Thermal Lens Spectroscopy by High Repetition Rate Femtosecond Lasers

OBJECTIVES: Each Ultra-short LASER pulses deposit small amount of energy in the sample. In the case of High Repetition Rate (HRR) Laser, this individual heat accumulates in the sample to the longer time scale. How this heating effect can be utilized to study the chemical system.

THEORY: Thermal lens (TL) spectroscopy is based on the photo-thermal effect where energy of a photon, fully or partially, is converted into heat energy. In this spectroscopy, a laser source is used whose wavelength is chosen such that it can couple either with electronic levels or vibration levels of the sample under study. Beside this, energy can be transferred to the sample by scattering but energy transferred to sample, generally, negligible small. So we can neglect this mode of energy transfer mechanism. Energy transferred in the former case, leads the system in the excited state. From there, system can relax to the ground through various mechanism such as radiative (fluorescence and phosphorescence) or non-radiative mechanism. If system relax via non-radiative mechanism, it completely transfer its absorbed energy  to the system in the form of heat while in case of radiative relaxation, some amount of energy is converted in the form of heat due the stokes shift. Because of all these heating mechanism, a spot of local heating is formed. As refractive index of medium is depend on the density and density changes with the change of temperature. All these factors make the local heats to be having smaller refractive index compare to the surrounding. Thermal conduction and convection try to equilibrate the heat generated in the sample and as we move away from the local heated area temperature decreases. In corollary to this, refractive index increases and system behave as Lens. As this lens is formed due to the heating (Thermal) in sample, so it is called Thermal Lens (TL). Also, it is based on light matter interaction, so it is called Thermal lens Spectroscopy.

TL spectroscopy is far more sensitive than the usual absorption based study in order of magnitude. TL spectroscopy depends number of parameters such absorption co- efficient (how strongly the sample absorbed the laser wavelength), thermal conductivity, thermal expansion coefficient, heat capacity, refractive index, thermo-optic coefficient etc.  The principle of TL spectroscopy is shown in fig.1

Fig1: Principle of Thermal Lens Signal

Fig. 2:- Schematic diagram of Thermal lens Spectroscopy

INSTRUMENTS REQUIRED:

1. Gold mirrors, mirror holders, lenses, lens holders, irises.
2. Motorized Translational stage, motion controller, oscilloscope, detectors and computer.
3. Laser glasses for eye safety.

SOFTWARE USED:

1. LabVIEW software for data acquisition.
2. Origin pro for data plotting and analysis.

Note: For user operation and usage no specific software needed.

EXPERIMENT PROCEDURE:

1. Turn the IMRA LASER to standby for 10-15 minutes.
2. Turn on the emission of 780nm (probe beam) and 1560nm (pump beam)
3. Make the Mach-Zehnder Interferometer set-up using 780nm and 1560 nm LASER beam.
4. Focus the final overlap beams of 780nm and 1560nm.
5.  Now place another lens in the 780nm beam path, such that, the final 780 nm beam becomes collimated.
6. Take the 40% of central portion of 780 nm by putting iris.
7. Put the sample in experimental set-up and measured the transmittance through the aperture by changing the position of sample with respect the focal point of the focused laser.
8. Plot the measured transmittance with respect to the position and calculate the Thermal Lens signal by following equation

where T_p(λ_e) and T_p0 are the probe transmittance through aperture in the presence and in the absence of pump-beam, respectively