A free-style essay is presented on such subjects as a historical view of ultrashort pulse generation, coherence of ultrashort light pulses, and studies of ultrafast phenomena not only by ultrashort light pulses.

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Two processes which are essential to generate ultrashort optical pulses are described. They are 1) producing/providing the broad-band optical spectra and 2) controlling the phase relation between the optical frequency components.
Additionally, typical methods of ultrashort optical pulse generation, i. e. modelocking, electrooptic gates, an optical pulse synthesizer, and…, are summarized.

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Recent investigations done in ETL on a femtosecond-pulse dye laser system are reviewed. Experimental techniques for generation and application of femtosecond pulses are described.

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It is quite important to generate optical pulses from semiconductor diode lasers with short pulsewidth and high peak power. Due to the compactness and capability of carrier injection of the diode laser, short optical pulses are well generated using a compact and simple system. These pulses are expected to be utilized not only in the optical communication, but also a wide variety of optoelectronic applications. This paper describes the generation of short optical pulses from semiconductor diode lasers, as well as the parameter dependence of the diode laser on the characteristics of short pulse generation.

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The present status of short pulse excimer lasers is described with an enphasis on XeCl, KrF and ArF. Main contents are (1) the gain spectra of excimer lasers, (2) short pulse generation and pulusewidth measurementat the excimer wavelengths, (3) scaling of peak power with regard to absorption, amplified spontaneous emission, and dispersion of optical elements. Typical short pulse excimer laser systems are summalized. Finally the prospect of applications such as multiphoton process are presented.

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This paper descrives the overview of recent developments on ultrashort pulse generation in solid state lasers.Traditional lasers such as Nd: glass lasers and Nd: YAG lasers generate more and more stable, higher power, and shorter optical pulses. “New lasers” such as Nd: YLF lasers, alexandrite lasers, and so on have become ultrashort pulse lasers. Pumping technique using laser diodes has been providing new horizen on ultrashort solid state lasers.

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Recent progress on optical pulse compression techniques, optical solitons, and soliton lasers has been discussed. It is shown that nonlinear pulse propagation in single-mode fibers, which includes self-phase modulation and group velocity dispersion, plays an important role in making linearly chirped rectangular pulses, resuiting in efficient compression with a grating pair. The self steepening effect, a third-order dispersion, and the stimulated Raman scattering, which would deteriorate the compression characteristics, have been discussed in dctail. For a negative GVD region, an optical soliton described with a nonlinear schrodinger equation can be creiatcd by balancing SPM with GVD. A soliton laser by means of a color center Iaser and a fiber Raman soliton laser are discussed in terms of a perturbation theory.

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Recent progress of ultrashort pulse lasers and pulse compression techniques has allowed us to generate pulses with duration of the order of 10 fs or less.
The development of measurement techniques is also necessary for characterizing the ultrashort pulses. Linear direct measurement combined with photodetectors and oscilloscopes or ultrafast streak camera is no longer adequate to temporally resolve these pulses. Instead, various nonlinear correlation methods based on the nonlinear optical processes with time resolution of subpicosecond or femtosecond are proposed and demonstrated.
The basic concept, advantages and limitations of the methods including the recent development are discussed here.

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Recent measuring technology for various types of ultrashort optical pulses using ultrafast streak cameras and optical oscilloscopes are reviewed. The optical oscilloscope is a conceptually new electron-optical instrument, in which two advanced technologies: streak camera and oscilloscope, are combined in one unit.

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In spite of recent advance in high-speed electronics reaching pico-second time regime, instrumentation means for their evaluations and testings are lacking. Pure electrical means is pointed out to have an inevitable limitation on its speed. Optical means can provide very short light pulses, which might be utilized to overcome the limitation in pure electrical means. Optical sampling technique is based upon recent availability of this very short optical pulse. Various optical sampling techniques are reveiewed on the general common concept of samplingtechnique and advantages of optical sampling are stressed.

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Measurements of the relaxation times T₁ and T₂ of a two-level system by nonlinear transient spectroscopic methods are discussed, where delayed incident pulses are assumed to have correlation times much shorter than T₁ or T₂ but varied widths. In particular, a delayed degenerate four-wave mixing (photon echoe) and apump-and-probe type transmission measurement with incoherent long pulses are considered in detail, and a principle of their use for ultrafast spectroscopy is clarified. If the field is steady and T₁>>T₂, the time resolutionof T₂measurement by the delayed four-wave mixlng is determined by the field correlation, therefore by the spectral width of the incident light irrespective of the pulse width. The time resoution of the T₁ measurement by the pump-and-probe method is determined by the intensity correlation of the incident light, whereasthe one by the delayed four-wave mixing by a higher order correlation.

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Three types of short-time measurements using temporally incoherent light instead of short pulses are described. Electronic dephasing in a polydiacetylene film, vibrational dephasing in dimethylsulfoxide liquid, and relaxation of optical Kerr effect in CS₂ were observed with femtosecond time resolution. General applicability the ultrafast measument by various types of optical nonlinear processes using incoherent light was confirmed.

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Formation mechanism of point defects in alkali halide crystals is introduced from a point of view on theultra fast relaxation process. Point defects (color centers) are produced in the crystals under irradiation ofionizing radiation or photons with photon energy larger than the band gap energy of the crystals. The first productis the positive hole which is localized in a period of several lattice viblations to form an X₂^- molecule (center) in a <110> direction in the crystal. Capture of a conduction electron by the X₂^- center creates the socalledself trapped exciton (STE), from which the intrinsic Luminescence and the point defect formation occur.The process starting by the ultra short pulse excitation and ending by the creation of the color centers is anultra fast phenomenum within a range between several picoseconds and several hundred picoseconds, andgives fundamental knowledge on the defect formation induced by the hole-or electron-lattice interaction.

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Population mixing and time-resolution techniques were applid to study the dynamical carrier processes inGaAs-AlGaAs superlattices under the perpendicular electric field.Competitive nature of ultrafast processes ofphoto-excited electron-hole pairs, exciton formation and pair ionization followed by tunneling through barriers, are clarified under the various electric field.

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Time resolved photoluminescence spectra of two-dimensional excitons in a GaAs supperlattice were observed by a picosecond dye laser. The relaxation times of the photo-excited excitons and carriers depend on the excitaion laser wavelength, and they are on the order of picosecond.

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Study is reported on the picosecond spectroscopy of relaxation processes in exciton systems in semiconductors by using a single-photon-counting set up developed by the authors. Excitonic emissions in ZnTe is the first topic, which show the dynamical aspect of the polaritons near the bottleneck. Time evolution of the resonance Raman processes are observable in real time in the polariton emissions. The second topic is excitonic magnetic polarons in Cd_1-xMn_xTe, where the dynamics of the spin-correlated excitons with Mn ions is discussed.

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Studies on intramolecular vibrational energy redistribution in the lowest excited electronic manifold are reviewed based on absorption, fluorescence, and ionization spectroscopy. A new method of “photochemical timing”is described for the vibrational mode dynamics of supersonically cooled trans-stilbene.

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Fundamental aspects of inter-and intramolecular photoinduced electron transfer (ET) and related phenomena studied by means of ultrafast laser spectroscopy as well as some related theoretical problems are discussed. Picosecond transient absorption spectral studies have demonstrated the existence of various ET states depending upon the solvents in the typical intramolecular exciplex systems and TICT compounds. Behaviors of geminate radical ion-pairs (RIP) formed in the course of photoinduced ET and electron photoejection have been studied in various systems. In the case of the geminate RIP of aromatic donor-acceptor systems in polar solvents, recombination rate has showed a bell-shaped energy gap dependence contrary to the fact that the bimolecular rate constant of fluorescence quenching due to ET is quite insensitive to the, relevant energy gap. These results have been interpreted by a new theory for the energy gap dependence of ET.

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Excitation energy transfer in restricted molecular geometries has been studied with vesicle surface, Langmuir-Blodgett monolayers and multilayers and a biological antenna pigment system. Picosecond time-resolved fluorescence spectra and decay curves revealed time behaviors of the excitation transfer characteristic of two-dimensional and stacking architectures. Distribution of guest molecules forms Fractal or Fractal-like structures in artificial molecular assemblies.

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Deca-picosecond to nanosecond time-resolved, absorption spectroscopy applied to a set of cis-trans isomers of retinal, retinylideneacetaldehyde and β-carotene is summarized. No configurational changes have been detected in the singlet excited-state (S₁) of either retinylideneacetaldehyde or β-carotene. Configurational changes (isomerization) have been detected in the triplet excited-state (T₁) of retinal, retinylidene acetaldehyde and β-carotene. The rate of triplet isomerization was dependent on the starting ground-state configuration; it is the highest for the central-bent, unmethylated-cis configuration, and it is the lowest for the all-trans configuration.

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Biological functions of proteins can be understood in terms of protein dynamics. The results by theoretical considerations according to the method of molecular dynamics have predicted that protein structure is inherently dynamic in the time region of subpicosecond to picosecond. To observe such a fast dynamics of proteins a picosecond laser photolysis and a time-resolved fluorescence spectroscopy in the picosecond region are considered to be the most useful technique. In this review article, firstly the biological significance of protein dynamics and secondly, experimental evidences for picosecond dynamics of flavoproteins, and finally, the protein dynamics studied by the time-resolved fluorescence of tryptophanes are documented.

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Optical fiber communication systems are widely introduced into telecommunication networks since 1980's. The sub-nanosecond optical pulses are used in recent practical single-mode fiber transmission systems. This paper discusses the future possibility of ultra-short pulses application to communication systems. High-speed signal processing capability is discussed, and optical short-pulse generation, detection and multiplexing technology are described. It is expected that the future ultra-high speed communication systems will have the very large video communication service capability.

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In this paper, high-speed optical devices having quantum well structures are discussed. These include quantum well lasers, quantum-confined Stark effect devices, and Optical-Stark effect devices.
It is expected that use of the quantum well structure leads to new types of ultrafast optical devices.

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Various types of optically bistable semiconductor devices are reviewed. Characteristics of these devices desirous for their use in optical computers are discussed.

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Merits and existing problems of using pulsed semiconductor lasers as optical clock for optoelectronic integrate circuits (OEIC) are discussed. Examples are taken from the research efforts of the author's group on opto-electronic demultiplexing and multiplexing based either on fast photoconductive switch or on bistable semiconductor laser. Importance of developing optical circuit technology enabling multiple input/output optical connection is pointed out.

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