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.

Sub-second hyper-spectral low-frequency vibrational imaging via impulsive Raman excitation.

Real-time vibrational microscopy has been recently demonstrated by various techniques, most of them utilizing the well-known schemes of coherent anti-stokes Raman scattering and stimulated Raman scattering. These techniques readily provide valuable chemical information mostly in the higher vibrational frequency regime (>400 cm-1). Addressing the low vibrational frequency regime (<200 cm-1) is challenging due to the usage of spectral filters that are required to isolate the signal from the Rayleigh scattered excitation field. In this Letter, we report on rapid, high-resolution, low-frequency (<130 cm-1) vibrational microscopy using impulsive coherent Raman excitation. By combining impulsive excitation with a fast acousto-optic delay line, we detect the Raman-induced optical Kerr lensing and spectral shift effects with a 25 µs pixel dwell time to produce shot-noise limited, low-frequency hyper-spectral images of various samples.

Impulsive Raman spectroscopy via precision measurement of frequency shift with low energy excitation.

Stimulated Raman scattering (SRS) has recently become useful for chemically selective bioimaging. It is usually measured via modulation transfer from the pump beam to the Stokes beam. Impulsive stimulated Raman spectroscopy, on the other hand, relies on the spectral shift of ultrashort pulses as they propagate in a Raman active sample. This method was considered impractical with low energy pulses since the observed shifts are very small compared to the excitation pulse bandwidth, spanning many terahertz. Here we present a new apparatus, using tools borrowed from the field of precision measurement, for the detection of low-frequency Raman lines via stimulated-Raman-scattering-induced spectral shifts. This method does not require any spectral filtration and is therefore an excellent candidate to resolve low-lying Raman lines (<200 cm-1), which are commonly masked by the strong Rayleigh scattering peak. Having the advantage of the high repetition rate of the ultrafast oscillator, we reduce the noise level by implementing a lock-in detection scheme with a wavelength shift sensitivity well below 100 fm. This is demonstrated by the measurement of low-frequency Raman lines of various liquid samples.

Flashlamp-pumped nanoparticle dispersion laser.

A flashlamp-pumped nanoparticle dispersion laser was demonstrated for the first time. The nanoparticles were Nd(2)O(3) modified with dimethyldichlorosilane (DMDCS) and dispersed in dimethyl sulfoxide-d(6) (DMSO-d(6)). The nanoparticle dispersion was pumped using four flashlamps to yield energy of 4.3 J in a 200 µs pulse, at a wavelength of 1057 nm.

Nanoparticle dispersion laser.

Lasing action from a dispersion of nanoparticles is reported for the first time to our knowledge. The nanoparticles are Nd(2)O(3) modified with dimethyldichlorosilane (DMDCS) in dimethylsulfoxide. The laser was pumped with a pulsed laser at 802 nm and yielded 2.7 mJ with a slope efficiency of 50%. This was compared to a standard Nd-doped phosphate glass that yielded very similar results in the same setup.