Creating and utilizing spatially varying light intensity profiles to enhance nonlinear responses

Full text: The modification of the optical and material properties of material systems is reliant on high light intensity, and is usually only possible by utilising laser light. Typical laser systems produce a beam with an intensity characterised by a Gaussian distribution in cross section, such as the rectilinear Hermite-Gaussian or cylindrical Laguerre-Gaussian modes. Thus the bulk of research performed into investigating intensity dependent nonlinear optical effects utilizes or assumes a 𝑇𝐸𝑀₀₀ fundamental Gaussian mode light intensity distribution. However, this particular distribution may not be optimal for inducing a specific nonlinear effect or may lead to other unwanted outcomes such as sample damage. In this paper, we seek to generate superimposed combinations of higher order Gaussian (HOG), or non-Gaussian (NG) beams and understand their effect on the sample and in inducing nonlinear optical response. Here we use spatially varying beam profiles produced by HOG or NG beams in the particular application of zscan, a common experimental technique that involves scanning a sample material along the direction of beam propagation through a tightly focussed laser beam. Since nonlinear effects are intensity dependent, a stronger nonlinear response will occur at the beam focus than away from this location and by measuring the difference the materials nonlinear properties can be characterized. This widely adopted technique is useful as it is easy to implement and can quite quickly and accurately determine the sample materials nonlinear absorption and refraction. To induce significant nonlinear optical response in a sample often requires peak intensities that can approach the damage threshold of the material. Therefore, when conducting a zscan experiment there exists the real risk of increasing the incident laser light intensity too high, damaging the sample through ablative or thermal effects. Often then there needs to be a balance between the intensity needs of the nonlinear process against the possibility of damaging the sample. In this paper it is proposed that modified Gaussian beam profiles such as the 'doughnut' or 𝑇𝐸𝑀^∗₀₁ mode have the potential to produce significant nonlinear response while lowering the risk of sample damage. Compared to a 𝑇𝐸𝑀₀₀ fundamental mode Gaussian beam, the doughnut has lower peak intensity while maintaining a higher average intensity. This combination of attributes ensures that sufficient intensity is applied to the sample that the desired nonlinear effect can be evoked, while maintaining intensity below the damage threshold. In this paper we demonstrate techniques and methods to realize the doughnut mode and other user defined beam profiles. Initially, these profiles were developed computationally using a nematic SLM via diffractive holography. However the SLM has a low damage threshold, limiting its use. Given that zscan typically uses intense pulsed radiation intensities one solution that allows use of the SLM is continuous wave version of zscan. However, this would be problematic in samples with weaker nonlinearities. Other solutions discussed here for the physical realisation of HOG or NG beams are phase plates, novel lenses (axicon), etc. Gauss-Bessel beams have some potential utility in enhancing the zscan process, and are readily usable with the simple addition of an axicon lens into the zscan array. By experiments both physical and numerical this paper will investigate and compare the effectiveness or not of these different beam profiles on the nonlinear diffraction and refraction using the zscan approach. (author)