|M.Sc Student||Granot Dafna|
|Subject||Efficient Tenfold Up-Conversion through Steady-|
State Non-Thermal Equilibrium Excitation
|Department||Department of Energy||Supervisor||Professor Carmel Rotschild|
|Full Thesis text|
Frequency up-conversion of a few low-energy photons into a single high-energy photon can contribute to different fields such as imaging, new light sources development, detection and solar energy harvesting. Conventional optical frequency up-conversion methods, including coherent and incoherent processes, have been extensively researched and used for decades. Yet, up-conversion of many photons (~10) by current methods exhibits negligible efficiency, due, e.g., to the large momentum mismatch between the pump and the produced frequencies in parametric processes, and the high intensities required for many photons to interact simultaneously in multi-photons absorption processes. Therefore, the record efficiency of ten-fold up-conversion is less than 0.01%, achieved under pulse excitation at intensities of 1015 W/cm2 - many orders of magnitude above currently available continuous wave sources. Our aim is to present a new multi-photon up-conversion mechanism operating efficiently at low pump intensities.
Laser heating to high temperatures can be considered an up conversion process if the energy of the photons, emitted thermally, exceeds the laser photon energy. Such up-conversion can be an efficient means to generate energetic photons, yet the spectrally broad thermal-emission and the challenge of operating at high temperatures limit its practicality. The advantages of this process can potentially be exploited by heating only specific modes, thereby generating narrow up-converted emission; however, so far such ‘hot-carriers’ have been observed only in down-conversion processes and as having negligible lifetime, due to fast thermalization.
My group experimentally demonstrated up-conversion from 10.6µm CO2 laser to 980nm narrow emission with 4% total efficiency. In my research, I explored the physical mechanism of this efficient up-conversion and empirically proved it is based on excitation of a steady-state non-thermal-equilibrium population, which induces steady, narrow emission at a practical bulk temperature. The emitting modes are heated to a high brightness-temperature, and therefore the up-converted radiance far exceeds the device’s possible black-body radiation. A model, based on the temperature dependence of rare-earth emission, also confirms the results.
In future research, the efficiency of this process can be further improved using different materials and excitation wavelengths, along with conventional methods for tailoring thermal emission such as photonic bandgap structures. This method may open new directions in various fields of research, such as new light sources and solar energy, where efficiency can be enhanced by up-converting sub-bandgap solar photons to wavelengths where photovoltaics are most efficient.