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This paper reviews the different multimodal applications based on a large extent of label-free imaging modalities, ranging from linear to nonlinear optics, while also including spectroscopic measurements. We put specific emphasis on multimodal measurements going across the usual boundaries between imaging modalities, whereas most multimodal platforms combine techniques based on similar light interactions or similar hardware implementations. In this review, we limit the scope to focus on applications for biology such as live cells or tissues, since by their nature of being alive or fragile, we are often not free to take liberties with the image acquisition times and are forced to gather the maximum amount of information possible at one time. For such samples, imaging by a given label-free method usually presents a challenge in obtaining sufficient optical signal or is limited in terms of the types of observable targets. Multimodal imaging is then particularly attractive for these samples in order to maximize the amount of measured information. While multimodal imaging is always useful in the sense of acquiring additional information from additional modes, at times it is possible to attain information that could not be discovered using any single mode alone, which is the essence of the progress that is possible using a multimodal approach.

Multiphoton microscopy (MPM) is an invaluable tool for visualizing subcellular structures in biomedical and life sciences. High-numerical-aperture (NA) immersion objective lenses are used to deliver excitation light to focus inside the biological tissue. The refractive index of tissue is commonly different from that of the immersion medium, which introduces spherical aberration, leading to signal and resolution degradation as imaging depth increases. However, the explicit dependence of this index mismatch-induced aberration on the involved physical parameters is not clear, especially its dependence on index mismatch. Here, from the vectorial equations for focusing through a planar interface between materials of mismatched refractive indices, we derive an approximate analytical expression for the spherical aberration. The analytical expression explicitly reveals the dependence of spherical aberration on index mismatch, imaging depth and excitation wavelength, from which we can expect the following qualitative behaviors: (1) Multiphoton signal and resolution degradation is less for longer excitation wavelength, (2) a longer wavelength tolerates a higher index mismatch, (3) a longer wavelength tolerates a larger imaging depth and (4) both signal and resolution degradations show the same dependence on imaging depth, regardless of NA or immersion on the condition that the integration angle is the same. Detailed numerical simulation results agree quite well with the above expectations based on the analytical approximation. These theoretical results suggest the use of long excitation wavelength to better suppress index mismatch-induced signal and resolution degradation in deep-tissue MPM.

A multi-frequency second harmonic generation (SHG) in one dimensional nonlinear aperiodic optical superlattice in the presence of linear spacers is considered. The effect of random fluctuations in the length of each layer on the output efficiency of SHG process is investigated, and the favorable percentage of fluctuation in the fabrication of aperiodic optical superlattice is obtained. Comparing perfect and fluctuated aperiodic structures, the researcher realized the decrease in the amount of output efficiency with the increase of fluctuation in the linear-nonlinear aperiodic crystals.

We investigate two dimensional nonlinear aperiodic structures leading to multi quasi-phase-matching conditions and multi-frequency second-harmonic generation (SHG) in one chip. For arranging devices close to an experimental situation the intensities of SHG in dependence from frequencies and direction of radiation are studied for irregular structures with random fluctuations of the sizes of each poling domain. Comparing the regular and irregular two dimensional nonlinear aperiodic structures we study the role of imperfection effects of random fluctuations in multi-frequency SHG.

We discuss the main features and our ongoing of work in the field on analogue gravity in photon fluids.