NPG Asia Materials research highlight | doi:

Graphene: Mode-locked lasers

Although graphene potentially has many applications, reports on exploiting its optical properties are rare. But the versatility of this material is truly remarkable, as demonstrated by Dingyuan Tang from Nanyang Technological University in Singapore and colleagues¹, who fabricated an essential component of a pulsed laser using a graphene–polymer composite.

The ‘cavity’ of a laser ensures that light is emitted at well-defined wavelengths, known as modes. These modes naturally oscillate in time, each with a different phase, and the superposition of multiple modes results in a constant output known as continuous-wave laser light. By introducing a mode locker into the cavity, all modes can be made to oscillate in phase, giving rise to short and very intense light pulses.

Tang and his collaborators realized that graphene could be used as a mode locker because it is a ‘saturable absorber’, that is, its light absorbance decreases rapidly with increasing light intensity — an essential property for passive mode lockers.

The team approached this application by introducing a graphene–polymer composite film into an erbium-doped fiber laser. “Encapsulating graphene in the polymer host passivates the graphene from ambient oxygen, which prevents photooxidation and optical bleaching by high-powered lasers,” says Tang. The researchers observed efficient mode locking with pulses as short as 700 fs at a wavelength of 1590 nm with a pulse energy of 3 nJ. These characteristics surpass those achieved by the same laser using other types of mode lockers such as carbon nanotubes and semiconductor saturable absorber mirrors. Furthermore, careful examination of the pulse shape revealed that the pulse was composed of one soliton — a nonlinear localized wave. This is a major improvement compared with standard mode lockers, which usually give rise to multiple solitons.

The results represent a real advance in photonics applications of graphene. “Graphene–polymer composite is so far the only mode locker that can be potentially employed for different wavelengths of lasers, ranging from visible to infrared, due to graphene’s super-broadband saturable absorption,” says Tang.

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