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Conference on Lasers and Electro-Optics

New coherent transient technique for femtosecond single-shot homogeneous spectrum measurement

Olavi Ollikainen, Jürgen Gallus, Urs P. Wild, Alexander Rebane

Published Nov 27, 2002• 0 citations
Publisher

Abstract

Summary form only given. Coherent optical transients such as photon echo are a versatile method far measurement of the homogeneous absorption spectrum of molecular systems in the presence of strong inhomogeneous broadening. Such experiments require measurement of the decay of the optical coherence, typically accomplished by gradually varying the time delay between the excitation pulses. Time-domain information is then Fourier transformed to frequency domain by a computer. In this paper we demonstrate a new method that yields the homogeneous spectrum in a single shot of femtosecond laser pulse illumination by performing Fourier transformation of coherent optical transient signal with a lens. We apply this method to measure the homogeneous spectrum of dye molecules in a polymer film in a broad range of temperatures.

DOI
10.1109/iqec.1998.680480

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A technique for the numerical simulation of coherent optical responses of the photon echo type formed in resonance media with strong inhomogeneous broadening under the action of femtosecond laser pulses is developed. This approach is based on solving the Maxwell-Bloch equations using the finite-difference time-domain (FDTD) method without application of a fast rotating field and slowly varying envelope approximations. The method can be used to simulate coherent responses in different resonance media with a complex structure of energy levels. The technique was validated using the example of describing experiments on narrowing a photon echo pulse and femtosecond echo processing.

Bulletin of the Russian Academy of Sciences Physics•Mar 1, 2012•1 citation
Summary form only given. Coherent optical transients such as photon echo are a versatile method far measurement of the homogeneous absorption spectrum of molecular systems in the presence of strong inhomogeneous broadening. Such experiments require measurement of the decay of the optical coherence, typically accomplished by gradually varying the time delay between the excitation pulses. Time-domain information is then Fourier transformed to frequency domain by a computer. In this paper we demonstrate a new method that yields the homogeneous spectrum in a single shot of femtosecond laser pulse illumination by performing Fourier transformation of coherent optical transient signal with a lens. We apply this method to measure the homogeneous spectrum of dye molecules in a polymer film in a broad range of temperatures.
A novel transient three-pulse scattering technique for measuring ultrafast dephasing times in condensed matter is analyzed using a perturbative solution of the density matrix equation. The advantages of this technique include subpulsewidth resolution, a clear distinction between homogeneous and inhomogeneous broadening, and sensitivity to spectral cross-relaxation. Its application to the case of a multilevel resonance is also considered. We report results of femtosecond dephasing experiments with dye molecules in liquids and in a polymer host. The dephasing time is determined to be less than 20 fsec for dyes in solution at room temperature. At low temperatures in polymers, a transition from homogeneous to inhomogeneous broadening has been observed and studied as a function of temperature.
It has been recently pointed out that incoherent light can be used to generate coherent optical transients and that there are decided advantages in using such broad-band non-laser source to achieve very high time resolution {1,2}. When a light pulse and its time-delayed replica interact with a sample to produce the non-linear optical response, the ultimate time resolution should identify with the correlation time τC of the light field rather than with the pulse duration τL. The situation where τC ≪ τL is easily obtained with broad-band incoherent light sources. Time resolution in the femtosecond range is then attainable using nanosecond light pulses. This has been recently demonstrated through the observation of the relaxation of photon echoes in Nd3+-doped silicate glass {1,2}. In these cases, attention is focused on the measurement of fast homogeneous decay rates and the inhomogeneous broadening ΩD is assumed to be much larger than the spectral width of the excitation light.
A simple method for observing coherent optical-transient phenomena is described. Effects such as photon echoes and free induction decay (FID) should allow studies of the dynamic properties of atoms, molecules, and solids on a time scale ranging from milliseconds to \ensuremath{\sim} 50 ps. The frequency of a cw dye laser is switched with an intracavity electro-optic modulator, producing in an external sample coherent transients which are detected in the forward beam. Measurements of long dephasing times are possible, as in FID, because the laser can oscillate indefinitely at the new frequency following a step-function switching pulse. Measurements of short decay times are also practical because the switching time is not restricted by the laser gain or cavity ringing time. In addition, the dye's dephasing time of a few picoseconds or less is too rapid to interfere with these observations. Since the pulse sequence can be preselected, the entire class of coherent optical transients is accessible. The method thus incorporates all of the advantages inherent in Stark switching, including heterodyne detection and high sensitivity, without being restricted to Stark-tunable systems. Laser frequency switching is discussed also in terms of a theoretical model which exposes the time-dependent properties of a phase-modulated cavity mode. Dephasing studies have been carried out thus far in atomic and molecular gases and low-temperature solids containing inorganic impurity ions or large organic molecules. The utility of the FID effect is emphasized where precise dephasing times are obtained from time-frequency Fourier transformation of FID signals using a digital computer. The first quantitative and detailed test of FID theory is provided by these experimental techniques.
Recently we have reported[1] on the measurements of electronic dephasing (T_2) and radiationless decay (T_1) in molecules using emission spectroscopy as a probe for the detection of the photon echo. This right-angle photon echo utilizes a single mode of a narrow-band (~MHz) cw dye laser and an intracavity electro-optical (EO) element that modulates the frequency of the laser.[2] Applying voltage pulses into the EO crystal switches the laser frequency from ω_0 (the peak frequency of the transition resonance) into another frequency ω. This switching procedure introduced by Brewer et al. for the observation of transients in the forward direction of the laser allows one to observe coherent transients in gases,[1,2] solids[3-5] and molecular beams.[6] Typically, the dispersion of the nonlinear EO crystal (e.g., AD*P) gives a 60 MHz frequency shift within the inhomogeneous resonance of the optical transition (width ~GHz or larger). Therefore, to switch the laser frequency further, one must use high-voltage signals that are usually hard to obtain especially for multiple-pulse experiments. Secondly, there is a limit on the frequency shift which is conditioned by the mode spacing of the cavity. Thirdly, because the laser is switched into a new group of molecules some complications may arise in interpreting the transient spectra especially in large molecules. This is because in large molecules the density of excited states is high and thus the buildup of transients of w molecules (which could be complex) will be superimposed on the decay of ω_0 molecules. Finally, off-resonance effects (e.g., beats) between the molecules at frequency ω and ω_0 cannot be avoided.
The signals of primary and stimulated femtosecond photon echoes are investigated in a dye-doped polymer film at room temperature. The homogeneous S 0 → S 1 spectral line width, which is due to the interaction between the impurity molecules and the quasi-local low-frequency modes, is estimated (≈5 × 1012 Hz). Special attention is paid to the study of spectra of femtosecond echo signals. The short-wave shifts of these spectra, with respect to the spectrum of femtosecond exciting pulses, are observed. These shifts indicate that the anti-Stokes regime of femtosecond pulse emission is realized. Therefore, the coherent regime of laser cooling of solids appears to be possible. The prospects of using of this new cooling regime in the function of a solid-state optical refrigerator are discussed.
A technique for the numerical simulation of coherent optical responses of the photon echo type formed in resonance media with strong inhomogeneous broadening under the action of femtosecond laser pulses is developed. This approach is based on solving the Maxwell-Bloch equations using the finite-difference time-domain (FDTD) method without application of a fast rotating field and slowly varying envelope approximations. The method can be used to simulate coherent responses in different resonance media with a complex structure of energy levels. The technique was validated using the example of describing experiments on narrowing a photon echo pulse and femtosecond echo processing.