Sub-picosecond Resolution Research Articles

Picosecond ultrasonics in thin films of conducting polymers

The polarized picosecond pump and probe technique with subpicosecond resolution has been used to spectroscopically generate and detect coherent vibrations in films of conducting polymers in the frequency range of 10 – 200 GHz. From the strain oscillations in real time we have deduced the sound velocity in thin films of cis-rich (CH) x at different cis/ trans percentages.

The Dynamics of Polar Solvation

AbstractFluorescence upconversion experiments with subpicosecond resolution are described. The time dependent fluorescence spectra of several probe molecules in polar solvents are measured and used to construct the Stokes shift correlation function C(t). The correlation functions decay nonexponentially in contrast to the predictions of simple Debye/Onsager continuum theory. At high values of the static dielectric constants the solvation times are much slower than predicted from standard continuum theory. Generalizations of continuum theory to include non‐spherical shapes, non‐Debye dielectric response and saturation effects are discussed. The experimental data are qualitatively in accord with predictions based on a frequency and position dependent dielectric response.

Time-resolved Raman spectroscopy with subpicosecond resolution: vibrational cooling and delocalization of strain energy in photodissociated (carbonmonoxy)hemoglobin.

A Raman spectrometer that provides both subpicosecond resolution and independent, tunable pump and probe pulses is described. The spectrometer is employed to obtain time-resolved spectra of (carbonmonoxy)hemoglobin (HbCO) at times from 0.2 to 95 ps subsequent to ligand photodissociation. The spectra are interpreted in terms of a vibrationally hot heme that cools substantially in 10 ps. Concomitant with the proposed vibrational cooling is a slower relaxation, which we suggest results from a protein response to heme doming induced by ligand detachment. Results and interpretations are discussed in the context of current models of the heme photophysics and of hemoglobin reactivity.

Comparative study of the gain dynamics of XeCl and KrF with subpicosecond resolution

We have studied the subpicosecond gain dynamics of XeCl and KrF with the following results for XeCl (KrF): saturation energy density 0.85 (2.0) mJ/cm2, small-signal-gain coefficient 0.12–0.15 (0.20–0.22) cm−1, gain recovery of 62% (25%) with a time constant of 52 (54) psec. A molecular reorientation time of ≈1 psec was observed in both excimers.

Polarization selectivity in time-resolved transient phase grating

A selective polarization approach to transient phase grating experiments is presented. The high potentiality of such a technique lies in the fact that nonlinear signals show not only a temporal behaviour but also symmetry properties associated to each physical process involved, through the polarization states of the four coming beams. Subpicosecond resolution measurements of the kinetics of liquid CS 2 transient anisotropy have been performed in this way. We demonstrate that even using ultra short pulses, polarization analysis alone allows full discrimination between electronic and molecular processes, both of which intervene in the induced nonlinearity. Futhermore, a temporal delay in the onset of the molecular processes is evidenced for the first time. It is accounted for by a non instantaneous orientation of the molecules along the actinic optical field.

Picosecond dynamics of a gain-switched InGaAsP laser

Optical pulses are observed, with subpicosecond resolution, from an InGaAsP laser driven with 17 ps wide electrical pulses. The temporal profile of these gain-switched pulses is analyzed to obtain the transient and steady-state injected carrier densities in the active layer for dc biases above and below threshold. Gain-switched pulses as short as 13 ps FWHM are presented, and empirical results are compared to computer simulations. The implications of these results to pulse code modulation at very high bit rates are also discussed.

A QUARTER CENTURY OF STIMULATED RAMAN SCATTERING

To round out a quarter century of SRS the timing of this writing (1986) requires a look ahead of only one year into the future. The proceedings of the 10th International Conference on Raman Spectroscopy present a picture of current activity. Further progress will be made in time-resolved spectroscopy with subpicosecond resolution, in the study of hyper-Raman and other higher order effects with CARS, in extension of resonant Raman excitation in the UV region of spectrum, and in the development of Raman laser sources. During past few years extensive theoretical investigations have been made for four-wave light mixing in the case of one or more very strong light beams. The perturbation approach for those fields ceases to be valid. If only one light field is strong, the usual approach is to make a transformation to a rotating coordinate system so that the strong Hamiltonian for this light field becomes time-independent. Very recently these techniques have been extended to the case of two or more strong fields. CARS-type experiments with strong beams are likely to receive more attention. Extrapolation of the current activities instills confidence in the vitality of stimulated Raman scattering for the foreseeable future.

Measurement of optical phase with subpicosecond resolution by time-domain interferometry

A new and extremely general interferometric technique has been experimentally demonstrated that permits the direct measurement of optical phase on a subpicosecond time scale. The intensity is characterized by cross correlation, and thus the optical field is completely determined.

Subpicosecond resolution studies of solvation dynamics in polar aprotic and alcohol solvents

Subpicosecond resolution measurements of the kinetics of dipolar solvation have been made. The time resolved Stokes shift of a dye molecule, LDS-750 was measured using the fluorescence upconversion technique in the solvents acetonitrile, DMSO, nitrobenzene, methanol, and n-butanol. The solvation dynamics in both aprotic and alcohol solvents occur on a time scale roughly given by the longitudinal relaxation time as predicted by simple continuum theories. The relaxation in nitrobenzene and butanol is nonexponential and the relaxation in methanol is significantly faster than the calculated time. These deviations from simple theory are discussed in the context of (i) the significance of high frequency dispersions in the dielectric response, (ii) translational contributions to the solvent relaxation, and (iii) molecular aspects of the solvation not accounted in the continuum description.

A Quarter century of stimulated Raman scattering

Abstract

Amplification et émission paramétriques à l'échelle subpicoseconde dans un cristal organique : - application à la spectroscopie infra-rouge

High peak power subpicosecond optical pulses have been generated by parametric amplification of a white femtosecond continuum in a new organic crystal, N-(4-nitrophenyl)-L-prolinol (NPP). The relation between this observation and the structural properties, both at microscopic and macroscopic levels of NPP are exposed. Special emphasis is put on the renewal of the traditional concept of non-critical phase-matching in view of the present requirements linked to the observation of ultrafast subpicosecond nonlinear phenomena. The notions of θ-non critical and λ-non critical phase-matching together with their implications are exposed. A new method for time-resolved luminescence spectroscopy with subpicosecond resolution is simply derived as an extension of the NPP nonlinear measurement set-up where a previously used infra-red emitting continuum water cell is simply replaced by any sample of interest with luminescence in the 1.0-1.6 μm range. This new technique, defined as PASS, a tag for Parametric Amplification and Sampling Spectroscopy, is demonstrated at 1.44 μm in a InGaAs-InAlAs multiple quantum well structure. The future of nonlinear organic materials in infra-red optical signal processing systems is briefly discussed in conclusion Impulsions optiques de forte puissance crete obtenues par amplification parametrique d'un continuum de lumiere blanche dans le N-(4-nitrophenyl)-(L)-prolinol) ou NPP. Proprietes du NPP. Revision du concept «accord de phase non critique». Concept «d'accord de phase λ- non critique». Presentation d'une nouvelle methode spectroscopique autorisant une resolution temporelle inferieure a la picoseconde, methode «PASS» pour «Parametric Amplication and Sampling Spectroscopy». Application de PASS a l'etude d'une structure de type puits quantiques multiples

Full intensity profile analysis of ultrashort laser pulses using four-wave mixing or third harmonic generation

New methods are proposed for the measurement of the full temporal intensity profile I( t) of ultrashort laser based on recording of a triple intensity-autocorrelation function. The possibilities offered by two third-order nonlinear processes, degenerate four-wave mixing (DFWM) and noncollinear third-harmonic generation (THG) are investigated from this aspect. In both cases double time-delay methods are considered first, where three replica pulses of profile I( t) with adjustable delays between them are used as input and the integral output intensity is monitored. The same information may be obtained in single-delay or single-shot schemes as well by using appropriate input beam expansions and a one- or two-dimensional detector array respectively, The methods considered have at least subpicosecond resolution. DFWM methods, in particular, are very convenient due to their wavelength-independence in a broad interval including UV.

Parametric amplification sampling spectroscopy of luminescence at the subpicosecond time scale in the 1–1.6 μm spectral range

A new method for time-resolved luminescence spectroscopy with subpicosecond resolution is proposed in the 1–1.6 μm spectral range, using parametric amplification in a highly nonlinear organic crystal. This technique is used to characterize the luminescence of an infrared dye and of an InGaAs-InAlAs multiple quantum well structure.

Optical properties of dye molecules adsorbed on fractal structures

We present preliminary measurements of the optical properties of Rhodamine B molecules adsorbed or trapped in porous matrices such as Vycor 7930 glass, silica xerogels and aerogels. The electronic transfer between donor and acceptor molecules adsorbed on such fractal structures is also discussed; a fast sub-picosecond resolution experiment is proposed to study this process.

Subpicosecond electrooptic sampling: Principles and applications

This paper reviews the principles and applications of electrooptic sampling for the characterization of repetitive ultrafast electrical transients. Electrooptic sampling is an electric field sensitive technique that utilizes ultrashort optical pulses as "sampling gates" via the Pockels effect in electrooptic media and has demonstrated subpicosecond temporal resolution and microvolt sensitivity. The technique can be adapted to characterize a wide variety of picosecond electronic devices such as field-effect transistors and photodetectors as well as probe complete integrated circuits with high temporal and spatial resolution.

Rotational diffusion of stilbene in alkane and alcohol solutions

An amplified colliding pulse modelocked laser system is described which is capable of subpicosecond resolution flash-photolysis studies. As an application of this system we used the anisotropic absorption technique (polarization spectroscopy) to measure the rotational reorientation of trans-stilbene. These measurements give a more accurate estimate of the microscopic friction involved in the isomerization dynamics of this molecule. When the friction is assumed proportional to the reorientation times, the isomerization rates in alkane solvents are adequately described by activated barrier-crossing theory.

Blue Shift of the Exciton Resonance due to Exciton-Exciton Interactions in a Multiple-Quantum-Well Structure

Multiple-quantum-well structures (MQWS) are attracting much interest for their potential use as efficient active devices in optoelectronics and nonlinear optical signal processing. Based on their strong excitonic optical nonlinearity, operation of MQWS as optical bistable elements and logical gates has been demonstrated both at low and room temperatures. To obtain information about the intrinsic response time of such devices, we have investigated the dynamics of exciton screening of a MQWS with subpicosecond resolution, at T = 15 K and T = 220 K. We have observed an unexpected shift of the exciton resonance to higher energy as a result of exciton-exciton interactions.

Subpicosecond time-resolved luminescence spectroscopy of highly excited CuCl

An apparatus for the detection of luminescence spectra with subpicosecond resolution is described. It is applied to the measurement of the formation time of excitonic particles in CuCl.

Femtosecond Orientational Relaxation of Photoexcited Carriers in GaAs

The anisotropy of optical absorption saturation by intense linearly polarized light pulses is measured with subpicosecond resolution in GaAs at 77 K, at a photon energy slightly above the band gap. For an electron-hole pair density of about 6\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, the momentum orientational relaxation time is determined to be 190 fs, most likely due to carrier-carrier scattering.

Time difference measurement with subpicosecond resolution

Circuitry and results of a simple device for the measurement of time differences with subpicosecond resolution are given. Originally intended for the measurement of extremely fast luminescence decay, the device may be of interest for many other applications. The half width of the pulse height distribution corresponds to 0.7 ps.