Scientists led by Xiang Zhang, a physicist with joint appointments at Lawrence Berkeley National Laboratory and UC Berkeley, have developed a microring laser cavity that can produce single-mode lasing from a conventional multi-mode laser cavity.
The ability to provide single-mode lasing on demand could benefit a wide range of applications including optical metrology and interferometry, optical data storage, high-resolution spectroscopy and optical communications.
‘Losses are typically undesirable in optics but, by deliberately exploiting the interplay between optical loss and gain based on the concept of parity-time symmetry, we have designed a microring laser cavity that exhibits intrinsic single-mode lasing regardless of the gain spectral bandwidth,’ said Zhang, who directs Berkeley Lab's Materials Sciences Division and is UC Berkeley's Ernest S. Kuh Endowed Chair Professor. ‘This approach also provides an experimental platform to study parity-time symmetry and phase transition phenomena that originated from quantum field theory yet have been inaccessible so far in experiments. It can fundamentally broaden optical science at both semi-classical and quantum levels.’
The work is described in a paper published in Science titled: ‘Single-Mode Laser by Parity-time Symmetry Breaking’.
A laser cavity or resonator is the mirrored component of a laser in which light reflected multiple times yields a standing wave at certain resonance frequencies called modes. Laser cavities typically support multiple modes because their dimensions are much larger than optical wavelengths. Competition between modes limits the optical gain in amplitude and results in random fluctuations and instabilities in the emitted laser beams.
‘For many applications, single-mode lasing is desirable for its stable operation, better beam quality, and easier manipulation,’ Zhang said. ‘Light emission from a single-mode laser is monochromatic with low phase and intensity noises, but creating sufficiently modulated optical gain and loss to obtain single-mode lasing has been a challenge.’
While mode manipulation and selection strategies have been developed to achieve single-mode lasing, each of these strategies has only been applicable to specific configurations. The microring laser cavity developed by Zhang's group is the first successful concept for a general design.
The key to their success is using the concept of the breaking of parity-time (PT) symmetry. The law of parity-time symmetry dictates that the properties of a system, like a beam of light, remain the same even if the system's spatial configuration is reversed, like a mirror image, or the direction of time runs backward.
Zhang and his group discovered a phenomenon called ‘thresholdless parity-time symmetry breaking’ that provides them with unprecedented control over the resonant modes of their microring laser cavity, a critical requirement for emission control in laser physics and applications.
The cavity consists of bilayered structures of chromium/germanium arranged periodically in the azimuthal direction on top of a microring resonator made from an indium-gallium-arsenide-phosphide compound on a substrate of indium phosphide. The diameter of the microring is 9µm.
In their Science paper, the researchers suggest that single-mode lasing through PT-symmetry breaking could pave the way to next generation optoelectronic devices for communications and computing as it enables the independent manipulation of multiple laser beams without the crosstalk problems that plague today's systems. Their microring laser cavity concept might also be used to engineer optical modes in a typical multi-mode laser cavity to create a desired lasing mode and emission pattern.
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