Mid-infrared spectroscopy is one of the most important techniques in chemical analysis. However, the detectors for the mid-infrared range suffer from lower specific detectivities in comparison to their visible counterparts, cost more and often require cryogenic cooling. Nonlinear interferometers allow measuring mid-infrared spectra by detecting only visible light using the induced coherence effect. In our work, we realize a nonlinear interferometer designed for broadband mid-infrared spectra with high resolution, which is easily tunable, and in analogy to classical Fourier transform infrared (FTIR) spectrometers requires no additional spectral selection.
In this paper, we present the implementation of a differential-absorption measurement-technique for surface moisture detection into a laser scanner aiding modern tunnel inspections. The use of laser scanners for tunnel inspections can reduce costly tunnel closures and provide digital data, compliant with modern building information modeling (BIM). Unfortunately, available systems are typically limited to pure 3D mapping. The Fraunhofer Institute for Physical Measurement Techniques IPM is developing a novel multi-parameter laser scanning system. For the first time, this system allows the simultaneous measurement of 3D-geometry, remission and surface moisture. The scanner measures simultaneously with two collinear laser beams with distinct wavelengths. One is centered at the absorption band of water at 1450 nm wavelength, while the other, with 1320 nm wavelength, is used as an intensity reference. The intensity ratio gives a good estimate of the surface water content. Additionally, the power of both lasers is modulated with a high frequency. This enables simultaneous measurement of the distance by comparing the phase difference of the backscattered light with a local reference. With this approach, we are able to record up to two million points per second containing distance, intensity and moisture information. Besides the technical implementation, we present point clouds from multiple test objects and surfaces. The presented data nicely demonstrates the ability to differentiate between absolute intensity variations, e.g. caused by dirt, and actual water contamination.
Upconversion of sub-band-gap photons is a promising approach to increase the efficiency of solar cells. In this paper, we review the recent progress in upconverter material development and realization of efficient upconverter silicon solar cell devices. Current published record values for the increase in the short-circuit current density due to upconversion are 13.1 mA/cm2 at a solar concentration of 210 suns determined in a sun simulator measurement. This increase is equivalent to a relative efficiency enhancement of 0.19% for the silicon solar cell. Although this is a considerable enhancement by more than one order of magnitude from values published only a few years ago, further enhancement of the upconversion performance is necessary. To this end, we investigate theoretically the application of resonant cavity and grating photonic structures. Our simulation based analysis considers irradiance enhancement and modified density of photon states due to the photonic structures and their impact on the upconversion dynamics in β-NaYF4: 20%Er3+. It shows that an optimized grating can increase upconversion luminescence by a factor of 3 averaged over the whole structure in comparison to an unstructured reference with the same amount of upconverter material.
For high band gap solar cells, organic molecule based upconverter materials are promising to reduce transmission losses of photons with energies below the absorption threshold. We investigate the approach of embedding the organic upconverter DPA:PtOEP directly into each second layer of a Bragg stack to achieve an enhancement of upconversion performance. The two major effects that influence the upconversion process within the Bragg stack are simulated based on experimentally determined input parameters. The locally increased irradiance is simulated using the scattering matrix method. The variation of the density of photon states is obtained from calculations of the eigenmodes of the photonic crystal using the plane wave expansion method. A relative irradiance enhancement of 3.23 has been found for a Bragg stack of 31 layers including λ/8-layers on both sides. For suppressing the loss mechanism of direct sensitizer triplet decay via variations of the density of photon states, a different design of the Bragg stack is necessary than for maximum irradiance enhancement. In order to find the optimum design to increase upconversion quantum yield, both simulation results need to be coupled in a rate-equation model. The irradiance enhancement found in our simulation is significantly higher than the one found in the simulation of a grating-waveguide structure, which achieved an increase of upconversion quantum yield by a factor of 1.8. Thus, the Bragg structure is very promising for upconversion quantum yield enhancement.
Upconverter materials and upconverter solar devices were recently investigated with broad-band excitation revealing the great potential of upconversion to enhance the efficiency of solar cell at comparatively low solar concentration factors. In this work first attempts are made to simulate the behavior of the upconverter β-NaYF4 doped with Er3+ under broad-band excitation. An existing model was adapted to account for the lower absorption of broader excitation spectra. While the same trends as observed for the experiments were found in the simulation, the absolute values are fairly different. This makes an upconversion model that specifically considers the line shape function of the ground state absorption indispensable to achieve accurate simulations of upconverter materials and upconverter solar cell devices with broadband excitations, such as the solar radiation.
Upconversion of low-energy photons presents a possibility to overcome the Shockley-Queisser efficiency limit for solar cells. In silicon 20% of the incident energy is lost due to transmission of these photons with energies below the band gap. Unfortunately, upconversion materials known today show pretty low absorption and quantum yields which are too low for this application. One possibility to boost the upconversion luminescence and even the quantum yield could be the embedding of the material in a suitable photonic structure environment. This influences the local irradiance onto the upconverter and the local density of states at the transition wavelengths. Thus, the radiative recombination from a specific energy level can be influenced. Hence, this approach has the potential to beneficially influence the upconversion quantum yield. For the buried grating structure shown here, a luminescence enhancement by a factor of 1.85 could be achieved, averaged over the grating.
Photonic crystals modify the local density of photon states. These variations influence the emission properties of a dipole
embedded within the photonic crystal. Furthermore, field enhancement can be observed within photonic crystals. In this
paper, we investigate how these effects influence upconversion processes in β-NaYF4:Er3+. For this purpose we use
finite-difference time-domain (FDTD) simulations of a grating-waveguide-structure in combination with a rate equation
model of the upconversion processes in β-NaYF4:Er3+. The grating parameters are optimized to achieve large field
enhancements within the structure for the combination of s- and p-polarized light. Furthermore, the variation of the
spontaneous emission rates for dipole emitters within the structure is simulated. The varied transition rates, as well as the
field enhancement, serve as input parameters for the rate equation model for upconversion. Using this approach, the
influence of the structure on the upconversion quantum yield is calculated. For a simulated initial irradiance of
1000 W/m2, we find enhancement factors of up to four for the field enhancement in the upconverter region and up to a
factor of three for the upconversion quantum yield. In consequence, the incorporation of upconverting material in
photonic structures in very promising to increase upconversion efficiencies.
Upconversion (UC) of sub-band-gap photons can increase solar cell efficiencies. Up to now, the achieved efficiencies are
too low, to make UC relevant for photovoltaics. Therefore, additional means of increasing UC efficiency are necessary.
In this paper, we investigate both metal and dielectric photonic nanostructures for this purpose. The theoretical analysis
is based on a rate equation model that describes the UC dynamics in β-NaYF4 : 20% Er3+. The model considers ground
state and excited state absorption, spontaneous and stimulated emission, energy transfer, and multi phonon relaxation.
For one, this model is coupled with results of Mie theory and exact electrodynamic theory calculations of plasmon
resonance in gold nanoparticles. The effects of a 200 nm gold nanoparticle on the local field density and on the transition
rates within in the upconverter are considered. Calculations are performed in high resolution for a three dimensional
simulation volume. Furthermore, the effect of changed local fields in the proximity of grating waveguide dielectric
nanostructure is investigated. For this purpose FDTD simulation models of such structures are coupled with the rate
equation model of the upconverter. The results suggest that both metal nanoparticles and dielectric nanostructures can
increase UC efficiency.