Scientists in the field of optical science have successfully brought together opposing forces, despite the many challenges this brings.
As a maiden effort, researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a single device that can function not only as a laser but also as an anti-laser. They even demonstrated the two opposing functions at a frequency within the telecommunications band.
The outcomes of their work were recently published in the Nature Photonics journal, and form the preparatory work for creating a new kind of integrated device that has the workability in operating as a laser, a modulator, an amplifier, and an absorber or detector.
Reversing the laser
In the recent past, the idea of anti-lasers, or coherent perfect absorber (CPA), has become evident as something that operates to reverse the work of a laser. In contrast to strong amplification of a light beam, an anti-laser entirely absorbs the coherent incoming light beams.
Although lasers are already omnipresent in present-day life, applications that involve anti-lasers, which were demonstrated by Yale University scientists as a maiden attempt five years ago, are yet underexplored. The ability of an anti-laser to detect even weak coherent signals amidst a “noisy” incoherent background enables its usage as a highly sensitive biological or chemical detector.
The scientists expressed that a device incorporating both these potentials can be a valuable building block developing photonic integrated circuits.
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. This device can potentially enable a very large contrast in modulation with no theoretical limits.
Zi Jing Wong, study lead author, a postdoctoral researcher in Zhang’s lab
Sophisticated nanofabrication technology was employed by the scientists to construct 824 repeating pairs of loss and gain materials to develop the device, which had a width of 1.5µm and a length of 200µm. For the purpose of comparison, the diameter of a single strand of human hair is approximately 100µm.
Chromium fused with germanium was used to form the loss medium. Indium gallium arsenide phosphide, a familiar material used in optical communications as an amplifier, was used to make the gain medium. The pattern was repeated to produce a resonant system in which light bounces back and forth through whole of the device to intensify the absorption or amplification magnitude.
When light is passed via such a gain-loss repeating system, one can expect that the light undergoes equal amounts of amplification and absorption, thus retaining the original light intensity. On the contrary, if parity-time symmetry conditions are satisfied by the system, this property does not hold good, which is one of the essential capacities of the device design.
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 This is important for high-speed modulation of light pulses in optical communication.
Xiang Zhang, study principal investigator and senior faculty scientist at Berkeley Lab’s Materials Sciences Division
Balance and symmetry
Parity-time symmetry is an idea derived from quantum mechanics. Positions are flipped in a parity operation, that is, the right hand becomes the left hand, and vice versa.
To this the time-reversal operation, which is similar to watching the action backward by rewinding a video, is added. For instance, the time-reversed action of inflating a balloon will be deflating of the same balloon. Similarly, the time-reversed equivalent of an amplifying gain medium in optics is an absorbing loss medium.
The conditions for parity-time symmetry are said to be met by a system if it returns to its original configuration when both time-reversal and parity operations are performed.
Immediately following the discovery of anti-laser, researchers had proposed that a system which demonstrates parity-time symmetry can support both anti-lasers and lasers in the same space at the same frequency. In the device developed by Zhang and his team, the size of the building blocks, the loss and gain magnitudes, and the wavelength of the light merge to form the parity-time symmetry conditions.
There is no net absorption or amplification of light if the loss and gain are equal and the system is balanced. However, upon perturbation of conditions, that is when the symmetry is disturbed, coherent absorption and amplification can be observed.
During the experiments, the researchers directed two light beams with equal intensity into opposite ends of the device and observed that when the phase of one light source was tweaked, the amount of time spent by the light waves in absorbing or amplifying materials could be controlled.
When the phase of one light source was accelerated, an interference pattern that favors the gain medium and results in amplified coherent light emission (i.e. a lasing mode) occurred. The opposite effect was noted when the phase of one light source was slowed down, consequently resulting in an anti-lasing mode in which more time is spent in the loss medium and the coherent absorption of the light beams occur.
Equal phase of both the wavelengths causes the sources to enter the device at the same instance, thus resulting in neither absorption nor amplification as the light spends the same amount of time in each region.
The researchers proposed a wavelength of about 1556nm, which is well within the optical telecommunications band.
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. The successful attainment of both lasing and anti-lasing within a single integrated device is a significant step towards the ultimate light control limit.
Liang Feng, study co-author and assistant professor of electrical engineering at the University at Buffalo
The DOE Office of Science primarily funded the study which made use of the Molecular Foundry, a DOE Office of Science User Facility at Berkeley Lab.