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Optics Express

Spontaneous feedback for the simultaneous narrowing and elevation of fluorescence spectral lines

Chen Yang Huang, Xiangming Hu, Qingping Hu

Published Feb 14, 2018• 1 citation
Publisher

Abstract

Narrow linewidth and high intensity of the fluorescence spectra are two important elements to improve the accuracy and efficiency of related practical measurements, but so far they have not been achievable simultaneously. We propose a new approach to sharpen the linewidth and to heighten the intensity simultaneously. Rather than double coherent mechanisms, it uses the spontaneous emission of inverted atoms in a cavity below the threshold as a resource for feedback control. The spontaneous feedback is simpler in principle and easier to realize technologically, and represents a kind of new way of controlling spontaneous emission.

DOI
10.1364/oe.26.004807

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Narrow linewidth and high intensity of the fluorescence spectra are two important elements to improve the accuracy and efficiency of related practical measurements, but so far they have not been achievable simultaneously. We propose a new approach to sharpen the linewidth and to heighten the intensity simultaneously. Rather than double coherent mechanisms, it uses the spontaneous emission of inverted atoms in a cavity below the threshold as a resource for feedback control. The spontaneous feedback is simpler in principle and easier to realize technologically, and represents a kind of new way of controlling spontaneous emission.
We show that it is possible at least in principle to have unbounded line narrowing in the spontaneous emission spectrum simultaneously with control of the corresponding intensity. For this purpose we employ a laser driven doublet of excited atomic, molecular, or semiconductor states with almost parallel dipole orientation, which is decaying to a ground state. Incoherent pumping is applied to control the population and thus the spontaneous emission intensity of the dressed state associated with the ultranarrow spectral line.
We calculate the spontaneous-emission spectrum and the spectrum of weakly driven fluorescence for a two-level atom coupled to a resonant-cavity mode. For strong atom-cavity coupling the spectra split into two peaks that can have subnatural linewidths. If the cavity linewidth is negligible, the spontaneous-emission spectrum has half the radiative linewidth of the atom; the spectrum of weakly driven fluorescence shows an additional 36% squeezing-induced narrowing. These effects can be observed using coupled-field and collective-polarization oscillators excited in a cavity containing N two-level atoms.
The interaction of a four-level atom consisting of one excited state, two upper levels and one ground state is considered. We use one field to drive the transition between the excited state and the ground state, and simultaneously apply another field to couple two transitions from the upper-level doublet to the ground state. We investigate how to control the central linewidth of resonance fluorescence spectrum of the driven transition via quantum interference between two transitions from the upper doublet to the ground state of the atom. In the case of orthogonal dipole moments for the upper doublet and the ground state the widths of the central line and the sidebands of the resonance fluorescence spectrum are totally controlled by the spontaneous emission rate of the upper doublet when the applying field is sufficiently strong. When the dipole moments are parallel, the central linewidth can be much narrower than the linewidths of spontaneous emission of the excited state and of the upper doublet due to the quantum interference. The smaller the upper doublet splitting, the narrower the central linewidth. We also notice that when the upper levels are degenerate the resonance fluorescence spectrum may depend on the initial condition because of the quantum interference.
For a three-level atom, two nondegenerate (even microwave and optical) electric dipole transitions are usually allowed; for either of these, the fluorescence spectra are well-described in terms of spontaneous transitions from a triplet of dressed sublevels to an adjacent lower-lying triplet. When the three dressed sublevels are equally spaced from each other, a remarkable feature known as degenerate cascade fluorescence takes place, which displays a five-peaked structure. We show that a single cavity can make all the spectral lines extremely narrow, whether they arise from cavity-coupled or cavity-free transitions. This effect is based on intrinsic cascade lasing feedback and makes it possible to use a single microwave cavity (even a bad cavity) to narrow the spectral lines in the optical frequency regime.
We investigate theoretically the features of the spontaneous emission spectrum of a four-level atomic system driven by a coherent probe field,a coupling laser field and a microwave field.The two upper levels are coupled by a microwave field with lower frequency.The time evolution of the populations is calculated.It is shown that the spectra behavior,such as spectral-line narrowing,spectral-line enhancement,spectral-line suppression,and total cancellation of fluorescence quenching,can be easily controlled by changing the phase and the amplitude of the microwave field.
The spontaneous emission property of a five-level atom with a loop structure driven by three optical fields and a microwave field is intensively studied. Several quantum interference effects can be observed in the spontaneous emission spectra, such as spectral-line narrowing, spectral-line enhancement, spectral-line suppression, partial cancellation and fluorescence quenching. The number of emission peaks, the positions of dark lines can be controlled by varying the intensities of driving fields or the phase of the microwave field coupling the two lower levels in the atomic system. Furthermore, since no rigorous conditions are required, the expected phenomena may easily be achieved experimentally in atomic systems.