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arXiv:1109.2841 · Laser Physics Letters

Coherence enhanced transient lasing in XUV regime

P K Jha, Anatoly A. Svidzinsky, Marlan O. Scully

Published Apr 19, 2012• 9 citations
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Abstract

We report the effect of a coherent drive on transient lasing in three-level Λ and Ξ configurations ( c ↔ a ↔ b ). We show that the presence of a resonant coherent drive on the a ↔ c optical transition can yield an order of magnitude enhancement of the output laser energy on a a → b extreme ultraviolet (XUV) or X-ray transition than with no coherent drive. We demonstrate the crucial role of coherence ϱ ac for the laser power enhancement. Contrary to the forward direction (with respect to the pump laser), where forward gain can be enhanced for some choice of the drive Rabi frequency Ω c , coherent drive on the ac transition always suppresses the backward gain.

DOI
10.7452/lapl.201110144

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We report the effect of a coherent drive on transient lasing in three-level Λ and Ξ configurations ( c ↔ a ↔ b ). We show that the presence of a resonant coherent drive on the a ↔ c optical transition can yield an order of magnitude enhancement of the output laser energy on a a → b extreme ultraviolet (XUV) or X-ray transition than with no coherent drive. We demonstrate the crucial role of coherence ϱ ac for the laser power enhancement. Contrary to the forward direction (with respect to the pump laser), where forward gain can be enhanced for some choice of the drive Rabi frequency Ω c , coherent drive on the ac transition always suppresses the backward gain.
We report the effect of a coherent drive on transient lasing in three-level $\Lambda $ and Ξ\XiΞ configurations (c↔a↔bc\leftrightarrow a\leftrightarrow bc↔a↔b). We show that the presence of a resonant coherent drive on the a↔ca\leftrightarrow ca↔c optical transition can yield an order of magnitude enhancement of the output laser energy on a a→ba\rightarrow ba→b XUV or X-ray transition than with no coherent drive. We demonstrate the crucial role of coherence ϱac\varrho_{ac}ϱac​ for the laser power enhancement. Contrary to the forward direction (with respect to the pump), where forward gain can be enhanced for some choice of the drive Rabi frequency Ωc\Omega_{c}Ωc​, coherent drive on the acacac transition always suppresses the backward gain.
It has been known for two decades that coherence can yield lasing without inversion (LWI), which could be useful for making lasers at shorter wavelength. However, excitation of extreme ultraviolet and x-ray atomic transitions typically requires a plasma medium with rapid collisions, which destroy atomic coherence. Here we demonstrate LWI on a time scale shorter than the decoherence time. We show that in such a regime LWI is possible in a V -scheme with a strong coherent drive on the low-frequency transition and obtain an analytical expression for the gain of the laser pulse at high frequency. We propose an experiment in which such LWI can be realized in He plasma.
It was shown some time ago that when the excitation of an ensemble of two-level atoms is swept in the direction of lasing, so that atoms are prepared in the excited state just as the radiation from previously excited atoms reaches them, the resulting laser amplifier is “highly anomalous” and yields superradiant emission without population inversion. We here show that transient gain in a three-level system has common features with Dicke superradiance and can yield strong extreme ultraviolet lasing in, for example, He atoms (at 58 nm) or He-like ions such asB3+(at 6.1 nm).
A scheme for obtaining laser in the deep x-ray regime is proposed based on the principle of gain without inversion. A ladder scheme is proposed in Ar+8{\mathrm{Ar}}^{+8}Ar+8 system. In this system, conventional lasing with population inversion has been obtained in a neonlike fashion. This lasing field is proposed for the drive field on the higher transition of the ladder. A coherent field obtained in the high harmonics ultrashort laser provides the probe field to be amplified. This will be selected to best match the frequency needed for the transition from lower level of the present x-ray lasing to the ground state. Numerical calculations in the bare state picture exhibit possible gain at resonance, and at the Rabi side bands, for appropriate choice of system parameters, i.e., the strength of the driving field, the strength of the incoherent pump that provides the required energy needed for the inversionless amplification and the deexcitation rate. The latter rate is detrimental to lasing as it increases. It is quite interesting to note that the system exhibits surprisingly high stability for gain without inversion even for quite high rate of the incoherent deexcitation processes. Real and imaginary parts of the coherences were also obtained for detuned drive and probe fields. These provide insight into the role played by the quantum interference. The present theoretical calculations give high confidence in the quantum interference process as a candidate for realizing very short wavelength laser.
Quantum coherence and interference effects in atomic and molecular physics has been extensively studied due to intriguing counterintuitive physics and potential important applications. Here we present one such application of using quantum coherence to generate and enhance gain in extreme ultra-violet(XUV)(@58.4nm in Helium) and infra-red(@794.76nm in Rubidium) regime of electromagnetic radiation. We show that using moderate external coherent drive, a substantial enhancement in the energy of the lasing pulse can be achieved under optimal conditions. We also discuss the role of coherence. The present paper is intended to be pedagogical on this subject of coherence-enhanced lasing.
A scheme for obtaining a laser without inversion in the deep X-ray regime is proposed based on the principle of gain without inversion. A ladder scheme is proposed in the Ar 8+ system. In this system, conventional lasing with population inversion in a neonlike scheme is used. This lasing field is proposed for the drive field on the higher transition of the ladder. A coherent field obtained in the high harmonics ultrashort laser provides the probe field to be amplified. The ground level is incoherently pumped into the highest level, while the latter is incoherently depumped into the ground level, at a different adjustable rate. A detailed density matrix calculation of the ladder configuration is performed [3], and the conditions under which the system exhibits inversionless gain, as well as regular gain, are derived. Numerical calculations in the bare state picture exhibits gain without inversion both at resonance and at the generalized Rabi side bands. The probe lineshape is shown to be constructed of two symmetrical dispersionlike features each centered at the generalized Rabi side bands. This is evidence of the quantum interference occurring at these locations and the basis for the gain without inversion.