Shakil Rehman | American International University-Bangladesh (original) (raw)

Papers by Shakil Rehman

Luminescence, 2005

Fluorescence, absorption and vibrational spectroscopic techniques were used to study spinach at t... more Fluorescence, absorption and vibrational spectroscopic techniques were used to study spinach at the photosystem II (PS II), chloroplast and cellular levels and to determine the effects and mechanisms of ultraviolet-B (UV-B) photoinhibition of these structures. Two-photon fluorescence spectroscopic imaging of intact chloroplasts shows significant spatial variations in the component fluorescence spectra in the range 640–740 nm, indicating that the type and distribution of chlorophylls vary markedly with position in the chloroplast. The chlorophyll distributions and excitonic behaviour in chloroplasts and whole plant tissue were studied using picosecond time-gated fluorescence imaging, which also showed UV-induced kinetic changes that clearly indicate that UV-B induces both structural and excitonic uncoupling of chlorophylls within the light-harvesting complexes. Transient absorption measurements and low-frequency infrared and Raman spectroscopy show that the predominant sites of UV-B damage in PS II are at the oxygen-evolving centre (OEC) itself, as well as at specific locations near the OEC-binding sites. Copyright © 2005 John Wiley & Sons, Ltd.

Applications of phase microscopy based on either coherent or partially coherent sources are widel... more Applications of phase microscopy based on either coherent or partially coherent sources are widely distributed in today's biological and biomedical research laboratories. But the quantitative phase information derivable from these techniques is often not fully understood, because in general, no universal theoretical model can be set up, and each of the techniques has to be treated specifically. This chapter is dedicated to the fundamental understanding of the methodologies that derive optical phase information using imaging techniques and microscopic instrumentation. Several of the latest and most significant techniques are thoroughly studied through the theoretical formalism of the optical transfer function. In particular, we classify these systems into two main categories: those based on coherent illumination, such as digital holographic microscopy (DHM) and its extension into tomography, and those based on partially coherent illumination, such as differential interference contrast (DIC) and differential phase contrast (DPC). Our intention is that the models described in this chapter give an insight into the behaviour of these phase imaging techniques, so that better instrumentation can be designed and improved phase retrieval algorithms can be devised.

Abstract. We describe the two-photon optical microscopy techniques of second-harmonic generation ... more Abstract. We describe the two-photon optical microscopy techniques of second-harmonic generation imaging and two-photon excited fluorescence imaging, and compare the principles, characteristics, imaging properties and performances of these techniques. We ...

Optics Communications, 2001

When the energy density at the focal spot of a microscope is sufficiently large, nonlinear optica... more When the energy density at the focal spot of a microscope is sufficiently large, nonlinear optical effects such as, harmonic generation, sum-frequency generation, coherent Raman scattering, parametric oscillations, and multi-photon fluorescence can be observed. These optical phenomena can be used in a nonlinear optical microscope to study the biological material. Nonlinear optical microscopy may be divided into incoherent and coherent modes. Incoherent nonlinear microscopy is characterized by the optical signal (like fluorescence) with a random phase, whose power is proportional to the concentration of radiating molecules.

Optics Communications, 2009

New performance parameters are presented for electromagnetic focusing systems of high numerical a... more New performance parameters are presented for electromagnetic focusing systems of high numerical aperture. The field in the focal region is expanded into either electric/magnetic dipole components, or transverse electric/magnetic components. The treatments can be applied to the important case of electric dipole waves, where the focused polarization distribution is the same as that of an electric dipole. For the limiting case of a system of unity numerical aperture, the transverse focus for a focused electric dipole field is sharpened up by 19% compared with an aplanatic system with plane-polarized input, while at the same time the focal intensity for a given input power is increased by 17%. This should be contrasted with using a pupil filter, in which case, as the focal spot is made smaller the focal intensity also decreases. Pure TE polarization results in an even smaller central lobe. Other inhomogeneous polarizations can also be investigated as linear combinations of electric/magnetic or TE/TM components.

Optics Communications, 2009

Optics Communications, 2009

Optics Communications, 2009

An approximation to a Bessel beam produced by tightly focusing linearly polarized light is known ... more An approximation to a Bessel beam produced by tightly focusing linearly polarized light is known to produce a smaller central lobe than focusing plane polarized light. This is because the plane polarized wave gives a broad central lobe caused mainly by a parasitic longitudinal field component. It is known that this problem can be overcome by focusing radially polarized light. Here we demonstrate that other polarization distributions based on a linear combination of transverse electric (TE1) and transverse magnetic (TM1) fields can give a beam even narrower than for the radially polarized case. Special cases of this combination are identified, corresponding to the smallest width (TE1), and the maximum peak intensity compared with the side lobes (electric dipole polarization). Axially-symmetric forms can be generated by illumination with elliptically polarized light. A particular case is azimuthal polarization with a phase singularity, which is equivalent to TE1. For a semi-angular aperture of 60°, the TE1 case gives a central lobe width 9% narrower than for radially polarized illumination, while for plane polarized illumination it is 12% wider than the radially polarized case.

Luminescence, 2005

Fluorescence, absorption and vibrational spectroscopic techniques were used to study spinach at t... more Fluorescence, absorption and vibrational spectroscopic techniques were used to study spinach at the photosystem II (PS II), chloroplast and cellular levels and to determine the effects and mechanisms of ultraviolet-B (UV-B) photoinhibition of these structures. Two-photon fluorescence spectroscopic imaging of intact chloroplasts shows significant spatial variations in the component fluorescence spectra in the range 640–740 nm, indicating that the type and distribution of chlorophylls vary markedly with position in the chloroplast. The chlorophyll distributions and excitonic behaviour in chloroplasts and whole plant tissue were studied using picosecond time-gated fluorescence imaging, which also showed UV-induced kinetic changes that clearly indicate that UV-B induces both structural and excitonic uncoupling of chlorophylls within the light-harvesting complexes. Transient absorption measurements and low-frequency infrared and Raman spectroscopy show that the predominant sites of UV-B damage in PS II are at the oxygen-evolving centre (OEC) itself, as well as at specific locations near the OEC-binding sites. Copyright © 2005 John Wiley & Sons, Ltd.

Applications of phase microscopy based on either coherent or partially coherent sources are widel... more Applications of phase microscopy based on either coherent or partially coherent sources are widely distributed in today's biological and biomedical research laboratories. But the quantitative phase information derivable from these techniques is often not fully understood, because in general, no universal theoretical model can be set up, and each of the techniques has to be treated specifically. This chapter is dedicated to the fundamental understanding of the methodologies that derive optical phase information using imaging techniques and microscopic instrumentation. Several of the latest and most significant techniques are thoroughly studied through the theoretical formalism of the optical transfer function. In particular, we classify these systems into two main categories: those based on coherent illumination, such as digital holographic microscopy (DHM) and its extension into tomography, and those based on partially coherent illumination, such as differential interference contrast (DIC) and differential phase contrast (DPC). Our intention is that the models described in this chapter give an insight into the behaviour of these phase imaging techniques, so that better instrumentation can be designed and improved phase retrieval algorithms can be devised.

Abstract. We describe the two-photon optical microscopy techniques of second-harmonic generation ... more Abstract. We describe the two-photon optical microscopy techniques of second-harmonic generation imaging and two-photon excited fluorescence imaging, and compare the principles, characteristics, imaging properties and performances of these techniques. We ...

Optics Communications, 2001

When the energy density at the focal spot of a microscope is sufficiently large, nonlinear optica... more When the energy density at the focal spot of a microscope is sufficiently large, nonlinear optical effects such as, harmonic generation, sum-frequency generation, coherent Raman scattering, parametric oscillations, and multi-photon fluorescence can be observed. These optical phenomena can be used in a nonlinear optical microscope to study the biological material. Nonlinear optical microscopy may be divided into incoherent and coherent modes. Incoherent nonlinear microscopy is characterized by the optical signal (like fluorescence) with a random phase, whose power is proportional to the concentration of radiating molecules.

Optics Communications, 2009

New performance parameters are presented for electromagnetic focusing systems of high numerical a... more New performance parameters are presented for electromagnetic focusing systems of high numerical aperture. The field in the focal region is expanded into either electric/magnetic dipole components, or transverse electric/magnetic components. The treatments can be applied to the important case of electric dipole waves, where the focused polarization distribution is the same as that of an electric dipole. For the limiting case of a system of unity numerical aperture, the transverse focus for a focused electric dipole field is sharpened up by 19% compared with an aplanatic system with plane-polarized input, while at the same time the focal intensity for a given input power is increased by 17%. This should be contrasted with using a pupil filter, in which case, as the focal spot is made smaller the focal intensity also decreases. Pure TE polarization results in an even smaller central lobe. Other inhomogeneous polarizations can also be investigated as linear combinations of electric/magnetic or TE/TM components.

Optics Communications, 2009

Optics Communications, 2009

Optics Communications, 2009

An approximation to a Bessel beam produced by tightly focusing linearly polarized light is known ... more An approximation to a Bessel beam produced by tightly focusing linearly polarized light is known to produce a smaller central lobe than focusing plane polarized light. This is because the plane polarized wave gives a broad central lobe caused mainly by a parasitic longitudinal field component. It is known that this problem can be overcome by focusing radially polarized light. Here we demonstrate that other polarization distributions based on a linear combination of transverse electric (TE1) and transverse magnetic (TM1) fields can give a beam even narrower than for the radially polarized case. Special cases of this combination are identified, corresponding to the smallest width (TE1), and the maximum peak intensity compared with the side lobes (electric dipole polarization). Axially-symmetric forms can be generated by illumination with elliptically polarized light. A particular case is azimuthal polarization with a phase singularity, which is equivalent to TE1. For a semi-angular aperture of 60°, the TE1 case gives a central lobe width 9% narrower than for radially polarized illumination, while for plane polarized illumination it is 12% wider than the radially polarized case.