Optical Vacuum Sensor Based on Lanthanide Upconversion—Luminescence Thermometry as a Tool for Ultralow Pressure Sensing

Abstract

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...

Luminescent Nd3+‐Based Microresonators Working as Optical Vacuum Sensors

“…For a pressure range from 3.8 to 2.3 × 10 −2 Torr, the spectral position of the WGM had a total shift of about 4 nm. Similarly to Runowski et al., [ 21 ] when pressure decreases below 1–2 Torr, an increase in the response of the sensor can be observed. With a rate change of ≈0.26 to 1 nm Torr −1 .…”

“…The new sensor showed an improvement, in terms of limit of detection when compared to the previous YVO 4 :Yb 3+ ‐Er 3+ inorganic material. [ 21 ] At 7.5 × 10 −1 Torr, this sensor presents a pressure resolution of 1.1 × 10 −2 Torr whereas YVO 4 :Yb 3+ ‐Er 3+ has 1.9 × 10 −2 Torr. For lower pressure, the improvement is more significant with a limit of detection of 6.3 × 10 −4 Torr at 7.5 × 10 −2 Torr for this sensor, against the 1.5 × 10 −3 Torr of its predecessor.…”

“…This last step created a gel that was fired at 400 °C for 6 h in order to remove the residual nitrates and organic compounds and the subsequently obtained powder sample was finally burnt out at 1200 °C for 20 h. The second thermal treatment was performed at 1550 °C for 12 h. The relatively high Nd 3+ concentration (2.5 mol%) was chosen because the materials heat up more when they contain higher concentration of the optically active ions, mainly due to the enhanced cross‐relaxation processes, leading to the improved light‐to‐heat conversion efficiency. [ 21 ]…”

“…Furthermore, luminescent materials play an important role as phosphors, contrast agent, labels, optical heaters/coolers, temperature and pressure sensors, etc. [ 20,21 ] In the field of luminescent materials, the lanthanide based organic and inorganic compounds are of special importance. This is mostly due to their favorable optical properties, such as multicolor emission and up‐conversion luminescence, narrow absorption and emission bands, large Stokes shift, long radiative lifetimes, etc.…”

“…Recently, in our previous publication, [ 21 ] we showed for the first time the possibility of converting luminescence thermometer into the optical vacuum sensor. To present this concept we used YVO 4 :Yb 3+ ‐Er 3+ inorganic material, working as up‐converting phosphor, luminescence thermometer and heater under NIR (975 nm) laser irradiation.…”