Tirtha received his B.S. degree in Electrical Engineering from Bangladesh University of Engineering and Technology (BUET) in 2014. Right afterwards, he joined OMEMS group to explore his interets in experimental work.
Tirtha’s research interests include particle behavior in laser, engineer microparticles to manipulate their optical and thermal properties. Still finding his foothold in research community, he is interested in any interesting experimental setup that will come along. Outside lab, he enjoys Tennis, Biking, Soccer, Volleyball, Table Tennis, Hiking, Camping etc. He also has interests in Politics, Business and Technology of all sorts. When these are not enough to waste his time, he watches TV shows and documentaries (a lot).
Materials with high bandgaps are resistant to particle-induced breakdown under CW illumination because evaporating contaminants then generate fewer free carriers near the surface. Laser-accelerated atmospheric particles induce failure even more strongly than fixed surface particles.
A Yb-doped fiber laser is used to accelerate and evaporate absorbing particles in air. Optical intensities of 1MW/cm2 and 2MW/cm2 illuminate stainless steel particles. These particles are accelerated to velocities of tens of meters per second before evaporating within a few tenths of a millisecond. Position measurements are made using direct imaging with a high-speed camera. A fundamental system of coupled differential equations to track particle momentum, velocity, mass, radius, temperature, vapor opacity, and temperature distribution is developed and shown to accurately model the trajectories and lifetimes of laser heated particles. Atoms evaporating from the particle impart momentum to the larger particle, which accelerates until it is slowed by drag forces. Heat transfer within the evaporating particles is dominated by radiation diffusion, a process that usually only dominates in astrophysical objects, for example in the photospheres of stars.
It is well-known that continuous-wave lasers can accelerate small particles to high velocities, and, more recently, it has been observed that laser damage of optical materials occurs at laser intensities that are orders of magnitude lower in the presence of accelerated absorbing particles. At these high powers the primary interactions are ablative, and a complete set of force balance equations in this regime have only recently been described. In our experiments, small groups of 35-41m diameter stainless steel particles are accelerated using a high-power continuous wave (CW) Yb-doped fiber laser illuminating at intensities between 1MW/cm2 and 2MW/cm2. The trajectories of the particles are tracked using a high-speed camera until they leave the camera field of view or evaporate, a process typically occurring on timescales of a few milliseconds. The lifetimes and trajectories of the superheated particles are calculated using a system of coupled equations to track particle velocity, mass, momentum, radius, vapor opacity, temperature, and temperature distribution. Upon illumination, the particles heat to their vaporization temperatures, with evaporating atoms transferring momentum to the remaining particle. Significant laser attenuation occurs due to an opaque glow region around the particle consisting of dense evaporated atoms and ions. Since the particle is not uniform in temperature, the number of evaporating atoms varies with position and imparts a net acceleration to the particle until a terminal velocity on the order of a few tens of meters per second is reached where drag forces offset further acceleration. Based on the calculated interplay between temperature gradient and acceleration, heat transfer within the evaporating particles must be dominated by radiation diffusion, a process that usually only dominates in astrophysical objects, for example in the photospheres of stars. It is further observed that absorbing particles accelerated by continuous wave (CW) lasers initiate catastrophic failure in the form of micromachined drill holes. This process was tested using stainless steel, PMMA, and silica particles with fused silica, sapphire, and spinel substrates. Hole drilling occurred at laser power densities as low as 250kW/cm2. A potential dependence of accelerated particle breakdown on substrate bandgap may suggest that the underlying physical process is similar to that seen for CW laser breakdown of contaminated optical coatings.