Srivathsan Vasudevan

Photothermal imaging of nanoparticles beyond the diffraction limit

In this paper, a full-field photothermal imaging technique, which does not require a time consuming scan as used in the conventional photothermal imaging system, is reported. Imaging on gold nanoparticles (70 nm) and a blue polystyrene bead (193 nm) were conducted and the experimental results demonstrate the visualization ability of the photothermal imaging technique on nanotargets that are below the diffraction limit. The photothermal imaging system can be operated in an ambient environment where vacuum is not required.

Excitation temporal pulse shape and probe beam size effect on pulsed photothermal lens of single particle

We developed the theoretical model of the time-resolved thermal lens spectroscopy of a single particle with various pulse shape optical excitations. To account for the pulse shape optical excitation in the model, a heat diffusion equation of two media (the particle and liquid solvent) is solved using the numerical Laplace transform method. The model also incorporates the propagation of a diffracted Gaussian probe beam due to the thermal lens effect. Numerical results are presented to illustrate the effects of the excitation pulse shape and probe beam size on the evolution of the photothermal lens signal. The developed model is utilized for the thermal diffusivity and size extraction of a red polystyrene particle.

Application of thermal lens response to monitor health status of red blood cells: A quantitative study of the cell death process by extracting thermal diffusivity and size

We explore monitoring the death process of individual red blood cells (RBC) quantitatively by using thermal lens (TL) response. TL response is a noninvasive excitation/probe technique that reflects photothermal parameters (e.g., absorption, thermal diffusivity, size, etc.). Since these parameters of cells change significantly during certain biological processes, real-time TL response was performed to monitor RBC death process when incubated with ionomycin. Theoretical model developed was applied to curve-fit the TL response for extracting thermal diffusivity and size of cells. Thermal diffusivity of dying RBC is found increased by 1.7 times in comparison with healthy cell.

Integration of laser trapping for continuous and selective monitoring of photothermal response of a single microparticle

Photothermal response (PTR) is an established pump and probe technique for real-time sensing of biological assays. Continuous and selective PTR monitoring is difficult owing to the Brownian motion changing the relative position of the target with respect to the beams. Integration of laser trapping with PTR is proposed as a solution. The proposed method is verified on red polystyrene microparticles. PTR is continuously monitored for 30 min. Results show that the mean relaxation time variation of the acquired signals is less than 5%. The proposed method is then applied to human red blood cells for continuous and selective PTR.

Photothermal Imaging (PTI) System for the Imaging of Gold Nano-Particles and Cellular Organelles

Photo-thermal imaging (PTI) is an imaging technique that can provide a resolution exceeding the diffraction limit. The theoretical resolution limit is less than 1 nm, therefore it is particularly suited to analyse particles in nano-size targets. It is also a non-labelling, high temporal resolution technique that can be conducted in a normal live cell environment. In this paper, a PT imaging microscopy system has been demonstrated and the integrity of the system is verified through a series of experiments on different samples, including red polystyrene bead and carbon nano-tube. The PTI technique can find further applications in biology studies for the visualization of cell organelles and apoptosis studies.

Applications of super-resolution in photothermal imaging

This paper focuses on applying Super-Resolution (SR) on Photothermal images of the 4.9μm Red Polystyrene Beads (RPB) obtained from a Photothermal imaging (PTI) system. Since the images acquired from the PTI system are noisy and it can be costly to achieve a higher resolution from the hardware side, it is efficient to do so by SR. In this paper, the experimental procedures are presented and the SR shows significant improvement in the processed images, mainly in terms of resolution, sharpness and contrast.

Temperature Distribution in Red Blood Cells using Photothermal Imaging Integrated with Digital Holography

Integration of digital holographic microscope with photothermal microscope is proposed. Besides obtaining 3D images, temperature distribution of red blood cells can be obtained, aiding real-time monitoring of biological assays.

Photothermal Techniques for Materials Characterization and Live Cells Monitoring

Photothermal technology can be applied for thermal characterization of thin films and studying biological live cells. Using pulsed photothermal reflectance technique, thermal conductivity of thin films ranging from 20 nm to a few microns can be determined. Photothermal imaging (PTI) technique can provide ultra high spatial resolution and no fluorescence molecules are required. Comparison with normal phase contrast images, PT images reveal temperature distribution and more information on the intracellular components. Photothermal response (PTR) technique can provide high temporal resolutions. In monitoring the dying process of MDCK cells, significant changes in the PTR signals can be observed whereas unnoticeable changes are found in the images.

Integration of photothermal response with optical tweezers for accurate extraction of thermal diffusivity of micro-particle

Photothermal Response (PTR) is a non-invasive method for real-time monitoring of biological processes. Accurate extraction of thermal parameters from the PTR results of a coalition of targets is difficult as the PTR signal is shape and size dependent. We propose optical tweezers to isolate a single particle from a group and perform PTR. The proposed technique is applied to red polystyrene micro-particles and the thermal diffusivity extracted is very much comparable to the standard values.