Raanan Bavli

Optical pulse compressor composed of saturable and reverse saturable absorbers

Abstract It is shown that an optical pulse compressor can be made of a first stage composed of a saturable absorber to reduce the leading edge of the pulse and a second stage composed of a reverse saturable absorber (a substance whose excited-state cross section is larger than its ground-state cross section) to reduce the trailing edge of the pulse. Criteria are given for choosing appropriate cross sections, concentrations, lifetimes, and optical paths, for the saturable and reverse saturable absorber. A numerical example is presented.

The sensitivity of absorption experiments with phase locked ultrashort pulses to the excited state potential energy surface

It is often believed, with some justification, that the excitation of a molecule with an ultrashort pulse generates wave packets whose behavior is well approximated by classical mechanics. Therefore, it seems that such pulses are not particularly useful for accurate studies of quantum dynamics of an excited molecule; at least not when compared to cw spectroscopy. In this paper we show that this shortcoming can be overcome if one studies the emission of a molecule which absorbs one photon by interacting with two phase locked ultrashort pulses. The excited state population is affected by quantum interference and the dependence of the emission on the delay time between the pulses is very sensitive to small changes in the excited state potential. This sensitivity is as high at room temperature as it is at zero degrees Kelvin. The use of phase locked pulses can distinguish between two potential energy surfaces for which the traditional pump–probe experiments give practically identical results. These statements...

Nonlinear optical response of three‐level systems

We investigate the properties of a homogeneously broadened three‐level system interacting with two optical fields, Eω1(x,t) and Eω2(x,t), with central frequencies ω1 and ω2 whose amplitudes are assumed to be slowly varying in time compared with dephasing times of the optical transitions. We calculate Raman susceptibilities allowing each transition to be induced by both fields and without employing the rotating wave approximation (RWA); this enables correct calculation of the optical response of the medium when the fields are on‐resonance or arbitrarily far off‐resonance with the Raman transition. In some frequency regions the RWA causes substantial errors. A distinction is drawn between two different types of RWA: (a) a neglect of off‐resonant components of the polarization for direct optical transitions, and (b) neglect of off‐resonant components of off‐diagonal density matrix elements of order Eω1(x,t)Eω2(x,t)* in the field amplitudes. The first RWA is known to become inaccurate when the pump frequency ...

Multiphoton transitions in molecules with permanent dipole moments

Abstract We use a nonperturbative treatment to study multiphoton transitions in molecules with permanent dipole moments. This generalizes the perturbative treatment of Scharf and Band (Chem. Phys. Letters 144 (1988) 165) for two-photon transitions in molecules with permanent dipole moments in the presence of electromagnetic fields of two frequencies, one near resonance and the other small, Our nonperturbative density matrix treatment accounts for level population decay ( T 1 ) effects and optical transition dephasing ( T 2 ) effects, both neglected in earlier approaches. Our results are applicable even in the regime where perturbation theory breaks down. In particular, when the sum of the field frequencies approaches a molecular resonance a substantial enhancement of the photo-absorption rate (by a two-photon process) occurs even when one frequency becomes as small as the transition linewidth.

On the use of time domain methods to study the excitation of a molecule by a strong, long laser pulse

Calculations which solve the time dependent Schrodinger equation on a grid are generally useful for studying the cw excitation of a molecule by a weak laser or the excitation by a short, strong laser pulse. Here we show how time dependent methods can be applied to study excitation by strong, long pulses. Moreover, even though the molecule is driven by a time dependent field, one can identify energy eigenstates that provide the time scales on which various observables evolve. The calculations which illustrate the methodology are performed on a model of the bound‐to‐bound electronic excitation of the I2 molecule. We study the dependence of the excitation probability on time, power, and laser frequency. We find that the I2 molecule driven by a strong laser emits at many frequencies below and above the laser frequency.

Extracavity Pulse Compressor and Intracavity Mode-Locking with Saturable and Reverse Saturable Absorbers

We show that an optical pulse compressor, a device to remove the leading and trailing edges of a subnanosecond pulse, can be made of a first stage composed of a saturable absorber (SA) whose function is to reduce the leading edge of the pulse, and a second stage composed of a reverse saturable absorber (RSA) which functions to reduce the trailing edge of the pulse. An RSA is a substance with a larger excited state absorption cross section, σ*(λ), than ground state cross section, σ(λ), at a particular wavelength λ of interest, and which fulfills four other criteria.1,2 A numerical example of pulse compression of a ps pulse whose central wavelength is 266 nm using a Coumerine dye as the RSA and quaterphenyl as the SA is presented. We then consider the effects of introducing SA and RSA within a laser cavity to mode-lock lasers. The introduction of SA and RSA into the laser cavity is shown to be particularly useful for passive mode-locking of lasers with gain media with high gain saturation energies wherein individual pulses cannot saturate the gain. Moreover, introduction of a RSA into the laser cavity stabilizes the laser against the onset of relaxation oscillations.