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AIP conference proceedings

Doppler shift effect in storage of terahertz frequency signals in Rydberg atoms

Jayanta Bera

Published Jan 1, 2020• 1 citation
Publisher

Abstract

Motivated from a recent theoretical work [1] on efficient storage of terahertz frequency signals in ultracold Rydberg atoms, I have studied the above mentioned storage process in the case when Doppler effect is present. It is present when the atoms are moving with a velocity which is coming from the thermal effect. Here, I have shown that this effect has made a prominent change in susceptibility and relative difference of the slope of dispersion which is the key factor of group index and group velocity [2].

DOI
10.1063/5.0001251

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Motivated from a recent theoretical work [1] on efficient storage of terahertz frequency signals in ultracold Rydberg atoms, I have studied the above mentioned storage process in the case when Doppler effect is present. It is present when the atoms are moving with a velocity which is coming from the thermal effect. Here, I have shown that this effect has made a prominent change in susceptibility and relative difference of the slope of dispersion which is the key factor of group index and group velocity [2].
It is shown that the dispersion dependence of the rotation of the plane of polarisation of weak radiation passing through a three-level atomic medium may be free of Doppler broadening, on the account of a velocity-dependent Stark-shift which is created by saturating the wave on an adjacent transition. In the case of two-photon resonance the use of opposing light beams with different frequencies gives the possibility for an essential increase of the signal on account of the resonance gain on an intermediate transition, without additional Doppler broadening.
In Chaps. 8 and 9, we considered a laser medium consisting of homo­ geneously broadened, stationary atoms. When the lasing atoms move as in a gas, they see an electric field with shifted frequency due to the Doppler effect, as shown in Fig. 10-1. One might argue that it is merely necessary to average the complex polarization or the susceptibility Xn over the frequency range corresponding to the velocity distribution, that is, calculate a new ^ n , for which Here the frequency co = coo + Kv, where coo is the frequency at atomic line (1) center, v is the component of velocity along the laser (z) axis, and the frequen­ cy distribution is determined by a Maxwell-Boltzmann velocity distribution W(y) = ( w)-1 exp [ - (v/w)2]. (2) Here u is the most probable speed of the atom, and K is the wave number. The simple recipe (1) is, in fact, valid for an inhomogeneously broadened mediumt consisting of stationary atoms such as ruby at low temperatures. However, in the standing-wave laser, the atoms not only see Doppler-shifted frequencies, but also move through the standing-wave electric field, effectively seeing an amplitude-modulated field. Equivalently, each atom sees two frequencies, for the standing-wave field is the sum of oppositely directed running waves, one of which appears to be Doppler upshifted, one down­ shifted. Hence it is necessary to consider both the frequency shift and the time dependence of the atomic z coordinate when calculating the polarization of a gaseous medium.
Abstract We have theoretically studied the multi-window electromagnetically induced transparency (EIT) and the related dispersive signals in multi-level V-type systems which are formed by the co-propagating circularly polarized pump ( σ − ) and probe ( σ + ) laser fields in the Zeeman sublevels of hyperfine states of 87 Rb-D 2 line. The optical Bloch equations (OBEs) are derived and numerically solved under the steady-state condition to obtain the probe absorption and dispersion spectra. We present the study of a single-window EIT for the three-level, double-window EIT for the four-level and triple-window EIT for the five-level V-type systems under both Doppler-free and Doppler-broadened conditions. The dispersive signals corresponding to the multi EIT windows for each system are also observed separately. From the knowledge of the slopes of the probe dispersion signals, we have calculated group index ( n g ) of the medium considering three different V-type schemes. The effect of temperature on the EIT, dispersion and group index profiles is also studied in detail. It is found that the amplitudes of EIT and dispersion signals and also the positive and negative values of n g are decreased as the temperature of the medium increases. To understand the Doppler-broadening effect on the group index profiles, we have considered three different velocity groups of atoms in the calculation of group index profiles. Moreover, the effects of probe intensity variation on the Doppler-broadened EIT and dispersive signals are studied extensively, and the conversions of EIT to EIA and dispersive positive slope to negative slope are observed under strong probe regime. Also, we have investigated the variation of dispersive slope and group index with the variation of probe intensity and probe frequency, respectively, at the multi-frequency position of the multi-window EIT. Finally, the switching between subluminal (slow light) to superluminal (fast light) propagation of light with the variation of both probe intensity and probe frequency is discussed in detail.
Abstract We propose monolayer graphene system and analyze the effects of giant Kerr nonlinearity and Doppler broadening. The focus is to construct and design experimentally viable model by which we can enhance the group index of the medium so that we have slow group velocity. For obtaining the desired goal, we consider Landau level of graphene monolayer placed perpendicular to magnetic field M Tokman et al (2013, Phys. Rev. lett. 110, 077404). Giant Kerr nonlinearity and Doppler broadening enhance the medium group index greatly, resulting in a slow group velocity that has uses in storage devices, optical buffering, quantum memory, and data processing. We also investigate the behavior exhibited by the weak probe’s transmission spectrum, which can change when the terahertz signal frequency is turned on and off. Advantages of under consideration model are its a 2D material whereas other materials are semiconductor or gaseous state. Also, Landau level of graphene has domains tuneable in tera hertz and infra-red region while we cannot tune the semiconductor and atomic media once the transition is chosen. Therefore, the proposed magnetized graphene system offers a tremendous opportunity for incorporating quantum interference into the development of optical solid-state devices.