H. Friedmann

Enhanced index of refraction: a comparison between two- and three-level systems

Abstract We show that the phenomena of enhancement of the refractive index and third-order susceptibility at a point of vanishing absorption, recently described is multilevel systems, are also present in two-level systems interacting with a strong pump and a weak to moderately strong probe laser. In addition we show that in the three-level system discussed by Scully and Zhu, the refractive index can be enhanced even when the probe is of comparable intensity to the pump. In both systems, the effects can be increased by detuning the pump.

Enhanced index of refraction in a population-trapped three-level system

Abstract It is shown that the phenomenon of enhancement of the refractive index at a point of zero probe absorption can be achieved in a population-trapped degenerate three-level Λ-system interacting resonantly with a single strong pump and a tunable probe. Non-absorption of the pump will ensure that the phenomenon persists over a long optical path without changing the frequency at which the medium is transparent to the probe.

Positive and negative dispersion in a three-level Λ system driven by a single pump

We extend our previous study of a nearly degenerate Λ three-level system that exhibits population trapping when driven by a single pump and produces an absorption or gain doublet in the probe absorption spectrum. In particular, we show that the probe dispersion is positive or negative between the doublet lines, and can survive Doppler broadening. For certain values of the pump Rabi frequency, the sign of the dispersion can be changed by altering the ratio of the splitting between the lower levels to the Doppler width.

Effective two-level Hamiltonian for coherent multiphoton processes

Abstract The adiabatic following model of two-photon coherent excitation of Grischkowsky, Loy and Liao has been extended to multiphoton excitation using projection operator techniques. The solutions of the generalized Bloch equations are given in the sudden switching, adiabatic following and steady state limits. The theory is used to discuss various models of collisionless multiphoton dissociation of polyatomic molecules.

Ultranarrow extraresonant antiholes in pump–probe spectroscopy induced by inelastic collisions

Pump-probe spectroscopy with copropagating pump and probe beams of an open two-level system where, as a result of inelastic collisions, the lower state of an optical transition relaxes faster to reservoir states than the upper state does, may yield an antihole in the probe spectral profile whose linewidth is subnatural and unaffected by Doppler broadening.

Two-photon coherence and steady-state saturated and inverted populations in three-level systems

Abstract We study the correlation between level populations, absorption and two-photon coherence in closed Λ- and V-shaped three-level systems interacting with two lasers of arbitrary intensity. Whereas it is well known that maximum two-photon coherence leads to population trapping in Λ-shaped three-level systems interacting with two equally detuned lasers, we demonstrate that minimum two-photon coherence, under saturation conditions, can lead to equal populations in all the levels of a Λ- or V-shaped system when the lasers are symmetrically detuned. Moreover we show that steady-state population inversion between one of the upper levels of a V-shaped system and the ground state can occur when either the detunings are asymmetrical or the upper states decay at different rates. It is shown that this population inversion does not lead to stimulated emission into the lasing modes. These effects are also correlated with two-photon coherence and are explained physically by a cross three-photon scattering process...

Extra resonances in degenerate four-wave mixing triggered by pressure-induced decay of the lower state.

By using the Schenzle-Brewer model, in which the ground state of a two-level system also has a finite lifetime, it is shown that two pressure-induced extraresonant degenerate four-wave mixing signals are obtained. The first extra resonance is similar to that predicted and observed by Bogdan et al. [Opt. Lett. 6, 82, 348 (1981)]. It is characterized by a linewidth equal to the inverse lifetime of the upper state and appears only in the presence of collisional redistribution. The second extra resonance is characterized by a linewidth equal to the inverse lifetime of the ground state and does not depend on collisional redistribution for its appearance.

Effect of two-photon saturation on ordinary Raman scattering

The excitation profile (EP) of ordinary Raman spattering under steady-state conditions and the time- and frequency-resolved emission spectra (TRS and FRS) of ordinary Raman scattering under transient excitation conditions undergo considerable changes in the presence of two-photon absorption [1,2].

Polarization dependence of resonance-enhanced three-photon scattering

The polarization properties of stimulated three-photon scattering in a three-level atomic system are presented. In our investigation of the stimulated emission spectrum of Na, we find that interference between the amplitudes of the resonant contributions of the Na doublet states results in interesting polarization phenomena. Particularly striking is the behavior when the laser is tuned near the dispersion-free point. With the laser linearly polarized the stimulated three-photon scattering is polarized perpendicular to the laser polarization. For circularly polarized excitation, total extinction of three-photon scattering is observed.

Coherence effects in ordinary Raman scattering in the presence of two-photon resonance

In the presence of coherent saturated near-resonant two-photon excitation, the ordinary Raman scattering line is replaced by a dynamic Stark shifted or Stark split doublet, and Rabi nutations appear in the time-resolved ordinary Raman spectrum. The relative intensity of the peaks of the doublet depends on the shape and on the average frequency of the exciting pulse. Striking new features arise from the interfering contributions of the upper and lower states of the two-photon transition to the Raman scattering process. The theory is applied to Raman scattering in atomic Na in the presence of the 32S12/(F=1, MF=1) to 42D32,5/2/(F=3,MF=3) two-photon resonances.