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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 27 — Dec. 17, 2012
  • pp: 28119–28124
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High-quality LiNbO3 microdisk resonators by undercut etching and surface tension reshaping

Tzyy-Jiann Wang, Jheng-Yu He, Cheng-An Lee, and Huan Niu  »View Author Affiliations


Optics Express, Vol. 20, Issue 27, pp. 28119-28124 (2012)
http://dx.doi.org/10.1364/OE.20.028119


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Abstract

We report ultra-smooth LiNbO3 microdisk resonators fabricated by selective ion implantation, chemical etching, and thermal treatment. The undercut microdisk structure is produced by chemically etching the buried lattice damage layer formed by selective ion implantation. By thermal treatment, surface tension smoothes and reshapes microdisk surface topography. The resonant characteristics of microdisk resonators are simulated by finite element method and are well consistent with the experimental results. The 20μm-diameter microdisk resonator has the FSR of 16.43nm and the Q factor of 2.60 × 104. The produced LiNbO3 microdisk resonators can be utilized in new microdisk applications with electro-optic and nonlinear-optic effects.

© 2012 OSA

1. Introduction

Lithium niobate (LiNbO3) is an important ferroelectric material in photonics due to its excellent physical effects, such as electro-optic effect, nonlinear-optic effect, photoelastic effect, and piezoelectric effect. Utilization of electro-optic effect on microdisk resonators produces fast and effective tuning of the resonant wavelength, and thus the tunability on light intensity and filtered wavelength. The nonlinear frequency conversion response is enhanced by using the microdisk resonator structure. However, limited by the LiNbO3 microdisk fabrication techniques, the ideal LiNbO3 microdisk resonators with several μm diameter and smooth surface have not yet been produced. Previously, LiNbO3 microresonators are produced in the microring form [7

7. 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). [CrossRef]

,8

8. Y. S. Lee, G.-D. Kim, W.-J. Kim, S.-S. Lee, W.-G. Lee, and W. H. Steier, “Hybrid Si-LiNbO₃ microring electro-optically tunable resonators for active photonic devices,” Opt. Lett. 36(7), 1119–1121 (2011). [CrossRef] [PubMed]

] by crystal ion slicing [9

9. T. A. Ramadan, M. Levy, and R. M. Osgood Jr., “Electro-optic modulation in crystal-ion-sliced z-cut LiNbO3 thin films,” Appl. Phys. Lett. 76(11), 1407–1409 (2000). [CrossRef]

]. In order to combine the unique WGM properties with electro-optic effect and nonlinear-optic effect, LiNbO3 disk resonators with diameter 3~6.3mm are produced by mechanical polishing. Their usage finds new photonic applications, such as high-order tunable filter [10

10. A. A. Savchenkov, V. S. Ilchenko, A. B. Matsko, and L. Maleki, “High-order tunable filters based on a chain of coupled crystalline whispering gallery mode resonators,” IEEE Photon. Technol. Lett. 17(1), 136–138 (2005). [CrossRef]

], electric field sensor [11

11. K. Sasagawa and M. Tsuchiya, “Lithium niobate disk sensor using photonic heterodyning,” Appl. Phys. Express 2, 082201 (2009). [CrossRef]

], planar coupling to disk resonator [12

12. G. Nunzi Conti, S. Berneschi, F. Cosi, S. Pelli, S. Soria, G. C. Righini, M. Dispenza, and A. Secchi, “Planar coupling to high-Q lithium niobate disk resonators,” Opt. Express 19(4), 3651–3656 (2011). [CrossRef] [PubMed]

], frequency doubling at a wide frequency range [13

13. V. S. Ilchenko, A. A. Savchenkov, A. B. Matsko, and L. Maleki, “Nonlinear optics and crystalline whispering gallery mode cavities,” Phys. Rev. Lett. 92(4), 043903 (2004). [CrossRef] [PubMed]

], naturally phase-matched second harmonic generation [14

14. J. U. Fürst, D. V. Strekalov, D. Elser, M. Lassen, U. L. Andersen, C. Marquardt, and G. Leuchs, “Naturally phase-matched second-harmonic generation in a whispering-gallery-mode resonator,” Phys. Rev. Lett. 104(15), 153901 (2010). [CrossRef] [PubMed]

], simultaneous second harmonic and third harmonic generation [15

15. K. Sasagawa and M. Tsuchiya, “Highly efficient third harmonic generation in a periodically poled MgO:LiNbO3 disk resonator,” Appl. Phys. Express 2(12), 122401 (2009). [CrossRef]

]. Although large-diameter LiNbO3 disk resonators have smaller radiation loss and thus higher Q factor, the large-disk problems, such as narrow FSR, large device size, and the difficulty in device integration, would restrict the application range.

In this work, we present the first LiNbO3 microdisk resonator with several μm diameter and smooth surface. The undercut structure of the microdisk is produced by selective ion implantation and chemical etching [16

16. T.-J. Wang, Y.-H. Tsou, W.-C. Chang, and H. Niu, “Fabrication of three-dimensional crystalline microstructures by selective ion implantation and chemical etching,” Appl. Phys., A Mater. Sci. Process. 102(2), 463–467 (2011). [CrossRef]

]. By surface tension reshaping, the rough etched surface is smoothed during thermal treatment. The characteristics of resonant modes, such as resonant wavelength, effective index, and field profile, are calculated by finite element method and compared with the experimental results. The resonator measurement demonstrates the large FSR and the high Q factor of the produced LiNbO3 microdisk resonators.

2. Device fabrication

Figure 1(a)
Fig. 1 (a) Fabrication process of the LiNbO3 undercut microdisk; (b) the vacancy concentration distribution in the three regions on LiNbO3 (LN) labeled in Fig. 1(a)-4.
shows the fabrication process of LiNbO3 microdisk resonators. First, a 120nm-thick Cr film is deposited on the –z face of the LiNbO3 substrate by RF magnetron sputtering. By photolithography, the 1.2μm-thick photoresist (PR) with a 20μm-diameter circle shape (microdisk pattern) is formed over the Cr film. The Cr film without PR protection is etched by the Cr-7 etchant. After removing the PR, the patterned Cr film is used as chemical-etching mask. Over the circular Cr film, the 2.5μm-thick PR is patterned with a 10μm-diameter circle shape (pillar pattern) and is used as implantation mask. According to the mask deployment in Figs. 1(a)-(4), the substrate is separated into three regions: (I) PR/Cr/LiNbO3; (II) Cr/LiNbO3; (III) exposed LiNbO3. The substrate is implanted by He+ ions with the inclination angle 7þ. The implantation energy is 350 keV and the implantation dose is 1.3 × 1016 ions/cm2. The implanted He+ ions produce the buried lattice damage layer in LiNbO3, labeled as dot lines in Figs. 1(a)-(5). The SRIM (Stopping and Range of Ions in Matter) program is used to calculate the vacancy concentration distributions after implantation, as shown in Fig. 1(b). In region I, the 2.5μm-thick PR effectively resists the implanted He+ ions such that no lattice damage occurs in LiNbO3. Because the Cr film is thin, the vacancy concentration distribution in region II is 0.12μm shallower than that in region III.

After implantation, the substrate is immersed in the diluted HF solution (HF/H2O = 1/0.5) for 10hrs with the temperature controlled at 18þC. This etching solution can etch the -z face of LiNbO3 and the lattice damage layer. Initially, the region without the protection of Cr film is etched. When the etching reaches the depth of the lattice damage layer, the etching would proceed laterally until the entire lattice damage layer is removed. The etching time after removing the entire lattice damage layer determines the gap between microdisk and substrate (or pillar height). After etching, the PR and the underlay Cr film are removed. Because the rough microdisk surface increases the scattering loss, surface tension reshaping is used to smooth the etched surface. As the LiNbO3 substrate is treated at the temperature approaching the melting point (1257þC), the surface would become slight molten state. Surface tension would modify the substrate surface such that the etched surface becomes smooth. The treatment temperature is chosen at 1120þC, which is smaller than the Curie temperature (1142þC), in order to maintain ferroelectric phase. The slightly melted surface layer re-crystallizes with the seeding of un-melted bulk crystal during cooling and maintains crystalline state after cooling. During thermal treatment, the LiNbO3 substrate is put in the crucible with LiNbO3 powder to suppress Li2O out-diffusion. The heating starts at a rate 5°/min, dwells at 1120þC for 3hrs, and ends with natural cooling. This process is under flowing N2 gas to avoid contamination.

Figure 2
Fig. 2 Photographs of the 20μm-diameter LiNbO3 microdisks (a-d) without thermal treatment; (e-h) with thermal treatment at 1120þC for 3hrs. (a,b,e,f), (c,g), and (d,h) are measured by scanning electron microscope, reflection microscope, and transmission microscope.
shows the photographs of undercut LiNbO3 microdisks without and with thermal treatment. The thermal treatment time is 3hrs, which is far shorter than 50hrs in [17

17. C. Y. J. Ying, C. L. Sones, A. C. Peacock, F. Johann, E. Soergel, R. W. Eason, M. N. Zervas, and S. Mailis, “Ultra-smooth lithium niobate photonic micro-structures by surface tension reshaping,” Opt. Express 18(11), 11508–11513 (2010). [CrossRef] [PubMed]

], to avoid microdisk deformation. The scanning electron microscope (SEM) photographs in Fig. 2(a,b) and 2(e,f) demonstrate the thermal treatment effect on enhancing surface smoothness. Not only the entire microdisk surface becomes smoother but also the substrate surface does. It is noted that thermal treatment reshapes the microdisk rim from the angled edge to the round edge. Due to the shadowing effect of implantation mask, the pillar diameter is larger than that of the implantation mask. Comparison of the reflection microscope photographs in Fig. 2(c) and 2(g) shows that thermal treatment makes the microdisk periphery becomes smoother and more like circular. The bottom surface of the microdisk also becomes smoother and uniform. The transmission microscope photographs in Fig. 2(d) and 2(h) display the detailed pillar structure. Because of the hexagonal nature of LiNbO3 crystal, chemical etching produces a pillar with the hexagonal pedestal. Thermal treatment reshapes the pillar pedestal to the circular shape and makes the pillar near microdisk become vertical, as shown in Fig. 2(f). The atomic force microscope (AFM) measurement shows that thermal treatment reduces the root-mean-square roughness of the etched surface from 1.17nm to 0.41nm, which is close to that of the original substrate surface (0.4nm).

The geometric structure of the produced microdisks has good repeatability as the etching temperature is precisely controlled. The geometry of the LiNbO3 microdisk can be varied by adjusting process parameters. The diameters of microdisk and pillar are determined by the photolithographic mask pattern. The implantation depth increases with the implantation energy. The level and the range of lattice damage become larger as the implantation dose increases. Therefore, the microdisk thickness increases with the implantation energy but decreases with the implantation dose. The former has the dominant effect. The pillar height increases with the chemical etching time. The microdisk thickness is designed at 0.4μm such that the microdisk has a single vertical resonant mode.

3. Device characterization

4. Results and discussion

The microdisk resonant modes and the tapered fiber mode are calculated by the full-vectorial mode solver using finite element method in cylindrical coordinates. The microdisk resonant modes for TM and TE polarizations are labeled as TMp,m and TEp,m, where p and m are the radial and azimuthal orders. Figure 4(a)
Fig. 4 (a) Comparison of effective index for the microdisk resonant modes and the tapered fiber mode; (b) the simulated field profiles of microdisk resonant modes.
shows the comparison of effective index for the first six TE radial modes (TE1~TE6) and the first three TM radial modes (TM1~TM3) of microdisk resonator, and the fundamental mode HE11 of tapered fiber. The marker positions correspond to the specific resonant modes. Because the ordinary index is larger than the extraordinary index in LiNbO3, the TE modes have larger effective indices than the TM modes at the same radial order. The effective coupling from tapered fiber to microdisk resonator occurs as the phase matching condition is satisfied. It is found that the tapered fiber coupling mainly excites the TM1 and TE4 modes. Figure 4(b) shows the field profiles of the TM1,57 and TE4,57 modes for the microdisk resonators with angled edge (without thermal treatment) and round edge (with thermal treatment). Because the TE4,57 mode has a larger field profile than the TM1,57 mode, the TE4,57 mode has smaller field overlap with the tapered fiber mode and larger resonator loss, which result in weaker coupling and resonance.

Figure 5
Fig. 5 Transmission spectrums of the LiNbO3 microdisk resonators labeled with loaded quality factor Q and mode order TM1,m (a) without thermal treatment; (b) with thermal treatment.
shows the measured transmission spectrums of the microdisk resonators without and with thermal treatment. The radial and azimuthal orders of the resonant modes are discriminated by comparing the measured resonant wavelength with the one calculated in simulation. The resonant wavelengths of the three main resonant modes (TM1,56, TM1,57, TM1,58) in Fig. 5 are very close to the simulated ones shown in Fig. 4(a). The main resonant mode TM1,56 of the microdisk resonator without thermal treatment has the FSR of 16.01nm, the T of −3.50dB, the QL of 3.09 × 103, the Q0 of 3.70 × 103, and the Qex of 1.86 × 104. After thermal treatment, the main resonant mode TM1,57 has the FSR of 16.43nm, the T of −7.74dB, the QL of 1.83 × 104. The Q0 and the Qex are 2.60 × 104 and 6.21 × 104. The blue shift 2.79nm of resonant wavelength is due to the slight shrinkage of microdisk diameter after thermal treatment. It demonstrates that thermal treatment not only enhances surface smoothness but also can be used in tuning the resonant wavelength. The obvious enhancement of the Q0 after thermal treatment is attributed to the reduction of surface roughness and the smoothness enhancement of microdisk rim. Efficient coupling between microdisk resonator and tapered fiber requires phase matching and large field overlap between microdisk resonant mode and tapered fiber mode. Although the TM1 and TE4 modes achieve the similar phase matching level, the TM1 modes are mainly excited due to its larger field overlap with the tapered fiber mode. The excitation of TM modes in the LiNbO3 microdisk resonator has the advantage of using the largest electro-optic coefficient r33 and the largest nonlinear-optic coefficient d33 of LiNbO3. It is noted that the larger pillar diameter increases the leakage loss of high-radial-order modes and facilitates to suppress their resonance. Because a large pillar diameter is used in this work, the first-radial-order TM modes are mainly excited. Elimination of high-radial-order resonant modes effectively extends the FSR of the microdisk resonator. The Q factors of LiNbO3 microdisk resonators are restricted by small index contrast (compared to semiconductor microdisk), small diameter (compared to LiNbO3 disk), and non-perfect circular shape. The last can be improved by photoresist reflow during the process of producing chemical-etching mask.

5. Conclusion

LiNbO3 microdisk resonators with large FSR and high Q factor are demonstrated for the first time. The ultra-smooth undercut microdisk structure with roughness of 0.41nm is successfully produced by selective ion implantation, chemical etching, and surface tension reshaping. The proposed fabrication method has good repeatability and the potential to produce microdisks on the other ferroelectric crystals with good nonlinear-optic effect. The microdisk geometry is designed such that the single vertical modes are excited and the high-radial-order modes are suppressed. This design consideration and the small microdisk diameter effectively extend the FSR of the microdisk resonator. The simulated resonant characteristics by finite element method are well consistent with the experimental results. The produced 20μm-diameter microdisk resonator has the FSR of 16.43nm and the Q factor of 2.60 × 104. The largest FSR value of the LiNbO3 resonator to date will facilitate new device design. The first produced LiNbO3 microdisk resonator has the potential to initiates new photonic applications with electro-optic and nonlinear-optic effects.

Acknowledgments

The authors would like to thank the support from National Science Council of the Republic of China, Taiwan, under grants NSC100-2221-E-027-060 and NSC101-2221-E-027-096.

References and links

1.

L. Zhou and A. W. Poon, “Silicon electro-optic modulators using p-i-n diodes embedded 10-micron-diameter microdisk resonators,” Opt. Express 14(15), 6851–6857 (2006). [CrossRef] [PubMed]

2.

S. J. Choi, Z. Peng, Q. Yang, S. J. Choi, and P. D. Dapkus, “Tunable microdisk resonators vertically coupled to bus waveguides using epitaxial regrowth and wafer bonding techniques,” Appl. Phys. Lett. 84(5), 651–653 (2004). [CrossRef]

3.

A. C. Tamboli1, E. D. Haberer, R. Sharma, K. H. Lee, S. Nakamura, and E. L. Hu, “Room-temperature continuous-wave lasing in GaN/InGaN microdisks,” Nat. Photonics 1, 61–64 (2007).

4.

J. Renner, L. Worschech, A. Forchel, S. Mahapatra, and K. Brunner, “CdSe quantum dot microdisk laser,” Appl. Phys. Lett. 89(23), 231104 (2006). [CrossRef]

5.

L. Stern, I. Goykhman, B. Desiatov, and U. Levy, “Frequency locked microdisk resonator for real time and precise monitoring of refractive index,” Opt. Lett. 37(8), 1313–1315 (2012). [CrossRef] [PubMed]

6.

T. Grossmann, S. Schleede, M. Hauser, T. Beck, M. Thiel, G. von Freymann, T. Mappes, and H. Kalt, “Direct laser writing for active and passive high-Q polymer microdisks on silicon,” Opt. Express 19(12), 11451–11456 (2011). [CrossRef] [PubMed]

7.

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). [CrossRef]

8.

Y. S. Lee, G.-D. Kim, W.-J. Kim, S.-S. Lee, W.-G. Lee, and W. H. Steier, “Hybrid Si-LiNbO₃ microring electro-optically tunable resonators for active photonic devices,” Opt. Lett. 36(7), 1119–1121 (2011). [CrossRef] [PubMed]

9.

T. A. Ramadan, M. Levy, and R. M. Osgood Jr., “Electro-optic modulation in crystal-ion-sliced z-cut LiNbO3 thin films,” Appl. Phys. Lett. 76(11), 1407–1409 (2000). [CrossRef]

10.

A. A. Savchenkov, V. S. Ilchenko, A. B. Matsko, and L. Maleki, “High-order tunable filters based on a chain of coupled crystalline whispering gallery mode resonators,” IEEE Photon. Technol. Lett. 17(1), 136–138 (2005). [CrossRef]

11.

K. Sasagawa and M. Tsuchiya, “Lithium niobate disk sensor using photonic heterodyning,” Appl. Phys. Express 2, 082201 (2009). [CrossRef]

12.

G. Nunzi Conti, S. Berneschi, F. Cosi, S. Pelli, S. Soria, G. C. Righini, M. Dispenza, and A. Secchi, “Planar coupling to high-Q lithium niobate disk resonators,” Opt. Express 19(4), 3651–3656 (2011). [CrossRef] [PubMed]

13.

V. S. Ilchenko, A. A. Savchenkov, A. B. Matsko, and L. Maleki, “Nonlinear optics and crystalline whispering gallery mode cavities,” Phys. Rev. Lett. 92(4), 043903 (2004). [CrossRef] [PubMed]

14.

J. U. Fürst, D. V. Strekalov, D. Elser, M. Lassen, U. L. Andersen, C. Marquardt, and G. Leuchs, “Naturally phase-matched second-harmonic generation in a whispering-gallery-mode resonator,” Phys. Rev. Lett. 104(15), 153901 (2010). [CrossRef] [PubMed]

15.

K. Sasagawa and M. Tsuchiya, “Highly efficient third harmonic generation in a periodically poled MgO:LiNbO3 disk resonator,” Appl. Phys. Express 2(12), 122401 (2009). [CrossRef]

16.

T.-J. Wang, Y.-H. Tsou, W.-C. Chang, and H. Niu, “Fabrication of three-dimensional crystalline microstructures by selective ion implantation and chemical etching,” Appl. Phys., A Mater. Sci. Process. 102(2), 463–467 (2011). [CrossRef]

17.

C. Y. J. Ying, C. L. Sones, A. C. Peacock, F. Johann, E. Soergel, R. W. Eason, M. N. Zervas, and S. Mailis, “Ultra-smooth lithium niobate photonic micro-structures by surface tension reshaping,” Opt. Express 18(11), 11508–11513 (2010). [CrossRef] [PubMed]

OCIS Codes
(130.3730) Integrated optics : Lithium niobate
(230.3120) Optical devices : Integrated optics devices
(230.5750) Optical devices : Resonators

ToC Category:
Integrated Optics

History
Original Manuscript: October 10, 2012
Revised Manuscript: November 21, 2012
Manuscript Accepted: November 23, 2012
Published: December 4, 2012

Citation
Tzyy-Jiann Wang, Jheng-Yu He, Cheng-An Lee, and Huan Niu, "High-quality LiNbO3 microdisk resonators by undercut etching and surface tension reshaping," Opt. Express 20, 28119-28124 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-27-28119


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References

  1. L. Zhou and A. W. Poon, “Silicon electro-optic modulators using p-i-n diodes embedded 10-micron-diameter microdisk resonators,” Opt. Express14(15), 6851–6857 (2006). [CrossRef] [PubMed]
  2. S. J. Choi, Z. Peng, Q. Yang, S. J. Choi, and P. D. Dapkus, “Tunable microdisk resonators vertically coupled to bus waveguides using epitaxial regrowth and wafer bonding techniques,” Appl. Phys. Lett.84(5), 651–653 (2004). [CrossRef]
  3. A. C. Tamboli1, E. D. Haberer, R. Sharma, K. H. Lee, S. Nakamura, and E. L. Hu, “Room-temperature continuous-wave lasing in GaN/InGaN microdisks,” Nat. Photonics1, 61–64 (2007).
  4. J. Renner, L. Worschech, A. Forchel, S. Mahapatra, and K. Brunner, “CdSe quantum dot microdisk laser,” Appl. Phys. Lett.89(23), 231104 (2006). [CrossRef]
  5. L. Stern, I. Goykhman, B. Desiatov, and U. Levy, “Frequency locked microdisk resonator for real time and precise monitoring of refractive index,” Opt. Lett.37(8), 1313–1315 (2012). [CrossRef] [PubMed]
  6. T. Grossmann, S. Schleede, M. Hauser, T. Beck, M. Thiel, G. von Freymann, T. Mappes, and H. Kalt, “Direct laser writing for active and passive high-Q polymer microdisks on silicon,” Opt. Express19(12), 11451–11456 (2011). [CrossRef] [PubMed]
  7. A. Guarino, G. Poberaj, D. Rezzonico, R. Degl’innocenti, and P. Günter, “Electro-optically tunable microring resonators in lithium niobate,” Nat. Photonics1(7), 407–410 (2007). [CrossRef]
  8. Y. S. Lee, G.-D. Kim, W.-J. Kim, S.-S. Lee, W.-G. Lee, and W. H. Steier, “Hybrid Si-LiNbO₃ microring electro-optically tunable resonators for active photonic devices,” Opt. Lett.36(7), 1119–1121 (2011). [CrossRef] [PubMed]
  9. T. A. Ramadan, M. Levy, and R. M. Osgood., “Electro-optic modulation in crystal-ion-sliced z-cut LiNbO3 thin films,” Appl. Phys. Lett.76(11), 1407–1409 (2000). [CrossRef]
  10. A. A. Savchenkov, V. S. Ilchenko, A. B. Matsko, and L. Maleki, “High-order tunable filters based on a chain of coupled crystalline whispering gallery mode resonators,” IEEE Photon. Technol. Lett.17(1), 136–138 (2005). [CrossRef]
  11. K. Sasagawa and M. Tsuchiya, “Lithium niobate disk sensor using photonic heterodyning,” Appl. Phys. Express2, 082201 (2009). [CrossRef]
  12. G. Nunzi Conti, S. Berneschi, F. Cosi, S. Pelli, S. Soria, G. C. Righini, M. Dispenza, and A. Secchi, “Planar coupling to high-Q lithium niobate disk resonators,” Opt. Express19(4), 3651–3656 (2011). [CrossRef] [PubMed]
  13. V. S. Ilchenko, A. A. Savchenkov, A. B. Matsko, and L. Maleki, “Nonlinear optics and crystalline whispering gallery mode cavities,” Phys. Rev. Lett.92(4), 043903 (2004). [CrossRef] [PubMed]
  14. J. U. Fürst, D. V. Strekalov, D. Elser, M. Lassen, U. L. Andersen, C. Marquardt, and G. Leuchs, “Naturally phase-matched second-harmonic generation in a whispering-gallery-mode resonator,” Phys. Rev. Lett.104(15), 153901 (2010). [CrossRef] [PubMed]
  15. K. Sasagawa and M. Tsuchiya, “Highly efficient third harmonic generation in a periodically poled MgO:LiNbO3 disk resonator,” Appl. Phys. Express2(12), 122401 (2009). [CrossRef]
  16. T.-J. Wang, Y.-H. Tsou, W.-C. Chang, and H. Niu, “Fabrication of three-dimensional crystalline microstructures by selective ion implantation and chemical etching,” Appl. Phys., A Mater. Sci. Process.102(2), 463–467 (2011). [CrossRef]
  17. C. Y. J. Ying, C. L. Sones, A. C. Peacock, F. Johann, E. Soergel, R. W. Eason, M. N. Zervas, and S. Mailis, “Ultra-smooth lithium niobate photonic micro-structures by surface tension reshaping,” Opt. Express18(11), 11508–11513 (2010). [CrossRef] [PubMed]

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