Picosecond time-resolved fluorescence spectroscopy of hematoporphyrin derivative

The picosecond time-resolved fluorescence decaysI(t)and spectraI(lambda, t)for hematoporphyrin derivative (HPD) in a phosphate buffer saline aqueous solution at different concentrations (8.4 times 10^{-6}sim 8.4 times 10^{-3}M) are measured by a two-dimensional synchroscan streak camera with a mode-locked CW dye laser, and a new emission band (which we call theY-band) is found at high concentration. It is shown that the fluorescence decays composed of fast and slow components at high concentration are due to theY-band (120 ps lifetime) from head-to-tail aggregates including equilibrium dimer and stable dimer, and the usual band (3.6 ns lifetime) from monomer, respectively, and the latter band is dynamically quenched by the Förster type resonance energy transfer from the monomer to the aggregate. Furthermore, the measurement of static fluorescence spectra from human gastric cancers and the surrounding in vivo after HPD injection shows that a band corresponding to theY-band from the aggregate appears at only the cancerous cells.     

Picosecond laser pulses

The broad bandwidth and long storage lifetimes of Nd³⁺: glass and ruby lasers have made possible the generation of picosecond laser pulses having peak powers it excess of one gigawatt and repetition rates in the microwave range. The numerous application areas of these pulses include research in nonlinear optics, transient response of atomic and molecular systems, optically generated plasmas, spectroscopy, ranging, optical information processing, and high-speed photography. This paper reviews several experimental techniques for generating, measuring, and utilizing these ultrashort laser pulses.     

Picosecond spectroscopy of semiconductors

The time-resolved reflectivity of picosecond pulses from optically excited carrier distributions can provide important information about the energy relaxation rates of hot electrons and holes in semiconductors. The basic optical properties of non-equilibrium carrier distributions are discussed, and in the specific case of GaAs, a semi-empirical analysis of the reflectivity spectrum is described which estimates the contributions from the principal critical points of the band structure. Using Boltzmann factors to approximate the hot carrier distributions, it is found that the non-equilibrium reflectivity spectrum is a sensitive function of carrier temperature and that it can reverse its sign as the distribution relaxes. These results are in good qualitative agreement with recent experiments employing a mode-locked cw dye laser.     

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.     

Picosecond pulse measurements using the active laser medium

A simple method for measuring the pulse lengths of synchronously pumped dye lasers which does not require the use of an external nonlinear medium, such as a doubling crystal or two-photon fluorescence cell, to autocorrelate the pulses is discussed. The technique involves feeding the laser pulses back into the dye jet, thus correlating the output pulses with the intracavity pulses to obtain pulse length signatures in the resulting time-averaged laser power. Experimental measurements were performed using a rhodamine 6G dye laser pumped by a mode-locked frequency-doubled Nd:YAG laser. The results agree well with numerical computations, and the method proves effective in determining lengths of picosecond laser pulses     

Picosecond magnetic spectroscopy of two- and three-dimensionaldiluted magnetic semiconductors

The authors have developed a novel type of magnetic spectroscopy which relies on an integrated multichip SQUID (superconducting quantum interference devices) microsusceptometer to obtain nearly quantum-limited spin resolution. The susceptometer, including two DC SQUIDs, pickup loops, and field coils, was fabricated using VLSI technology to achieve an unprecedented scale of magnetic sensitivity, optimized for the study of microscopic samples or thin layers. Furthermore, in combination with an ultrafast pulsed laser system, measurements can be made at picosecond time scales, providing an understanding of the dynamics of the magnetic phenomena in these materials and the effects of quantum confinement on magnetic behavior     

Picosecond pulse response of a travelling-wave semiconductor laser amplifier

The dynamic pulse response of a travelling-wave semiconductor laser amplifier has been investigated. Peak output powers greater than 100 mW, with a fibre-to-fibre gain of 10 dB, have been observed for narrow pulses (<50ps FWHM) at low repetition rates. The dynamic bandwidth of the amplifier has been shown to be at least 50 GHz.     

Picosecond switching dynamics of a bistable-wavelength-latchtwo-segment distributed feedback laser

The bistable-wavelength-latch operation of a two-segment distributed-feedback laser was observed and characterized. Short electrical trigger pulses were used to demonstrate mode power switching in less than 200 ps with only 450-ps delay between set and reset pulses. Spectrally resolved measurements show transient spectral broadening during the bistable wavelength switching. Fast laser wavelength switching is important for wavelength-division-multiplex photonic switching systems     

Picosecond optical pulse generation from mode-locked phased laser diode array

Actively mode-locked 26 ps optical pulses are generated from a gain-guided, 10-stripe, phased laser GaAs diode array with an external cavity consisting of two cylindrical lenses and a corner reflector. To our knowledge, this is the shortest pulse width yet demonstrated from a mode-locked phased laser diode array. The detuning bandwidth of the mode-locking is measured as 2.5 MHz.<>     

Picosecond internal Q-switching mode correlates with laser diode breakdown voltage

The record picosecond power density recently achieved with a current-pumped laser diode turned our attention to a still unexplained 50-year-old phenomenon termed “internal Q-switching”. The correlation found experimentally here between the relatively high breakdown voltage (～5–11 V) in a heavily doped single-heterostructure laser diode and its high-power picosecond lasing provides a means for solving the puzzle. Together with the experimental fact that picosecond lasing occurs from the p-n junction, this implies that internal Q-switching is determined by the compensated layer rather than by “traditional” single-heterostructure waveguide. This finding is valid for various growth technologies independently of whether the high break-down voltage and picosecond lasing are achieved by exact compensation of shallow donors by shallow acceptors, or by doping profile gradients.     

Picosecond synchronisation on mode-locked laser pulses by double constant-fraction triggering

A novel technique is introduced for the synchronisation with better-than-30 ps jitter of electrical pulses to ultrafast light pulses at high repetition rate (74 MHz and more). The technique makes it possible to perform time-correlated single-photon counting measurements at picosecond resolution using mode-locked lasers without cavity dumping.<>     

Picosecond optical properties of a grating surface emittingtwo-dimensional coherent laser array

The picosecond dynamical properties of a two-dimensional grating surface emitting (GSE) laser array are investigated by recording with a streak camera the optical emission under low duty cycle, 200-ns pulsed operation. The elements of the two-dimensional GSE arrays are index-guided ridge lasers. The array (a 10×2 array) consists of two groups of ten laterally coupled laser elements which are coupled longitudinally by an intervening grating, and gratings on either end of the two groups which provide the necessary feedback for the laser cavities. A variety of nonlinear dynamic behavior regimes, ranging from quiescent through periodic, to erratic is observed during the course of a single-drive current pulse. The transition from one regime to another may be due to transient heating during the pulse     

Application of linear prediction least-squares fitting to time-resolved optical spectroscopy

Linear prediction least-squares fitting is shown to be a powerful tool for the quantitative analysis of pump-probe-type experiments. When used correctly, it is capable of extracting multiple exponential or exponentially damped sinusoidal components from noisy data. Furthermore, it can be used to analyze data that are not simply the response of the system being studied, but rather the convolution of a nonnegligible instrument function with the desired response. In pump-probe experiments, it can be used to deduce time constants that are considerably shorter than the laser pulse itself.     

Picosecond and femtosecond pulse generation in a regenerativelymode-locked Ti:sapphire laser

The authors have produced transform-limited pulses ranging from 100 ps to 40 fs duration from a Ti:sapphire laser. Output powers in excess of 1 W and peak powers of 0.5 MW have been observed. They describe the technique of regenerative mode locking and present evidence that a transient with a peak power of more than 10 kW is required to initiate mode locking. The role of group velocity dispersion is highlighted and a value of -750 fs² is measured for the group delay dispersion in an operating laser. The authors describe the limits on both the power and pulsewidth obtainable from this laser and present pulse compression experiments which produce 17 fs pulses with 70 mW of average power     

Picosecond pulse analysis of gain-switched 1.55 µm InGaAsP laser

Results of computer simulation of gain-switched pulses from InGaAsP lasers emitting at 1.55 μm are reported. Multimode rate equations are solved for short triangular current pulses of large amplitude superimposed on a dc bias close to threshold. Lasers of conventional length (200μm) and with short cavity (50 μm) are analyzed. The effect of wavelength chirp on time-resolved and the time-averaged multimode spectra is demonstrated. Optical pulses as short as 10 ps are predicted for short-cavity lasers with reflective coatings. While the number of longitudinal modes in gain-switched spectrum is efficiently reduced for short-cavity lasers, the dynamic linewidth of each mode remains of a similar magnitude (up to 140 GHz) as for standard-cavity lasers. It is expected that gain saturation effects could reduce the calculated values of dynamic linewidth.     

Picosecond pulses, TEM00mode, mode-locked ruby laser

Picosecond pulses with a single transverse-mode operation have been obtained in a mode-locked ruby laser by using a special rooftop prism cavity. Addition of rhodamine 6G to the dye (DDI in methanol) enabled us to obtain consistently pulses as short as 5 ps.     

Technique for measuring true time-resolved spectra of a semiconductor laser

Using a multiple fibre array where each fibre is of a different length, a new technique is presented for examining the true time-resolved spectrum of a semiconductor laser. As an example, the separate contributions of partitioning and multi-mode operation in the spectral output of five longitudinal modes of a 1.3 ?m buried channel laser are identified.     

Picosecond ultrasonics

The generation and detection of ultrasonic waves with picosecond time resolution are described. With picosecond ultrasonics, the frequency range of acoustic waves is extended into the 100-GHz range. It allows the investigation of thin-film materials, microstructures, and materials with large acoustic attenuation. Velocity and attenuation measurements on a variety of thin films are reviewed. The results on complex microstructures, including multilayers, are presented     

Picosecond spectrochronography

The general problem of extracting complete (amplitude and phase) information out of an optical signal is discussed. We have shown that the best one can do to determine all essential features of light pulses is to apply simultaneous temporal and spectral analysis to take spectrochronograms with the appropriate shape of the resolution cell on theomegat-plane. We use the term "spectrochronogram" instead of the much broader term "time-resolved spectrum" for a specific measurement result where the resolutionsDeltaomegaandDeltatused are transform correlated. A novel subtractive mount of monochromators has been proposed to overcome the obstacles to experimental realization of uncertainty-principle-limited setups for high spectral resolution picosecond spectrochronography. For the examples of perylene and anthracene molecules, experimental spectrochronograms revealing temporal behavior of hot luminescence lines and, correspondingly, picosecond kinetics of intramolecular vibrational relaxation have been presented. Some further applications of picosecond spectrochronography have been discussed.     

Radial and time-resolved measurement of cuprous bromideconcentration in a Cu-HBr laser

Radial and time-resolved measurements of cuprous bromide (CuBr) molecules have been performed in the Cu-HBr laser for the first time. The results show that under lasing conditions as well as under oven conditions the concentration of CuBr is very small (<10¹³ molecules/cm³) in the laser plasma tube