Single beam low frequency 2D Raman spectroscopy.

Low frequency Raman spectroscopy resolves the slow vibrations resulting from collective motions of molecular structures. This frequency region is extremely challenging to access via other multidimensional methods such as 2D-IR. In this paper, we describe a new scheme which measures 2D Raman spectra in the low frequency regime. We separate the pulse into a spectrally shaped pump and a transform-limited probe, which can be distinguished by their polarization states. Low frequency 2D Raman spectra in liquid tetrabromoethane are presented, revealing coupling dynamics at frequencies as low as 115 cm-1. The experimental results are supported by numerical simulations which replicate the key features of the measurement. This method opens the door for the deeper exploration of vibrational energy surfaces in complex molecular structures.

High-speed low-frequency chirped coherent anti-Stokes Raman scattering microscopy using an ultra-steep long-pass filter.

Coherent anti-Stokes Raman scattering (CARS) microscopy is becoming a more common tool in biomedical research. High-speed CARS microscopy has important applications in live cell imaging and in label-free pathology. However, only a few realizations exist of CARS imaging applied in the few terahertz spectral range (<300 cm-1), in which much is unknown to date. Although single-beam CARS microscopy proved to be robust in this low-frequency region, pixel-dwell time using presently available schemes is still relatively long, in the millisecond scale. Single-beam notch-shaped chirped-CARS (C-CARS) microscopy in the fingerprint region can be performed without using lock-in detection, yet it necessitates double-notch shaping, resulting in a relatively complex system. Here, we demonstrate that C-CARS in the low-frequency regime can be achieved using a sharp-edge, which is created by an ultra-steep long-pass filter (ULPF). Furthermore, we demonstrate that this variant of C-CARS spectroscopy can be performed without post-processing analyses. This is used to image collagen in a biological sample with a pixel dwell time of 200 microseconds. This sharp-edge C-CARS method may find important application in rapid low-frequency CARS imaging of live cells or for imaging of fast flowing objects such as in microfluidic channels.

Experimental generation of arbitrarily shaped diffractionless superoscillatory optical beams.

We present, theoretically and experimentally, diffractionless optical beams displaying arbitrarily-shaped sub-diffraction-limited features known as superoscillations. We devise an analytic method to generate such beams and experimentally demonstrate optical superoscillations propagating without changing their intensity distribution for distances as large as 250 Rayleigh lengths. Finally, we find the general conditions on the fraction of power that can be carried by these superoscillations as function of their spatial extent and their Fourier decomposition. Fundamentally, these new type of beams can be utilized to carry sub-wavelength information for very large distances.