Spectral focusing: High spectral resolution spectroscopy with broad-bandwidth laser pulses

Abstract

In this work, we show how broad-bandwidth femtosecond pulses can be used to achieve high spectral resolution in nonlinear spectroscopy and microscopy. Our approach is based on chirping the excitation pulses in order to focus their entire bandwidth into a narrow spectral region. We show that spectral features which are 100 times narrower than the excitation light can be resolved with this simple spectral focusing. The gain in spectral selectivity and sensitivity makes its application to nonlinear microscopy very convenient. This is demonstrated with diffraction-limited coherent anti-Stokes Raman scattering microscopy.

Chemical Imaging with Frequency Modulation Coherent Anti-Stokes Raman Scattering Microscopy at the Vibrational Fingerprint Region

“…As emphasized earlier, the key requirement of the spectral focusing CARS method is the identical GVD of the pump and Stokes pulses. 18 In our experiments, the blue part of the laser pulse acts as the pump and the red part as the Stokes to excite CARS signals at the fingerprint region. We characterize the balance of GVD in the pump and Stokes pulses by the spectral resolution of the CARS spectrum from cyclohexane.…”

“…[18][19][20] Note that this condition depends on both the bandwidth of the laser pulses and the amount of GVD. It also implies that the optimal laser configuration is where the pump and Stokes pulses have the same bandwidths.…”

“…16 Recently, new CARS microscopy methods have emerged based on the so-called "spectral focusing" mechanism. 14,15,[17][18][19][20][21] These methods can excite a single vibrational level with high spectral resolution by "chirped broad-band" laser pulse pairs (pump and Stokes pulses) and the excitation vibrational frequency can be switched by the time delay between the pulse pair. 22 Cell and tissue imaging with these methods have already been demonstrated.…”

Spectrally-broad coherent anti-Stokes Raman scattering hyper-microscopy utilizing a Stokes supercontinuum pumped at 800 nm

“…Such frequency-scanning CARS can thus be thought of as "imaging first", where rapid-even video rate [13]-image collection enables live-cell imaging, albeit at the expense of sparse or time-consuming hyperspectral capabilities. A hybrid technique that utilizes both single-wavelength detection and broadband laser excitation is known as spectralfocusing CARS (SF-CARS) [5,[14][15][16]. With SF-CARS, a narrowband pump pulse and broadband Stokes pulse are stretched in time such that their frequency-time relationship (i.e.…”

“…SF-CARS has been implemented using three distinct experimental approaches: by using two synchronized laser sources to generate pump/probe and Stokes [15]; by using a single spectrally-broad sub-25-fs laser source with intra-pulse filtering to generate the pump/probe and Stokes [16,18]; or by using a fs-laser-pumped photonic crystal fibre (PCF) source to generate a supercontinuum from which some (or all) of the pump/probe/Stokes beams are derived [5,9,10,14,19]. Implementations of SF-CARS that use a single spectrally-broad femtosecond laser source have been successful in performing both CARS imaging and dense spectral acquisition, albeit with the added challenges and costs of utilizing an ultra-broadband laser system.…”