Tatsuo Yajima

Spatial Optical Parametric Coupling of Picosecond Light Pulses and Transverse Relaxation Effect in Resonant Media

The dynamical properties of a third-order nonlinear optical process, in which two incident light pulses with wave vectors k 1 and k 2 generate a light wave with 2 k 2 - k 1 in resonant media, are discussed with particular attention to the effect of transverse relaxation time T 2 . It is shown that the correlation trace (output light energy vs incident pulse separation) for this process provides a general means of obtaining information on ultrashort T 2 for both the homogeneously and inhomogeneously broadened transitions, and for both t p (pulse width) T 2 . An experimental demonstration for t p > T 2 is given by using a dye molecule and a picosecond light source.

Non-linear optical spectroscopy of an inhomogeneously broadened resonant transition by means of three-wave mixing

Abstract A calculation shows that the dispersion of optical mixing of the type 2ω1 − ω2 provides a new means of determining directly both the longitudinal and transverse relaxation time associated with an inhomogeneously broadened optical transition of materials having very short relaxation times of a picosecond or less.

Time-delayed four-wave mixing using incoherent light for observation of ultrafast population relaxation

Time-delayed four-wave mixing using incoherent light has been proposed as a new method to measure the ultrafast population relaxation time T1. Assuming three input beams of incoherent light with much longer duration than T1, the theoretical calculation has shown that the decay curve by T1 appears in the signal profile and that the time resolution is determined by the correlation time of the light. The decay time to be observed is twice that of the transient four-wave mixing with short pulses. We have experimentally confirmed the prediction by observing picosecond population relaxations of organic dye solutions, using nanosecond pulses from a normal broadband dye laser. An advantage of the present method over the pump-probe method using incoherent light is also discussed.

Far-Infrared Difference-Frequency Generation by Picosecond Laser Pulses

A method of far-infrared generation by optical difference-frequency mixing between the modes of a mode-locked laser has been studied both theoretically and experimentally. General formulae for the spectrum and the intensity of the beat radiation for both mode-locked and non-mode-locked inputs were derived. The generation of a broad-band far-infrared beat in the range of 1–20 cm-1 was observed when the output of a mode-locked Nd:glass laser was incident on LiNbO3, ZnTe, ZnSe, CdS and quartz crystals without collinear phase matching conditions. The efficiency of beat generation was found to be enhanced by a factor of 10 due to mode-locking. The beat intensity and its anisotropy were measured for each crystal and were found to be in reasonable agreement with the theory. By using metal-mesh filters, a major portion of the beat energy was found to exist in the spectral region above 10 cm-1. The possibility of this scheme as a tunable submillimeter-wave source is discussed.

Characteristics of a new-type SHG crystala β-BaB2O4 in the femtosecond region

Abstract The optical characteristics of a new nonlinear crystal β-BaB 2 O 4 , including the group-velocity dispersion and the nonlinear coefficient in the femtosecond region, are investigated through the second-harmonic generation (SGH). The performances of β-BaB 2 O 4 relative to KDP are evaluated from both calculations and measurements. From the measurements using the crystals with different thicknesses, the influence of the group-velocity mismatch of β-BaB 2 O 4 on the broadening of the correlation width is revealed, and is found to be smaller than that of KDP. Overall performance of the SHG conversion efficiency of β-BaB 2 O 4 including all affecting factors is evaluated to show higher values by a factor of 10 to 10 2 than that of KDP depending on operating conditions.

Three-Level Maser Action in Gas I. Theory of Multiple Quantum Transition and Doppler Effect in Three-Level Gas Maser

The multiple quantum process in the off-resonant condition of the three-level gas maser is discussed. The intensity of the emission produced by this process is calculated by a perturbation theory and also by a more general treatment which is the application of Javans theory to the off-resonant case. Some graphical representations of the signal line under off-resonant pumping are given for the purpose of comparison with experiment. The Doppler effect in three-level maser action is analyzed from both semiclassical and quantum mechanical points of view under the presence of collision relaxation. It was shown that in a saturation process between two levels, Doppler effect behaves as to contribute to both line-width and relaxation time for the transition, and that under strong pumping, Doppler effect in pumping transition has a marked influence on both intensity and shape of the signal line through the effect of multiple quantum transition.

Ultrafast Relaxation Study by Resonant Rayleigh-Type Mixing Spectroscopy Using Picosecond Light Pulses

The dispersion characteristics of the nonlinear susceptibility χ (3) (ω 3 =2ω 1 -ω 2 ) under the Rayleigh-type and one-photon resonant conditions have been measured for dye molecules in both liquids and solids to determine ultrafast relaxation times associated with electronic excited states. Picosecond dye lasers were used for damage-free measurement in solids. The comparison of the results for the same dye in liquid and solid revealed two types of behavior concerning the dephasing time ( T 2 ). Temperature dependence of T 2 in solid was also measured in a wide range 10∼300 K. Anomalous enhancement of the output light for lower range of |ω 1 -ω 2 | was found and attributed to an effect of spectral cross relaxation process. On the basis of these results underlying relaxation mechanisms are discussed.

Generation of Broadly Tunable Subpicosecond Light Pulses from a Synchronously and Passively Mode-Locked CW Dye Laser

Transform-limited pulses with a minimum width as short as 0.30 ps have been obtained from a synchronously pumped rhodamine 6G dye laser with a dye solution of mixed gain and saturable absorber (DODCI) media. Stable subpicosecond pulses (0.30–0.85 ps) were generated in a wide tuning range of 574–611 nm using a broad-band wedged-etalon tuning element. With the frequency tuning element inside the cavity, the cavity-length detuning range to maintain perfect mode-locking was ±10 µm which is less critical than for synchronous mode-locking alone, and the range extended to ±1.5 mm without the frequency tuning element.

Rapid-Scan Autocorrelator for Monitoring cw Mode-Locked Dye Laser Pulses

A second-order autocorrelator operating at a high repetition rate of 25 Hz was developed for the measurement of the picosecond pulse width of a cw mode-locked dye laser. The simple system consists of a Michelson-like interferometer and an assembly of retardation plates of different thickness which are mounted on an optical chopper. The pulse width is instantaneously displayed on an oscilloscope. Using this instrument, pulses as short as 1.3 ps can easily and reproducibly be obtained from a synchronously-pumped rhodamine-6G dye laser, even though the pulse-width varies critically with cavity length and other laser parameters.

The Three-Level Gas Maser as a Microwave Spectrometer

A method of microwave spectroscopy using three-level maser action is described, and some preliminary experiment on HDCO is given. A b -type transition, 3 03 ←2 12 , which is connected with a a -type transition 2 11 ←2 12 , has been definitely assigned by the observation of three-level master action. Moreover, the weak transition, 3 03 ←2 12 , was observed with very good signal to noise ratio by using the strong transition, 2 11 ←2 12 , as a detecting transition. Theories of the three-level maser action in gases are also compared with experiment and shown to be in good agreement. Further applications of the three-level gas maser as a spectrometer as well as an amplifier are discussed.