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In this paper, temperature variations are detected thanks to an enhanced nano-optical pyroelectric sensor. Sensing is obtained with the pyroelectric effect of lithium niobate (LN) in which, a suitable air-membrane photonic crystal cavity has been fabricated. The wavelength position of the cavity mode is tuned 11.5 nm for a temperature variation of only 32 °C. These results agree quite well with 3D-FDTD simulations that predict tunability of 12.5 nm for 32 °C. This photonic crystal temperature sensor shows a sensitivity of 0.359 nm/°C for an active length of only ~5.2 μm.
©2013 Optical Society of America
1. Introduction
Temperature sensors utilizing optical techniques offer a promising direction in the development of sensor technologies due to several advantages as compared to other temperature measurement techniques like for example high sensitivity, large temperature range, stability and immunity of optical signal to environmental disturbances [1–3]. Fiber-optic temperature sensors are the most widely used. One of the most popular are the fiber Bragg gratings (FBG) based sensors. They possess sensitivities up to 20 pm/°C with a fiber probe tapered to a point [4]. By using long-period gratings the sensitivity can reach 0.6 nm/°C for a length of 1 mm [5]. High birefringence fiber-loop mirrors (HiBi-FLM) have a higher temperature sensitivity (0.9435 nm/°C) but at the expense of a long active length (720 mm) [6]. Since several years, and thanks to the development of photonic crystal fibers (PCF), PCF-based temperature sensors have been developed showing high sensitivities (6.6 nm/°C) but requiring a PCF length of 6.1 cm [7]. All these fiber-based sensors can reach high sensitivities but sensors are generally bulky and cannot be used as integrated chip sensors. In the quest of integration and compactness, some studies concerning temperature sensing based on surface plasmon and ring resonator devices have recently appeared [8–10]. In particular, a 4 μm radius silicon ring resonator has been recently published showing a sensitivity of 83 pm/°C [10]. Very recently, one-dimensional photonic crystal (PhC) surface plasmon waveguides have been theoretically proposed as highly sensitive temperature sensors [11]. Sensitivity of 70 pm/K has been theoretically predicted around the telecommunication window. In terms of the materials that can be used for the sensor fabrication, pyroelectric materials are excellent candidates for this purpose [12] since their spontaneous polarization presents strong temperature dependence.
In this paper, Focused Ion Beam (FIB) technology combined with optical grade dicing is used to create a temperature sensor based on a lithium niobate photonic crystal on an air-suspended membrane. Pyroelectric effect on lithium niobate will be used to detect temperature variations. In order to enhance the pyroelectric effect, we have designed a slow light geometry based on a 2D Fabry-Perot photonic crystal (Fig. 1). It consists of a triangular lattice of air holes oriented along the ΓΜ direction. The Fabry Perot configuration, one line of defect perpendicular to the light propagation direction, is composed of 5 rows of holes from each side of the defect line. Plane Wave Expansion (PWE) simulations using the supercell technique have been performed in order to obtain the dispersion diagram of the structure and to find the geometrical parameters that provide a flat band (corresponding to the defect line) falling at the working wavelength of 1550 nm. This mode is contained inside the photonic bandgap which is comprised between 1380 nm and 1940 nm. The result of these simulations (that are fully described in [13]) show that a flat band at the wavelength λ of 1544 nm is obtained for a/λ = 0.36, being a the hole periodicity and for r/a = 0.3 being r the hole radius. We have also calculated the group velocity for this particular band and show that it corresponds to a value equal to c/20, being c the light speed in vacuum. This value ensures slow light propagation conditions and we will see in what follows that it will allow having an enhancement on the lithium niobate pyroelectric effect when the photonic crystal device is used at the operating wavelength around 1544 nm. The enhancement factor has been calculated to be equal to ~18 as described in References [13–15]. In these previous works, the photonic crystal FP structure was used to enhance the electro-optic effect on lithium niobate waveguides that did not have a suspended membrane. The work presented here offers two major novelties. First is the fact that we have greatly improved the waveguiding conditions by engraving the photonic crystal on a lithium niobate air supended membrane. The second novelty is that the photonic crystal is used as a temperature detector by enhancing lithium niobate pyroelectricity.
Indeed, lithium niobate is a typical polar material that exhibits a large pyroelectric effect. It can emit electrons with energies up to 105 eV and can generate electric fields as high as 106 V/cm [16]. Some recent publications have shown its enormous potential to generate significant physical effects at low energy cost if slow light geometries are engineered for example in a photonic crystal. Thus, electro-optical intensity modulators of only 13 μm x 13 μm with 300 fold enhanced performances [17], acousto-optic photonic crystal modulators [18], tunable superprisms [19] and micro-ring resonators [20] have shown the potential to have lithium niobate active and nonlinear optical devices-on chip.
In order to go a step further and to fabricate highly performing optical functions based on photonic crystals, a sharp optical transmission edge with large rejection ratios is essential and therefore, it is mandatory to develop etching techniques for the fabrication of sub-micrometric structures in which guided light through the lithium niobate substrate interacts in an optimum way with the photonic crystal structure. The optimum solution to obtain tight optical mode confinement in only several micrometers size would be photonic crystal devices fabricated on suspended lithium niobate membranes. Applications requiring ultra-high switching speed that would be impossible to achieve in other materials become a reality in such architectures.
Most of membrane waveguide designs with photonic crystals are limited to SOI-based structures [21–24]. Some attempts have been already done in lithium niobate but experimental optical demonstration of the devices is still missing [25–27].
2. Device design
In order to analyze the optical behavior of the device, 3D-FDTD simulations have been performed for a structure with 5 rows of holes at each side of the line defect. Figure 2(a) shows the transmission spectrum corresponding to the photonic crystal in the suspended membrane and in a conventional Annealed Proton Exchange (APE) lithium niobate with a ridge waveguide. Transmission band gap goes from 1350 nm to 1900 nm with the Fabry-Perot resonance peak located at 1518 nm. Peak transmission is 78% in the suspended membrane compared with the 13% obtained in a ridge APE waveguide without air window clearly showing an improved light confinement within the membrane.
Fig. 2 (a) Calculated transmission spectrum of the PhC with the air window (pink solid line) and in the APE waveguide without the air window (blue dotted line) (b)Horizontal and vertical cross section of the electric field distribution at the peak resonance wavelength (λ = 1518 nm) and inside the photonic band gap (λ = 1460 nm) calculated by 3D-FDTD.
The electric field distribution has been calculated for two different spectral locations: at the peak central wavelength (λ = 1518 nm) and at a wavelength inside the photonic band gap (λ = 1460 nm). Figure 2(b) depicts the horizontal cross section (top view of the device) and a vertical cross-section of the field distribution, which allows us to see the field distribution inside the LN PhC membrane. Bragg resonances are observed at the entrance of the photonic crystal. They correspond to the interference pattern between the incident light and the reflected light from the photonic crystals entrance facet. At the peak resonance wavelength, light is transmitted through the photonic crystal. This wavelength can serve then as the working wavelength of the tunable device.
3. Fabrication
Fabrication of the structure has been done in several steps. Firstly, a planar Annealed Proton Exchange waveguide has been fabricated on an X-cut lithium niobate sample allowing vertical light confinement along the whole device length. Horizontal light confinement is provided by a ridge waveguide fabricated by optical grade dicing with a circular precision saw (DISCO DAD 321) [13,15]. The ridge width is 6 μm. The suspended air membrane has been fabricated with FIB, by opening up a window on the lateral side walls of the ridge resulting on a bridge-type membrane by the ridge LN structure. The membrane thickness is ~2 μm. End-fire light coupling with a lensed fiber has been used to characterize the ridge waveguiding properties. Light was coupled into the waveguide, (it is worth noticing that APE waveguides only allow light propagation with TE polarization). A Peltier element was fixed into the sample support for the temperature measurements. Figure 3 shows the SEM image of the membrane (Fig. 3(a)) and the experimental evidence of monomodal guiding at the wavelength of 1550 nm (Fig. 3(b)).
Fig. 3 (a) SEM image of the ridge membrane, (b) Image of the mode at the exit of the membrane at λ = 1550 nm. (c) SEM image of the membrane with the PhC.
Secondly, the Fabry-Perot 2D photonic crystal structure has been FIB etched on the membrane surface so that holes completely penetrate the membrane thickness. A SEM image of the complete structure can be seen in Fig. 3(c).
4. Sensing analysis and characterizations
Temperature tunability is achieved through the variation of the lithium niobate refractive index. If we consider that the index of refraction variation is due to the internal electric field generated by the pyroelectric effect, this variation can be described by the equation:
where n is the index of refraction of an annealed proton exchange waveguide and is equal to 2.143, r33 is the electro-optic coefficient on an X-cut substrate, f is the local field factor which is inversely proportional to the group velocity inside the PhC [13], and Ez is the electric field generated by the pyroelectric effect. This electric field depends on the temperature variation in the following way:where, εo and εr are the vacuum and relative dielectric constants (for lithium niobate εr = 28.7), and p is the pyroelectric coefficient (p = −6x10−5 C m−2 K−1). We have calculated by 3D-FDTD simulations the transmission spectra at different temperatures and in parallel, experimental transmission performed with an infrared DFB laser source (1500nm-1600nm) is performed on the fabricated device. The Fabry-Perot resonance peak, calculated and experimentally measured, is displayed in Fig. 4(a). In order to match the experimental and the theoretical predictions, the calculated spectrum was blue shifted by 0.78% in wavelength compared to the experimental results, which is a reasonable deviation given experimental fabrication uncertainties. Let us have a look now to the experimental measurements and the theoretical predictions as the temperature is varied. The experimental tunability of the resonance peak as a function of the temperature can be observed in Fig. 4(b). We can observe an almost linear red-shift variation of the peak as the temperature increases. For a temperature variation of 32 °C we observe a peak shift of 11.5 nm. 3D-FDTD simulations of the transmission spectra for the same temperature variations are displayed in Fig. 4(c). A red shift of 12.5 nm is theoretically predicted for a temperature variation of the device of 32 °C which is in agreement with the theoretical predictions. The experimental and theoretical device sensitivity is shown in Fig. 5 where we show the peak shift as a function of the temperature.Fig. 4 (a) Fabry-Perot resonance peak obtained by 3D-FDTD simulation of the device and the experimentally measured transmission, (b) experimentally measured transmission spectra of the peak as a function of the temperature, (c) 3D-FDTD simulation transmission spectra of the peak as a function of the temperature
Fig. 5 Experimental (pink circles) and theoretical (blue squares) plot showing the peak shift as a function of the temperature in the device.
From our simulation results, and corroborated from the experiments, we have found that the temperature variation of the transmission peak is ~18 times enhanced (f 3 in Eq. (1) with respect to bulk lithium niobate. This enhancement is due to a slow group velocity (c/20) of the Fabry-Perot cavity mode (directly obtained by the 3D-FDTD simulations) and as a consequence one can obtain a pyroelectric index of refraction variation of 0.0206. Thermal expansion in lithium niobate has not been taken into account in this study since the index variation induced by thermal effect (Δn = 0.0013) is one order of magnitude smaller than the one corresponding to the pyroelectric effect.
Temperature sensing is therefore equal to 0.359 nm/°C for a device of only ~5.2μm in interaction-length that can easily be integrated on a chip. In the case of unstructured bulk lithium niobate this sensitivity will of of 0.019 nm/°C. In addition, from Eq. (2) we can deduce that the equivalent electric field that is generated is −2.1x108 V/m.
5. Conclusion
In conclusion, we have designed and fabricated a photonic crystal cavity of 5.2 μm interaction-length on a lithium niobate air suspended membrane. The membrane greatly increases the light confinement. This device has been used for temperature sensing based on a slow light enhanced pyroelectric effect. A temperature sensitivity of 0.359 nm/°C has been experimentally obtained and theoretically validated by 3D-FDTD simulations.
Acknowledgment
This work was supported by the ANR project PhoXcry (contract ANR-09-NANO-004) and by the Labex ACTION program (contract ANR-11-LABX-01-01). The authors would like to thank B. Guichardaz for helpful support in clean room. Huihui Lu is also financially supported by the Conseil Régional de Franche-Comté, France.
References and links
1. E. Zrenner, “Artificial vision: Solar cells for the blind,” Nat. Photonics 6(6), 344–345 (2012). [CrossRef]
2. M. V. Romalis, “Atomic sensors: Chip-scale magnetometers,” Nat. Photonics 1(11), 613–614 (2007). [CrossRef]
3. P. W. Barone, S. Baik, D. A. Heller, and M. S. Strano, “Near-infrared optical sensors based on single-walled carbon nanotubes,” Nat. Mater. 4(1), 86–92 (2005). [CrossRef] [PubMed]
4. J.-L. Kou, S.-J. Qiu, F. Xu, and Y.-Q. Lu, “Demonstration of a compact temperature sensor based on first-order Bragg grating in a tapered fiber probe,” Opt. Express 19(19), 18452–18457 (2011). [CrossRef] [PubMed]
5. M. Chomát, J. Čtyroký, D. Berková, V. Matějec, J. Kaňka, J. Skokánková, F. Todorov, A. Jančárek, and P. Bittner, “Temperature sensitivity of long-period gratings inscribed with a CO2 laser in optical fiber with graded-index cladding,” Sens. Actuators B Chem. 119(2), 642–650 (2006). [CrossRef]
6. Y. Liu, B. Liu, X. Feng, W. Zhang, G. Zhou, S. Yuan, G. Kai, and X. Dong, “High-birefringence fiber loop mirrors and their applications as sensors,” Appl. Opt. 44(12), 2382–2390 (2005). [CrossRef] [PubMed]
7. W. Qian, C.-L. Zhao, S. He, X. Dong, S. Zhang, Z. Zhang, S. Jin, J. Guo, and H. Wei, “High-sensitivity temperature sensor based on an alcohol-filled photonic crystal fiber loop mirror,” Opt. Lett. 36(9), 1548–1550 (2011). [CrossRef] [PubMed]
8. X. Zhang and X. Li, “Design, fabrication and characterization of optical microring sensors on metal substrates,” J. Micromech. Microeng. 18(1), 015025 (2008). [CrossRef]
9. H. S. Lee, G. D. Kim, and S. S. Lee, “Temperature compensated glucose sensor exploiting ring resonators,” IEEE Photon. Technol. Lett. 21(16), 1136 (2009). [CrossRef]
10. G.-D. Kim, H.-S. Lee, C. H. Park, S.-S. Lee, B. T. Lim, H. K. Bae, and W.-G. Lee, “Silicon photonic temperature sensor employing a ring resonator manufactured using a standard CMOS process,” Opt. Express 18(21), 22215–22221 (2010). [CrossRef] [PubMed]
11. T. Srivastava, R. Das, and R. Jha, “Highly Sensitive Plasmonic Temperature Sensor Based on Photonic Crystal Surface Plasmon Waveguide,” Plasmonics 8(2), 515–521 (2013). [CrossRef]
12. R. W. Whatmore, “Pyroelectric devices and materials,” Rep. Prog. Phys. 49(12), 1335–1386 (1986). [CrossRef]
13. H. Lu, B. Sadani, G. Ulliac, N. Courjal, C. Guyot, J.-M. Merolla, M. Collet, F. I. Baida, and M.-P. Bernal, “6-micron interaction length electro-optic modulation based on lithium niobate photonic crystal cavity,” Opt. Express 20(19), 20884–20893 (2012). [CrossRef] [PubMed]
14. H. Lu, B. Sadani, N. Courjal, G. Ulliac, N. Smith, V. Stenger, M. Collet, F. I. Baida, and M.-P. Bernal, “Enhanced electro-optical lithium niobate photonic crystal wire waveguide on a smart-cut thin film,” Opt. Express 20(3), 2974–2981 (2012). [CrossRef] [PubMed]
15. H. Lu, B. Sadani, G. Ulliac, N. Courjal, C. Guyot, M. Collet, F. I. Baida, and M.-P. Bernal, “Lithium niobate photonic crystal wire cavity: Realization of a compact electro-optically tunable filter,” Appl. Phys. Lett. 101(15), 151117 (2012). [CrossRef]
16. J. D. Brownridge, “Pyroelectric X-ray generator,” Nature 358(6384), 287–288 (1992). [CrossRef] [PubMed]
17. M. Roussey, M.-P. Bernal, N. Courjal, D. Van Labeke, F. I. Baida, and R. Salut, “Electro-optic effect exaltation on lithium niobate photonic crystals due to slow photons,” Appl. Phys. Lett. 89(24), 241110 (2006). [CrossRef]
18. N. Courjal, S. Benchabane, J. Dahdah, G. Ulliac, Y. Gruson, and V. Laude, “Acousto-optically tunable lithium niobate photonic crystal,” Appl. Phys. Lett. 96(13), 131103 (2010). [CrossRef]
19. J. Amet, G. Ulliac, F. I. Baida, and M.-P. Bernal, “Experimental evidence of enhanced electro-optic control on a lithium niobate photonic crystal superprism,” Appl. Phys. Lett. 96(10), 103111 (2010). [CrossRef]
20. A. Guarino, G. Poberaj, D. Rezzonico, R. Degl'innocenti, and P. Günter, “Electro–optically tunable microring resonators in lithium niobate,” Nat. Photonics 1(7), 407–410 (2007).
21. Y. A. Vlasov, M. O’Boyle, H. F. Hamann, and S. J. McNab, “Active control of slow light on a chip with photonic crystal waveguides,” Nature 438(7064), 65–69 (2005). [CrossRef] [PubMed]
22. S. J. McNab, N. Moll, and Y. A. Vlasov, “Ultra-low loss photonic integrated circuit with membrane-type photonic crystal waveguides,” Opt. Express 11(22), 2927–2939 (2003). [CrossRef] [PubMed]
23. E. Dulkeith, S. J. McNab, and Y. A. Vlasov, “Mapping the optical properties of slab-type two-dimensional photonic crystal waveguides,” Phys. Rev. B 72(11), 115102 (2005). [CrossRef]
24. G. Poberaj, H. Hu, W. Sohler, and P. Günter, “Lithium niobate on insulator (LNOI) for micro-photonic devices,” Laser & Photon. Rev. 6(4), 488–503 (2012). [CrossRef]
25. G. Si, E. J. Teo, A. A. Bettiol, J. Teng, and A. J. Danner, “Suspended slab and photonic crystal waveguides in lithium niobate,” J. Vac. Sci. Technol. B 28(2), 316 (2010). [CrossRef]
26. R. Geiss, S. Diziain, R. Iliew, C. Etrich, H. Hartung, N. Januts, F. Schrempel, F. Lederer, T. Pertsch, and E.-B. Kley, “Light propagation in a free-standing lithium niobate photonic crystal waveguide,” Appl. Phys. Lett. 97(13), 131109 (2010). [CrossRef]
27. H. Hartung, E.-B. Kley, T. Gischkat, F. Schrempel, W. Wesch, and A. Tünnermann, “Ultra thin high index contrast photonic crystal slabs in lithium niobate,” Opt. Mater. 33(1), 19–21 (2010). [CrossRef]
