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Negative dispersion without absorption in atomic ensembles

Rocco, Alessandra

Published Jan 1, 2001• 1 citation
Publisher

Abstract

Dott. Alessandra Rocco Negative dispersion without absorption in atomic ensembles In this work the optical properties of a calcium atomic beam, strongly driven on the transition, are investigated, in order to obtain a coherently prepared medium with the right amount of negative dispersion at a point of zero absorption. From the theory it is well known that a strongly driven Two-Level system exhibits such properties. Such a medium could in particular be used to fill an optical resonator in order to obtain a broadband high finesse cavity (White Light Cavity). For the investigations a spectroscopic method had been used, consisting of a Mach-Zehnder interferometer with a phase modulated arm. In this way it is possible to perform a high sensitive simultaneous measurement of both phase shift and absorption profiles in a calcium atomic beam, for an increasing driving field, which frequency is stabilized on the transition 4s2 S0 4s4p P1 at 423nm. For the first time both absorption and phase shift were systematic investigated as function of the driving field intensity, while it was developed a new geometrical configuration for the atomic oven which permitted to reach a higher particle density in the interaction zone. It was then possible to calculate the values of derivative of phase shift and absorption at the transition frequency, where the dispersion results anomalous, having fitted the measured curves to the phase shift and absorption profiles for a strongly driven Two-Level Atom. An analysis of the evolution of the both the quantities as driving field function could be then performed. The theoretical analysis of the strongly driven two level system could then be experimentally verified, especially that the absorption decreases stronger than dispersion with an increasing driving field. It was in fact measured a decreasing of a factor 53 in the dispersion, between the not driven and driven atomic system while the corresponding absorption decreased of a factor 330, for a value of the driving field of D 2π54.4MHz. The possibility of inserting the medium inside an optical resonator to obtain a broadband high finesse cavity has also been discussed. The data analysis showed that even if the λ compensation was reachable in the achieved condition, the residual absorption was still too high to realize a White Light Cavity. The configuration for further development of the system which should allow this realization was also analyzed, together with a comparison with the performances of different atomic systems.

DOI
10.15488/5905

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Dott. Alessandra Rocco Negative dispersion without absorption in atomic ensembles In this work the optical properties of a calcium atomic beam, strongly driven on the transition, are investigated, in order to obtain a coherently prepared medium with the right amount of negative dispersion at a point of zero absorption. From the theory it is well known that a strongly driven Two-Level system exhibits such properties. Such a medium could in particular be used to fill an optical resonator in order to obtain a broadband high finesse cavity (White Light Cavity). For the investigations a spectroscopic method had been used, consisting of a Mach-Zehnder interferometer with a phase modulated arm. In this way it is possible to perform a high sensitive simultaneous measurement of both phase shift and absorption profiles in a calcium atomic beam, for an increasing driving field, which frequency is stabilized on the transition 4s2 S0 4s4p P1 at 423nm. For the first time both absorption and phase shift were systematic investigated as function of the driving field intensity, while it was developed a new geometrical configuration for the atomic oven which permitted to reach a higher particle density in the interaction zone. It was then possible to calculate the values of derivative of phase shift and absorption at the transition frequency, where the dispersion results anomalous, having fitted the measured curves to the phase shift and absorption profiles for a strongly driven Two-Level Atom. An analysis of the evolution of the both the quantities as driving field function could be then performed. The theoretical analysis of the strongly driven two level system could then be experimentally verified, especially that the absorption decreases stronger than dispersion with an increasing driving field. It was in fact measured a decreasing of a factor 53 in the dispersion, between the not driven and driven atomic system while the corresponding absorption decreased of a factor 330, for a value of the driving field of D 2π54.4MHz. The possibility of inserting the medium inside an optical resonator to obtain a broadband high finesse cavity has also been discussed. The data analysis showed that even if the λ compensation was reachable in the achieved condition, the residual absorption was still too high to realize a White Light Cavity. The configuration for further development of the system which should allow this realization was also analyzed, together with a comparison with the performances of different atomic systems.
Summary form only given. We study the optical properties of negative dispersive but nevertheless transparent media. Such medium can be generated using atomic phase coherence. Theoretical studies have shown that strongly driven two level systems may exhibit the desired optical properties, that is negative dispersion of a well defined amount at a point of vanishing absorption. When this medium is placed inside an optical resonator the linewidth is increased without sacrificing its build up. We present the experimental setup which is used to realize a negative dispersive non-absorbing medium. To obtain a strongly driven two level system a calcium atomic beam is pumped at the transition 4s/sup 21/S/sub 0//spl rarr/4s4p/sup 1/P/sub 1/. The optical properties of the beam are studied.
This study compares the experimental results of absorption and refractive index of a probe laser measured in a strongly driven two-level atom with the results measured in a degenerate two-level system using pump-probe spectroscopy. This work demonstrates the possibility of realization an anomalous dispersive transparent medium using the strongly driven 4s21S0- 4s4p1P1Ca transition. Absorption and phase shift profiles are measured as function of the driving field intensity using a Ca atomic beam. With increasing driving field intensities absorption and phase shift decrease, but the absorption decreases stronger than the dispersion. Results show that the resonant absorption and the anomalous dispersion are reduced with respect to the non-driving case by a factor 490 and 47, respectively. This means that a ~ 10-fold enhancement of the amount of dispersion provided per amount of absorption is attained.
The absorption and dispersion properties of a driven two-level atomic system probed by a weak field have been measured simultaneously with a phase modulated Mach-Zehnder interferometer. We find that the resonant absorption and the negative dispersion are reduced with respect to the nondriving case by a factor of 490 and 47, respectively, which is in good agreement with the theory. This means a $\ensuremath{\sim}10$-fold enhancement of the amount of dispersion provided per amount of residual absorption. The experimental values are compared to the requirements that have to be fulfilled for the realization of a broadband, high finesse optical cavity. Finally, we propose a specific degenerate three-level system that overcomes some of the shortcomings of the driven two-level atom.
The dispersive and absorptive features of a closed transition in atomic cesium are investigated by means of a heterodyne interferometer as a function of the powers of probe and pump laser. Coherent population trapping (CPT) phenomena have raised the interest of the community in recent years. Possible applications of such effects are in the field of slow light propagation and, on the other side, superluminal effects. In particular, special interest has been devoted to the investigation of the refractive index of coherently prepared atoms. The goal of this investigation is the design of new optical media which show either negative or positive dispersion without absorption or with increased anomalous absorption. Such media could be useful ui building white light cavities, opto-optical modulators, and in quantum non-demolition (QND) measurements which are crucial to the development of the future generations gravitational wave detectors. Among these coherency processes we have been studying the phenomenon of electromagnetically induced absorption (EIA) (2) in a closed transition in cesiumatoms. The transition under analysis was the 'CS DZ line at 852 nm, in particular the hyperfine transition 6s zS~/2 F=4 + 6p 2P~/2 F'=5. The system was analyzed by means of a three beam heterodyne interferometer (l). This experimental setup shows high sensitivity to phase measurements and strongly reduced sensitivity to acoustical or vibrational noise. Using a third reference laser which does not interact with the mediumone can perform simultaneous measurements of the phase shift and the absorption of either the probe or the pump laser. The dispersion and the absorption of probe and coupling lasers have been measured as a funcion of the power of the coupling and probe lasers themselves. The power of the lasers are changed in the range of several decades. The halfwidth of the EL4 signal shows an increase with respect to the power of the probe laser while the phase shift of the probe laser decreases. The possibility to extend EIA measurements to a closed four level system in cesium is under analysis and is discussed here. Such possible developments of the experimental setup using a fourth laser or modulating the present reference laser enhance the capability to exploit simultaneous measurements of dispersion and absorption even in the four level system. This work was supported by SFB 407 of the Deutsche Forschungsgemeinschaft (German Research Society).
We describe a concept to realize a broadband high finesse optical cavity (white light cavity) using low-loss negative dispersive media. Two atomic systems are discussed, which allow transparency and λ-compensation, i.e. the dispersion of the medium compensates the frequency dependence of the vacuum wavelength. These atomic systems are (i) the driven two-level system and (ii) two closely spaced gain lines. We explain why the second scheme should provide better performance in terms of the realization of a white light cavity.