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Non-classical light scattered by laser-pumped molecules possessing permanent dipoles

Alexandra Mîrzac, Viorel Ciornea, Mihai Macovei

Published Jun 15, 2018• 0 citations
Publisher

Abstract

Squeezing in resonance fluorescence processes of a laser-pumped molecule possessing permanent dipoles is shown. A weak low-frequency coherent field pumps the sample near resonance with the dynamically Stark-splitting states induced by a second stronger laser field driving the main two-level transition. The fluorescence spectrum consists of three coherently scattered spectral lines and up to nine incoherently scattered spectral lines. Compared with a similar problem yet in the absence of permanent dipoles, aditional squeezing regions are found.

DOI
10.5281/zenodo.4019166

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Squeezing in resonance fluorescence processes of a laser-pumped molecule possessing permanent dipoles is shown. A weak low-frequency coherent field pumps the sample near resonance with the dynamically Stark-splitting states induced by a second stronger laser field driving the main two-level transition. The fluorescence spectrum consists of three coherently scattered spectral lines and up to nine incoherently scattered spectral lines. Compared with a similar problem yet in the absence of permanent dipoles, aditional squeezing regions are found.
Bare-state population inversion is demonstrated in a two-level system with all dipole matrix elements nonzero. A laser field is resonantly driving the sample whereas a second weaker and lower frequency coherent field additionally pumps it near resonance with the dynamically-Stark-splitted states. Due to existence of differing permanent dipole moments in the excited and ground bare states, quantum coherences among the involved dressed-states are induced leading to inversion in the steady-state. Furthermore, large refractive indices are feasible as well as the determination of the diagonal matrix elements via the absorption or emission spectra. The results apply to available biomolecular, spin or asymmetric quantum dot systems.
Bare-state population inversion is demonstrated in a two-level system with all dipole matrix elements nonzero. A laser field is resonantly driving the sample, whereas a second weaker and lower frequency coherent field additionally pumps it near resonance with the dynamically Stark-splitted states. Due to the existence of differing permanent dipole moments in the excited and ground bare states, quantum coherences among the involved dressed states are induced, leading to inversion in the steady state. Furthermore, large refractive indices are feasible, as well as the determination of the diagonal matrix elements via the absorption or emission spectra. The results apply to available biomolecular, spin, or asymmetric quantum dot systems.
We investigate the resonance fluorescence features of a small ensemble of closely packed and moderately laser-pumped two-level emitters at resonance. The mean distance between any two-level radiators is smaller than the corresponding emission wavelength, such that the dipole-dipole interactions are not negligible. We have found that under the secular approximation, the collective resonance fluorescence spectrum consists of 2 N + 1 spectral lines, where N is the number of emitters from the sample. The 2 N spectral sidebands, symmetrically located around the generalized Rabi frequency with respect to the central line at the laser frequency, are distinguishable if the dipole-dipole coupling strength is larger than the collective spontaneous decay rate. This way, one can estimate the radiators number within the ensemble via measuring of the spontaneously scattered collective resonance fluorescence spectrum. Contrarily, if the dipole-dipole coupling is of the order of or smaller than the cooperative spontaneous decay rate, but still non-negligible, the spectrum turns into a Mollow-like fluorescence spectrum, where the two sidebands' spectral lines broaden proportional to the dipole-dipole coupling strength. Published by the American Physical Society 2024
The phenomenon of resonance fluorescence provides an interesting manifestation of the quantum theory of light and is a “real world” application of the material of the Chapters 8 and 9. In this process, a two-level atom is typically driven by a resonant continuous-wave laser field and the spectral and quantum statistical properties of the fluorescent light emitted by the atom are measured. Experimentally this can be achieved by scattering a laser off a collimated atomic beam such that the directions of the laser beam, atomic beam, and detector axis are mutually perpendicular as shown in Fig. 10.1. If the driving field is monochromatic, then at low excitation intensity the atom absorbs a photon at the excitation frequency and reemits it at the same frequency as a consequence of conservation of energy. The spectral width of the fluorescent light is therefore very narrow. The situation, however, is considerably more complicated when the excitation intensity increases and the Rabi frequency associated with the driving field becomes comparable to, or larger than, the atomic linewidth. At such intensity levels, the Rabi oscillations show up as a modulation of the quantum dipole moment and sidebands start emerging in the spectrum of the emitted radiation. This so-called dynamic Stark splitting is an interesting feature of the atom–field interaction. In addition to that, the fluorescent light exhibits certain nonclassical properties including photon antibunching and squeezing. In this chapter, we develop a theory of resonance fluorescence to explain these phenomena.
Abstract The squeezing spectrum of the fluorescence field emitted from a four-level atom in J = 1/2 to J = 1/2 configuration driven by two coherent fields is studied. We find that the squeezing properties of the fluorescence radiation are significantly influenced by the presence of vacuum-induced coherence in the atomic system. It is shown that such coherence induces spectral squeezing in phase quadratures of the fluorescence light for both weak and strong driving fields. The dependence of the squeezing spectrum on the relative phase of the driving fields is also investigated. Effects such as enhancement or suppression of the squeezing peaks are shown in the spectrum as the relative phase is varied. An analytical explanation of the numerical findings is presented using dressed-states of the atom-field system.
The resonance fluorescence of a two-level atom driven by a coherent laser field and damped by a finite bandwidth squeezed vacuum is analysed. We extend the Yeoman and Barnett technique to a non-zero detuning of the driving field from the atomic resonance and discuss the role of squeezing bandwidth and the detuning in the level shifts, widths and intensities of the spectral lines. The approach is valid for arbitrary values of the Rabi frequency and detuning but for the squeezing bandwidths larger than the natural line-width in order to satisfy the Markoff approximation. The narrowing of the spectral lines is interpreted in terms of the quadrature-noise spectrum. We find that, depending on the Rabi frequency, detuning and the squeezing phase, different factors contribute to the line narrowing. For a strong resonant driving field there is no squeezing in the emitted field and the fluorescence spectrum exactly reveals the noise spectrum. In this case the narrowing of the spectral lines arises from the noise reduction in the input squeezed vacuum. For a weak or detuned driving field the fluorescence exhibits a large squeezing and, as a consequence, the spectral lines have narrowed linewidths. Moreover, the fluorescence spectrum can be asymmetric about the central frequency despite the symmetrical distribution of the noise. The asymmetry arises from the absorption of photons by the squeezed vacuum which reduces the spontaneous emission. For an appropriate choice of the detuning some of the spectral lines can vanish despite that there is no population trapping. Again this process can be interpreted as arising from the absorption of photons by the squeezed vacuum. When the absorption is large it may compensate the spontaneous emission resulting in the vanishing of the fluorescence lines.