Microlaser Achieves Optical Gain Via Spin-Helix Lasing

A microlaser that is tunable in the range of 40 nm has been developed by researchers at the University of Warsaw, the Military University of Technology in Warsaw, and the University of Southampton. The broadly tunable laser emits two circularly polarized beams that are directed at different angles, due to a persistent spin helix on the surface of the laser’s microcavity.

The researchers introduced optical gain into this system by dispersing a molecular dye in a liquid crystal microcavity and demonstrated an optically persistent spin helix lasing in the Rashba-Dresselhaus regime. The dispersion of photons in microstructured optical systems could enable new applications in optoelectronics, ranging from information transfer and processing to quantum optics.

Obtained tunable microlaser emitting two beams. The beams are circularly polarized and directed at different angles. Courtesy of Mateusz Krol/Faculty of Physics, University of Warsaw.

The researchers filled the integrated photonic microcavity of the laser with a liquid crystal with high birefringence, and embedded a high-efficiency, organic semiconductor dye in the liquid crystal.

To construct the microcavity, the researchers placed two distributed Bragg reflectors between 2 and 3 µm apart. A standing electromagnetic wave formed inside the cavity. The space between the mirrors was filled with liquid crystal, which received a special mirror coating.

“The characteristic feature of liquid crystals is their elongated molecules,” researcher Marcin Muszynski said. “Figuratively speaking, they were ‘combed’ on the surface of the mirrors and could stand up under the influence of an external electric field.”

The researchers arranged the liquid crystal molecules inside the microcavity to produce two linearly polarized light modes in the cavity — that is, two standing waves of light with opposite linear polarizations.

Because the liquid crystal medium is birefringent, it can be characterized by two refractive indexes, depending on the direction of the electric field oscillations. The refractive index of the liquid crystal in the microlaser depended on the polarization of the electromagnetic wave inside the microcavity. The light in the laser microcavity interacted with the liquid crystal molecules differently, based on whether the electric field of the propagating wave oscillated along the molecules or perpendicular to them.

When the researchers optically stimulated the dye embedded in the liquid crystal molecules, they obtained a lasing effect — a coherent light radiation with a strictly defined energy. The gradual rotation of the liquid crystal molecules led to an unexpected result; that is, the laser emitted one linearly polarized beam perpendicular to the surface of the mirrors. One of the modes did not change its energy as the molecules rotated, while the energy of the other mode increased as the orientation of the molecules changed. The use of liquid crystals enabled smooth tuning of the wavelength over a range up to 40 nm by manipulating the electric field.

“However, when we rotated the liquid crystal molecules so that the energy of both modes — the one sensitive to the orientation of the molecules and the one that did not change its energy — overlapped (that is, they were in resonance), the light emitted from the cavity suddenly changed its polarization from linear to two circular, right- and left-handed, with both circular polarities propagating in different directions, at an angle of several degrees,” professor Jacek Szczytko said.

The horizontally polarized laser emission is repeatable and can be triggered by single pulses of a pump laser, the researchers said. The laser action takes place from the bottom of two oppositely polarized valleys shifted apart in reciprocal space. The researchers measured emissions in real space that showed the persistent spin helix lasing.

The researchers confirmed the phase coherence of the laser. “The so-called persistent spin helix — pattern of stripes with different polarizations of light, spaced 3 μm apart — appeared on the surface of the sample,” Muszynski said. “Theoretical calculations show that such a pattern can be formed when two oppositely polarized beams are phase coherent, and both modes of light are inseparable. This phenomenon is comparable to quantum entanglement.”

Currently, the tunable laser works in pulses, because the organic dye slowly photodegrades when exposed to intense light. The researchers believe that replacing the organic emitter with an emitter made from more durable polymers or inorganic materials (for example, perovskites) could allow for a longer lifetime.

The researchers believe that the proposed platform for microlasing could be used in quantum communications, in which information is encoded through light polarization.

“The obtained, precisely tunable laser can be used in many fields of physics, chemistry, medicine, and communication,” professor Barbara Pietka said. “We use nonlinear phenomena to create a fully optical neuromorphic network. This new photonic architecture can provide a powerful machine learning tool for solving complex classification and inference problems, and for processing large amounts of information with increasing speed and energy efficiency.”

The research was published in Physical Review Applied (www.doi.org/10.1103/PhysRevApplied.17.014041).