Nonlinear light-matter interactions have been key to advances in biological imaging and sensing. Naturally, customized light fields have been deployed to draw out and target these nonlinear phenomenon. In the case of Surface Plasmon Polaritons, electro-magnetic fields are confined to a nearly two dimensional region on a supporting metallic thin film. Here we explore consequences and applications of using gold films to support these propagating surface modes. The presence of sub-$\mu$m-sized particles in the excitation area distorts and confines the surface field, resulting in nonlinear electron-hole pair generation in the gold followed by radiative recombination. This mechanism, in which the gold film is both the source of strong near-fields as well as the antenna for coupling radiation to the far-field, holds promise as a label-free sensing mechanism of small particles near the surface. In addition, the ability to simultaneously support multiple SPP fields with well defined momenta allows for an unconventional method of generating four-wave mixing signals at the film surface. SPP mediated coherent anti-Stokes Raman scatterig (CARS) signals from biological samples are demonstrated, revealing unconventional imaging properties.

Surface plasmon polariton (SPP) modes on metallic films give rise to surface fields that can be used to enhance optical phenomena. Whereas SPP modes have been used to enhance the fluorescence and Raman response from molecules in the vicinity of the metallic surface, here we explore the use of SPP modes for detecting particles and structures that are non-fluorescent. In particular, we study the role of thin gold films in supporting SPP fields, which subsequently become distorted and confined by the presence of sub-micron-sized particles, resulting in nonlinear electron-hole pair generation in the gold followed by radiative recombination. This mechanism, in which the gold film is both the source of strong near-fields as well as the antenna for coupling radiation to the far-field, holds promise as a label-free sensing mechanism of small particles near the surface. In this work, the sensitivity and utility of this relatively unexplored property of thin gold films is investigated.

Sum frequency generation microscopy (SFG) is a nonlinear optical technique used to visualize interfaces and noncentrosymmetric structures that lack inversion symmetry. The advantage of SFG, which uses two collinearly coherent lasers over its degenerate counterpart, second harmonic generation (SHG), is that it enables chemical contrast through molecular vibrational absorptions in the infrared known as the fingerprint region. IR-active molecular modes are energized into a vibrationally excited state through a direct dipole excitation, and then probed with higher energy photon(s) before parametrically radiating a photon at the sum of the incident frequencies. Incorporating this technique into a multi-modal microscope is an important development as it allows for high resolution imaging with spectroscopic information without exogenous labels. Following the development of the SFG microscope came the discovery of vibrationally resonant third-order sum frequency generation (TSFG), a four-wave mixing process that probes resonant contributions of the chi_(3) tensor. This technique,comparable in sensitivity to the coherent anti-Stokes Raman scattering (CARS) modality,was applied to label-free imaging of cells rich in lipid droplet content. This work contributes to the study of high resolution IR-microscopy by merging it with the field of nonlinear optical microscopy.

Optical force microscopy is a novel technique where mechanical detection with a cantilevered probe replaces the detection of photons to investigate optically induced processes and states. A theoretical and experimental analysis is performed here of the forces present in optical force microscopy operated in tapping mode, which reveals two dominant optically induced forces, the gradient force and the scattering force. Force-distance curves are reconstructed from experimental amplitude and phase information for glass, gold nanowires and molecular clusters of silicon naphtalocyanine samples. The scattering force is shown to be insensitive to both nano-scale tip-sample distances and sample polarizability and is dependent on the form of the tip. The gradient force demonstrates a z^-4 tip-sample distance dependence, localized to a few nanometers, and is strongly dependent on the polarizability of the sample which enables spectroscopic imaging through force detection. The different distance-dependence and polarizability-dependence of the gradient and scattering forces give rise to a complex force-distance curve which determines imaging contrast along with the cantilever set-point, knowledge of which is essential for image interpretation.

The mid-infrared (MIR) part of the electromagnetic spectrum corresponds to an energy range that includes the resonances of molecular bond vibrations. For this reason, MIR light is extensively used to characterize samples, as the absorption of particular MIR frequencies results in a spectrum corresponding to the mode vibrations of chemical motifs. The ability to exploit this in the context of imaging would allow both the morphological character and chemical composition of samples to be visualized simultaneously. Unfortunately, MIR imaging technologies still struggle to gain widespread adoption as analytical tools, as they fail to provide high-definition images quickly over the whole MIR spectrum. The work contained in this thesis seeks to overcome this hurdle by transferring the information carried by the MIR radiation to the visible-to-near-infrared part of the electromagnetic spectrum. In the latter spectral range, camera technologies are fast, sensitive and ever increasing in definition. Although the linear spectral responsivity of such cameras excludes the direct detection of MIR radiation, a nonlinear absorption process can be used to convert MIR light to charge carriers in the camera's light sensitive element. In this approach, two pulses of different energy, one in the MIR and one in the near infrared (NIR) are timed to simultaneously reach the detector, enabling the excitation of charge carriers through nondegenerate two-photon absorption (NTA). The use of NTA effectively turns a visible/NIR camera into a MIR imager. This thesis explores this principle to generate MIR images of a variety of samples, where the image contrast is provided by the spectroscopic fingerprints of molecular vibrations. The work in this thesis includes the first demonstration of the NTA principle for the purpose of widefield MIR imaging, followed by specific implementations that leverage the physical properties of ultrashort laser pulses to provide 3D imaging as well as hyperspectral imaging capabilities. In addition, this work discusses a rigorous theory of NTA in indirect semiconductors such as silicon. Together, the work presented herein provides a comprehensive picture of the physics and applications of the NTA process for rapid MIR imaging of a broad range of sample targets.

We introduce a novel type of light-matter interaction force microscopy and spectroscopy, called as "Photo-induced force microscopy''. We visualize the optical interplay between tip and sample junction at molecular level in ambient condition. The linear and nonlinear optical responses such as localized permittivity, stimulated Raman and pump-probe are addressed at space--time limit. The ability of control the force at nano/molecular scale in ambient condition leads to a variety of potential application such as single molecule study,

nano-printing, nano-architecturing with chemical selectivity.