Bidyut Baran Das

Raman, fluorescence, and time-resolved light scattering as optical diagnostic techniques to separate diseased and normal biomedical media

Studies of Raman scattering, fluorescence and time-resolved light scattering were conducted on cancer and normal biomedical media. Fourier transform Raman spectroscopic measurements were performed on human normal, benign and cancerous tissues from gynecological (GYN) tracts. A comparison of the intensity differences between various Raman modes as well as the number of Raman lines, enables one to distinguish normal GYN tissues from diseased tissues. Fluorescence spectroscopic measurements on human breast tissues show that the ratio of fluorescence intensity at 340 nm to that at 440 nm can be used to distinguish between cancerous and non-cancerous tissues. Separate studies on normal and cancerous breast cell lines show spectral differences. The measurements of back-scattered ultrafast laser pulses from human breast tissues show differences in the scattered pulse profiles for different tissues. These studies show that various optical techniques have the potential to be used in medical diagnostic applications.

Ultrafast time-gated imaging in thick tissues: a step toward optical mammography

With an ultrafast time-gated optical detection method, a thin translucent strip of fat (2.5 mm thick) hidden inside a 4-cm-thick tissue is located with millimeter spatial resolution.

Imaging of a translucent object hidden in a highly scattering medium from the early portion of the diffuse component of a transmitted ultrafast laser pulse.

A translucent object hidden in a highly scattering medium is shown to be visible when the early portion of the transmitted diffuse pulse (snake photons) is detected. The use of the snake scattered photons to image objects depends on the scattering characteristics along a quasi-straight-line path that the photons traverse across the medium. A translucent object with different scattering characteristics compared with its surrounding medium will change the intensity of the snake photons. By scanning the medium across the laser beam and detecting only the snake photons, a translucent object hidden in a highly scattering medium could be located.

Imaging of diffusing media by a progressive iterative backprojection method using time-domain data

A method for the reconstruction of 3-D images of the interior of dense scattering media, based on the analysis of time-resolved backscattered signals is described. The method evaluates a linear perturbation equation by a progressive iterative backprojection scheme. A key feature of the method is the use of weighting functions which estimate the impact that absorption of photons in the interior have on the response of detectors located at the surface. Examples of reconstructed images shown are based on the analysis of simulated data for multilayered media and simulated and experimental data for media containing finite-volume absorbers. These results contain features which indicate that images having high resolution are obtainable even in the limiting case where the view angle is restricted to only backscattered signals and the absorption contrast across an interior boundary is 1%. A general scheme, similar to a layer- stripping approach, is described for the case where signals emerging about a target are measured.

Spectral optical-density measurements of small particles and breast tissues

The optical density of breast tissues without blood is found to be relatively constant from 320 to 800 nm, indicating a relatively independent scattering cross section over this wavelength region.

Three-dimensional image reconstruction in highly scattering turbid media

A novel inverse algorithm, which combines a 2D matrix inversion with a 1D Fourier transform inversion, is designed for obtaining an image of 3D hidden objects in scattering media. The existence of 2D boundaries, on which source- detector pairs are located around, violates the condition for using a 3D Fourier transform inverse imaging. This effect has been handled in our approach. This method greatly reduces the computational burden, compared to standard 3D matrix inversion methods. The result of image of hidden objects using time- resolved simulated data is presented.

Analysis of time-resolved data for tomographical image reconstruction of opaque phantoms and finite absorbers in diffusive media

Experimental time-resolved data was used for direct reconstruction of images of laboratory phantoms in highly scattering media. Using different time zones of the temporal profiles, computed images were calculated by solving a one-step linear perturbation equation derived from transport theory. In nearly all cases tested, high quality reconstructions were obtained even for highly undermined problems.

Radiative and nonradiative recombination processes in ZnCdSe∕ZnCdMgSe multi-quantum-wells

Carrier recombination through radiative and nonradiative processes in lattice-matched n-Zn0.5Cd0.5Se∕Zn0.21Cd0.19Mg0.6Se multi-quantum-wells (MQWs) was investigated by temperature-dependent time-resolved photoluminescence (PL) spectroscopy. The n-Zn0.5Cd0.5Se∕Zn0.21Cd0.19Mg0.6Se MQW samples with different well widths were grown on InP substrates by molecular beam epitaxy. The PL decay times and the PL intensities were measured as functions of temperature. For a doping level of 1×1018cm−3, the dominant mechanism of the radiative process was found to be free carrier recombination while excitonic recombination was absent due to the effect of strong carrier screening. The nonradiative mechanism was determined to be hole capture through multiphonon emission (MPE). The expressions of the nonradiative MPE recombination lifetime, the PL decay time, and the PL intensity were deduced as functions of temperature and were used to fit the measured temperature dependence of the PL decay times and the PL intensities. Th...

UV-fluorescence spectroscopic technique in the diagnosis of breast, ovarian, uterus, and cervix cancer

Malignant breast tumors can be separated from benign and normal tissues using uv-fluorescence spectroscopic technique. Using the same method one can also distinguish cancerous tissues from noncancerous ones in case of cervix, uterus and ovary.

Optical study of strongly coupled CdSe quantum dots

Electronic coupling of vertically stacked CdSe quantum dots is investigated by photoluminescence (PL), time-resolved photoluminescence (t-PL) techniques and Raman techniques. Five samples with each containing 30 layers of self-assembled CdSe quantum dots were grown over InP substrate by molecular beam epitaxy method. The nominal deposition thickness of CdSe layers is 3 ML (monolayer) for all samples. The barrier layers, ZnCdMgSe with MgSe fraction of 59%, were grown with different deposition times to produce different thicknesses of 14, 15, 20, 22, and 25 A. Strong scattering from lomgitudinal-optical phonons of CdSe quantum dots was observed at 228 cm-1 in Ramen spectroscopy. Photoluminescence spectra and time-resolved photoluminescence spectra were measured at 77 K. With decreasing of barrier thickness, the PL peak shifts to low energy side. This indicates that thinner spacing barrier results in stronger interdot coupling. The lifetime of electrons for each sample was determined by fitting the t-PL data...