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Whispering gallery modes in a holmium doped glass microsphere: Temperature sensor in the second biological window
Optical Materials 83 (2018) 207–211
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Whispering gallery modes in a holmium doped glass microsphere: Temperature sensor in the second biological window
T
L. de Sousa-Vieiraa, S. Ríosa, I.R. Martína,b,∗, L. García-Rodrígueza, V.N. Sigaevc, V.I. Savinkovc, G. Yu Shakhgildyanc a
Departamento de Física, Universidad de La Laguna, Apdo. 456, E-38200, San Cristóbal de La Laguna, Santa Cruz de Tenerife, Spain Instituto Universitario de Materiales y Nanotecnología (IMN), Universidad de La Laguna, Apdo. 456, E-38200, San Cristóbal de La Laguna, Santa Cruz de Tenerife, Spain c D. Mendeleev University of Chemical Technology, Miusskaya sq., 9, 125047, Moscow, Russia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Transparent microspheres Holmium Emission Whispering gallery modes Temperature sensor
The displacements on the whispering gallery modes (WGM) wavelengths of the three emission bands (660 nm, 760 nm and 1200 nm) of Ho3+ doped yttrium aluminosilicate microspheres were analysed as a function of the temperature. The displacements have been studied in order to evaluate the viability of these microspheres as thermal sensors. A 532 nm continuous wave laser was used to excite and heat the microspheres using a confocal microscope setup. Average displacement rates of the WGM peaks with the laser power of 5.3, 6.2 and 9.7 p.m./ mW were found for the emission bands centred at 660 nm, 760 nm and 1200 nm, respectively. Temperature uncertainties of 0.6 K for the band centred at 660 nm, 0.5 K for the 760 nm band and 0.3 K for the 1200 nm band have been obtained. It is interesting to note that the maximum resolution corresponds to the band centred at 1200 nm that is especially interesting because it is inside the second biological window, which has medical applications.
1. Introduction Optical sensors are devices capable of acquiring information about a surrounding media by analysing the change in the intensity or phase of a light beam. These devices present numerous advantages compared to traditional sensors, in terms of electrical passiveness, greater sensitivity, freedom from electromagnetic interference, wide dynamic range, point and distributed configurations and multiplexing capabilities [1]. Nowadays, Whispering Gallery Modes (WGM) resonators have a wide range of applications: as temperature [2,3] or humidity sensors [4], laser action [5], chemical vapor sensors [6], among others. Specifically, spherical WGM resonators, known as microspheres, are useful as temperature sensors [2,3] because of the high sensitivity of their resonance modes to any change of temperature. As it will be discussed later, the wavelengths of the WGM can be shifted if there is a change in the microsphere size or in the optical properties of the surrounding medium, for example due to a temperature variation. So, by monitoring the shift in the WGM peaks when a temperature change takes place, the temperature of the microsphere can be estimated. Another benefit of using microspheres as optical sensors is that the detection can be carried out remotely, so it is possible to make measurements inside the
∗
system causing just a small perturbation in the physical system [7]. Glasses in the Y2O3–Al2O3–SiO2 system, known as YAS glasses, are of great interest for applications in nuclear medicine. YAS microspheres have been used to treat liver cancer [8,9]. One way to treat malignant tumours is with radiation, but external radiation doses are limited because they damage the nearby healthy tissue. A high-effective solution to this issue is to place the radiation source inside the tumour by injecting radioactive microspheres [9]. On the other hand, microspheres are usually doped with lanthanides because these elements have numerous fluorescent emissions in the visible and the near infrared region of the electromagnetic spectrum [10]. The triple ionized holmium ions have an emission peak at about 1200 nm [11], which corresponds to the 5I6 → 5I8 transition, in the near infrared region. This emission is interesting because it lays in the second biological window, that extends from 1000 nm to 1400 nm, where the absorption of light by the constituents of biological tissue is minimized, i.e. tissues are partially transparent [12]. Two water absorption bands located at about 980 nm and 1500 nm limit this spectral range. Multiple researches relating the second biological window are being carried out for medical and biological applications [12–16]. The aim of this work is to use YAS microspheres doped with Ho3+
Corresponding author. Departamento de Física, Universidad de La Laguna, Apdo. 456, E-38200, San Cristóbal de La Laguna, Santa Cruz de Tenerife, Spain. E-mail address: [email protected] (I.R. Martín).
https://doi.org/10.1016/j.optmat.2018.06.014 Received 28 February 2018; Received in revised form 7 June 2018; Accepted 8 June 2018
Available online 18 June 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
Optical Materials 83 (2018) 207–211
L. de Sousa-Vieira et al.
3. Experimental methods
due to its promising emission at 1200 nm, as temperature sensors. A laser is used to excite the WGM and heat the microsphere at the same time, thus we can monitor the temperature of the microsphere analysing the displacement of the WGM for increasing temperatures.
3.1. Microspheres production The YAS microspheres were prepared by analogy with procedures described in Ref. [22]. Glasses with molar composition (in mol.%) 13.48Y2O3 – 37.56Al2O3 – 46.96SiO2 − 2Ho2O3 were obtained by melting-quenching process starting from Y2O3, Ho2O3, amorphous SiO2 and Al(OH)3 reagents with purity higher than 99.99%. YAS glasses were melted at temperatures of 1630 °C for about 3 h in a platinum crucible placed in an electric furnace with SiC heaters and working chamber in high-purity corundum. In order to obtain microspheres, plates not thicker than 200 μm were produced by the rolling method and grinded by corundum disks. Grains were then selected in size by means of an automated wet sieving technique (Retsch AS 200) in the range 20–32 μm. Spheroidization of glass particles was performed by injection of them in an Argon plasma generated by a streamlined plasma torch with a maximum working current of about 500 A.
2. Theoretical background 2.1. Resonance condition Under the geometrical optics approximation, it can be obtained that the WGM must satisfy the following resonance condition [17]:
λ ·l = 2πR n
(1)
with l the polar mode number, that is the number of wavelengths that fits into the resonator, n the microsphere refractive index and R its radius. These resonant wavelengths can suffer shifts due to changes in the refractive index, or in the size of the cavity. Such changes on the microsphere parameters may occur because of temperature variations, among others. The derivative of the resonant wavelength with respect to temperature, provides a relationship between both parameters:
dλ 1 ∂n 1 ∂R ⎞ =⎛ + λ = (β + α ) λ dT R ∂T ⎠ ⎝ n ∂T 1 ∂n
3.2. WGM displacement A Ho3+ doped YAS microsphere with a size of 27 μm was excited at its center with a 532 nm laser and its emission detected on the surface as explained in Ref. [7]. Its emission spectrum as a function of the laser power was obtained and recorded with an Andor CCD spectrograph. The experimental setup is shown in Fig. 1. A laser beam is diverted with a mirror (M1) to a pinhole (P) and a collimating lens (CL), to obtain a circularly collimated beam. Then, the light is reflected by a dichroic mirror (M2) to a microscope objective (MO), that focalizes the light in the microsphere. Between M2 and MO a beam splitter (BS) sends part of the light to a CCD camera that shows an image of the microspheres on a TV screen. The light transmitted by the dichroic mirror is reflected by a high precision mirror (M3) and focused on the entrance slit of the spectrograph Andor SR-3031-B with a CCD Newton DU920 N, used to obtain the microsphere emission spectrum on the visible region. Moreover, in order to obtain the emission spectrum on the near infrared region the mirror M3 is removed and the emission is focused to an optical fiber (F) that is connected to a spectrograph Andor SR-500i-B2 with a CCD InGaAs CCDDU490A-1.7. In order to calibrate the WGM displacements with temperature the microsphere was located inside of a cuvette (surrounded by air). The cuvette was immersed in a water bath reservoir and the temperature of the bath was changed with an accuracy of 0.1 °C using a resistor. Closed water cycled assured the homogeneity of the temperature in the bath before each measurement. The walls of the water bath are transparent in the visible range. The emission spectra were recorded for a set of temperature values.
(2) 1 ∂R
where β = n ∂T is the thermo-optic coefficient, and α = R ∂T the thermal expansion coefficient. The sign of these coefficients is positive [18]. If a temperature increase takes place, the peaks corresponding to the resonant wavelengths in the emission spectrum of the microsphere will shift to the red region of the spectra. 2.2. Sensitivity and temperature uncertainty Optical temperature sensors are often characterized in terms of their relative sensitivity, which is a parameter that represents the variation of the measured variable with the temperature relative to its magnitude [7]. Then, the sensitivity for the displacement of the WGM with temperature is:
SR =
1 dλ λ dT
(3)
Where dλ/dΤ is given by Eq. (2). On the other hand, the temperature uncertainty of the thermal sensor can be defined as [19,20]:
δT =
1 δλ SR λ
(4)
4. Results and discussion
where δT is the temperature uncertainty, which represents the minimum temperature variation that can be detected by a thermal sensor, δλ represents the uncertainty in the WGM peaks, λ the resonant wavelength and SR the relative sensitivity. Eq. (2) shows how the wavelength shift with temperature depends on the refractive index change through the thermal optical coefficient and on the changes on the microsphere size through the thermal expansion coefficient. Those parameters are not known for the glass under study but the thermal expansion coefficient has been measured by Hyatt (Ref. [21]) for a glass with a similar chemical composition YAS10, Y2O3 (35 w%), Al2O3 (35 w%), SiO2 (30 w%) having a constant value of α = 48·10−7 ºC−1. This coefficient does not depend on the microsphere radius, and so the relative sensitivity. Then the microsphere size can be changed to adapt to the application of interest without compromising its performance as a temperature sensor. If a better sensitivity in needed for a particular application, a material with higher values of α and β must be chosen.
The emission spectrum of a Ho3+ doped YAS microsphere obtained by exciting with a laser power with 3 mW at room temperature is shown in Fig. 2 (A). Placing the excitation and the detection at the center of the microsphere, three broad bands can be observed corresponding to typical transitions of the trivalent Ho3+ ions. The broad band centred at 660 nm can be assigned to the 5F5 → 5I8 holmium transition, the one at 760 nm to 5S2 (5F4) → 5I7 transition, and the 1200 nm band to 5I6 → 5I8 transition. However, if the detection is placed at the microsphere border, the WGM are observed overlapping the holmium emission bands, as can be seen in Fig. 2 (B). 4.1. Temperature calibration As was previously explained the WGM peaks can be displaced as function of the temperature. Therefore, the displacements of the WGM peaks of a Ho3+ doped YAS microsphere, when it is heated (as was 208
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Fig. 1. Experimental setup to obtain the emission spectrum of an excited microsphere with a laser at 532 nm. The scale bar in the inset photograph correspond to 30 μm.
Fig. 2. (A) Emission spectrum of a Ho3+ doped YAS microsphere obtained under excitation at 532 nm at room temperature, placing the detection at its center (showing the corresponding transitions). (B) Emission spectrum of a Ho3+ doped YAS microsphere showing the WGM under excitation at 532 nm at room temperature, placing the detection at the border of the microsphere. WGM are clearly visible. The inset shows the power dependence of the spectra for some selected WGM peaks.
Fig. 3. Displacement of the WGM peaks at the 660 nm emission of a Ho3+ doped YAS microsphere as a function of temperature.
way. Taking into account that β + α do not depend of the wavelength, the slopes for the other two bands can be estimated from the expression Δλ=(β + α)λΔΤ, obtaining slopes of 6.2 p.m./K and 9.7 p.m./K for the 760 nm and 1200 nm emission bands respectively. It can be observed that the slopes increase for increasing values of the wavelength. For a temperature increase of 30°, shifts of approximately 182 p.m. and 289 p.m. are predicted for the 760 nm and 1200 nm emission bands, respectively (see Fig. 4). These results for the WGM displacements with temperature in microspheres are similar to the ones obtained in other works [2,7]. It has to be noticed that the results obtained for the 760 and 1200 nm bands are pseudo-experimental.
explained in the experimental section), have been estimated from the emission spectra at different temperatures. Fig. 3 presents the average wavelength displacement of the WGM peaks of the 660 nm band, showing a linear dependence. For a temperature increase of 30°, the WGM peaks under study shifted an average of 158 p.m. on this temperature range. From these data, the relationship between the wavelength displacement and the temperature is Δλ= (5.3 ± 0.1)*T(1553 ± 30) pm obtained by linear fitting. The slope represents the variation of wavelength with temperature given by Eq. (2). The linear behaviour means that β + α are constant in this temperature range. Considering Eq. (2) and the temperature calibration for the 660 nm emission band experimentally obtained, the relationship between the WGM peaks displacement and the temperature for the 760 nm and the 1200 nm emission bands can be inferred, in a pseudo-experimental
4.2. WGM displacement due to laser power In this part of the work, the laser power was gradually increased from 3 to 64 mW, exciting and heating the microsphere at the same time, producing the displacement of the WGM peaks with the temperature. Several WGM have been measured for each band and a mean 209
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calculated from Eq. (4) taking into account that the relative sensitivity, estimated from Eq. (3), is 8.11 × 10−6 K−1, obtaining a value of δT = 0.6 K, 0.5 K and 0.3 K for the 660 nm, 760 nm and 1200 nm bands respectively, all lying in the range of uncertainties of single-center emission thermometers [22]. Considering that the limit of detection of our experimental set-up is a FWHM of 0.11 nm, uncertainties of 0.20 K, 0.18 and 0.11 K for the bands mentioned above could be obtained. This value can be further improved if a glass with higher sensitivity is used. Repeatibility of the experiment can be characterized by the repeatability coefficient given by 2.77δT, being δT the standard deviation of the temperature [24]. This coefficient has a value R = 1.6 K for the 660 nm band, 1.4 K for the 760 nm band and 0.9 K for the 1200 nm band. This means that if we perform two measurements, the absolute difference between them will be lower than the repeatability coefficient in a 95% of the occasions. 5. Conclusions Fig. 4. Displacement of the WGM wavelengths for the three emission bands.
On the first part of the experiment, displacement rates of the WGM peaks with temperature of 5.3 p.m./K, 6.2 p.m./K and 9.7 p.m./K were found for the 660 nm, 760 nm and 1200 nm bands, respectively. On the other hand, heating and exciting a microsphere with a 532 nm laser, average displacement rates of the WGM peaks with the laser power of 17.6 p.m./mW, 20.9 p.m./mW and 35.2 p.m./mW were found for the emission bands centred at 660 nm, 760 nm and 1200 nm, respectively, under the same conditions. From these results the maximum value of the microsphere temperature was estimated to be 490.4 K. We have found that the temperature uncertainty of this method is 0.6 K for the band centred in 660 nm, 0.5 K for the 760 nm band and 0.3 K for the 1200 nm band, lying in the range of uncertainties of single-center emission thermometers (0.4–1 K) reported by C.D.S. Brites et al. [20]. These values can be further improved if a peak with a lower value of FWHM is obtained or a glass with higher α and β is used. Taking all the results into account, it can be concluded that when a calibrated Ho3+ doped YAS microsphere is excited and heated up with a laser, its temperature can be estimated by measuring the WGM peaks displacement, and that the emission band centred at 1200 nm can be used if medical applications are considered, since that emission lays in the second biological window, where the biological tissues are partially transparent.
Fig. 5. Displacement of the WGM peaks at the three emissions of a Ho3+ doped YAS microsphere as a function of the laser power.
value is represented in Fig. 5. The error bars are omitted because they can not be distinguished (the errors are about 6 p.m.). Average displacement rates of 17.6 ± 0.3 p.m./mW, 20.9 ± 0.3 p.m./mW and 35.2 ± 0.7 p.m./mW were found, for the emission bands centred at 660 nm, 760 nm and 1200 nm, respectively. Using the temperature calibration previously obtained for this microsphere for the 660 nm centred band (see linear fit parameters in section 4.1), the temperature reached with the laser can be estimated, obtaining a value of 490.4 K (217.4 °C). On the other hand, WGM displacements of 1200 p.m. and 2100 nm were obtained experimentally with the laser excitation for the 760 nm and 1200 nm bands, respectively. Using the pseudo-experimental relationships between the WGM displacement and the temperature described in section 4.1, displacements of 1230 p.m. and 1950 p.m. are found for a temperature of 490.4 K, in good agreement with the experimental values above. It can be concluded that with this method, the temperature of a Ho3+ doped YAS microsphere can be estimated using different emission bands, studying the WGM wavelengths displacement when it is heated. Temperature uncertainty has been experimentally obtained by measuring 100 emission spectra under the same conditions and estimating the uncertainty of the WGM position through the standard deviation of its centroid position. A value of δλ = 0.003 nm has been obtained for the 660 nm band, confirming that the uncertainty in the peak position is about 1% of its FWHM as reported by F. Vollmer et al. [23] and independent on the wavelength. Temperature uncertainty was
Acknowledgments This research was partially supported by Ministry of Education and Science of Russia (grant 14.Z50.31.0009), by the Spanish MINECO (MAT2015-71070-REDC, and MAT2016-75586-C4-4-P, FIS201677319-C2-1-R), and by the EU-FEDER funds. References [1] V.K. Rai, Appl. Phys. B 88 (2) (2007) 297. [2] C. Pérez-Rodríguez, L. Labrador-Páez, I.R. Martín, S. Ríos, Laser Phys. Lett. 12 (2015) 046003. [3] M. Horn, J. Opt. Soc. Am. B 23 (2006) 212. [4] L. Labrador-Paez, K. Soler-Carracedo, M. Hernandez-Rodriguez, I.R. Martin, T. Carmon, L.L. Martin, Optic Express 25 (2017) 1165. [5] L. Yang, K.J. Vahala, Opt. Lett. 28 (2003) 592. [6] G. Miaomiao, W. Cong, L. Xianqing, L. Yuan, H. Fengqin, S.Z. Yong, Chem. Commun. 53 (2017) 3102. [7] L.L. Martín, C. Pérez-Rodríguez, P. Haro-González, I.R. Martín, Optic Express 19 (2011) 25792. [8] M.B. Bortot, S. Prastalo, M. Prado, Procedia Materials Science 1 (2012) 351. [9] L. Hench, D. Day, W. Höland, V. Rheinberger, Int. J. Appl. Glass Sci. 1 (2010) 104. [10] J.-C.G. Bünzli, Chem. Rev. 110 (2010) 2729. [11] T. Schweizer, B.N. Samson, J.R. Hector, W.S. Brocklesby, D.W. Hewak, D.N. Payne, Infrared Phys. Technol. 40 (1999) 329. [12] U. Rocha, K.U. Kumar, C. Jacinto, I. Villa, F. Sanz-Rodríguez, M.C. Iglesias-de-laCruz, A. Juarranz, E. Carrasco, F.C.J.M. van Veggel, E. Bovero, J. García-Solé, D. Jaque, Small 10 (6) (2014) 1141. [13] M. Quintanilla, Y. Zhang, L.M. Liz-Marzán, Chem. Mater. 30 (2018) 2819.
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[20] C.D.S. Brites, A. Millan, L.D. Carlos, Lanthanides in luminescent thermometry, in: J.V.G. Bünzli, V.K. Pecharsky (Eds.), Handbook on the Physics Chemistry of the Rare Earths: Including Actinides, Elsevier B.V., Amsterdam, 2016, pp. 339–427. [21] M.J. Hyatt, J. Am. Ceram. Soc. 70 (1987) 283. [22] V.N. Sigaev, G.N. Atroschenko, V.I. Savinkov, P.D. Sarkisov, G. Babajew, K. Lingel, R. Lorenzi, A. Palearim, Mater. Chem. Phys. 133 (2012) 24. [23] F. Vollmer, S. Arnold, Nat. Methods 5 (7) (2008) 591. [24] J.W. Bartlett, C. Frost Ultrasound, Obstet. Gynecol. 31 (2008) 466.
[14] Qiwen Pan, Suiying Ye, Dandan Yang, Jianrong Qiu, Guoping Dong, Mater. Res. Bull. 93 (2017) 310. [15] W. Yin, T. Bao, X. Zhang, Q. Gao, J. Yu, X. Dong, L. Yan, Z. Gu, Y. Zhaoab, Nanoescale 3 (2018) 1517. [16] Ol Savchuk, J.J. Carvajal, L.G. De la Cruz, P. Haro-González, M. Aguiló, F. Díaz, J. Mater. Chem. C 4 (2016) 7397. [17] T. Ioppolo, N. Das, M. Volkan Ötügen, J. Appl. Phys. 107 (2010) 103105. [18] D.S. Sanditov, B.S. Sydykov, Phys. Solid State 56 (2014) 1006. [19] Q. Ma, T. Rossmann, Z. Guo, J. Phys. D Appl. Phys. 41 (24) (2008) 245111.
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Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Whispering gallery modes in a holmium doped glass microsphere: Temperature sensor in the second biological window
T
L. de Sousa-Vieiraa, S. Ríosa, I.R. Martína,b,∗, L. García-Rodrígueza, V.N. Sigaevc, V.I. Savinkovc, G. Yu Shakhgildyanc a
Departamento de Física, Universidad de La Laguna, Apdo. 456, E-38200, San Cristóbal de La Laguna, Santa Cruz de Tenerife, Spain Instituto Universitario de Materiales y Nanotecnología (IMN), Universidad de La Laguna, Apdo. 456, E-38200, San Cristóbal de La Laguna, Santa Cruz de Tenerife, Spain c D. Mendeleev University of Chemical Technology, Miusskaya sq., 9, 125047, Moscow, Russia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Transparent microspheres Holmium Emission Whispering gallery modes Temperature sensor
The displacements on the whispering gallery modes (WGM) wavelengths of the three emission bands (660 nm, 760 nm and 1200 nm) of Ho3+ doped yttrium aluminosilicate microspheres were analysed as a function of the temperature. The displacements have been studied in order to evaluate the viability of these microspheres as thermal sensors. A 532 nm continuous wave laser was used to excite and heat the microspheres using a confocal microscope setup. Average displacement rates of the WGM peaks with the laser power of 5.3, 6.2 and 9.7 p.m./ mW were found for the emission bands centred at 660 nm, 760 nm and 1200 nm, respectively. Temperature uncertainties of 0.6 K for the band centred at 660 nm, 0.5 K for the 760 nm band and 0.3 K for the 1200 nm band have been obtained. It is interesting to note that the maximum resolution corresponds to the band centred at 1200 nm that is especially interesting because it is inside the second biological window, which has medical applications.
1. Introduction Optical sensors are devices capable of acquiring information about a surrounding media by analysing the change in the intensity or phase of a light beam. These devices present numerous advantages compared to traditional sensors, in terms of electrical passiveness, greater sensitivity, freedom from electromagnetic interference, wide dynamic range, point and distributed configurations and multiplexing capabilities [1]. Nowadays, Whispering Gallery Modes (WGM) resonators have a wide range of applications: as temperature [2,3] or humidity sensors [4], laser action [5], chemical vapor sensors [6], among others. Specifically, spherical WGM resonators, known as microspheres, are useful as temperature sensors [2,3] because of the high sensitivity of their resonance modes to any change of temperature. As it will be discussed later, the wavelengths of the WGM can be shifted if there is a change in the microsphere size or in the optical properties of the surrounding medium, for example due to a temperature variation. So, by monitoring the shift in the WGM peaks when a temperature change takes place, the temperature of the microsphere can be estimated. Another benefit of using microspheres as optical sensors is that the detection can be carried out remotely, so it is possible to make measurements inside the
∗
system causing just a small perturbation in the physical system [7]. Glasses in the Y2O3–Al2O3–SiO2 system, known as YAS glasses, are of great interest for applications in nuclear medicine. YAS microspheres have been used to treat liver cancer [8,9]. One way to treat malignant tumours is with radiation, but external radiation doses are limited because they damage the nearby healthy tissue. A high-effective solution to this issue is to place the radiation source inside the tumour by injecting radioactive microspheres [9]. On the other hand, microspheres are usually doped with lanthanides because these elements have numerous fluorescent emissions in the visible and the near infrared region of the electromagnetic spectrum [10]. The triple ionized holmium ions have an emission peak at about 1200 nm [11], which corresponds to the 5I6 → 5I8 transition, in the near infrared region. This emission is interesting because it lays in the second biological window, that extends from 1000 nm to 1400 nm, where the absorption of light by the constituents of biological tissue is minimized, i.e. tissues are partially transparent [12]. Two water absorption bands located at about 980 nm and 1500 nm limit this spectral range. Multiple researches relating the second biological window are being carried out for medical and biological applications [12–16]. The aim of this work is to use YAS microspheres doped with Ho3+
Corresponding author. Departamento de Física, Universidad de La Laguna, Apdo. 456, E-38200, San Cristóbal de La Laguna, Santa Cruz de Tenerife, Spain. E-mail address: [email protected] (I.R. Martín).
https://doi.org/10.1016/j.optmat.2018.06.014 Received 28 February 2018; Received in revised form 7 June 2018; Accepted 8 June 2018
Available online 18 June 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
Optical Materials 83 (2018) 207–211
L. de Sousa-Vieira et al.
3. Experimental methods
due to its promising emission at 1200 nm, as temperature sensors. A laser is used to excite the WGM and heat the microsphere at the same time, thus we can monitor the temperature of the microsphere analysing the displacement of the WGM for increasing temperatures.
3.1. Microspheres production The YAS microspheres were prepared by analogy with procedures described in Ref. [22]. Glasses with molar composition (in mol.%) 13.48Y2O3 – 37.56Al2O3 – 46.96SiO2 − 2Ho2O3 were obtained by melting-quenching process starting from Y2O3, Ho2O3, amorphous SiO2 and Al(OH)3 reagents with purity higher than 99.99%. YAS glasses were melted at temperatures of 1630 °C for about 3 h in a platinum crucible placed in an electric furnace with SiC heaters and working chamber in high-purity corundum. In order to obtain microspheres, plates not thicker than 200 μm were produced by the rolling method and grinded by corundum disks. Grains were then selected in size by means of an automated wet sieving technique (Retsch AS 200) in the range 20–32 μm. Spheroidization of glass particles was performed by injection of them in an Argon plasma generated by a streamlined plasma torch with a maximum working current of about 500 A.
2. Theoretical background 2.1. Resonance condition Under the geometrical optics approximation, it can be obtained that the WGM must satisfy the following resonance condition [17]:
λ ·l = 2πR n
(1)
with l the polar mode number, that is the number of wavelengths that fits into the resonator, n the microsphere refractive index and R its radius. These resonant wavelengths can suffer shifts due to changes in the refractive index, or in the size of the cavity. Such changes on the microsphere parameters may occur because of temperature variations, among others. The derivative of the resonant wavelength with respect to temperature, provides a relationship between both parameters:
dλ 1 ∂n 1 ∂R ⎞ =⎛ + λ = (β + α ) λ dT R ∂T ⎠ ⎝ n ∂T 1 ∂n
3.2. WGM displacement A Ho3+ doped YAS microsphere with a size of 27 μm was excited at its center with a 532 nm laser and its emission detected on the surface as explained in Ref. [7]. Its emission spectrum as a function of the laser power was obtained and recorded with an Andor CCD spectrograph. The experimental setup is shown in Fig. 1. A laser beam is diverted with a mirror (M1) to a pinhole (P) and a collimating lens (CL), to obtain a circularly collimated beam. Then, the light is reflected by a dichroic mirror (M2) to a microscope objective (MO), that focalizes the light in the microsphere. Between M2 and MO a beam splitter (BS) sends part of the light to a CCD camera that shows an image of the microspheres on a TV screen. The light transmitted by the dichroic mirror is reflected by a high precision mirror (M3) and focused on the entrance slit of the spectrograph Andor SR-3031-B with a CCD Newton DU920 N, used to obtain the microsphere emission spectrum on the visible region. Moreover, in order to obtain the emission spectrum on the near infrared region the mirror M3 is removed and the emission is focused to an optical fiber (F) that is connected to a spectrograph Andor SR-500i-B2 with a CCD InGaAs CCDDU490A-1.7. In order to calibrate the WGM displacements with temperature the microsphere was located inside of a cuvette (surrounded by air). The cuvette was immersed in a water bath reservoir and the temperature of the bath was changed with an accuracy of 0.1 °C using a resistor. Closed water cycled assured the homogeneity of the temperature in the bath before each measurement. The walls of the water bath are transparent in the visible range. The emission spectra were recorded for a set of temperature values.
(2) 1 ∂R
where β = n ∂T is the thermo-optic coefficient, and α = R ∂T the thermal expansion coefficient. The sign of these coefficients is positive [18]. If a temperature increase takes place, the peaks corresponding to the resonant wavelengths in the emission spectrum of the microsphere will shift to the red region of the spectra. 2.2. Sensitivity and temperature uncertainty Optical temperature sensors are often characterized in terms of their relative sensitivity, which is a parameter that represents the variation of the measured variable with the temperature relative to its magnitude [7]. Then, the sensitivity for the displacement of the WGM with temperature is:
SR =
1 dλ λ dT
(3)
Where dλ/dΤ is given by Eq. (2). On the other hand, the temperature uncertainty of the thermal sensor can be defined as [19,20]:
δT =
1 δλ SR λ
(4)
4. Results and discussion
where δT is the temperature uncertainty, which represents the minimum temperature variation that can be detected by a thermal sensor, δλ represents the uncertainty in the WGM peaks, λ the resonant wavelength and SR the relative sensitivity. Eq. (2) shows how the wavelength shift with temperature depends on the refractive index change through the thermal optical coefficient and on the changes on the microsphere size through the thermal expansion coefficient. Those parameters are not known for the glass under study but the thermal expansion coefficient has been measured by Hyatt (Ref. [21]) for a glass with a similar chemical composition YAS10, Y2O3 (35 w%), Al2O3 (35 w%), SiO2 (30 w%) having a constant value of α = 48·10−7 ºC−1. This coefficient does not depend on the microsphere radius, and so the relative sensitivity. Then the microsphere size can be changed to adapt to the application of interest without compromising its performance as a temperature sensor. If a better sensitivity in needed for a particular application, a material with higher values of α and β must be chosen.
The emission spectrum of a Ho3+ doped YAS microsphere obtained by exciting with a laser power with 3 mW at room temperature is shown in Fig. 2 (A). Placing the excitation and the detection at the center of the microsphere, three broad bands can be observed corresponding to typical transitions of the trivalent Ho3+ ions. The broad band centred at 660 nm can be assigned to the 5F5 → 5I8 holmium transition, the one at 760 nm to 5S2 (5F4) → 5I7 transition, and the 1200 nm band to 5I6 → 5I8 transition. However, if the detection is placed at the microsphere border, the WGM are observed overlapping the holmium emission bands, as can be seen in Fig. 2 (B). 4.1. Temperature calibration As was previously explained the WGM peaks can be displaced as function of the temperature. Therefore, the displacements of the WGM peaks of a Ho3+ doped YAS microsphere, when it is heated (as was 208
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Fig. 1. Experimental setup to obtain the emission spectrum of an excited microsphere with a laser at 532 nm. The scale bar in the inset photograph correspond to 30 μm.
Fig. 2. (A) Emission spectrum of a Ho3+ doped YAS microsphere obtained under excitation at 532 nm at room temperature, placing the detection at its center (showing the corresponding transitions). (B) Emission spectrum of a Ho3+ doped YAS microsphere showing the WGM under excitation at 532 nm at room temperature, placing the detection at the border of the microsphere. WGM are clearly visible. The inset shows the power dependence of the spectra for some selected WGM peaks.
Fig. 3. Displacement of the WGM peaks at the 660 nm emission of a Ho3+ doped YAS microsphere as a function of temperature.
way. Taking into account that β + α do not depend of the wavelength, the slopes for the other two bands can be estimated from the expression Δλ=(β + α)λΔΤ, obtaining slopes of 6.2 p.m./K and 9.7 p.m./K for the 760 nm and 1200 nm emission bands respectively. It can be observed that the slopes increase for increasing values of the wavelength. For a temperature increase of 30°, shifts of approximately 182 p.m. and 289 p.m. are predicted for the 760 nm and 1200 nm emission bands, respectively (see Fig. 4). These results for the WGM displacements with temperature in microspheres are similar to the ones obtained in other works [2,7]. It has to be noticed that the results obtained for the 760 and 1200 nm bands are pseudo-experimental.
explained in the experimental section), have been estimated from the emission spectra at different temperatures. Fig. 3 presents the average wavelength displacement of the WGM peaks of the 660 nm band, showing a linear dependence. For a temperature increase of 30°, the WGM peaks under study shifted an average of 158 p.m. on this temperature range. From these data, the relationship between the wavelength displacement and the temperature is Δλ= (5.3 ± 0.1)*T(1553 ± 30) pm obtained by linear fitting. The slope represents the variation of wavelength with temperature given by Eq. (2). The linear behaviour means that β + α are constant in this temperature range. Considering Eq. (2) and the temperature calibration for the 660 nm emission band experimentally obtained, the relationship between the WGM peaks displacement and the temperature for the 760 nm and the 1200 nm emission bands can be inferred, in a pseudo-experimental
4.2. WGM displacement due to laser power In this part of the work, the laser power was gradually increased from 3 to 64 mW, exciting and heating the microsphere at the same time, producing the displacement of the WGM peaks with the temperature. Several WGM have been measured for each band and a mean 209
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calculated from Eq. (4) taking into account that the relative sensitivity, estimated from Eq. (3), is 8.11 × 10−6 K−1, obtaining a value of δT = 0.6 K, 0.5 K and 0.3 K for the 660 nm, 760 nm and 1200 nm bands respectively, all lying in the range of uncertainties of single-center emission thermometers [22]. Considering that the limit of detection of our experimental set-up is a FWHM of 0.11 nm, uncertainties of 0.20 K, 0.18 and 0.11 K for the bands mentioned above could be obtained. This value can be further improved if a glass with higher sensitivity is used. Repeatibility of the experiment can be characterized by the repeatability coefficient given by 2.77δT, being δT the standard deviation of the temperature [24]. This coefficient has a value R = 1.6 K for the 660 nm band, 1.4 K for the 760 nm band and 0.9 K for the 1200 nm band. This means that if we perform two measurements, the absolute difference between them will be lower than the repeatability coefficient in a 95% of the occasions. 5. Conclusions Fig. 4. Displacement of the WGM wavelengths for the three emission bands.
On the first part of the experiment, displacement rates of the WGM peaks with temperature of 5.3 p.m./K, 6.2 p.m./K and 9.7 p.m./K were found for the 660 nm, 760 nm and 1200 nm bands, respectively. On the other hand, heating and exciting a microsphere with a 532 nm laser, average displacement rates of the WGM peaks with the laser power of 17.6 p.m./mW, 20.9 p.m./mW and 35.2 p.m./mW were found for the emission bands centred at 660 nm, 760 nm and 1200 nm, respectively, under the same conditions. From these results the maximum value of the microsphere temperature was estimated to be 490.4 K. We have found that the temperature uncertainty of this method is 0.6 K for the band centred in 660 nm, 0.5 K for the 760 nm band and 0.3 K for the 1200 nm band, lying in the range of uncertainties of single-center emission thermometers (0.4–1 K) reported by C.D.S. Brites et al. [20]. These values can be further improved if a peak with a lower value of FWHM is obtained or a glass with higher α and β is used. Taking all the results into account, it can be concluded that when a calibrated Ho3+ doped YAS microsphere is excited and heated up with a laser, its temperature can be estimated by measuring the WGM peaks displacement, and that the emission band centred at 1200 nm can be used if medical applications are considered, since that emission lays in the second biological window, where the biological tissues are partially transparent.
Fig. 5. Displacement of the WGM peaks at the three emissions of a Ho3+ doped YAS microsphere as a function of the laser power.
value is represented in Fig. 5. The error bars are omitted because they can not be distinguished (the errors are about 6 p.m.). Average displacement rates of 17.6 ± 0.3 p.m./mW, 20.9 ± 0.3 p.m./mW and 35.2 ± 0.7 p.m./mW were found, for the emission bands centred at 660 nm, 760 nm and 1200 nm, respectively. Using the temperature calibration previously obtained for this microsphere for the 660 nm centred band (see linear fit parameters in section 4.1), the temperature reached with the laser can be estimated, obtaining a value of 490.4 K (217.4 °C). On the other hand, WGM displacements of 1200 p.m. and 2100 nm were obtained experimentally with the laser excitation for the 760 nm and 1200 nm bands, respectively. Using the pseudo-experimental relationships between the WGM displacement and the temperature described in section 4.1, displacements of 1230 p.m. and 1950 p.m. are found for a temperature of 490.4 K, in good agreement with the experimental values above. It can be concluded that with this method, the temperature of a Ho3+ doped YAS microsphere can be estimated using different emission bands, studying the WGM wavelengths displacement when it is heated. Temperature uncertainty has been experimentally obtained by measuring 100 emission spectra under the same conditions and estimating the uncertainty of the WGM position through the standard deviation of its centroid position. A value of δλ = 0.003 nm has been obtained for the 660 nm band, confirming that the uncertainty in the peak position is about 1% of its FWHM as reported by F. Vollmer et al. [23] and independent on the wavelength. Temperature uncertainty was
Acknowledgments This research was partially supported by Ministry of Education and Science of Russia (grant 14.Z50.31.0009), by the Spanish MINECO (MAT2015-71070-REDC, and MAT2016-75586-C4-4-P, FIS201677319-C2-1-R), and by the EU-FEDER funds. References [1] V.K. Rai, Appl. Phys. B 88 (2) (2007) 297. [2] C. Pérez-Rodríguez, L. Labrador-Páez, I.R. Martín, S. Ríos, Laser Phys. Lett. 12 (2015) 046003. [3] M. Horn, J. Opt. Soc. Am. B 23 (2006) 212. [4] L. Labrador-Paez, K. Soler-Carracedo, M. Hernandez-Rodriguez, I.R. Martin, T. Carmon, L.L. Martin, Optic Express 25 (2017) 1165. [5] L. Yang, K.J. Vahala, Opt. Lett. 28 (2003) 592. [6] G. Miaomiao, W. Cong, L. Xianqing, L. Yuan, H. Fengqin, S.Z. Yong, Chem. Commun. 53 (2017) 3102. [7] L.L. Martín, C. Pérez-Rodríguez, P. Haro-González, I.R. Martín, Optic Express 19 (2011) 25792. [8] M.B. Bortot, S. Prastalo, M. Prado, Procedia Materials Science 1 (2012) 351. [9] L. Hench, D. Day, W. Höland, V. Rheinberger, Int. J. Appl. Glass Sci. 1 (2010) 104. [10] J.-C.G. Bünzli, Chem. Rev. 110 (2010) 2729. [11] T. Schweizer, B.N. Samson, J.R. Hector, W.S. Brocklesby, D.W. Hewak, D.N. Payne, Infrared Phys. Technol. 40 (1999) 329. [12] U. Rocha, K.U. Kumar, C. Jacinto, I. Villa, F. Sanz-Rodríguez, M.C. Iglesias-de-laCruz, A. Juarranz, E. Carrasco, F.C.J.M. van Veggel, E. Bovero, J. García-Solé, D. Jaque, Small 10 (6) (2014) 1141. [13] M. Quintanilla, Y. Zhang, L.M. Liz-Marzán, Chem. Mater. 30 (2018) 2819.
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