Self-Phase-Stabilized Heterodyne Vibrational Sum Frequency Generation Microscopy

Abstract

Vibrational sum frequency generation (VSFG) spectroscopy has been a powerful technique to probe molecular structures at non-centrosymmetric media. Recent developed heterodyne (HD) detection can further reveal spectral phase and molecular orientations. Adding imaging capability to an HD VSFG signal can bring spatial visualization capability into this non-linear optical technique. However, it has been a challenge to build an HD VSFG microscope that is both easy to align and has good spectral phase stability -two necessary criterions for the broad application of this technique into various areas of science. Here, we report a fully-collinear HD VSFG microscope, which meets both phase stability and optical alignment requirements that can spatially resolve images of molecular interfaces and domains, with chemical and structural sensitivities. The phase stability is more than nine times better than a Michelson Interferometric HD VSFG microscope. Using this HD VSFG microscope, we study the structures of molecular selfassembly films. Because of the superior phase sensitivity, we successfully identify two molecular domains with different molecular orientations, which we show is not possible to extract from an ensemble-averaged VSFG spectrum or homodyne-detected VSFG image.

Enhancing second-harmonic generation using dipolar-parity modes in non-planar plasmonic nanocavities

“…Finally, the ability to control the spatial distribution and symmetry of optical modes at the nanoscale is important for the engineering of strong nonlinearities [7,8].In this context, plasmonic systems have been used extensively to confine the optical fields into deep subwavelength volumes and to enhance the nonlinear optical phenomena at the nanoscale. Various nonlinear processes have been shown to benefit from plasmonic enhancement including SHG, third harmonic generation, four wave mixing, optical Kerr effect [9][10][11][12][13][14][15][16][17][18][19][20][21][22]. MIM nanocavities are of particular interest as they can reproducibly achieve strong field enhancement in the thin insulating layer [23,24].…”

“…MIM nanocavities are of particular interest as they can reproducibly achieve strong field enhancement in the thin insulating layer [23,24]. Moreover, the plethora of modes supported by MIM nanoresonators allows to design multi-resonant structures enhancing the optical fields at all the frequencies involved in the nonlinear process [10].2nd order processes such as SHG require inversion symmetry breaking for electron motion. The pure plasmonic structures are made of noble metals which possess centrosymmetric crystal lattice and thus the symmetry breaking is achieved at the material interface.…”

“…Thus, dipolar-like distribution for both fundamental and SHG modes would interfere destructively in the far field resulting in zero SHG [13]. This leads to the necessity of having even-parity modes such as quadrupoles at the SHG wavelength which may not be highly radiative due to the poor coupling to the free space radiation or not accessible because of the gap plasmon dispersion [10].An alternative approach for using plasmonics for efficient frequency conversion is to couple non-plasmonic, highly nonlinear materials to metallic nanoparticles [26][27][28]. In such a scheme, plasmonic near fields concentrate light inside the adjacent nanomaterial thus generating strong nonlinearities in those materials.…”

“…MIM nanocavities are of particular interest as they can reproducibly achieve strong field enhancement in the thin insulating layer [23,24]. Moreover, the plethora of modes supported by MIM nanoresonators allows to design multi-resonant structures enhancing the optical fields at all the frequencies involved in the nonlinear process [10].…”

“…Thus, dipolar-like distribution for both fundamental and SHG modes would interfere destructively in the far field resulting in zero SHG [13]. This leads to the necessity of having even-parity modes such as quadrupoles at the SHG wavelength which may not be highly radiative due to the poor coupling to the free space radiation or not accessible because of the gap plasmon dispersion [10].…”