Dr. Khalil Ur Rehman Profile

Dr. Khalil Ur Rehman

at National Yang-Ming Univ

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Publications (2)

SPIE Journal Paper | 13 March 2020 Open Access

Spontaneous loss versus stimulation gain in pump-probe microscopy: a proof of concept demonstration

Subir Das, Khalil Ur Rehman, Guan-Yu Zhuo, Fu-Jen Kao

JBO, Vol. 25, Issue 03, 036501, (March 2020) https://doi.org/10.1117/12.10.1117/1.JBO.25.3.036501

KEYWORDS: Laser beam diagnostics, Seaborgium, Signal detection, Microscopy, Luminescence, Signal to noise ratio, Modulation, Sensors, Stimulated emission depletion microscopy, Photodetectors

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Significance: The large background, narrow dynamic range, and detector saturation have been the common limiting factors in stimulated emission (SE)-based pump-probe microscopy, attributed to the very small signal overriding the very intense laser probe beam. To better differentiate the signal of interest from the background, lock-in detection is used to measure the fluorescence quenching, which is termed spontaneous loss (SL). The advantages are manifold. The spontaneous fluorescence signal can be well separated from both the pump and the probe beams with filters, thus eliminating the background, enlarging the dynamic range, and avoiding the saturation of the detector. Aim: We propose and demonstrate an integrated pump-probe microscopy technique based on lock-in detection for background removal and dynamic range enhancement through SL detection. Approach: The experimental setup is configured with a pulsed diode laser at a wavelength λpu = 635 nm, acting as a pump (excitation) and a mode-locked Ti:sapphire laser at a central wavelength λpr = 780 nm, serving as the probe beam (stimulation). Both pulse trains are temporally synchronized through high precision delay control by adjusting the length of the triggering cables. The pump and probe beams are alternatively modulated at different frequencies f1 and f2 to extract the stimulated gain (SG) and SL signal. Results: SG signal shows saturation due to the irradiation of the intense probe beam onto the photodetector. However, the detector saturation does not occur at high probe beam power for SL detection. The fluorescence lifetime images are acquired with reduced background. The theoretical signal-to-noise ratios for SG and SL are also estimated by photon statistics. Conclusion: We have confirmed that the detection of SL allows the elimination of the large background without photodetector saturation, which commonly exists in SG configuration. This modality would allow unprecedented manipulation and investigation of fluorophores in fluorescence imaging.

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Proceedings Article | 22 December 2015 Paper

Optical parameter measurement of highly diffusive tissue body phantoms with specifically designed sample holder for photo diagnostic and PDT applications

A. Rehman, K. Rehman, S. Anwar, S. Firdous, M. Nawaz

Proceedings Volume 9668, 966842 (2015) https://doi.org/10.1117/12.2202497

KEYWORDS: Tissue optics, Scattering, Optical properties, Absorption, Tissues, Integrating spheres, Light scattering, Optical testing, Signal to noise ratio, Photodynamic therapy

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