particular approaches to meeting the need are described and the results obtained to date in the implementation of these techniques are discussed.

A low dark current, high speed InGaAs PIN photodiode suitable for optoelectroinc integration has been fabricated by LPE method on sem-insulating InP substrate. The photodiode has a low dark current density of 2.5x10-6A/cm2 at -10V. At the operating voltage of -5V, external quantum efficiency of >90% at 1.3um and >83% at 1.55um, a rise time of <35ps and a FWHM of <45 ps have been measured. The approach and limitations to the design of a higher speed PIN photodiode will be discussed. The importance of packaging is emphasized, and pulse distortion due to dispersion in miscrostrip line will be discussed.

Applications of picosecond optoelectronic technique to advance the microwave and millimeter-wave technologies are discussed. The direct conversion of dc energy to rf pulses with high efficiency has been achieved. The laser induced electron-hole plasma in a semi-conductor waveguide is also used to control the propagation of millimeter-waves. The performance of a high speed millimeter-wave modulator is reported.

We have developed integrated photoconductors for picosecond pulse generation and sampling on Si integrated circuits. Here we describe the device implementation.

We report a new technique for directly sampling electrical waveforms in GaAs integrated circuits with picosecond time resolution. This noninvasive sampling system provides a powerful new tool for the design and diagnosis of GaAs integrated circuits. The technique is based on the intrinsic electro-optic effect in GaAs and utilizes a mode-locked and compressed Nd:YAG laser to electro-optically sample the fringing fields of microstrip transmission lines in GaAs integrated circuits. The frequency doubled output of the laser can be used to excite on-chip photodetectors which serve as test signal generators, or the circuits can be driven by external signal generators. We also demonstrate how the sampling system can be operated as a harmonic mixer for evaluation of pulse-to-pulse timing jitter in mode-locked lasers.

We describe a completely passive technique for limiting the power of light beams. The techniques we use are based on optical self-action (self-focusing or defocusing) and non-linear absorption or a combination of both. These processes are passive and thus require no external mechanism to induce limiting other than the light beam itself. We demonstrate picosecond turn on times while 10 secresponse times are possible depending on the material. Specifically we demonstrate optical limiting in CS2 and other organic liquids, and in liquid crystals.

A cw-pumped mode-locked and simultaneously Q-switched Nd:YAG laser system has been characterized using a high-speed photodiode, sampling oscilloscope technique and the results compared with those obtained employing streak camera techniques.