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What is CARS spectroscopy? Why is it so special?
In today's scientific field, CARS spectroscopy (Coherent Anti-Stokes Raman Spectroscopy) has emerged in chemical and physical research with its unique advantages. This technology is mainly used to detect the vibration signals of molecules, similar to traditional Raman spectroscopy, but its sensitivity and signal strength far exceed the former. CARS spectroscopy is performed using multiphoton technology, which enables it to provide clearer molecular images and thus become an important tool in many research fields.
Historical BackgroundCARS spectroscopy is a third-order nonlinear optical process involving the interaction of three laser beams.
CARS spectroscopy was first proposed in 1965 when P. D. Maker and R. W. Terhune from Ford Motor Company published a study on the phenomenon. They used pulsed ruby lasers to probe the third-order response of a variety of materials and observed that a blue-shifted CARS signal was generated when the pulses of the two beams overlapped in space and time. This technique was given the name "CARS spectroscopy" by Begley et al. at Stanford University in 1974.
Basic Principles
The working principle of CARS spectroscopy can be explained by classical and quantum mechanical models. Classically, a molecule can be viewed as a (damped) oscillator with a characteristic frequency ωv. In CARS, this oscillator is driven by the frequency difference between the pump beam and the Stokes beam. This driving mechanism is similar to how the ear is sensitive to the difference frequency between two different notes struck on a piano.
Comparison of CARS and Raman spectroscopyIn the CARS process, the pump beam first excites the molecule into a virtual state that is not an intrinsic state of the molecule but allows transitions to other real energy levels.
CARS and Raman spectroscopy share similarities in probing the vibrational modes of molecules, but there are also significant differences. CARS requires two pulsed laser sources, while Raman spectroscopy requires only a single continuous wave (CW) laser. Since the CARS signal is observed on the blue side, it does not have to compete with the fluorescence phenomenon, which makes CARS more advantageous in practical applications.
Application Areas
CARS microscopy
CARS has a wide range of applications in species-selective microscopy and combustion diagnostics, especially in the area of non-invasive imaging in biological samples. Many researchers use CARS microscopy to observe lipids in biological samples, providing a new method for studying biology.
Combustion Diagnosis
CARS spectroscopy is also used for temperature monitoring of gases and flames because its signal has a nonlinear dependence on temperature. The CARS signal reflects the thermal state of the system because it is related to the number of particles in the ground state and vibrationally excited states.
Other Applications
In addition to the above applications, CARS technology is currently being developed for use in security monitoring areas such as detecting roadside bombs. This highlights its potential value in public safety.
In summary, CARS spectroscopy has become a hot technology in current research due to its superior signal intensity and high sensitivity to molecular vibration modes. As the technology develops further, will we be able to see its application in more areas in the future?
