Current research / employment

Short abstract of IMPRS PhD project:

As modern-day optoelectronics and related technologies are constantly moving towards smaller dimensions, there is an increasing need to develop efficient miniature light sources. Microcavity lasers are very promising in this respect due to their microscale sizes, low lasing thresholds, and narrow output linewidths. This work focuses on the development and study of three such lasers, namely, the ZnO tetrapod laser, the glycerol microdrop Raman laser, and the CdSe/ZnS quantum dot microdrop laser. An “end-cap” type electrodynamic trap is used to spatially confine the lasing microparticles. A Q-switched Nd:YAG laser (10 Hz, τ ∼10 ns) is used for optical excitation. We experimentally show the viability of electrodynamically isolating and micropositioning ZnO-based nanostructures to investigate their intrinsic optical nature under atmospheric conditions. An electrospray technique is used to spray a dilute solution of ZnO tetrapods (in methanol) into the electrodynamic trap. Subsequent tuning of trapping parameters, as the methanol evaporates, leads to the stable confinement of a single ZnO tetrapod in free space. UV lasing (around 390 nm), with threshold fluence around 10 mJ/cm2, is observed from single and multiple trapped ZnO tetrapods with typical leg lengths of 15-25 μm. Moreover, precise translational micromanipulation of a trapped tetrapod is shown up to a range of 100 μm. We further demonstrate Raman lasing from a trapped pure glycerol microdrop and present long-term measurements of the lasing blinking (on/off) behavior. Single and multimode Raman lasing (around 630 nm) are achieved and shown for glycerol drops of 10.3 μm and 44.7 μm in diameter, respectively. Typical threshold fluences are measured to be between 200-390 mJ/cm2. Lasing is found to occur in temporally separated and nearly symmetric bursts which increase in frequency and decrease in duration as the evaporation rate of the drop is increased. By using drops of different glycerol concentrations and by varying the pump fluence, we conclusively demonstrate that the Raman lasing blinking is caused by double resonances in the evaporating drop and that it can be manipulated by controlling the drop’s evaporation rate. Finally, we demonstrate single and multimode lasing (around 640 nm) from CdSe/ZnS doped microdrops, of diameters 9 μm and 34 μm, respectively, at threshold pump fluences of around 50 mJ/cm2. Blue-shifts of up to 2 nm for the lasing modes and 3.2 nm for the quantum dot gain profile are observed with increasing pump fluences. Moreover, our results indicate that the minimum quantum dot concentration required for lasing can be more than two orders of magnitude lower than the previously reported theoretical limit.

Publications R. Sharma, J. P. Mondia, J. Schäfer, Z. H. Lu, L. J. Wang, Evaporation effects on blinking properties of the glycerol microdrop Raman Laser, J. Appl. Phys., 105, 113104, (2009)

R. Sharma, J. P. Mondia, J. Schäfer, W. Smith, S.-H. Li, Y.-P. Zhao, Z. H. Lu, L. J. Wang, Measuring the optical properties of a trapped ZnO tetrapod, Microelectronics Journal, 40, 520, (2009)

J. P. Mondia, R. Sharma, J. Schäfer, W. Smith, Y. P. Zhao, Z. H. Lu, L. J. Wang, An electrodynamically confined single ZnO tetrapod laser, Appl. Phys. Lett., 93, 121102, (2008)

J. Schäfer, J. P. Mondia, R. Sharma, Z. H. Lu, A. S. Susha, A. L. Rogachand, L. J. Wang, Quantum dot microdrop laser, Nano Lett., 8, 1709, (2008)

J. Schäfer, J. P. Mondia, R. Sharma, Z. H. Lu, L. J. Wang, Modular microdrop generator, Review of Scientific Instruments, 78, 066102, (2007)