Laser and Anti-Laser in an Optical Cavity

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. (Source: Z. Jing Wong / UC Berkeley)

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. (Source: Z. Jing Wong, UCB)

Scientists at the Department of Energy’s Lawrence Berkeley National Laboratory have for the first time created a single device that acts as both a laser and an anti-laser, and they demonstrated these two opposite functions at a frequency within the tele­communi­cations band. Their findings lay the groundwork for developing a new type of integrated device with the flexi­bility to operate as a laser, an amplifier, a modulator, and an absorber or detector.

“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 principal inves­tigator Xiang Zhang, senior faculty scientist at Berkeley Lab’s Materials Sciences Division. “This is important for high-speed modu­lation of light pulses in optical communi­cation.”

The concept of anti-lasers, or coherent perfect absorber (CPA), emerged in recent years as something that reverses what a laser does. Instead of strongly amplifying a beam of light, an anti-laser can completely absorb incoming coherent light beams. While lasers are already ubiquitous in modern life, applications for anti-lasers – first demonstrated five years ago by Yale University researchers – are still being explored. Because anti-lasers can pick up weak coherent signals in the midst of a “noisy” inco­herent background, it could be used as an extremely sensitive chemical or biological detector.

A device that can incorporate both capabilities could become a valuable building block for the construction of photonic integrated circuits, the researchers said. “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 Zi Jing Wong, a post­doctoral researcher in Zhang’s lab. “This device can poten­tially enable a very large contrast in modulation with no theoretical limits.”

The researchers utilized sophis­ticated nano­fabrication techno­logy to build 824 repeating pairs of gain and loss materials to form the device, which measured 200 micrometers long and 1.5 micrometers wide. A single strand of human hair, by comparison, is about 100 micro­meters in diameter. The gain medium was made out of indium gallium arsenide phosphide, a well-known material used as an amplifier in optical communi­cations. Chromium paired with germanium formed the loss medium. Repeating the pattern created a resonant system in which light bounces back and forth throughout the device to build up the ampli­fication or absorption magnitude.

If one is to send light through such a gain-loss repeating system, an educated guess is that light will experience equal amounts of ampli­fication and absorp­tion, and the light will not change in intensity. However, this is not the case if the system satisfies conditions of parity-time symmetry, which is the key requirement in the device design.

In optics, the time-reversed counterpart of an amplifying gain medium is an absorbing loss medium. A system that returns to its original confi­guration upon performing both parity and time-reversal operations is said to fulfill the condition for parity-time symmetry.

Schematics above 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. (Source: Z. Jing Wong / UC Berkeley)

Schematics above 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. (Source: Z. Jing Wong / UC Berkeley)

Soon after the discovery of the anti-laser, scientists had predicted that a system exhi­biting parity-time symmetry could support both lasers and anti-lasers at the same frequency in the same space. In the device created by Zhang and his group, the magnitude of the gain and loss, the size of the building blocks, and the wave­length of the light moving through 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. In the experiments, two light beams of equal intensity were directed into opposite ends of the device. The researchers found that by tweaking the phase of one light source, they were able to control whether the light waves spent more time in amplifying or absorbing materials.

Speeding up the phase of one light source results in an inter­ference pattern favoring the gain medium and the emission of amplified coherent light, or a lasing mode. Slowing down the phase of one light source has the opposite effect, resulting 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 are equal and they enter the device at the same time, there is neither ampli­fication nor absorption because the light spends equal time in each region. The researchers targeted a wavelength of about 1,556 nanometers, which is within the band used for optical tele­communi­cations.

“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 Liang Feng, former post­doctoral researcher in Zhang’s Lab, and now an assistant professor of electrical engineering at the University at 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.” (Source: LBNL)

Reference: Z. Jing Wong et al.: Lasing and anti-lasing in a single cavity, Nat. Phot., online 

Link: Zhang Lab, NSF Nanoscale Science and Engineering Center, University of California, Berkeley, USA

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