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Physical Review Letters

Cavity-Free Mode Control of Superfluorescence from Thermal Gas

H. Maeda, K. Kitano

Published May 11, 2026• 0 citations
Publisher

Abstract

Transverse-mode control of light has traditionally relied on optical cavities, whereas recent cavity-free approaches based on periodically arranged cold atoms that exploit collective radiation have attracted increasing attention. Here, we demonstrate a new cavity-free method applicable to thermal gases. Using crossed femtosecond laser pulses, we excite rubidium atoms and observe superfluorescence (SF) emitted during an ultrafast relaxation process. We demonstrate successful control of the transverse modes of SF when the interference fringes formed by the crossed beams are resonant with half the SF wavelength. This ultrafast, cavity-free approach provides a versatile playground for studying interactions between quantum many-body systems and well-defined electromagnetic fields in free space.

DOI
10.1103/cbxq-8n45

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Transverse-mode control of light has traditionally relied on optical cavities, whereas recent cavity-free approaches based on periodically arranged cold atoms that exploit collective radiation have attracted increasing attention. Here, we demonstrate a new cavity-free method applicable to thermal gases. Using crossed femtosecond laser pulses, we excite rubidium atoms and observe superfluorescence (SF) emitted during an ultrafast relaxation process. We demonstrate successful control of the transverse modes of SF when the interference fringes formed by the crossed beams are resonant with half the SF wavelength. This ultrafast, cavity-free approach provides a versatile playground for studying interactions between quantum many-body systems and well-defined electromagnetic fields in free space.
We study the superfluorescence (SF) from a gas of rubidium atoms. The atoms of a dense vapor are excited to the 5D5D5D state from the 5S5S5S state by a two-photon process driven by 100-fs laser pulses. The atoms decay to the 6P6P6P state and then to the 5S5S5S state. The SF emission at 420 nm on the 6P6P6P--5S5S5S transition is recorded by a streak camera with picosecond time resolution. The time duration of the generated SF is tens of picoseconds, which is much shorter than the time scale of the usual relaxation processes, including spontaneous emission and atomic coherence dephasing. The dependence of the time delay between the reference input pulse and SF is measured as a function of laser power. The experimental data are described quantitatively by a simulation based on the semiclassical atom-field interaction theory. The observed change in scaling laws for the peak intensity and delay time can be elucidated by an SF theory in which the sample length is larger than the cooperation length.
We investigated nonlinear optical processes driven by superfluorescent (SF) fields and their controllability with external laser fields. Cesium atoms were excited from the 6S6S6S ground state to the 8P8P8P state by a femtosecond laser pulse. The cascaded SF decay, $8P\ensuremath{\rightarrow}8S\ensuremath{\rightarrow}7P$, created coherence between the 7P7P7P and ground states, and led to the emission of a 455-nm pulse. A nanosecond laser pulse was applied to switch the $8S\ensuremath{\rightarrow}7P$ transition to the $8S\ensuremath{\rightarrow}6P$ one and newly generate an 852-nm pulse. At the same time, the nanosecond pulse suppressed the 852-nm emission through the inhibition of the development of the SF field between the 8P8P8P and 8S8S8S states, which led to the saturation of the 852-nm power with the increase of the nanosecond pulse power. The presented results offer an approach to investigate cooperative radiation processes in many-body systems and to develop an alternate light source.
We demonstrate that collective atomic interferences can be investigated by measuring the superfluorescence (SF) time delay. A pair of broadband (≈20 nm), ultrashort (≈80 fs), collinear pulses with a variable delay coherently excites rubidium (Rb) atoms. The generated superfluorescent pulses at 420 nm on the cascade transition are recorded by a picosecond streak camera. Both intensity and SF time delay of the 420 nm pulse are altered as the delay between input pulses varies. In particular, the SF time delay of the normalized 420 nm pulse exhibits oscillations with different periods. This can be understood in terms of atomic and quantum interferences due to two possible two-photon excitation pathways through the intermediate levels (Rb D-lines).
Superfluorescence (SF) originates from the spontaneous buildup of macroscopic coherence among initially incoherent emitters. Its formation is highly sensitive to dephasing and collective coupling in the system. Here, we observe room-temperature SF in CdS/CdSe/CdS spherical quantum wells. With increasing excitation fluence, the peak emission intensity shows a nearly quadratic increase, while the emission delay and pulse width decrease. Burnham-Chiao ringing is also observed, revealing the characteristic collective radiation dynamics of SF. Further experiments with stripe excitation show that shortening the excitation length L changes the dominant fast emission from amplified spontaneous emission to SF. More importantly, the threshold for SF under stripe excitation is only about 1/30 of that under spot excitation. This large reduction indicates that the propagating radiation field provided by spatially extended excitation favors the buildup of cooperative coherence. These findings provide direct experimental evidence for understanding ultrafast many-body coherence dynamics and offer a route to actively control collective emission at room temperature.
Extreme ultraviolet (XUV) light is a valuable frequency range for ultrafast optics experiments. The short wavelength is a requirement for ultrashort pulses, which are widely used to characterize fast dynamics. Additionally, the high photon energy enables quantum control of high frequency transitions in atoms. These transitions are of interest for ultrafast quantum control as the bandwidth of the control pulses needs to be less than the transition frequency. Presently, however, no good modulators exist for these frequencies, which significantly reduces the possible ultrafast optics experiments or applications in the XUV regime. This thesis addresses this problem and focuses on the control of XUV light in time and space. To enable control of XUV light a method is described which modulates the phase of XUV light emitted from a gas of noble atoms. The atoms are resonantly excited with a coherent XUV pulse generated through high order harmonic generation. After the excitation pulse has passed through the gas, the atoms will emit light in the absence of an external field, known as free induction. The phase of the emitted light is changed by shifting the resonance frequencies through the AC Stark shift with a non-resonant infrared control pulse. This phase-control of the emitting atoms also translates into a control of the phase of the emitted XUV field.With the method described in the previous paragraph, the direction of XUV light emitted from noble gases was controlled, and the temporal dynamics of the emission was studied. The redirection of the XUV emission, through the use of a spatially offset IR pulse, was found to change both with intensity of the control pulse and by changing the spatial offset between the pulses. By varying the delay between the excitation and the control pulse the time of redirection was controlled, which enabled high signal to noise study of the temporal dynamics of the emission. Furthermore, with two control pulses the emission was redirected twice, resulting in an XUV pulse shape with controlled duration emitted with a certain angle. Hopefully the presented work will result in more research into the development of this technique, to fully realize the possibility to shape the amplitude and phase of XUV pulses. Such a development would open the door for ultrafast XUV quantum control.
Collective coupling of multiple atoms with a cavity mode produces two normal modes that are separated in energy by Vacuum Rabi splitting. We show that quantum coherence and interference can be produced by a control laser that couples the atoms confined in the cavity mode from free space, which leads to suppression of the normal mode excitation, or polariton excitation of the cavity-atom system. The control laser splits the normal mode of the cavity-atoms system and opens two excitation channels. The destructive quantum interference between the two channels renders the cavity-atoms system opaque to the light coupled into the cavity mode. We demonstrate suppression of the normal mode (polariton) excitation by the destructive quantum interference in an experiment with cold Rb atoms confined in an optical cavity.