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Chinese Physics Letters

Measurement of ultrafast dephasing time of ruby by transient grating with incoherent light

Ruihua Zhang, Qian Jiang, Zuhe Yu, Panming Fu, Peixian Ye

Published Dec 1, 1987• 1 citation
Publisher

Abstract

Dephasing time T 2 of ruby in the absorption band 4 T 2 has been measured for the first time using the method of transient grating with incoherent light. The population grating was induced in the material by two broad-band nanosecond pulses of a dye laser and probed by a Continuous Wave (CW) Ar + laser. At room temperature, T 2 =0.04 similar 0.08 ps was obtained when the sample was excited at λ = 5640Å.

DOI
10.1088/0256-307x/4/12/008

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Optics Letters•May 1, 1990•8 citations
Dephasing time T 2 of ruby in the absorption band 4 T 2 has been measured for the first time using the method of transient grating with incoherent light. The population grating was induced in the material by two broad-band nanosecond pulses of a dye laser and probed by a Continuous Wave (CW) Ar + laser. At room temperature, T 2 =0.04 similar 0.08 ps was obtained when the sample was excited at λ = 5640Å.
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.
In recent years, a series of nonlinear optical processes with incoherent light, such as time-delay four-wave mixing, optical Kerr effect and coherent Stokes Raman scattering, were developed and attracted much attention. It has been demonstrated that these processes can be used to measure the ultrafast dephasing times of excitations (shorter than 1 psec.) with nanosecond pulse lasers or even CW lasers. In this talk, after a short review of the works done by others previously theoretical and experimental investigations in this field in my laboratory are presented.
Until now all reported studies of transient spectroscopy with incoherent light, including photon echo and pump-probe spectroscopy, were concerned with two-level systems where quasi-monochromatic light of only one frequency couples the two levels.[1-4] However, there exist various coherent transient phenomena where light of two or more frequencies are concerned, and time resolution of the transient spectroscopies using these phenomena are also expected to be limited not by the duration of used light pulses but by the correlation time of the radiation fields.[5]
Ultrafast homogeneous transverse relaxation time (dephasing time) T2 of Cresyl Fast Violet in cellulose has been measured in the S0 ↔ S1 transition by photon echoes with temporally incoherent nanosecond laser pulses. At 10 K, T2 was 0.7 psec when the sample was excited 6250 Å near the long-wavelength edge of the absorption band, but it was several tens of femtoseconds at 5940 Å. The fact that T2 is shorter at shorter wavelengths can attributed to the intramolecular relaxation of Cresyl Fast Violet molecules. T2 was measured also at 80 and 300 K. At 10 K persistent nonphotochemical holes (PNPH’s) were observed, and the relations between T2 and the shapes of the PNPH’s have been considered.
A series of experiments of time-delay four-wave mixing with incoherent light (TDFWM-IL) in absorption bands of ruby and dye molecules at 300 K-20 K are reported, which demonstrate that two-level theory fails in the interpretation of the results, and that our multilevel theory of TDFWM-IL should be applied. The following experimental results are emphasized: the normalized delay-time dependences of the signal intensity IS (τ) are not dependent on the sample temperature from 80 K to 300 K for the pump-beam wavelengths not closed to the zero-phonon line or the absorption edge as the power spectrum S0(ω) of the incoherent light source kept invariable; at low temperature of 20 K, IS(τ) is modulated periodically with a period equal to the interval of two adjacent vibrational transitions in the absorption band 4T2 of ruby, as predicted by the multilevel theory. The experiments performed at the zero-phonon line of ruby and at the absorption edge of Cresyl Violet in PMMA demonstrate that the effective two-level model is suitable to approximately analyze not only the experimental results of TDFWM-IL in the unresolved continuous absorption bands with homogeneous broadening, but also the effects of inhomogeneous broadening on the behavior of TDFWM-IL in these bands. Additionally, some data of the subpicosecond dephasing processes of these materials are also obtained for the first time.
With the method of self-diffraction of incoherent light, molecular vibrations were observed in real time in various dye molecules pumped near the absorption edge. In Rhodamine 560, 35-fsec oscillations in the signal were measured, which corresponds to a vibrational frequency of 1000 cm(-1). Simultaneously, this technique can be used to deduce the ultrafast dephasing times in the molecular vibrational quasi-continuum. Dephasing times of 20 and 150 fsec were observed in Rhodamine 590 and Rhodamine 560, respectively.