טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentNechayev Sergey
SubjectSolar Powered Laser
DepartmentDepartment of Nanoscience and Nanotechnology
Supervisor Professor Carmel Rotschild
Full Thesis textFull thesis text - English Version


Abstract

Solar energy is a vast inexhaustible clean alternative to fossil fuels. Sun is the most abundant energy source available on Earth. The utilization of solar photovoltaics is limited due to the lack of an efficient storage, conversion and transportation mechanisms. The lack of the efficient storage capability for solar energy induces undesired power fluctuation over the electric grid, which limits the maximum portion of solar energy as part of the energy sources portfolio. A solar powered laser is an optical device that converts incoherent, spectrally broad sunlight into a coherent monochromatic light beam. Such lasers have great potential in solar energy conversion and storage. Unfortunately, sunlight is a weak spectrally broad pump and inorganic gain media such as neodymium have low absorption coefficients and poor spectral overlap with sunlight, which requires high solar concentrations for these lasers. Consequently solar pumped lasers are currently non-practical as they operate at high solar concentrations, necessitating cumbersome cooling and sun tracking setups. In this thesis we present a novel optical pumping system for solar pumped lasers that enables efficient coupling of incoherent and spectrally broad sunlight to the lasing gain medium. This method utilizes Cascade Energy Transfer for solar pumped lasers. In this concept high-quality factor laser cavities are sensitized by a combination of materials that form an energetic cascade of near- and ultimately far-field energy transfer. These properties reduce material losses in the cavity and transform incoherent spectrally broad light to coherent monochromatic laser emission. We report a solar powered laser with a record low equivalent solar lasing threshold of 230 suns. The laser consists of a 750-µm-thick Nd3-doped YAG planar waveguide with remote far-field sensitization, showing peak cascade energy transfer of 14% and broad spectral response in the visible portion of the solar spectrum. If the cavity was pumped via cascade energy transfer only, i.e. excluding the contribution of the residual direct pump of the gain medium, the lasing threshold would be 415 suns. The efficient absorption of incoherent, spectrally broad sunlight in small mode volumes paves the way to efficient broadband pumping of high-quality micro-lasers.

However, cascade energy transfer is challenging to model due to the complexity involved with the wide range of angles and wavelengths. Here, we present a generic theoretical framework for computing the absorption, emission and energy transfer of incoherent radiation between cascade sensitizer and laser gain media. Our model is based on linear equations of the modified net radiation method and is therefore robust, fast converging and has low complexity. We apply this formalism to compute the optimal parameters of low-threshold solar-pumped lasers.  It is revealed that the interplay between the absorption and self-absorption of such lasers defines the optimal pump absorption below the maximal value, which is in contrast to conventional lasers for which full pump absorption is desired. Experimental results on the energy transfer between luminescent materials and  agree with the net radiation model. These generic tools modularize gain and sensitizing components, and pave the way to the optimal design of broadband-pumped high-quality micro-lasers.