Single Device Can Function as Both a Laser and Anti-Laser

An integrated device that demonstrates lasing and anti-lasing at the same frequency in a single cavity has been developed using parity–time symmetry. The lasing and anti-lasing resonances that were demonstrated share common resonant features such as identical frequency dependence, coherent in-phase response, and fine spectral resolution. Lasing and anti-lasing in a single device could offer a novel path for enabling light modulation with high contrast approaching the ultimate limit.

Scanning electron microscope image of the single device capable of lasing and anti-lasing. Indium gallium arsenide phosphide (InGaAsP) material functions as the gain medium, while the chromium (Cr) and germanium (Ge) structures introduce the right amount of loss to satisfy the condition of parity-time symmetry that is required for lasing and anti-lasing. Courtesy of Zi Jing Wong/UC Berkeley.

To form the device, which measures 200 ‎µM long and 1.5 ‎µM wide, a research team at Lawrence Berkeley National Laboratory (Berkeley Lab) built 824 repeating pairs of gain and loss materials using nanofabrication technology. The gain medium was made out of indium gallium arsenide phosphide (InGaAsP), a material commonly used as an amplifier in optical communications. The loss medium was formed by pairing Chromium (Cr) with germanium (Ge). The team repeated this pattern to create a resonant system in which light bounces back and forth throughout the device to build up the magnitude of amplification (or absorption).

“In a single optical cavity we achieved both coherent light amplification and absorption at the same frequency, a counterintuitive phenomenon because these two states fundamentally contradict each other,” said senior faculty scientist Xiang Zhang. “This is important for high-speed modulation of light pulses in optical communication.”

In experiments, researchers directed two light beams of equal intensity into opposite ends of the device. They found that by tweaking the phase of one light source, they were able to control whether the light waves spent more time amplifying or absorbing materials.

Schematics show light input (green) entering opposite ends of a single device. When the phase of light input 1 is faster than that of input 2 (left panel), the gain medium dominates, resulting in coherent amplification of the light, or a lasing mode. When the phase of light input 1 is slower than input 2 (right panel), the loss medium dominates, leading to coherent absorption of the input light beams, or an anti-lasing mode. Courtesy of Zi Jing Wong/UC Berkeley.

Speeding up the phase of one light source resulted in an interference pattern favoring the gain medium and the emission of amplified coherent light, or a lasing mode. Conversely, slowing down the phase of one light source resulted in more time spent in the loss medium and the coherent absorption of the beams of light, or an anti-lasing mode.

If the phase of the two wavelengths was equal and the wavelengths entered the device at the same time, neither amplification nor absorption occurred, because the light occupied each region for an equal amount of time.

The magnitude of the gain and loss, the size of the building blocks, and the wavelength of the light moving through the device combine to create conditions of parity-time symmetry.

When the system is balanced and the gain and loss are equal, there is no net amplification or absorption of the light. But if conditions are perturbed such that the symmetry is broken, coherent amplification and absorption can be observed.

(From left) Berkeley researchers Xiang Zhang, Zi Jing Wong, Jeongmin Kim and Yuan Wang stand next to the optical setup they designed to demonstrate both lasing and anti-lasing in a single device. Courtesy of Marilyn Chung/Berkeley Lab.

"This work is the first demonstration of balanced gain and loss that strictly satisfies conditions of parity-time symmetry, leading to the realization of simultaneous lasing and anti-lasing," said professor Liang Feng at the University of Buffalo. "The successful attainment of both lasing and anti-lasing within a single integrated device is a significant step towards the ultimate light control limit."

The researchers targeted a wavelength of about 1,556 nm, which is within the band used for optical telecommunications. The device has the flexibility to operate as a laser, an amplifier, a modulator, and an absorber or detector.

“On-demand control of light from coherent absorption to coherent amplification was never imagined before, and it remains highly sought after in the scientific community,” said researcher Zi Jing Wong. “This device can potentially enable a very large contrast in modulation with no theoretical limits.”

The research was published in Nature Photonics (doi: 10.1038/nphoton.2016.216)