Articles written in Journal of Chemical Sciences

    • Exploring control parameters of two photon processes in solutions

      Debabrata Goswami Amit Nag

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      Two-photon microscopy depends extensively on the two-photon absorption cross-sections of biologically relevant chromophores. High repetition rate (HRR) lasers are essential in multiphoton microscopy for generating satisfactory signal to noise at low average powers. However, HRR lasers generate thermal distortions in samples even with the slightest single photon absorption. We use an optical chopper with HRR lasers to intermittently `blank’ irradiation and effectively minimize thermal effects to result in a femtosecond z-scan setup that precisely measures the two-photon absorption (TPA) cross-sections of chromophores. Though several experimental factors impact such TPA measurements, a systematic effort to modulate and influence TPA characteristics is yet to evolve. Here, we present the effect of several control parameters on the TPA process that are independent of chromophore characteristics for femtosecond laser pulse based measurements; and demonstrate how the femtosecond laser pulse repetition rate, chromophore environment and incident laser polarization can become effective control parameters for such nonlinear optical properties.

    • Spectrally resolved photon-echo spectroscopy of CdSe quantum dots at far from resonance excitation condition


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      Spectrally resolved photon echo spectroscopy in the off-resonance condition is reported for the first time to study the coherence and population dynamics of CdSe quantum dots. In this case, the information related to the system dynamics can be inferred indirectly. This is especially useful when such dynamical information might be hidden under the absorption maxima of the sample. We observe that a substantial intensity of the photon echo signal was obtained in two different CdSe quantum dot samples (CdSe 610 and CdSe 640), whichhave absorption maxima at 620 nm and 590 nm, respectively. Due to the difference in sizes of these two quantum dots, a small change is observed in the molecular dynamics of these two quantum-dot samples. Specifically, thespectral diffusion of CdSe 640 occurs within the first 50 fs, whereas that for CdSe 610 occurs at about 100 fs timescale. The integrated plots of the photon echo signal, as a function of population time, result in two decay constants. The faster among the two decay components is pulse width limited and is in between 30 and 40 fs at different fixed coherence times for both samples. The slower decay component for the CdSe 610 sample is found to be in the range of 75–85 fs, while that for CdSe 640, it is between 82 and 92 fs at different fixed coherencetimes.

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