Click to view
The experimental setup for the 2D-CARS measurements displaying the 2D imaging spectrometer system and the alignment of the laser beams. The details of the time-synchronized fs and ps laser systems employed are described in the supplementary material. 28 The structured probe beam is formed by passing the probe beam through a mask (4 mm × 4 mm) which is relay imaged to the crossing of the pump/Stokes beam with a 2:1 magnification to increase the probe pulse irradiance. In light blue is the 2D-CARS image from the resolved J = 8 rotational transition. The optical detection system consists of a single “effective” lens used in combination with a diffraction grating to disperse the 2D-CARS light. L1, L2, and L3 – spherical lenses with f1 = 1 m, f2 = 0.5 m, f3 = 0.75 m, respectively. M – mirror, HWP – half wave plate, CL – cylindrical lens (f = 1 m / 0.3 m), BD – beam dump, PBS – polarizing beam splitter cube, SP – short wave pass filter, G – grating (3600 l/mm), CCD – charge coupled device camera, RF – 100 MHz radio frequency source to which both the fs and ps seed lasers are phase-locked.
Click to view
(a) 2D-CARS single-shot spectra recorded in pure N2 at room temperature. In the second row of 2D spectra, the center structure of the mask is removed to provide a defined 2D imaging field. (b) Recorded spectra in pure N2 and N2 in a mixture with O2 (Air), detected with a larger dispersion, are shown to indicate the potential for separately analyzing the 2D spatial field in mixtures of species. A vector diagram is used to orientate each spatial location of the measured 2D field. The image consists of ∼125 × 120 pixels which give about 15 000 spectra provided by a single-laser-shot.
Click to view
Extracted rotational CARS spectra taken from a single height (Y) in the image from Fig. 2(a) . The spectra originate from a specific height in the probed 2D plane and cover 20 mm (X) of space at this height. From the intensity spectra, the average temperature was evaluated to be 299 K with a standard deviation of 1.4%, this yields an accuracy error of 1.5%. There are 125 such pixel rows in the single laser shot 2D-CARS image, which each provide another set of data like that presented in this figure.
Article metrics loading...
Coherent anti-Stokes Raman spectroscopy (CARS) has been widely used as a powerful tool for chemical sensing, molecular dynamics measurements, and rovibrational spectroscopy since its development over 30 years ago, finding use in fields of study as diverse as combustion diagnostics, cell biology, plasma physics, and the standoff detection of explosives. The capability for acquiring resolved CARS spectra in multiple spatial dimensions within a single laser shot has been a long-standing goal for the study of dynamical processes, but has proven elusive because of both phase-matching and detection considerations. Here, by combining new phase matching and detection schemes with the high efficiency of femtosecond excitation of Raman coherences, we introduce a technique for single-shot two-dimensional (2D) spatial measurements of gas phase CARS spectra. We demonstrate a spectrometer enabling both 2D plane imaging and spectroscopy simultaneously, and present the instantaneous measurement of 15 000 spatially correlated rotational CARS spectra in N2 and air over a 2D field of 40 mm2.
Full text loading...