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Ultrafast Laser Spectroscopy

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In our laboratory at IICT, we use Time Correlated Single Photon Counting (TCSPC) in nanosecond time domain, femtosecond Fluorescence Up-conversion Spectroscopy in sub-picosecond to picosecond time domain and Ultra-fast Transient Absorption Spectroscopy in sub-picosecond to nanosecond time domain.

Time-correlated single photon counting (TCSPC)

Time-correlated single photon counting (TCSPC) is well established technique in nanosecond time domain and commonly used to analyze the relaxation of molecules from an excited state to a lower energy state. Since various molecules in a sample will emit photons at different times following their simultaneous excitation, the decay must be thought of as having a certain rate rather than occurring at a specific time after excitation. By observing how long individual molecules take to emit their photons, and then combining all these data points, an intensity vs. time graph can be generated that displays theexponential decay curve typical to these processes.

Femtosecond Fluorescence Up-conversion (FU)

Femtosecond up-conversion is a special type of pump-probe spectroscopic technique where nonlinear optical crystal play a crucial role to add up the fluorescence signal and probe signal to generate a signal with a new frequency via photon up-conversion, which is subsequently detected. The probe with constant intensity scans through delay times after the pump excites the sample, generating a plot of intensity over time which is proportional to the fluorescence intensity of excited samples. This technique is used for measuring very fast reaction dynamic for fluorescent molecules/materials. Examples of these include dynamic of cis-trans photoisomerization process, excited state and population dynamics of DNA, and the charge transfer processes in photosynthetic reaction centres. Charge transfer dynamics in photosynthetic reaction centres has a direct bearing on man’s ability to develop light harvesting technology, while the excited state dynamics of DNA has implications in diseases such as skin cancer. Advances in femtosecond methods are crucial to the understanding of ultrafast phenomena in nature.

Ultrafast Transient Absorption Spectroscopy (TAS)

This method is typical of 'pump-probe' experiments, where a very short pulsed laseris used to excite a molecule's electrons from their ground statesto higher-energyexcited states. A probing light source, typically a white light continuum, is used to obtain anabsorption spectrum of the compound at various times following its excitation. As the excited molecules absorb the probe light, they are further excited to even higher states. After passing through the sample, the unabsorbed white light intensity is monitored, and the data is processed to generate an absorption spectrum of the excited state. Unlike TCSPC, with the help of TA measurements, one can look into non-radiative relaxation of higher electronic states (~femtoseconds), vibrational relaxations (~picoseconds) and radiative relaxation of excited singlet state (occurs typically on nanoseconds time scale) Furthermore, this technique can be carried for kinetic measurements of species specifically for observing species that have short-lived and non-phosphorescent populations within the triplet manifold as part of their decay path. Transient Absorption Spectroscopy also be used to trace the intermediate states in a photo-chemical reaction; energy, charge or electron transfer process; conformational changes, thermal relaxation, fluorescence or phosphorescence processes, optical gain spectroscopy of semiconductor laser materials. etc. It has become an important tool for characterizing various electronic states and energy transfer processes in nanoparticles, to locate trap states and further helps in characterizing the efficient passivation strategies.

Fluorescence Correlation Spectroscopy

Last but not the least we do Fluorescence Correlation Spectroscopy , single molecule level spectroscopic technique, which determine diffusion dynamic of macromolecules, nanoparticles which helps measuring hydrodynamic size of the particles and chemical kinetics of diffusion control processes.

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