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1. A precise photometric ratio via laser excitation of the sodium layer – II. Two-photon excitation using lasers detuned from 589.16 and 819.71 nm resonances

   https://doi.org/10.1093/mnras/stab1619  

   Albert, Justin E; Budker, Dmitry; Chance, Kelly; Gordon, Iouli E; Bustos, Felipe Pedreros; Pospelov, Maxim; Rochester, Simon M; Sadeghpour, H R (October 2021, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT This paper is the second in a pair of papers on the topic of the generation of a two-colour artificial star [which we term a laser photometric ratio star (LPRS)] of de-excitation light from neutral sodium atoms in the mesosphere, for use in precision telescopic measurements in astronomy and atmospheric physics, and more specifically for the calibration of measurements of dark energy using type Ia supernovae. The two techniques, respectively, described in both this and the previous paper would each generate an LPRS with a precisely 1:1 ratio of yellow (589/590 nm) photons to near-infrared (819/820 nm) photons produced in the mesosphere. Both techniques would provide novel mechanisms for establishing a spectrophotometric calibration ratio of unprecedented precision, from above most of Earth’s atmosphere, for upcoming telescopic observations across astronomy and atmospheric physics; thus greatly improving the performance of upcoming measurements of dark energy parameters using type Ia supernovae. The technique described in this paper has the advantage of producing a much brighter (specifically, brighter by approximately a factor of 103) LPRS, using lower power (≤30 W average power) lasers, than the technique using a single 500 W average power laser described in the first paper of this pair. However, the technique described here would require polarization filters to be installed into the telescope camera in order to sufficiently remove laser atmospheric Rayleigh backscatter from telescope images, whereas the technique described in the first paper would only require more typical wavelength filters in order to sufficiently remove laser Rayleigh backscatter. 

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2. Measurement of three-photon excitation cross-sections of fluorescein from 1154 nm to 1500 nm

   https://doi.org/10.1364/BOE.498214  

   LaViolette, Aaron_K; Ouzounov, Dimitre_G; Xu, Chris (July 2023, Biomedical Optics Express)

   Measurements of three-photon action cross-sections for fluorescein (dissolved in water, pH ∼11.5) are presented in the excitation wavelength range from 1154 to 1500 nm in ∼50 nm steps. The excitation source is a femtosecond wavelength tunable non-collinear optical parametric amplifier, which has been spectrally filtered with 50 nm full width at half maximum band pass filters. Cube-law power dependance is confirmed at the measurement wavelengths. The three-photon excitation spectrum is found to differ from both the one- and two-photon excitation spectra. The three-photon action cross-section at 1154 nm is more than an order of magnitude larger than those at 1450 and 1500 nm (approximately three times the wavelength of the one-photon excitation peak), which possibly indicates the presence of resonance enhancement. 

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3. Radiative decay of the ${}^{229m}\mathrm{Th}$ nuclear clock isomer in different host materials

   https://doi.org/10.1103/PhysRevResearch.7.013052  

   Pineda, S V; Chhetri, P; Bara, S; Elskens, Y; Casci, S; Alexandrova, A N; Au, M; Athanasakis-Kaklamanakis, M; Bartokos, M; Beeks, K; et al (January 2025, Physical Review Research)

   A comparative vacuum ultraviolet spectroscopy study conducted at ISOLDE-CERN of the radiative decay of the ${}^{229m}\mathrm{Th}$nuclear clock isomer embedded in different host materials is reported. The ratio of the number of radiative decay photons and the number of ${}^{229m}\mathrm{Th}$embedded are determined for single crystalline ${\mathrm{CaF}}_2, {\mathrm{MgF}}_2, {\mathrm{LiSrAlF}}_6$, AlN, and amorphous ${\mathrm{SiO}}_2$. For the latter two materials, no radiative decay signal was observed and an upper limit of the ratio is reported. The radiative decay wavelength was determined in ${\mathrm{LiSrAlF}}_6$and ${\mathrm{CaF}}_2$, reducing its uncertainty by a factor of 2.5 relative to our previous measurement. This value is in agreement with the recently reported improved values from laser excitation. Published by the American Physical Society2025 

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4. Photoluminescence Excitation Spectroscopy Characterization of Surface and Bulk Quality for Early-Stage Potential of Material Systems

   https://doi.org/10.1109/PVSC40753.2019.8980637  

   Grubbs, Elizabeth K.; Moore, James; Bermel, Peter A. (June 2019, Photoluminescence Excitation Spectroscopy Characterization of Surface and Bulk Quality for Early-Stage Potential of Material Systems)

   Abstract — Photoluminescence Excitation Spectroscopy (PLE) is a contactless characterization technique to quantify Shockley-Reed-Hall (SRH) lifetimes and recombination velocities in direct band gap experimental semiconductor materials and devices. It is also useful as to evaluate surface passivation and intermediate fabrication processes, since it can be implemented without the need for development of effective contact technologies. In this paper, we present a novel experimental PLE system for precision-based quantification of the aforementioned parameters as well as a system for which absolute PLE characterization may occur. Absolute PLE measurements can be used to directly calculate VOC for new photovoltaic (PV) material systems and devices. Key system capabilities include a continuous excitation spectrum from 300 nm –1.1 μm, automated characterization, up to 1 nm wavelength resolution (up to 60x higher than prior work), and a reduced ellipsometry requirement for post-processing of data. We utilize a GaAs double heterostructure (DH) and an InP crystalline wafer as calibration standards in comparison with data from an LED-based PLE to demonstrate the validity of the results obtained from this new system. Index Terms – photovoltaic cells, photoluminescence, charge carrier lifetime, gallium arsenide, indium phosphide. 

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5. Dynamic spectral tailoring of a 10 GHz laser frequency comb for enhanced calibration of astronomical spectrographs

   https://doi.org/10.1364/OE.557365  

   Sekhar, Pooja; Fredrick, Connor; Zhong, Peter; Kowligy, Abijith_S; Cingöz, Arman; Diddams, Scott_A (April 2025, Optics Express)

   Laser frequency combs (LFCs) are an important component of Doppler radial velocity (RV) spectroscopy that pushes fractional precision to the 10⁻¹⁰level, as required to identify and characterize Earth-like exoplanets. However, large intensity variations across the LFC spectrum that arise in the nonlinear broadening limit the range of comb modes that can be used for optimal wavelength calibration with sufficient signal-to-noise ratio. Furthermore, temporal spectral-intensity fluctuations of the LFC, that are coupled to flux-dependent detector defects, alter the instrumental point spread function (PSF) and result in spurious RV shifts. To address these issues and improve calibration precision, spectral flattening is crucial for LFCs to maintain a constant photon flux per comb mode. In this work, we demonstrate a dynamic spectral shaping setup using a spatial light modulator (SLM) over the wavelength range of 800–1300 nm. The custom shaping compensates for amplitude fluctuations in real time and can also correct for wavelength-dependent spectrograph transmission, achieving a spectral profile that delivers the constant readout necessary for maximizing precision. Importantly, we characterize the out-of-loop properties of the spectral flattener to verify a twofold improvement in spectral stability. This technique, combined with our approach of pumping the waveguide spectral broadener out-of-band at 1550 nm, reduces the required dynamic range. While this spectral region is tailored for the LFC employed at the Habitable-zone Planet Finder (HPF) spectrograph, the method is broadly applicable to any LFC used for astronomical spectrograph calibration. 

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