Krill Prize Laureate 2017
Dr. Carmel Rotchild
Technion – Israel Institute of Technology
- New thermodynamic ideas for solar energy: In principle, converting inefficient parts of the solar spectrum to wavelengths most suitable for photovoltaics should double their efficiency. Confronting this problem using conventional optical frequency conversion methods is known to be very challenging because they require huge intensities and high coherence far above solar radiation.
Recently, we discovered what we believe is a breakthrough in both fundamental and applied science. The radiance of thermal emission, as described by Planck’s law, depends only on the emissivity and temperature of a body, and increases monotonically with temperature rise at any emitted wavelength. Nonthermal radiation, such as photoluminescence (PL), is a fundamental light–matter interaction that conventionally involves the absorption of an energetic photon, thermalization, and the emission of a red-shifted photon. In this quantum process, radiation is governed by the photon rate conservation and thermodynamically described by the chemical potential. Until recently, the role of rate conservation when thermal excitation is significant had not been studied in any nonthermal radiation, leaving open many questions; for example, what is the overall emission rate if a high quantum efficiency PL material is heated to a temperature
where it thermally emits a rate of 50 photons/sec at its bend edge, while in parallel, the PL is excited at a rate of 100 photons/sec? We discovered that the answer is an overall rate of 100 blue-shifted photons/sec. In contrast to thermal emission, the PL rate is conserved if the temperature increases, while each photon is blue-shifted. A further rise in temperature leads to an abrupt transition to thermal emission where the photon rate increases sharply. We also demonstrated how endothermic-PL generates orders of magnitude more energetic photons than thermal emission at similar temperatures. These findings show that PL is an ideal optical heat pump, and can harvest thermal losses in photovoltaics with theoretical maximal efficiency of 70%, and a practical device that aims to reach 48% efficiency. These discoveries have already led to two publications in leading journals (Optica, Nature Communication and its Optics and Photonics News). A paper was also published in the proceedings as an invited talk at SPIE, the largest conference on optics. Based on our proposed device, we were invited to join in publishing with leading researchers in the field (Eli Yablonovich, Gang Chen, Martin Green, and more) on the “Roadmap on the Optical Energy Conversion”. These publications of my group put us in the vanguard of research on the thermodynamics of radiation. As a result of these achievements, I have been invited to give numerous invited talks and my student, Dr. Assaf Manor, was awarded an ‘Adams Fellowship’. We are currently aiming to demonstrate a fully operational energy conversion device with record efficiency as part of our ERC grant on “new thermodynamic ideas for frequency conversion and photovoltaics”. Due to its ‘all optical’ nature, and low temperature operation, such a device has a significant chance of becoming a disruptive technology in photovoltaics.
- New thermodynamic idea for extreme optical frequency up-conversion: Frequency up- conversion of a few low-energy photons into a single high-energy photon contributes to imaging, light sources, and detection. The up-converting of many photons, however, exhibits negligible efficiency. Up-conversion through laser heating is an efficient means to generate energetic photons, yet the spectrally broad thermal emission and the challenge of operating at high temperatures limit its practicality. We recently developed and demonstrated experimentally up- conversion by excitation of a steady-state nonthermal-equilibrium population, which induces steady, narrow emission at a practical bulk temperature. Specifically, we used a 10.6mm laser to resonantly excite vibronic states in silica that are coupled to emitters operating at 980nm. The result is a narrow emission with 4% total efficiency and up-converted radiance that far exceeds the device’s possible black-body radiation. Such efficiency is better by many orders of magnitude than ever reported and opens the way for the development of new light sources with record efficiencies. We recently published these results in ASC Photonics. In another paper, currently under review, we use the same method to demonstrate experimentally that in contrast to thermal excitation, different co-doped emitters compete with each other for energy transferred from the ‘hot’ vibronic modes in a similar way as in photoluminescence (down conversion), even though the photon energy is increased 10-fold. In addition, the same method is used to generate 13-, 16- and 20-fold narrow up-conversion at record efficiencies. Further development of this method using tailored materials will put us in a position to present new and more efficient UV and X-ray
- New thermodynamic idea for solar powered laser and on chip high-Q lasers: The optical conversion of incoherent solar radiation into a bright, coherent laser beam enables the application of nonlinear optics to solar energy conversion and energy storage. Prior to our work, solar powered lasers only operated under highly concentrated sunlight (above 3000 suns), which makes them inapplicable. As Yablonovich presented in his paper ‘Thermodynamics of Daylight- pumped Lasers’, for lasers to operate with non-concentrated solar radiation, it requires a super lasing material that has a ratio between pump absorption and absorption at lasing wavelength that exceeds 105. This ratio is nearly three orders of magnitude higher than known lasing materials. In my postdoc, I showed that such a material can be tailored by cascaded energy transfer between available materials. Now, using this method, we were able to have demonstrate low threshold solar lasers operating at 200 suns. This work was published in Scientific Reports, and co- authored with my MIT supervisor, Marc Balo. Although this concept is my idea, and my student, Sergey Nechayev, did most of the experiments, it was done in Marc’s lab at MIT as a result of the delay in my lab In a second paper recently published in Scientific
Reports, with the same authors, I developed a new analytic tool for an incoherent pump for photonic devices. The simulation is based on a modified net radiation method where we incorporated photoluminescence and is much simpler and more reliable than existing Monte- Carlo or coherent simulations. We used this method for a solar powered laser and revealed, for the first time, that pump absorption cannot be maximal as in conventional lasers, but must be optimized due to the tradeoff between pump absorption and absorption at lasing wavelength. A third paper on the thermodynamic efficiency limit of solar powered lasers, and its analog to solar cells is currently under review. These publications put us in the forefront of incoherent pumped laser research.