Development of new method for controlling light propagation modes without a cavity

A research group including Lecturer Kenta Kitano of Ibaraki University and Professor Haruka Maeda of Aoyama Gakuin University has succeeded in developing a new method to control the propagation mode (directionality) of light without the use of a cavity, which was conventionally considered essential. This was achieved by controlling a collective emission phenomenon known as superfluorescence using a unique approach. Utilizing this method enables the ultrafast control of propagation modes. The findings were published in the journal Physical Review Letters.

Controlling the propagation mode of light is an indispensable technology for many optical devices, and cavities have typically been used to achieve this. For example, a laser oscillates by placing a gain medium inside a cavity and controlling stimulated emission through optical feedback. Furthermore, in cavity quantum electrodynamics, placing matter inside a specialized cavity allows for precise control over the interaction between quantum systems and single-mode electromagnetic waves.

On the other hand, cavities carry the experimental constraint of inherently possessing a finite size. To fundamentally resolve this issue, research aimed at controlling the propagation mode of light without using a cavity has advanced in recent years.

In the optical tweezers method, laser light is used to trap individual atoms one by one, controlling the spacing between them with a precision significantly higher than the wavelength of the light. Such an atomic ensemble behaves as a single light-emitting material. When the atomic spacing resonates with the radiation wavelength, high-intensity light pulses known as superfluorescence are emitted in specific directions. However, this technique can only be applied to ensembles of extremely slow atoms, known as cold atoms.

Superfluorescence is a collective emission phenomenon in which atoms interact with each other via the vacuum field, causing them to behave as a single emitter. This interaction occurs most efficiently when the atomic spacing corresponds to half the wavelength of the superfluorescence.

The research group adopted a completely different approach: instead of trapping atoms with a laser, they used light with a spatial periodic structure to position-selectively excite atoms.

They split a femtosecond laser pulse into two and crossed them to form interference fringes. By precisely controlling the intersection angle, they adjusted the spacing of the interference fringes with sub-nanometer precision, exciting only the atoms located at the antinodes of the fringes. In experiments using rubidium atoms in a heated glass cell, sweeping the intersection angle caused the intensity of the superfluorescence to surge sharply at a specific angle.

The interference fringe spacing calculated backwards from this angle matched the theoretical resonance condition. Furthermore, it was confirmed that under resonance conditions, the beam profile of the superfluorescence took on a single, distinct shape, whereas under non-resonance conditions, it exhibited significant fluctuations and became unclear.

Because this method is cavity-free and does not require cold atoms, it can be applied to a much broader range of materials. It has the potential to serve as an ideal platform for studying the interactions between quantum many-body systems and single-mode electromagnetic waves. Additionally, as many aspects of the quantum nature of superfluorescence remain unexplained, combining this method with cold atom technology and single-photon detection technology opens the door to new approaches.

In the future, this breakthrough is expected to lead to the development of quantum many-body optical devices.

Journal Information
Publication: Physical Review Letters
Title: Cavity-Free Mode Control of Superfluorescence from Thermal Gas
DOI: 10.1103/cbxq-8n45