Optical microscopy enables non-destructive characterization of material properties, which improves the fundamental understanding of materials and facilitates technological development. However, with current technological advancements it is becoming increasingly critical to understand materials at the nanoscale. While the resolution of optical microscopy is limited by diffraction, superresolution techniques allow for optical resolution at the nanoscale. Generally, superresolution techniques are fluorescence based, which limits the types of optical processes that can be utilized. We are developing multimodal superresolution techniques for nanoscale sample characterization that are suitable for a wide range of optical processes.
One approach to superresolution microscopy is to engineer the excitation and collection volumes to maximize the resolution of the optical system. Generally, this is accomplished by applying and amplitude or phase mask to the excitation beam. One common type of phase mask that is utilized are azimuthally symmetry concentric phase steps, known as Toraldo-style phase masks. We apply a pi phase step to the center of the excitation beam, which results in a narrowed central spot and increased intensity in the side lobe compared to a conventional point spread function (PSF) from an unmasked beam with a uniform phase front. Compared to many superresolution techniques, this approach is highly versatile and can be applied to variety of optical techniques.
To implement this approach for imaging the challenge is to suppress the side lobes while retaining the narrowed central spot that provides the improved resolution. This can be accomplished by engineering the detection volume, such as implementing confocal detection to suppress the side lobes while detecting the central spot. An alternative approach for nonlinear optical processes is to apply different phase masks to each excitation beam, since the total PSF is a multiplication of the excitation beams. In this case, the additional beams can be used to suppress the side lobes while maintaining the narrowed central lobe. For nonlinear processes both confocal detection and beam multiplication can be implemented to maximize the resolution improvement with minimized side lobes, which can be tailored to the specific superresolution application. Thus far we have demonstrated this technique for fluorescence lifetime imaging, second harmonic generation, and coherent Raman scattering.
Kim, Hyunmin, Garnett W. Bryant, and Stephan J. Stranick. "Superresolution four-wave mixing microscopy." Optics express 20.6 (2012): 6042-6051.
Beams, Ryan, Jeremiah W. Woodcock, Jeffrey W. Gilman, and Stephan J. Stranick. "Phase mask-based multimodal superresolution microscopy." Photonics. Vol. 4. No. 3. Multidisciplinary Digital Publishing Institute, 2017.
Beams, Ryan, and Stephan J. Stranick. "Side lobe suppression in phase mask-based nonlinear superresolution microscopy." Nanoimaging and Nanospectroscopy V. Vol. 10350. International Society for Optics and Photonics, 2017.