Ion-beam sputtered (IBS) thin-film interference filters for nonlinear optical imaging

Nonlinear optical (NLO) microscopy is emerging as a powerful technique for the study of biological samples. By combining several different imaging modalities such as multiphoton (MP) fluorescence, second-harmonic and thirdharmonic generation (SHG and THG), and coherent Raman scattering techniques such as coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS), it is possible to combine the best practices of label and label-free imaging into a single platform capable of imaging structures within single cells and elucidating the health of biological tissue samples, even at the submicron level. Single-substrate, ion-beam-sputtered (IBS) thinfilm interference filters are a key enabling technology in laser-based optical microscopy and play a critical role in multimodal NLO imaging. In microscopy applications, optical filters are used to select and discriminate exactly which wavelengths of light are to be transmitted, reflected and suppressed. In this paper we discuss various important characteristics of hard-coated thin-film interference filters, such as high light throughput, steep edges, and high out-of-band blocking, all of which require careful consideration when designing and manufacturing optical filters for NLO imaging applications. To understand the true performance of hard-coated IBS filters, a simple CARS imaging experiment was performed. We found a 2.6 times increase in signal enhancement and 70% improvement in image contrast when compared to a commercially available filter commonly used in CARS microscopy applications.