Dark solitons – regions of optical extinction against bright backgrounds – have been seen spontaneously forming in ring semiconductor lasers. Made by an international team of researchers, the observation could lead to improvements in molecular spectroscopy and integrated optoelectronics.
Frequency combs – pulsed lasers that output light with equally-spaced frequencies – are one of the most important achievements in the history of laser physics. Sometimes referred to as optical rulers, they are the basis of time and frequency standards and are used to define many fundamental quantities in science. However, traditional frequency comb lasers are bulky, complex and expensive and laser experts are keen on developing simpler versions that can be integrated in chips.
While undertaking one such attempt in 2020, researchers in Federico Capasso’s group at Harvard University discovered accidentally that, after initially entering a highly turbulent regime, a quantum cascade ring laser settled down to a stable frequency comb – albeit one with only nine teeth – in the mid-infrared “fingerprint” region widely used in molecular spectroscopy.
A ring laser has an optical cavity in which light is guided around a closed loop and a quantum cascade laser is a semiconductor device that emits infrared radiation.
Unexpected results
“All those interesting results came out from a control device – we were not expecting this to happen,” says Harvard’s Marco Piccardo. After months of head-scratching, the researchers worked out that the effect can be understood in terms of an instability in the non-linear differential equation that describes the system – the complex Ginzberg–Landau equation.
In the new work, Capasso and colleagues teamed up with researchers in Benedikt Schwarz’s group at Vienna University of Technology. The Austrian team had developed several designs for frequency combs based on quantum cascade lasers. The researchers integrated a waveguide coupler into the same chip. This makes it much easier to extract light and achieves greater output power. It also allows the scientists to tune the coupling losses, nudging the laser between its frequency-comb regime and the regime where it should operate as a continuous-wave laser that outputs radiation continuously.
In the “continuous wave” regime, however, something even stranger happens. Sometimes when the laser is switched on it behaves simply as a continuous-wave laser, but flicking the laser off and on may cause one or more dark solitons to appear randomly.
Solitons are non-linear, non-dispersive, self-reinforcing wave packets of radiation that can propagate through space indefinitely and pass through one another effectively unchanged. They were first observed in 1834 in water waves but have subsequently been seen in numerous other physical systems including optics.
Solitons in tiny gaps
The surprising thing about this latest observation is that the solitons appear as tiny gaps in the continuous laser light. This apparently small change in the laser emission makes a tremendous change to its frequency spectrum.
“When you talk about a continuous wave laser, it means that in the spectral domain you have a single monochromatic peak,” explains Piccardo. “This dip means the whole world…These two pictures are related by the uncertainty principle, so when you have something very, very narrow in space or time, that means that in the spectral domain you have many, many modes, and having many, many modes means you can do spectroscopy and look at molecules that emit over a very, very large spectral range.”
Dark solitons have occasionally been seen before, but never in a small, electrically-injected laser like this. Piccardo says that spectrally speaking, a dark soliton is as useful as a bright one. Some applications such as pump-probe spectroscopy require bright pulses, however. The techniques needed to produce bright solitons from dark ones will be the subject of further work. The researchers are also studying how to produce solitons deterministically.
Optical frequency comb fits in your back pocket
A crucial advantage of this comb design for integration is that, as light circulates in only one direction in the ring waveguide, the researchers believe the laser is inherently immune to the feedback that can disrupt many other lasers. It would therefore not require magnetic isolators, which are often impossible to integrate into silicon chips at commercial scale.
With integration in mind, the researchers want to extend the technique beyond quantum cascade lasers. “Despite the chip being really compact, quantum cascade lasers typically require high voltages to operate, so they’re not really a way to put the electronics on the chip,” says Piccardo. “If this could work in other lasers such as interband cascade lasers, then we could miniaturize the whole thing and it could really be battery operated.”
Laser physicist Peter Delfyett of the University Of Central Florida in Orlando believes the work holds promise for future work. “This dark pulse in the frequency domain is a bank of colours and, while their spectral purity is quite good, their exact positioning has not been achieved – yet,” he says. “However, the fact that they can do this – making solitons on chip with an electrically pumped device – that is in fact an extremely significant advance. Without a doubt.”
The research is described in Nature.
