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Physical Review A

Nonlinear optical processes driven by superfluorescent fields and their controllability with external laser fields

Kenta Kitano, Masataka Satô, Yuki Komasaka, H. Maeda

Published Sep 11, 2020• 5 citations
Publisher

Abstract

We investigated nonlinear optical processes driven by superfluorescent (SF) fields and their controllability with external laser fields. Cesium atoms were excited from the $6S$ ground state to the $8P$ state by a femtosecond laser pulse. The cascaded SF decay, $8P\ensuremath{\rightarrow}8S\ensuremath{\rightarrow}7P$, created coherence between the $7P$ 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 $8P$ and $8S$ 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.

DOI
10.1103/physreva.102.031101

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We investigated the polarization properties of superfluorescence (SF) emitted from dense cesium atomic vapor in a cell. The atoms were excited from the 6 S ground to the 8 P state using a femtosecond laser pulse. The SF fields generated on the cascaded decay, 8 P → 8 S → 7 P , mediated the nonlinear optical process. We observed 4.2-µm and 456-nm forward directional emissions generated on the 8 S → 7 P and 7 P → 6 S transitions, respectively. The polarizations of the two fields were correlated in each laser shot, and their directions fluctuated from shot to shot, reflecting the noise that initiated the 4.2-µm emission.
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.
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.
Superfluorescence (SF) is collective spontaneous emission wherein radiators spontaneously synchronize, resulting in an intense single-pulse emission. The avalanche radiation of photons is initiated by the first photon emitted into the SF propagation mode. Because this process is stochastic, the absolute phase of the SF changes randomly from shot to shot. We demonstrate that this phase can be controlled by seeding the SF with a resonant continuous-wave (CW) laser. The seed light was weak enough not to cause the stimulated emission but strong enough to inject the first photon into the SF propagation mode prior to injection by the radiators themselves. Cross-correlation measurements between the seeded SF and CW laser revealed that the seed light was coherently amplified by the SF. The amplification factor for the instantaneous intensity was estimated to be 7 orders of magnitude. These results will pave the way for the development of new types of quantum optical amplifiers.
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).
We investigated the superfluorescent decay process of dense rubidium atomic vapor in a cell. Using a femtosecond laser pulse, the atoms were excited from the 5 S ground state to the 6 P state. The 2.73 μ m and 1.37 μ m fields were generated on the cascaded decay, 6 P → 6 S → 5 P , which further stimulated the 780 nm forward emission on the 5 P → 5 S transition. Using sum frequency generation (SFG) spectroscopy, we observed all emission fields and the time delay between them, with sufficient temporal resolution. The experimental results were successfully reproduced using semiclassical simulations.