Slow light, produced in a medium with two widely spaced absorption resonances, is analytically studied. We show that a pulse with frequency tuned in the middle of such a doublet structure slows down due to its energy storage in the excited atomic states. If the lifetime of the atomic excitation is much longer than the pulse duration, the pulse shape is nicely reproduced at the output of a thick sample for pulses with smooth envelopes. Pulses with sharp edges are fairly reproduced at the output. Each abrupt change of the pulse amplitude is followed by transients, which are delayed together with the pulse. A possibility to slow down a single photon is also considered.
Slow light, produced in a medium with two widely spaced absorption resonances, is analytically studied. We show that a pulse with frequency tuned in the middle of such a doublet structure slows down due to its energy storage in the excited atomic states. If the lifetime of the atomic excitation is much longer than the pulse duration, the pulse shape is nicely reproduced at the output of a thick sample for pulses with smooth envelopes. Pulses with sharp edges are fairly reproduced at the output. Each abrupt change of the pulse amplitude is followed by transients, which are delayed together with the pulse. A possibility to slow down a single photon is also considered.
We consider the propagation of a light pulse in a medium with a single resonance. If the frequency of the pulse is tuned far from resonance and the pulse duration is much shorter than the lifetime of the excited state of the resonant particles in the medium (atoms in a gas, impurity ions in a solid, etc.), the group velocity of the pulse is appreciably reduced. It is shown that the slowing down of the group velocity of the pulse is accompanied with a pulse chirp, which produces a pulse broadening in time. It is proposed to use two samples in sequence with opposite chirps (up chirp and down chirp or vice versa) compensating the pulse broadening. Then the pulse can be delayed with almost no losses, distortion, and broadening. However, there is a maximum distance, beyond which the pulse experiences corruption. Pumping with an auxiliary laser beam can control the delay time of the light pulse in the medium. Conditions to eliminate the contribution of the dephasing processes in the pulse propagation are considered.
Slow light formation in media with (i) an electromagnetically induced transparency, (ii) a doublet structure, and (iii) a single absorption line, detuned from resonance, is considered with the help of a simple model. The model is based on the description of particles in these media by a pseudospin 1/2, which is subject to two ‘orthogonal fields’. We mainly focus on the analysis of the reversible process of the particle excitation–de-excitation resulting in the temporal storage of the light-pulse energy without the pulse corruption. The influence of irreversible relaxation processes on the slow light formation is studied.
Single $\ensuremath{\gamma}$ photon propagation in a dense absorptive medium with two widely spaced resonances is experimentally studied. After an initial fast decay, a revival of the photon amplitude in the form of a bump, exceeding the probability amplitude of the incident photon, is observed. The irradiation time of this bump delays approximately by the lifetime of the excited nuclei in the absorber. This effect is explained by the interference of the incoming radiation with the collectively scattered radiation, the phase of which is modulated with the frequency of the doublet splitting. Initially, the destructive interference changes to a constructive one, distinguishing the storage and retrieval stages of the photon propagation in a dense medium, i.e., the collective absorption and collective re-emission processes.
Summary form only given. The recent observation of slow light propagation by Harris and Hau (1999) motivates the study of nonlinear optical processes under conditions where slow group velocity, or equivalently large nonlinearity, is the dominant feature of the problem. By combining electromagnetically induced transparency (EIT) and cold atom technology, a sharp resonance is created with a transmission linewidth much less than the natural linewidth of the atoms. A pulse of light propagating in this medium has a phase velocity of c and a group velocity that can be less than 10/sup -7/ c. As a light pulse of sufficiently wide bandwidth enters a cold-atom, EIT medium, the pulse shape, electric field strength, and power density remain nominally unchanged. Concurrently, because the group velocity is decreased, the stored energy per volume must increase. This increase occurs by spatial compression of the travelling pulse. A pulse measuring 300 m in free space compresses to 30 /spl mu/m within the cold-atom medium.
This thesis deals with the study of propagation effects experienced by weak light pulses as they propagate in atomic media driven by strong pulses. We explore both the ultra-short and long pulse regime and investigate the phenomena that arise at these different time scales. In the short pulse regime, a strongly driven two level atomic system presents transient light shifts, and non-adiabatic transitions occur between these adiabatic levels. We have studied a method to probe these light shifts in real time by propagating a weak probe through the medium. The light shifts enrich the spectrum of the probe and the probe gets shaped as a result. In this way a strongly driven two-level system can act as an active pulse shaper, and can introduce oscillations in the temporal profile of an ultra-short pulse at a time scale shorter than the pulse duration. We also show that by driving the system with two time delayed non-resonant strong fields, the non-adiabatic effects can be rendered phase dependent. This gives very sensitive phase control of the excited state population and can be used to formulate new techniques in interferometry. In the long pulse regime we present a new method of slowing light that can be realized in a double two-level system interacting with two orthogonally polarized light pulses that propagate along different axis. Spatio-temporal dependence of the total polarization induces a grating in the ground Zeeman coherence. The stronger of the two fields (the control field) is diffracted from this grating into the direction of the weak probe field compensating for the absorption of this latter field. A transparency window is thus created in the absorption spectrum of the probe leading to the slowing down of light. The transparency window exhibits characteristics similar to the one obtained by EIT (electromagnetic induced transparency) method. However, the important difference between our method and the traditional EIT method, is that ours doesn't rely on realizing dark state in the system. This may open the possibility of slowing down light in more complex atomic media. Moreover, when the linear absorptive response of the medium is cancelled in this manner, the nonlinear response becomes more important. In the situation where fields propagate in the same direction and have same frequency, two regimes have been investigated. For small optical depths, the effective susceptibility behaves as chi_lin exp[2 i phi] (with phi the phase difference between two fields). This renders phase control of the medium response, and the medium can be changed from an absorber to an amplifier, with normal or anomalous dispersion, by adjusting the relative phase phi. In the regime of large optical thickness, phase saturation takes place and the effective susceptibility turns into chi_lin*, changing an absorber into an amplifier without effecting the dispersive response (chi_lin is linear susceptibility).
We analyze the energy storage process of light propagating with slow group velocity in a sample where electromagnetically induced transparency (EIT) is created by a strong coupling field. We compare the formation of slow light in EIT and in self-induced transparency (SIT). For SIT, soliton-like propagation of light with essentially reduced group velocity takes place because of the temporary storage of an appreciable part of the pulse energy in the atoms. For EIT, no energy of the probe is stored in the atoms. This energy is transformed to the coupling field and leaves the sample with phase velocity c without absorption. Slow light is formed by a low frequency coherence induced at the input by the probe and coupling fields in a two-quantum excitation process. This coherence propagates as a “spin wave” with small group velocity, and at a large distance from the input, the coherence rules the process of the energy transformation from the coupling field to the probe, reproducing exactly the temporal profile of the probe at the input.