The industrial sector consumes one third of its total incoming energy and the remaining energy is discharged as wasted heat. Here, we experimentally demonstrate Thermally Enhanced Photoluminescence device for harvesting industrial wasted heat using low band gap photovoltaics. Specifically, the emission of a low band gap photoluminescence material excited next to its band edge and is heated in parallel, is characterized by a conserved photon flux of a blue-shift emission that can be coupled to a higher bandgap PV generating enhanced electricity due to the high operating voltage. The efficiency of such concept is orders of magnitude higher than the efficiency of thermal emission at the same temperature. Using a hot plate at 50oC-100oC as a heat source and Er:Tm doped silica, we have observed the absorption at a spectral range between 1.6 um and 1.9 um that is followed by a blue-shifted photoluminescence at 1.6 um that can be efficiently coupled to GaInAs or Ge solar cell. Using efficient LED and solar cell can result in a pseudo “perpetual motion” where the excessive heat together with the LED pump generates electric power exceeding the LED power consumption. This device can be extended for harvesting solar radiation between 1µm and 2 µm, which is considered as wasted radiation. Theoretical result shows that the ideal system can reach up to 28% efficiency cells, overcoming thermo-electrics concepts.
Frequency up-conversion is a technique for the generation of high energy photon from two or more lower energy photons. Although many up-conversion techniques have been demonstrated such as parametric up-conversion or multi-photon absorption, their conversion efficiencies become negligible for high-order up-conversion. Alternatively, in thermal emission very high temperatures are needed for reasonable efficiencies and the emission is spectrally broad rendering this up-conversion method impractical for most applications. We present a new efficient extreme up-conversion method for generating NIR and visible wavelengths using CW LWIR laser by non-thermal-equilibrium excitation through spontaneous reduction of the chemical potential. In this method we exploit the high chemical potential of the pump specific modes to excite the vibronic states of the host, subsequently transferring the energy to chosen emitters, resulting in narrow non-thermal steady-state emission. All while, only residual energy contributes to material's temperature, thus keeping it at a comparably low temperature. We experimentally demonstrate 7, 10, 13, 16, and 20-fold up-conversion at external efficiency of up to 4%, exceeding black-body radiation of the bulk temperature. Furthermore, we present energy transfer between emitters, a phenomenon in contrast to thermal emission, showing the photoluminescence behavior of this method. We use CW CO2 laser (10.6m) to excite silica vibronic states and transfer the energy to rare-earth emitters at the NIR and visible spectrum. This new outlook on up-conversion via energy transfer paves the way for developing new light sources and new methods of imaging and detection with high efficiencies.
While single-junction photovoltaics (PV's) are considered limited in conversion efficiency according to the Shockley-Queisser limit, concepts such as solar thermo-photovoltaics aim to harness lost heat and overcome this barrier. We claim the novel concept of Thermally Enhanced Photoluminescence (TEPL) as an easier route to achieve this goal.
Here we present a practical TEPL device where a thermally insulated photo-luminescent (PL) absorber, acts as a mediator between a photovoltaic cell and the sun. This high temperature absorber emits blue-shifted PL at constant flux, then coupled to a high band gap PV cell. This scheme promotes PV conversion efficiencies, under ideal conditions, higher than 62% at temperatures lower than 1300K. Moreover, for a PV and absorber band-gaps of 1.45eV (GaAs PV's) and 1.1eV respectively, under practical conditions, solar concentration of 1000 suns, and moderate thermal insulation; the conversion efficiencies potentially exceed 46%.
Some of these practical conditions belong to the realm of optical design; including high photon recycling (PR) and absorber external quantum efficiency (EQE). High EQE values, a product of the internal QE of the active PL materials and the extraction efficiency of each photon (determined by the absorber geometry and interfaces), have successfully been reached by experts in laser cooling technology. PR is the part of emitted low energy photons (in relation to the PV band-gap) that are reabsorbed and consequently reemitted with above band-gap energies. PV back-reflector reflectivity, also successfully achieved by those who design the cutting edge high efficiency PV cells, plays a major role here.
The present study discusses the photosensitivity of GeS2 chalcogenide glass in response to irradiation with femtosecond pulses at 1047 nm. Bulk GeS2 glasses are prepared by conventional melt quenching technique and the amorphous nature of the glass is confirmed using X-ray diffraction. Ultrafast laser inscription technique is used to fabricate the straight channel waveguides in the glass. Single scan and multi scan waveguides are inscribed in GeS2 glasses of length 0.65 cm using a master oscillator power amplifier Yb doped fiber laser (IMRA μjewel D400) with different pulse energy and translation speed. Diameters of the inscribed waveguides are measured and its dependence on the inscription parameters such as translation speed and pulse energy is studied. Butt coupling method is used to characterize the loss measurement of the inscribed optical waveguides. The mode field image of the waveguides is captured using CCD camera and compared with the mode field image of a standard SMF-28 fibers.
In this work, we synthesized bulk amorphous GeGaS glass by conventional melt quenching technique. Amorphous nature of
the glass is confirmed using X-ray diffraction. We fabricated the channel waveguides on this glass using the ultrafast laser
inscription technique. The waveguides are written on this glass 100 μm below the surface of the glass with a separation of 50
μm by focusing the laser beam into the material using 0.67 NA lens. The laser parameters are set to 350 fs pulse duration at
100 KHz repetition rate. A range of writing energies with translation speeds 1 mm/s, 2 mm/s, 3 mm/s and 4 mm/s were
investigated. After fabrication the waveguides facets were ground and polished to the optical quality to remove any tapering
of the waveguide close to the edges. We characterized the loss measurement by butt coupling method and the mode field
image of the waveguides has been captured to compare with the mode field image of fibers. Also we compared the
asymmetry in the shape of the waveguide and its photo structural change using Raman spectra.
We report here, a finite difference thermal diffusion (FDTD) model for controlling the cross-section and the guiding nature of the buried channel waveguides fabricated on GeGaS bulk glasses using the direct laser writing technique. Optimization of the laser parameters for guiding at wavelength 1550 nm is done experimentally and compared with the theoretical values estimated by FDTD model. The mode field diameter (MFD) between 5.294 μm and 24.706 μm were attained by suitable selection of writing speed (1mm/s to 4 mm/s) and pulse energy (623 nJ to 806 nJ) of the laser at a fixed repletion rate of 100 kHz. Transition from single-mode to multi-mode waveguide is observed at pulse energy 806nJ as a consequence of heat accumulation. The thermal diffusion model fits well for single-mode waveguides with the exception of multi-mode waveguides.
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