New Insight into How Semiconductor Lasers Work

The ultrafast detection capabilities of terahertz technology are used to watch laser emissions evolve from multiple colours to a single wavelength over less than a billionth of a second. (Source: U. Leeds)

Pioneering engineers working with terahertz frequency tech­nology have been researching how indi­vidual fre­quencies are selected when a laser is turned on, and how quickly the selection is made. The development of specific terahertz equipment has allowed them to inves­tigate this process for the first time. Their results will underpin the future development of semi­conductor lasers, including those used in public and private sector-owned telecommu­nications systems.

For many years, it has been predicted that operating fre­quencies within semi­conductor lasers stabilise on a timescale of a few nano­seconds and can be changed within a few hundreds of pico­seconds. Until now, though, no detector has been capable of measuring and proving this precisely, and the best results have only been achieved on nano­second time­scales, which are too slow to allow really efficient analysis or to be used to develop the most effec­tive new systems.

The Univer­sity of Leeds researchers, working with inter­national colleagues at École Normal Supérieure in Paris, France and the Univer­sity of Queensland in Brisbane, Australia have now used terahertz frequency quantum cascade lasers and a technique called terahertz time-domain spectro­scopy to understand this laser stabi­lisation process. The terahertz-powered technology can measure the wave­length of light in periods of femto­seconds giving unpre­cedented levels of detail. By knowing the speed at which wave­lengths change within lasers, and what happens during that process within miniscule time frames, more efficient devices and systems can be built.

Iman Kundu explaining the group’s findings, said: “We’ve exploited the ultra­fast detection capabi­lities of terahertz tech­nology to watch laser emissions evolve from multiple colours to a single wave­length over less than a billionth of a second. Now that we can see the detailed emission of the lasers over such incredibly small time frames, we can see how the wave­length of light changes as one moves from one steady state to a new steady state.”

The benefits for commercial systems designers are poten­tially signi­ficant. Terahertz technology isn’t available to many sectors, but the reseachers believe its value lies in being able to highlight trends and explain the detailed operation of integrated photonic devices, which are used in complex imaging systems which might be found in the pharma­ceutical or elec­tronics sectors. “Designers can then apply these findings to lasers operating at different parts of the electro­magnetic spectrum, as the underlying physics will be very similar”, says Kundu.

Edmund Linfield, Chair of Terahertz Electronics at the Univer­sity of Leeds, who was also involved in the study said: “We’re using the highly advanced capa­bilities of terahertz tech­nology to shine a light on the operation of lasers. Our research is aimed at showing engineers and developers where to look to drive increased per­formance in their own systems. By doing this, we will increase the global competi­tiveness of the UK’s science and engi­neering base.” (Source: U. Leeds)

Reference: I. Kundu et al.: Ultrafast switch-on dynamics of frequency-tuneable semiconductor lasers, Nat. Commun. 9, 3076 (2018); DOI: 10.1038/s41467-018-05601-x

Link: Terahertz Photonics Laboratory, University of Leeds, Leeds, UK

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