Shirshendu Dinda

Engineering Optical Spatiotemporal Properties to Impact Femtosecond Nonlinearities: Case Studies and Implementations

The subjects of this thesis revolve around processes that change or control Spatiotemporal properties of laser pulses and their interactions. In Chapter 1 brief history of light-matter interaction is described. Through their duality nature, one can comprehend such properties and interactions independently. The regimes where they act with each other differently are also described. Vectorial imaging is an integrated part of optical imaging and microscopy in general. The main goal in any kind of microscope is the enhancements of image quality, i.e. resolution, contrast, etc. One of the popular methods is to use high numerical aperture (NA) objective lens (OL). The properties in the focal region of a high NA OL is quite different from properties predicted by the Rayleigh postulate of imaging (i.e., diffraction limited spot-size). The vectorial properties of light (such as, the direction of electric field or polarization) plays a crucial role as the diffraction of light due to high NA OL changes intensity distribution of light in the focal region (hence the resolution) predicted by scalar imaging. The intensity distribution of light in the focal region of a high NA OL changes drastically as the shape of the light beam and the polarization change. In Chapter 2, mathematical and geometrical description of scalar and vector imaging are discussed. Depending on properties of the input beam one can generate several excitation spots in the focal region, which is quite useful in optical tweezers research. Changing the phase of the input beam creates an asymmetric distribution of light, which increases the spatial resolution. Chapter 3 is focused on optical pulse shaping. In general, optical pulses are very generic in their temporal shapes. Most common among them is the transform limited Gaussian pulse. Using basic principles of Fourier domain pulse modulation, one can transform properties of optical pulses into any arbitrary shapes and phases. There are several ways to implement this pulse shaping technique to generate desired pulses. AOM (acousto-optic modulator) method has been used to transfer desired information to the optical pulse, which is a fast modulator (much faster than the liquid crystal modulator, LCM) and generates background free shaped pulses. Though fast, AOM modulator cannot update the incoming modulation information faster than 100 kHz. In my work, I have shown that, though one cannot overcome this physical limit, there is a unique approach to generate average pulse shapes of the desired kind. In this technique one can generate an overwhelming number of output pulses with similar output envelops. Though envelops are same, many pulses are different in phase. This kind of pulse shaping is useful when one is performing experiments with fast sampling rate, where the exact phase of the optical pulse does not have a significant role. In Chapter 4, the generation process of femtosecond Supercontinuum (SC) in condensed (liquid) medium is explored. A brief description of the SC generation history and involved physical process are described in both graphical and mathematical representations. For better use of SC, we need to know how to control SC. In this chapter, we have generated SC in the near threshold regime and inspected the underlying processes involved in the SC generation process. Delicate interference structures appear in the supercontinuum in water and CCl4 as the energy of the input pulse crosses the threshold value. The fringe pattern in the SC spectrum gives information about the temporal separation and the quality of the SC sources.