We report on the development of a new primary thermometer based upon high-precision spectroscopy of mercury vapors in the deep-ultraviolet region for the practical realization of the new kelvin. The line profile of the (6s2)1S0 &gt; (6s6p)3P1 intercombination transition of the 200Hg bosonic isotope is observed with a high spectral fidelity using a coherent radiation source at 253.7 nm. This latter consists of a near-IR external cavity diode laser followed by a double-stage second-harmonic generation apparatus. Metrology grade UV spectroscopy is demonstrated by locking the diode laser to a self-referenced optical frequency comb synthesizer

Bertiglia F.

Cancio Pastor P.

Castrillo A.

Chishti N. A.

Clivati C.

Coluccelli N.

Fasci E.

Galzerano G.

Gianfrani L.

Gravina S.

Levi F.

Lopardo G.

Sorgi A.

English

Archivio istituzionale della ricerca - Politecnico di Milano

Journal of Physics: ConferenceSeries     PAPER • OPEN ACCESSComb-assisted mercury spectroscopy in the deep-ultraviolet: towards the development of a newprimary thermometerTo cite this article: S Gravina et al 2023 J. Phys.: Conf. Ser. 2439 012015 View the article online for updates and enhancements.You may also likeThe Co-evolution of Disks and Stars inEmbedded Stages: The Case of the Very-low-mass Protostar IRAS 15398–3359Yuki Okoda, Yoko Oya, Nami Sakai et al.-Filamentary Network and Magnetic FieldStructures Revealed with BISTRO in theHigh-mass Star-forming Region NGC2264: Global Properties and LocalMagnetogravitational ConfigurationsJia-Wei Wang, Patrick M. Koch, SeamusD. 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Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing Ltd25th International Conference on Spectral Line ShapesJournal of Physics: Conference Series 2439 (2023) 012015IOP Publishingdoi:10.1088/1742-6596/2439/1/0120151Comb-assisted mercury spectroscopy in thedeep-ultraviolet: towards the development of a newprimary thermometerS Gravina1, C Clivati2, N A Chishti1, A Castrillo1, E Fasci1, FBertiglia2, G Lopardo2, A Sorgi3, N Coluccelli4, G Galzerano5, PCancio Pastor3, F Levi2 and L Gianfrani11 Department of Mathematics and Physics, Università degli Studi della Campania “LuigiVanvitelli”, 81100, Caserta, Italy2 National Institute of Metrological Research (INRiM), 10135, Torino, Italy3 National Institute of Optics (INO) - National Research Council (CNR), 50125, Firenze, Italy4 Department of Physics, Politecnico di Milano, 20133, Milano, Italy5 Institute for Photonics and Nanotechnologies (IFN) - National Research Council (CNR),20133, Milano, ItalyE-mail: stefania.gravina@unicampania.itAbstract. We report on the development of a new primary thermometer based upon high-precision spectroscopy of mercury vapors in the deep-ultraviolet region for the practicalrealization of the new kelvin. The line profile of the (6s2)1S0 → (6s6p)3P1 intercombinationtransition of the 200Hg bosonic isotope is observed with a high spectral fidelity using a coherentradiation source at 253.7 nm. This latter consists of a near-IR external cavity diode laser followedby a double-stage second-harmonic generation apparatus. Metrology grade UV spectroscopy isdemonstrated by locking the diode laser to a self-referenced optical frequency comb synthesizer.1. IntroductionSince 20 May 2019, the fundamental units of the International System (SI) are defined in termsof fundamental Physics constants, whose values have been fixed. Such a revolution was madenecessary in order to assure the future stability of the SI [1, 2]. The implementation of the newdefinitions paves the way to the development of new methods and techniques, including thoseof quantum nature. The unit kelvin is linked to the Boltzmann constant, kB, whose value hasbeen set to 1.380649×10−23 J/K [3]. As it is well known, Doppler Broadening Thermometry(DBT) is a laser-based method that directly probes the thermal motion of the particles of agaseous sample at the thermodynamic equilibrium [4]. In its best implementation, performed ina gas of acetylene, thermodynamic temperature determinations have been demonstrated with aglobal relative uncertainty of about 10 ppm [5].Here, we report on the status of a new DBT experiment that is based upon the preciseobservation of the shape of the mercury intercombination transition (6s2) 1S0 → (6s6p) 3P 1 at253.7 nm, by using a comb-assisted laser spectrometer.Past literature shows a few examples of DBT implementation in which an atomic sample hasbeen used as thermometric substance. More particularly, low-pressure alkali-metal vapors, such25th International Conference on Spectral Line ShapesJournal of Physics: Conference Series 2439 (2023) 012015IOP Publishingdoi:10.1088/1742-6596/2439/1/0120152as rubidium and cesium, have been successfully tested [6, 7]. These atomic systems offer theadvantage of the simple structure of their absorption spectra, especially when compared withthose of polyatomic molecules. Furthermore, collisional perturbations of the absorption profilecan be neglected, the vapour pressure being extremely small.Mercury has a variety of favorable features for the aims of DBT. It is ideally suited fortemperature measurements between 230 and 300 K, the vapor pressure ranging from 10−4 to0.3 Pa, namely, orders of magnitude larger than that of Rb, K, or Sr in the same temperatureinterval. On the other hand, the pressure is small enough to completely neglect collisionalperturbations to the line profile and, at the same time, sufficiently large to produce a measurableabsorption signal in a compact gas cell, with a length of a few millimeters. Furthermore, mercuryincludes six stable isotopes, four of which having zero nuclear spin. In this respect, it is possibleto exploit the possibility of probing a line with no hyperfine structure, thus leading to a furthersimplification of the spectral analysis of Doppler-limited profiles.2. MethodDBT is based on the precise observation of the shape of a given atomic or molecular line ina gas at the thermodynamic equilibrium, in a laser-based absorption spectroscopy experiment,under the linear regime of radiation-matter interaction. The absorption process is described bythe Beer-Lambert law, which relates the attenuation of a laser beam travelling into a gaseousmedium to the properties of the gas, as it follows:I(ν) = I0e−NSLg(ν−ν0), (1)where I(ν) is the intensity of the transmitted beam at frequency ν, ν0 is the line center frequency,I0 is the incident intensity, N is the gas density, S is the line strength, L is the absorption path-length and g(ν − ν0) is the line shape function, normalized to 1 by convention.When the intensity of the probe laser is orders of magnitude smaller than the saturationintensity of the selected transition, the absorption profile is well described by a Voigt convolution,provided that the gas pressure is so small that collisional narrowing effects are totally negligible.This is the case of mercury vapors under the operation conditions of the present experiment.The thermodynamic temperature can be measured by retrieving the Doppler width and theabsolute center frequency from a non-linear least squares fit of the experimental line profile andusing the following equation:T =(∆νDν0)2 Mc22ln2kB, (2)where ∆νD is the Doppler half-width at half maximum (HWHM), c is the speed of light invacuum and M is the absorbers’ mass.3. The UV spectrometerThe deep-UV spectrometer is schematically shown in figure 1. It is based on a coherent,continuous wave, tunable, UV source, whose detailed description is reported in [8]. Such asource essentially consists in a double stage of second-harmonic generation (SHG) of the singlemode radiation from an external cavity diode laser (namely, the pump laser), this latter beingmounted in the common Littrow configuration and emitting at 1014.8 nm.The first-stage duplication (from 1014.8 nm to 507.4 nm) is performed in a fiber-coupledperiodically-poled lithium niobate (PPLN) crystal in a waveguide. The produced green radiationseeds a high-power multi-mode diode laser at the same wavelength, namely the slave laser, foramplifying the optical power up to five times by injection locking. The second-stage duplication(from 507.4 nm to 253.7 nm) occurs through a 12 mm long β-barium borate (BBO) crystal,25th International Conference on Spectral Line ShapesJournal of Physics: Conference Series 2439 (2023) 012015IOP Publishingdoi:10.1088/1742-6596/2439/1/0120153Figure 1. Sketch of the UV absorption spectrometer. AOM, acousto-optic modulator; BS, beamsplitter; Pd, photodiode; M, mirror; RF, radio frequency synthesizer; PC, personal computer.placed inside a bow-tie resonant cavity. The cavity is locked to the incoming beam through theHänsch-Couillaud technique [9].An intensity control feedback loop based on an acousto-optic modulator is implemented tostabilize the power of the probe laser beam. A portion of the first-oder diffracted beam issent to a 2-cm long quartz cell, in which low-pressure mercury vapors are in equilibrium withthe liquid phase. The spectroscopic cell is placed inside a thermostatic chamber so that the gastemperature is actively controlled and stabilized within 0.05 mK at the triple point of water overa time span of several hours [10]. After the interaction with mercury vapors, the transmittedUV beam is focused on a preamplified SiC photodetector, whose responsivity is linear withinthe uncertainty of 0.004% (1σ) [11].For the aims of DBT, the transmitted signal should be acquired while scanning the UVfrequency around the intercombination transition of mercury so as to cover a frequency intervalat least 3 times wider than the Doppler width of the line, namely, 3 GHz or larger. In thefree-running mode of operation, by tuning the near-IR pump laser, it is possible to satisfy thisrequirement. Figure 2 shows an example spectrum that is obtained by means of a continuous,mode-hop free, frequency scan of the UV radiation over the 198Hg, 200Hg and 202Hg line profiles,in coincidence with the intercombination transition. The weakest peak is perturbed by anhyperfine structure component of the fermionic isotope 201Hg. The scan width is as wide as16 GHz, which is a quite remarkable result for a sophisticated frequency chain like the oneof this work. Frequency calibration of the spectrum is achieved by using the absolute centerfrequencies of the three Hg isotopes that are reported in [12]. Nonetheless, a calibration basedon three points cannot be sufficiently accurate for a metrology grade experiment.For this reason, a novel comb-locked configuration was implemented, using a self-referencedoptical frequency comb synthesizer (OFCS, FC1500-250-WG, MENLO Systems) in conjunctionto a second-harmonic generation unit [13]. More particularly, the highest wavelength portion25th International Conference on Spectral Line ShapesJournal of Physics: Conference Series 2439 (2023) 012015IOP Publishingdoi:10.1088/1742-6596/2439/1/0120154Figure 2. Absorption spectrum for mercury isotopes at 253.7 nm. The upper panel shows theabsolute residuals, as a result of a nonlinear least-squares fit to Voigt convolutions.of the comb was frequency-doubled so as to make it possible the comparison with the pumplaser and the consequent observation of a beat note with a sufficiently high signal-to-noise ratio.After proper processing, the beat note was used to tightly lock the pump laser to the comb. It isworth mentioning that the comb repetition rate (frep) and the carrier-envelope offset frequencywere stabilized against a GPS-disciplined rubidium clock. Highly accurate and reproduciblefrequency scans of the pump laser frequency were obtained by finely tuning the repetition rate.A scan of frep of 750 Hz yielded a frequency tuning in the near-IR of about 900 MHz, whichtranslated into a 3.6-GHz wide scan in the UV domain. This is about a factor of 4 narrower thanthat achievable in the free-running mode. The current limitation is due to the locking electronicsacting on the pump laser which do not support larger scans in the comb-locked configuration.4. ResultsFigure 3 displays a set of 20 repeated acquisitions of absorption spectra, as recorded at thetemperature of the triple point of water across the intercombination transition of the 200Hgisotope. Each Doppler-limited profile is recovered with 751 points that are obtained from astep-by-step tuning of frep with a resolution of 1 Hz. The total acquisition time for the entireset amounts to 1 hour. The signal-to-noise ratio (SNR) results to be about 250. The data setallowed us to determine the line center frequency with a precision of 2×10−10, while the gastemperature could be retrieved with a relative statistical uncertainty of 1.1‰. It is clear thatthe noise level should be significantly reduced in order to approach an uncertainty smaller than10 ppm. In this regard, a further optimization of the UV laser source will be necessary to lowerthe intensity noise. Furthermore, spectral averaging over several hours can be a viable solutionthat can be implemented by exploiting the frequency stability of the spectrometer. In figure 4we report the outcome of spectral averaging over 20 consecutive scans. The signal to noise ratiois close to 1000. The Doppler width that is retrieved from the Voigt fit gives a thermodynamictemperature of 273.34 ± 0.11 K, the uncertainty being the internal error corresponding to 1σ.25th International Conference on Spectral Line ShapesJournal of Physics: Conference Series 2439 (2023) 012015IOP Publishingdoi:10.1088/1742-6596/2439/1/0120155This in good agreement with the set-point (273.16 K).Figure 3. Plot of 20 repeated acquisitions of the 200Hg absorption spectrum at 253.7 nm. Thex-axis represents the absolute frequency as measured by means of the OFCS.Figure 4. Example of spectral averaging over 20 consecutive scans. The upper panel shows theabsolute residuals resulting from a nonlinear least-squares fit to a Voigt profile.25th International Conference on Spectral Line ShapesJournal of Physics: Conference Series 2439 (2023) 012015IOP Publishingdoi:10.1088/1742-6596/2439/1/01201565. ConclusionWe have reported on the recent progress regarding the development of a new primarythermometer based upon Doppler-broadened precision spectroscopy of mercury atoms at 253.7nm. The apparatus is based on a frequency chain that links the UV domain to the near-IRregion. We have demonstrated highly-accurate and reproducible frequency scans around the Hgline by direct referencing of the near-IR pump laser to an optical frequency comb synthesizer andtuning the comb repetition rate. The results of the first test experiment are very encouragingand stimulate further work to be done to pursue the goal of a low-uncertainty realization of thenew kelvin.AcknowledgmentsThis research was supported by the Italian Ministry for University and Research (Programtype PRIN 2015) and by Università degli Studi della Campania “Luigi Vanvitelli” within theV:ALERE program (Call published in 2019, Project V:ANS).References[1] Machin G 2018 Measurement Science and Technology 29 022001[2] Newell D B and Eite T 2019 NIST Special Publication 330 1-138[3] Fischer J et al 2018 Metrologia 55 R1-R20[4] Gianfrani L 2016 Philosophical Transactions of the Royal Society A 374 20150047[5] Castrillo A, Fasci E, Dinesan H, Gravina S, Moretti L, and Gianfrani L 2019 Physical Review Applied 11064060[6] Truong G W, May E F, Stace T M and Luiten A N 2011 Physical Review A 83 033805[7] Truong G W, Stuart D, Anstie J D, May E F, Stace T M and Luiten, A. N. 2015 Metrologia 52 S324.[8] Clivati C, Gravina S, Castrillo A, Costanzo G A, Levi F and Gianfrani L 2020 Optics Letters 45 3693-96[9] Hänsch T W and Couillaud B 1980 Optics Communications 35 441-4[10] Lopardo G, Bertiglia F, Barbone A, Bertinetti M, Dematteis, R, Giraudi D and Gianfrani L 2021 Measurement173 108594[11] Dinesan H, Gravina S, Clivati C, Castrillo A, Levi F and Gianfrani L 2020 Metrologia 57 065001[12] Witkowski M, Kowzan G, Munoz-Rodriguez R, Ciury lo R, Żuchowski P S, Mas lowski P and Zawada M 2019Optics Express 27, 11069-83.[13] Gravina S, Clivati C, Castrillo A, Fasci E, Chishti N A, Galzerano G, Levi F and Gianfrani L 2022 PhysicalReview Research 4 033240.

Comb-assisted mercury spectroscopy in the deep-ultraviolet: towards the development of a new primary thermometer

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Comb-assisted mercury spectroscopy in the deep-ultraviolet: towards the development of a new primary thermometer

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