Liquid-phase exfoliation (LPE) is a low-cost and scalable technique for producing a wide range of van der Waals nanomaterials that can be incorporated into existing laboratory sample and industrial material production. Liquid-phase exfoliated nanomaterials have the potential to produce devices quickly and at low-cost, with colloidal dispersions easily adaptable to existing production methods. Other methods of nanomaterial production, such as chemical vapour deposition (CVD) and mechanical exfoliation can be costly, time-consuming and require complicated equipment to produce comparatively small area devices. Such methods are excellent for laboratory-scale samples to examine physical properties and produce proof of concept devices. However, LPE can and will bridge the gap towards real-world applications that require faster, easier and more cost-effective production methods. In this work, we investigate the saturable absorption and Kerr nonlinearity of graphene fabricated by LPE and CVD. 

Thin films of LPE graphene were produced on BK7 glass substrates, and compared to CVD graphene transferred onto an identical substrate. Through careful consideration of the concentration of graphene dispersion and deposition methods, very thin films of graphene can be prepared. Atomic force microscopy (AFM) measurements showed effective bi-layer graphene thickness for the LPE samples. Z-scan measurements performed with 180 fs pulses at a wavelength of 1030 nm reveal that both LPE and CVD graphene display strong saturable absorption characteristics, with a nonlinear absorption coefficient (*β*) approaching -104 cm/GW and a Kerr nonlinearity (*n2*) of -1 cm2/GW. Such strong saturable absorption is ideal for devices such as mode-lockers for ultrafast pulsed lasers. The magnitude of the nonlinear absorption coefficient of the LPE graphene increases with pulse duration, *τ*, up to 105 cm/GW at around *τ* = 10 ps. These results pave the way for the use of LPE graphene in nonlinear optical applications such as frequency generation and mode locking.

Nonlinear optical properties of graphene: liquid-phase exfoliation vs chemical vapour deposition

Supercontinuum generation in optical fiber is the result of an interplay between multiple nonlinear processes. Simulations to reproduce experimental observations are well documented: a differential equation known as the generalized nonlinear Schrödinger equation (GNLSE) is solved to determine the effect of propagation on the spectral and temporal profiles of the slowly-varying amplitude of a laser pulse. For the scalar case, the GNLSE has been shown to yield good results. In a more accurate case, it becomes necessary to take into account the polarization evolution of light as it travels through the fiber, for instance for a fiber with two orthogonal principal axes. This is typically added into the GNLSE to form coupled equations including the birefringence of the fiber and additional nonlinear terms coupling the slowly-varying amplitudes of two polarization components. Upon studying the experimental spectra obtained from a germania-doped photonic crystal fiber, we observe that the polarization state varies significantly across the spectrum, leaving the spectral edges with a greater degree of polarization than the areas near the input pump wavelength. We also observe that the spectral region around the pump maintains a low degree of polarization. These trends lead us to believe that a portion of the energy transfer from input polarization state to an orthogonal output state is proportional to the propagation length, consistent with a linear depolarization contribution that cannot be reproduced using only nonlinear equations. We therefore demonstrate the effect of an additional depolarization term to account for the redistribution of energy between polarization modes. Furthermore, the interaction between linear and nonlinear depolarization mechanisms leads to a more realistic estimate of the polarization maintaining capability of an optical fiber over a broadband spectrum.

Adding a linear contribution to depolarization in simulations of supercontinuum generation

W1-1 Photonics and Nano-Optics (DAMOPC) / Photonique et nano-optique (DPAMPC)

technical paper

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Nonlinear optical properties of graphene: liquid-phase exfoliation vs chemical vapour deposition

Adding a linear contribution to depolarization in simulations of supercontinuum generation

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