the measured spectroscopic parameter is usually a pressure-induced line shift, i.e., spectral shift of the emission bands of Cr 3+ (ruby) or Sm 2+ . [14,15,20,22] Whereas, in the case of temperature the commonly measured parameter is the luminescence intensity ratio (LIR), i.e., band ratio of two thermally coupled levels (TCLs; separated by ≈200-2000 cm −1 ) of, e.g., Nd 3+ , Er 3+ , or Tm 3+ , which is directly related to the local temperature of the system (probe), and conforms Boltzmann distribution. [3,[23][24][25] A great number of optically active functional materials is based on the Ln 2+/3+ , because of their unique spectroscopic properties, such as multicolor photoluminescence induced by UV or near-infrared (NIR) (energy up-conversion) irradiation, narrow absorption/emission bands, large spectral shift of the emission bands in relation to the absorption ones, long emission lifetimes, etc. [26][27][28][29][30][31][32][33][34][35] Matrices hosting Ln 3+ ions are usually fluorides, oxides, vanadates, phosphates, and borates. [3,4,11,12,[19][20][21][22][23][24][25][26] This is mainly because of their resistance to photobleaching and high temperature treatment, as well as relatively low phonon energy in contrast to organic compounds. [3][4][5][19][20][21][22][23][24][25][26] Moreover, the Ln 3+ -doped inorganic materials may exhibit up-conversion (UC) phenomena, i.e., anti-Stokes emission of higher-energy photons, generated by the absorption of two or more lowerenergy photons. [32,[36][37][38][39] Thanks to the high absorption cross-section of Yb 3+ in the NIR range, and the presence of a ladder-like structure of Ln 3+ energy levels, the upconverting materials codoped with Yb 3+ / Ln 3+ (Ln 3+ = Ho 3+ , Er 3+ , Tm 3+ ) may work not only as temperature sensors, but also as optical "heaters," as during their irradiation with a high-power NIR lasers they locally heat up. [40][41][42][43][44][45][46][47][48] This is due to the occurrence of various nonradiative processes between the Ln 3+ ions, quenching luminescence of the material and leading to heat generation. [43][44][45][46][47][48] Thanks to the efficient light-to-heat conversion, the optical heating phenomenon can be utilized in photothermal therapies, thermophotovoltaics, formation of new materials under extreme conditions, etc. [43][44][45][46][47][48][49] Currently, temperature of the system can be optically monitored in a relatively broad range, starting from cryogenic up to around ≈10 3 K, whereas pressure could be monitored only in the "high-pressure" range (≈10 2 -10 6 bar). These limitations are associated with the fundamental concept of pressure sensing, i.e., measurements of physical parameters directly Currently the lowest optically determinable pressure values are around 10 2 bar, making the pressure below inaccessible for optical detection. This work shows for the first time how to overcome these limitations, and optically monitor the low pressure values in a vacuum region (from ≈10 −5 to 10 −2 bar), utilizing the light-induced and pressure-g...

Lanthanide-based luminescent nanothermometers play a crucial role in optical temperature determination. However, because of the strong thermal quenching of the luminescence, as well as the deterioration of their sensitivity and resolution with temperature elevation, they can operate in a relatively low-temperature range, usually from cryogenic to ≈800 K. In this work, we show how to overcome these limitations and monitor very high-temperature values, with high sensitivity (≈2.1% K −1 ) and good thermal resolution (≈1.4 K) at around 1000 K. As an optical probe of temperature, we chose upconverting Yb 3+ −Tm 3+ codoped YVO 4 nanoparticles. For ratiometric sensing in the low-temperature range, we used the relative intensities of the Tm 3+ emissions associated with the 3 F 2,3 and 3 H 4 thermally coupled levels, that is, 3 F 2,3 → 3 H 6 / 3 H 4 → 3 H 6 (700/800 nm) band intensity ratio. In order to improve sensitivity and resolution in the high-temperature range, we used the 940/800 nm band intensity ratio of the nonthermally coupled levels of Yb 3+ ( 2 F 5/2 → 2 F 7/2 ) and Tm 3+ ( 3 H 4 → 3 H 6 ). These NIR bands are very intense, even at extreme temperature values, and their intensity ratio changes significantly, allowing accurate temperature sensing with high thermal and spatial resolutions. The results presented in this work may be particularly important for industrial applications, such as metallurgy, catalysis, high-temperature synthesis, materials processing and engineering, and so forth, which require rapid, contactless temperature monitoring at extreme conditions.

A novel, contactless optical sensor of pressure based on the luminescence red-shift and bandwidth (full width at half-maximum, fwhm) of the Ce 3+ -doped fluorapatite-Y 6 Ba 4 (SiO 4 ) 6 F 2 powder has been successfully synthesized via a facile solid-state method. The obtained material exhibits a bright blue emission under UV light excitation. It was characterized using powder X-ray diffraction, scanning electron microscopy and luminescence spectroscopy, including high-pressure measurements of excitation and emission spectra, up to above ∼30 GPa. Compression of the material resulted in a significant red-shift of the allowed 4f → 5d and 5d → 4f transitions of Ce 3+ in the excitation and emission spectra, respectively. The pressure-induced monotonic shift of the emission band, as well as changes in the excitation/emission band widths, have been correlated with pressure for sensing purposes. The material exhibits a high pressure sensitivity (dλ/dP ≈ 0.63 nm/GPa) and outstanding signal intensity at high-pressure conditions (∼90% of the initial intensity at around 20 GPa) with minimal pressure-induced quenching of luminescence. KEYWORDS: Ce 3+ doping, contactless pressure gauge, compression in DAC, lanthanide ions (Ln 3+ ), luminescent functional materials, Y 6 Ba 4 (SiO 4 ) 6 F 2 apatite phosphors

No abstract

The growing interest in the miniaturization of various devices and conducting experiments under extreme conditions of pressure and temperature causes the need for the development of small, contactless, precise, and accurate optical sensors without any electrical connections. In this work, YF 3 :Yb 3+ -Er 3+ upconverting microparticles are used as a bifunctional luminescence sensor for simultaneous temperature and pressure measurements. Different changes in the properties of Er 3+ green and red upconverted luminescence, after excitation of Yb 3+ ions in the near-infrared at ∼975 nm, are used to calibrate pressure and/or temperature inside the hydrostatic chamber of a diamond anvil cell (DAC). For temperature sensing, changes in the relative intensities of the Er 3+ green upconverted luminescence of 2 H 11/2 and 4 S 3/2 thermally coupled multiplets to the 4 I 15/2 ground state, whose relative populations follow a Boltzmann distribution, are calibrated. For pressure sensing, the spectral shift of the Er 3+ upconverted red emission peak at ∼665 nm, between the Stark sublevels of the 4 F 9/2 → 4 I 15/2 transition, is used. Experiments performed under simultaneous extreme conditions of pressure, up to ∼8 GPa, and temperature, up to ∼473 K, confirm the possibility of remote optical pressure and temperature sensing.