Graphene is a point-bandgap semiconductor with a linear dispersion of electrons with low energy that can be described by the Dirac equation involving relativistic effects. It also possesses large polarizability when illuminated by light, that is, large optical nonlinearity. These unique characteristics, in combination with the ultrafast relaxations of charge carriers (electrons and holes) in this material, in the femtosecond and picosecond time scale, suggest that graphene can be useful as an ultrafast saturable absorber (SA), that is, a material in which absorption of light decreases when the light intensity increases, and in a very short period of time. This property is fundamental for the generation of ultrashort light pulses in laser technology. Furthermore, graphene can be used for this application in a very broad spectral range without requiring modification of the electronic bandgap. F. Rotermund from Ajou University, South Korea, B.H. Hong from Sungkyunkwan University, South Korea, and their colleagues have reported in the October 15 issue of Optics Letters (DOI: 10.1364/OL.36.004089; p. 4089) the fabrication of high-quality, large-area graphene SAs and their application for efficient mode-locking of a solid-state laser operating near 1.25 μm.
The researchers synthesized monolayer graphene by chemical vapor deposition of a mixture of methane and hydrogen gases on Cu foils. They then spin-coated a layer of polymethylmethacrylate (PMMA) on the grown monolayer graphene, and etched the underlying Cu foil before transferring the supported graphene layer onto a quartz substrate and removing the PMMA layer with acetone. The size of the graphene layer transferred onto the substrate was over 1.2 cm2 × 1.2 cm2. This method could be extended using a layer-by-layer stacking approach to fabricate a bilayer graphene saturated absorber.
The linear transmission of the monolayer graphene was measured to be 97.6% at around 1.25 μm. The researchers detected two different behaviors in the decay curves for the saturable absorption: an instantaneous response of 155 fs followed by a slower recovery
time of 1.45 ps. They associated the fast decay with collision between charge carriers lying in the same band together with the emission of phonons, while the slow component was associated with relaxation of electrons and holes lying in different bands and the decrease of energy of long lifetime phonons (cooling of hot phonons). The researchers also estimated other important parameters for monolayer and bilayer graphene SAs necessary to generate ultrashort laser pulses (laser mode-locking). These included saturation fluences that determine the pulse energy required for extracting most of the energy stored in the gain medium of the laser; modulation depths that represent the maximum change in absorption which can be induced by the incident light at a particular wavelength; and nonsaturable losses that are the unwanted part of the losses.
The researchers considered that the values they measured for graphene were well suited to achieve stable mode-locking of bulk solid-state lasers. They demonstrated laser mode-locking with graphene SAs in a Cr:forsterite laser, delivering 94-fs pulses with a spectral bandwidth of 20 nm near 1.24 μm. These results yielded a time-bandwidth product of 0.37, which is close to the Fourier limit. The researchers achieved stable mode-locked operation for hours with an average output power up to 230 mW at 75 MHz, without the appearance of multiple pulsing and Q-switching instabilities, and without visible damage to the absorber.
The researchers consider that graphene can be further applied for other bulk solid-state lasers in the wide spectral region due to its unique band structure and superior nonlinear optical properties with modulation depth being tailored through appropriate layer-by-layer stacking of monolayer graphene.
