  Parity-time (PT) symmetry is a fundamental notion in quantum field theories.
It has opened a new paradigm for non-Hermitian Hamiltonians ranging from
quantum mechanics, electronics, to optics. In the realm of optics, optical loss
is responsible for power dissipation, therefore typically degrading device
performance such as attenuation of a laser beam. By carefully exploiting
optical loss in the complex dielectric permittivity, however, recent
exploration of PT symmetry revolutionizes our understandings in fundamental
physics and intriguing optical phenomena such as exceptional points and phase
transition that are critical for high-speed optical modulators. The interplay
between optical gain and loss in photonic PT synthetic matters offers a new
criterion of positively utilizing loss to efficiently manipulate gain and its
associated optical properties. Instead of simply compensating optical loss in
conventional lasers, for example, it is theoretically proposed that judiciously
designed delicate modulation of optical loss and gain can lead to PT synthetic
lasing that fundamentally broadens laser physics. Here, we report the first
experimental demonstration of PT synthetic lasers. By carefully exploiting the
interplay between gain and loss, we achieve degenerate eigen modes at the same
frequency but with complex conjugate gain and loss coefficients. In contrast to
conventional ring cavity lasers with multiple modes, the PT synthetic
micro-ring laser exhibits an intrinsic single mode lasing: the non-threshold PT
broken phase inherently associated in such a photonic system squeezes broadband
optical gain into a single lasing mode regardless of the gain spectral
bandwidth. This chip-scale semiconductor platform provides a unique route
towards fundamental explorations of PT physics and next generation of
optoelectronic devices for optical communications and computing.

Optics