The Planetary Spectroscopy Laboratory (PSL): wide spectral range, wider sample temperature range

The Planetary Spectroscopy Laboratory (PSL) of DLR in Berlin provides spectral measurements of primarily planetary analogues from the visible to the far-infrared range. PSL has supported the data analysis as well as the development and calibration of instruments for planetary missions from ESA, NASA and JAXA. For this purposes PSL provides reflection, transmission and emission spectroscopy of target materials. Currently PSL operates three identical Bruker Vertex 80V vacuum FTIR spectrometer (the third one just installed in June 2019), two spectrometers are equipped with aluminum mirrors optimized for the UV, visible and near-IR, the third features gold-coated mirrors for the near to far IR spectral range. External simulation chambers are attached to two of the instruments for emissivity measurements. The chamber at the near to far IR instrument allows emissivity measurements from 0.7-200 μm under vacuum for sample temperatures from 320K to above 900K, using an innovative induction system. The second chamber (purged with dry air and water cooled to ≤270K) allows emissivity measurements of samples with surface temperature from 290K to 420K. We measure bi-directional reflectance of samples; with variable incidence and emission angles between 0° and 85° (minimum phase angle is 26° to prevent damages to the mirrors). Samples are measured currently at room temperature and 170K, with a planned extension for temperatures below 100K, by means of a new external chamber, whose funding is accepted and will be available in 2020. Bi-directional and hemispherical reflectance is measured under purging/vacuum conditions, covering the 0.2 to above 200 μm spectral range. An FT-IR microscope installed at the end of 2018, allows microscopic analysis in transmission and reflectance in the VIS+VNIR+MIR spectral range. Transmission of thin slabs, optical filters, optical windows, pellets, and others is measured in the complete spectral range from UV to FIR using a parallel beam configuration to avoid refraction

Carbon-rich amorphous silicon carbide and silicon carbonitride films for silicon-based photoelectric devices and optical elements: Application from UV to mid-IR spectral range

Spectral behavior of sulfides in simulated daytime surface conditions of Mercury: Supporting past (MESSENGER) and future missions (BepiColombo)

Multispectral bidirectional reflectance of northern forest canopies with the advanced solid-state array spectroradiometer (ASAS)

Effect of grafite on emissivity and reflectance spectra for Mercury surface simulants

Hemispherical reflectance and internal quantum efficiency measurements have been employed to evaluate the response of Si nanostructured surfaces formed by using random and periodic reactive ion etching techniques. Random RIE-textured surfaces have demonstrated solar weighted reflectance of {approx} 3% over 300--1,200-nm spectral range even without the benefit of anti-reflection films. Random RIE-texturing has been found to be applicable over large areas ({approximately} 180 cm{sup 2}) of both single and multicrystalline Si surfaces. Due to the surface contamination and plasma-induced damage, RIE-textured surfaces did not initially provide increased short circuit current as expected from the enhanced absorption. Improved processing combined with wet-chemical damage removal etches resulted in significant improvement in the short circuit current with IQEs comparable to the random, wet-chemically textured surfaces. An interesting feature of the RIE-textured surfaces was their superior performance in the near IR spectral range. The response of RIE-textured periodic surfaces can be broadly classified into three distinct regimes. One-dimensional grating structures with triangular profiles are characterized by exceptionally low, polarization-independent reflective behavior. The reflectance response of such surfaces is similar to a graded-index anti-reflection film. The IQE response from these surfaces is severely degraded in the UV-Visible spectral region due to plasma-induced surface damage. One-dimensional grating structures with rectangular profiles exhibit spectrally selective absorptive behavior with somewhat similar IQE response. The third type of grating structure combines broadband anti-reflection behavior with significant IQE enhancement in 800--1,200-nm spectral region. The hemispherical reflectance of these 2D grating structures is comparable to random RIE-textured surfaces. The IQE enhancement in the long wavelength spectral region can be attributed to increased coupling into obliquely propagating transmitted diffracted orders inside the Si substrate. Random RIE texturing techniques are expected to find widespread commercial applicability in low-cost, large-area multicrystalline Si solar cells. Grating-texturing techniques are expected to find applications in thin-film and space solar cells.

Measurements have been made of the hemispherical and specular reflectance of metallic surfaces of controlled roughness. The surfaces, which were ground nickel rectangles, were irradiated at various angles of incidence by a beam of black-body radiation, the temperature of which was also varied. The instrumentation which was devised to perform the experiments is described. The measurements show that beyond a certain surface roughness, the hemispherical reflectance is virtually independent of further increases in roughness. On the other hand, the specular reflectance decreases steadily with increasing roughness. Additionally, the hemispherical reflectance is found to be quite insensitive to the angle of incidence, while the specular reflectance increases with angle of incidence for the rougher surfaces.

Generation of a coherent electromagnetic radiation in the far IR (THz) spectral range upon excitation of a semiconductor InAs crystal by 70-fs Ti: sapphire laser pulses is studied. The effect of a magnetic field of different orientation on generation in the submillimeter-wavelength range is analyzed. Placing the crystal into the magnetic field of an optimized permanent magnet with a strength of 5 kOe aligned along the surface of the semiconductor increased the power of generated radiation by a factor of six compared with that in the absence of the field. For the average pump-laser output power of 150 mW and repetition rate of 80 MHz, the average power of the THz radiation reached 100 nW. For detection of ultrashort pulses of the THz radiation, we used, for the first time, a highly sensitive uncooled optoacoustic detector, which detected signals with a power lower than 1 nW.

Characterization of the Reflectance Anisotropy of Three Boreal Forest Canopies in Spring–Summer

The bidirectional reflectance distribution function (BRDF) provided by the Li-Strahler geometric-optical forest canopy model has been integrated to provide spectral instantaneous hemispherical reflectances of sparsely vegetated surfaces. Further integration over the Sun's zenith angles can yield daily or longer interval hemispherical reflectances as well. A variety of simulated canopies were modeled with varying solar angles. In all cases, as the geometric-optical model introduced increased shadowing of the surface with increased solar zenith angle, the direct-beam hemispherical surface reflectance gradually decreased. The hemispherical reflectance values are direct beam calculations and do not directly include canopy multiple scattering and leaf specularity or consider the impact of diffuse irradiance. These limitations are acceptable for sparse canopies, in which 3D shadowing effects are large. However, radiative transfer calculations have shown that these phenomena must be incorporated before truly realistic modeling of hemispherical surface reflectances can be achieved for dense canopies. >

Total emissivity measurements without use of an absolute reference

This study proposes an ultraviolet-visible composite optical target simulation technique based on a liquid crystal display (LCD) spatial light modulation device to solve the problem of not being able to satisfy the demand for optical target simulation for both ultraviolet and visible light operating spectral ranges in a single system when composite simulation of multi-source spatial targets is performed. We establish a composite light source model of an ultraviolet light emitting diode (LED) and a xenon lamp to enhance the energy simulation of the ultraviolet portion, and the light is mixed and homogenized by an integrating sphere. We analyze the light transmission principle of LCD display devices and derive the equation for the relationship between its working band and transmittance. We design a transmission-type projection system with a wide spectral range and simulate the transmittance of the whole system, and demonstrate the optical target simulator can realize the simulation requirements of a wide working spectral range, high interstellar angular distance accuracy, and high magnitude accuracy.

Almost all of the detected hot Jupiters present clouds and hazes in their atmospheres. These ensembles of aerosols play an important role during primary transits blocking the stellar flux transmitted through the outer atmosphere of exoplanets, thus having an important influence on transmitted planetary spectra.Transmission spectra are very sensitive to the presence of clouds in the exoplanet atmospheres (Fortney, 2005). Depending on their size, aerosols can leave smooth effects on the observed spectra. Sometimes, cloud opacity can hide the spectral signature of different molecules that would be otherwise imprinted in the signal. When clouds become optically thick, they block all the flux that would cross the levels below them, similarly to a solid surface (Benneke & Seager, 2012). Thus, a good understanding of the properties of the aerosols in exoplanets is essential to understand their gaseous envelopes.Prior to James Webb Space Telescope (JWST) observations, spectral range was relatively limited and only combining different data sources, such as Hubble and Spitzer, it was possible to cover a big enough spectral range to reveal the smooth aerosol spectral signatures.  For simplicity, most of the previous studies assumed flat or Rayleigh-like cloud opacities. Figure 1 allows checking that these simplifications were valid enough for the spectral ranges covered by the Hubble Space Telescope (HST). Nevertheless, recent JWST/NIRSpec data with its wider spectral range are a game changer allowing disentanglement of gaseous absorptions and cloud extinctions.Figure 1: Simulated extinction for spherical aerosols of different radius distributions all over the complete spectral range of JWST/NIRSpec-PRISM. HST/STIS, HST/WFC3 and Spitzer/IRAC spectral ranges are delimited by grey background for comparisons. Note how some combinations of sizes and instruments would be indistinguishable from a flat extinction curve.In this work, we have studied the cloud properties of planet WASP-39b using the spectra of the primary transit obtained with JWST/NIRSpec (Rustamkulov et al., 2023). To do so, we have implemented a nested-sampling Bayesian approach, which has become popular for the atmospheric retrievals during the last decade (Benneke & Seager, 2012; Fisher & Heng, 2018). This framework does not just allow fitting the parameters of a certain model, but it also allows selecting which is the best model for fitting the data observed among a set, each one with a variety of atmospheric parameters and assumptions through Bayesian evidence.We tested a number of vertical distribution parameterisations found in the literature (Barstow, 2020). Following the Occam’s razor reasoning, for Bayesian evidences being equal, we favor models with the lesser number of free parameters. In our study, we find that a model with a semi-infinite bottom cloud and a clear atmosphere above is the best option, while there are many similarities between the retrieved parameters for all cases (e.g., similar cloud top pressures).Then, we have studied the effect that the aerosol extinction shape has on the observed spectra. We again tested a number of extinction models and compared their Bayesian evidences. In this case, in spite of all of them achieving good fits (see figure 2), realistic simulations of a complex cloud extinction dependence with wavelength are preferred versus models assuming simpler wavelength dependencies. Extinction produced by big sized aerosols with an opacity growing with wavelength fits better the observed data and it seems to be a robust conclusion from different competing models.Figure 2: Best fitted spectra for some cloud extinction models. Observational data are shown as black/grey dots with error bars.These results also have an impact on the retrieved molecular abundances (see figure 3) suggesting possible overestimations of some chemical species or even calling into question the detection of some of them, as proposed in Lueber et al., 2024.Figure 3: Values retrieved for some atmospheric parameters when using different models relative to a flat model retrieval. ReferencesBarstow, J. K. 2020, MNRAS, 497, 4183Benneke, B. & Seager, S. 2012, ApJ, 753, 100Fisher, C. & Heng, K. 2018, Monthly Notices of the Royal Astronomical Society, 481, 4698Fortney, J. J. 2005, Monthly Notices of the Royal Astronomical Society, 364, 649Lueber, A., Novais, A., Fisher, C. and Heng, K., arXiv:2405.02656.Rustamkulov, Z., Sing, D. K., Mukherjee, S., et al. 2023, Nature, 614, 659

The Darwin mission is a project of the European Space Agency that should allow around 2015 the search for extrasolar planets and a spectral analysis of their potential atmospheres in order to detect gases and particularly tracers of life. The basic concept of the instrument is a Bracewell nulling interferometer. It allows high angular resolution and high dynamic range. However, this concept, proposed 25 years ago, is very difficult to implement with high rejection factor and has to be demonstrated in laboratory before being applied in space. Theoretical and numerical approaches of the question highlight strong difficulties : - The need for very clean and homogeneous wavefronts, in terms of intensity, phase and polarisation distribution ; - The need for achromatic optical devices working in a wide spectral range (typically 6 to 18 microns for the space mission). A solution to the first point is the optical filtering which has been successfully experimentally demonstrated at 10 microns using a single mode laser. We focus now on the second point and operate a test bench working in the near infrared, where the background constraints are reduced. We present this test bench and the first encouraging results in the 2-4 microns spectral range. We particularly focus on recent optical developments concerning achromatic component, and particularly the beam combiners and the phase shifter, which are keypoints of the nulling interferometry principle. Finally, we present the future of this experimental demonstration, in the thermal infrared, covering the real and whole spectral range of Darwin.

An optical filter unit is demonstrated, which uses two successively arranged tunable thin-film optical band-pass filters and allows for simultaneous adjustment of the central wavelength in the spectral range 522-555 nm and of the spectral bandwidth in the range 3-16 nm with a wavelength switching time of 8 ms∕nm. Different spectral filter combinations can cover the complete visible spectral range. The transmitted intensity was found to decrease only linearly with the spectral bandwidth for bandwidths >6 nm, allowing a high maximum transmission efficiency of >75%. The image of a fiber bundle was spectrally filtered and analyzed in terms of position-dependency of the transmitted bandwidth and central wavelength.