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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 24 — Nov. 26, 2007
  • pp: 16090–16096
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Ultra-High Q/V Fabry-Perot microcavity on SOI substrate

P. Velha, E. Picard, T. Charvolin, E. Hadji, J. C. Rodier, P. Lalanne, and D. Peyrade  »View Author Affiliations


Optics Express, Vol. 15, Issue 24, pp. 16090-16096 (2007)
http://dx.doi.org/10.1364/OE.15.016090


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Abstract

We experimentally demonstrate an ultra high Q/V nanocavity on SOI substrate. The design is based on modal adaptation within the cavity and allows to measure a quality factor of 58.000 for a modal volume of 0.6(λ/n)3. This record Q/V value of 105 achieved for a structure standing on a physical substrate, rather than on membrane, is in very good agreement with theoretical predictions also shown. Based on these experimental results, we show that further refinements of the cavity design could lead to Q/V ratios close to 106.

© 2007 Optical Society of America

1. Introduction

2. Design and fabrication of Fabry-Perot microcavities

Silicon-On-Insulator wafers used in this work were manufactured by SOITEC. Stack layer consists of a 340 nm silicon top layer (waveguide core) and a 2 µm thick SiO2 insulator supported by a bulk silicon substrate. The ridge waveguide (500 nm width) and holes patterning is performed by electron-beam lithography with a Leica VB-6UHR on a single 400 nm thick NEB22 negative resist spin coated on top of the sample. The electron beam energy is 100 keV resulting in a 5-nm probe-beam diameter. After developing the resist, Inductive-Coupled-Plasma etching of the sample is performed in an industrial plasma chamber (DPS from Applied Materials Inc).

Fig.1. . EM picture of the lineic Fabry-Perot cavity inserted in the silicon waveguide. On both sides of the cavity, each mirror is composed of a taper and of a periodic section. The taper is located on the cavity side of the mirror and is made of four holes with increasing diameter (130, 170, 200, 200 nm) and separated by increasing distances (300, 320, 350 nm respectively). The periodic mirror is made of N holes (N=4 on the picture) with a diameter of 200 nm and a period of 370 nm. The inset presents a magnification of the tapered zone.

The native oxide is first removed by a short breakthrough with a CF4 gas mixture. Then, the silicon native layer is etched with a HBr, Cl2, O2 gas mixture, at a pressure of 4 mT. Finally, the resist is stripped by oxygen plasma. Then, each sample is manually cleaved in order to obtain optical facets. As an example, Fig. 1 presents a fabricated Fabry-Perot cavity where each mirror is composed of two sections. The periodic section of the mirror is formed of holes with identical diameter and with identication hole-separation distances. The tapered section is formed with four holes with different diameters and with periods decreasing as one approaches the cavity defect.

3. Optical resonance properties of the microcavities

After cleaving optical facets, the device is mounted on a XYZ translational stage regulated in temperature. TE-polarized light from a tunable laser source emitting from 1.45µm to 1.59 µm is launched into the 9 mm long ridge access waveguide. Light is injected through the cleaved facets in waveguides. We use a polarization maintaining optical fibber. This optical fibber is chemically etched in order to obtain an end diameter of about 2 µm. After passing through the device, the light is collected by a x15 reflective objective. The waveguide output is then imaged, either onto an IR camera for observing light at the output of the cleaved facet, or onto an InGaAs photodiode for transmission measurements. Additionally, when using the photodiode, the imaging system incorporates a diaphragm and a polarizer in order to spatially filter inevitable spurious substrate-guided light. Sample is observed by a microscope with an angle of collection of 30°, which permits to collect the light escaping vertically from the structure. This light is guided via an optical fibber from the microscope objective onto another InGaAs detector.

Fig. 2. Resonant cavity peak collected above the cavity. The insert d etails the resonant peak for two regulated temperature of the sample. Left-inset is fitted with a lorentzian curve replicated with a dashed line in the right-inset.

We have also performed measurements for other cavities for N ranging from N=3 to N=7. The spectra in Fig. 3 show the transmission of these cavities in black and the radiation losses measured by the top of the cavity in light grey. Transmission spectra through the device are normalized by a reference waveguide measurement. Spectra collected above the cavity show a resonant peak with the same spectral position and linewidth as observed in transmission spectra. This is consistent with the Fabry-Perot model [16

16. P. Yeh, “Optical Waves in Layered Media,” Wiley-Interscience (1988)

] and gives a second check for the estimation of the Q factor.

Fig. 3. Evolution of the resonant peak with increasing the number of holes in the periodic mirror (N). The black curve presents the normalized transmission across the cavities, the grey one shows the vertical losses collected by the top of the sample in arbitrary units with inverted axis.

As the number N of holes increases, the transmission peak (Tmax) at resonance shows a global significant decrease, meanwhile losses collected above the cavity increase (Fig. 3 and table 1). When the light is bouncing inside the cavity, it could experience two kinds of losses: In one hand, losses at each reflection due to the mode-profile mismatch and located at the tapered sections. This term is roughly constant and independent of the number of holes in the periodic section of the mirror. In a less significant way, the second source of loss corresponds to in-line losses due to the ridge waveguide roughness estimated to 6 dB/cm.

As the Q factors increase, both the number of reflections inside the cavity and the light path increase. This explains the decrease of Tmax and the increase of the light collected above the cavity.

Table 1. Experimental Cavity Q’s and Tmax for N ranging from 3 to 7 designed with the same tapering section.

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4. Cavity performances

Figure 4 shows that the Q factors (model and experimental) augment with N, which corresponds to the increase of the mirror reflectivity.

Fig. 4. Experimental (dots) evolution of the Q factor for the lineic Fabry-Perot cavity with increasing N. The shaded region represents the theoretical values for an increasing number of holes in the periodic mirror of the cavity and for a tolerance of +/- 10 nm on the nominal value of the hole diameters. The dot curve shows the optimum Q factor value calculated for an optimal cavity length for each N.

Fig. 5. Evolution of the normalized quality factor QNORM as a function of the resonant wavelength of for N=3 to N=7. The labeled numbers represent the experimental values (Qexpr). h is the cavity length given by the relation h=λ/neff.

Figure 5 details the evolution of calculated QNORM factor (QNORM being the normalised Q factor obtained by dividing the actual Q by the maximum Q achievable with the same number N of holes) as a function of the cavity defect length h. On the same picture, the experimental values for the resonant wavelengths and Q factors have been added for N ranging from 3 to 7.

We first observe that the calculated QNORM optimum (curves in Fig. 5) is shifting and is getting sharper as N increases. This shift is due to the fact that the Q factors for N<4 are mainly limited by the mirror transmission Tm, which is minimal for λ=1.55 µm. For N>4, the Q factor is limited by the losses Lm, which is minimal for λ=1,6 µm. Thus the maximum of Q factor achievable is shifted from 1,55 µm to 1,6 µm. Secondly, cavities have been designed and fabricated with a constant length h for all N’s. The experimental cavity length allows to obtain Q factors close to the optimum QNORM for N=3 and N=4, but a progressively-increasing wavelength shift is obtained for larger N values. Finally, we report on Fig. 4 (dash-line) the theoretical evolution of Q factor calculated with the optimum h value for every N. Figure 5 additionally shows that the experimental Q factors can be further improved by adjusting the cavity length for each N values in order to have the optimal Q factor.

To conclude, with such a design optimization, cavity Q factor’s could become closer and closer to the calculated value Q=150,000 for N=7 or Q=400,000 for N=10. Moreover, we note that the cavity geometry allows to achieve ultra-high Q/V on a SOI substrate over a broad spectral range (e.g. Q>58 000 over ~40 nm and Q>20 000 over ~100 nm).

5. Conclusion

Acknowledgments

This work is supported by the French national program PNANO from the “Agence Nationale de la Recherche” under the MIRAMAN contract and P. Velha benefit from a BDI-CNRS-CEA fellowship. CEA-LETI is acknowledged for providing us with an access to the micro/nanofabrication facility center.

References and links

1.

J. Poon, L. Zhu, G. DeRose, and A. Yariv, “Transmission and group delay of microring coupled-resonator optical waveguides,” Opt. Lett. 31, 456 (2006) [CrossRef] [PubMed]

2.

A. Melloni and M. Martinelli, “Synthesis of Direct-Coupled-Resonators Bandpass Filters for WDM Systems,” J. Lightwave Technol. 20, 296 (2002) [CrossRef]

3.

Bradley Schmidt, Qianfan Xu, Jagat Shakya, Sasikanth Manipatruni, and Michal Lipson, “Compact electro-optic modulator on silicon-on-insulator substrates using cavities with ultra-small modal volumes,” Opt. Express15, 3140 (2007), http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-6-3140 [CrossRef] [PubMed]

4.

K. Sakoda, “Optical Properties of Photonic Crystals,” Springer (2004)

5.

O. Painter, R. K. Lee, A. Scherer, A. Yariv, J. D. O. Brien, P. D. Dapkus, and I. Kim, “Two-Dimensional Photonic Band-Gap Defect Mode Laser,” Science 84, 1819 (1999) [CrossRef]

6.

D. K. Armani, T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Ultralow-threshold microcavity Raman laser on a microelectronic chip,” Nature 421, 925 (2003) [CrossRef] [PubMed]

7.

T. Tanabe, M. Notomi, E. Kuramochi, A. Shinya, and H. Taniyama, “Trapping and delaying photons for one nanosecond in an ultrasmall high-Q photonic-crystal nanocavity,” Nature Photonics 1, 49 (2007) [CrossRef]

8.

S. Noda, M. Fujita, and T. Asano, “Spontaneous-emission control by photonic crystals and nanocavities” Nature Photonics 1, 449 (2007) [CrossRef]

9.

M. W. Pruessner, T. H. Stievater, and W. S. Rabinovich,” Integrated waveguide Fabry-Perot microcavities with silicon/air Bragg mirrors,” Opt. Lett. 32, 533 (2007) [CrossRef] [PubMed]

10.

Y. Tanaka, T. Asano, R. Hatsuta, and S. Noda, “Investigation of point-defect cavity formed in twodimensional photonic crystal slab with one-sided dielectric cladding,” Appl. Phys. Lett. 88, 011112 (2006) [CrossRef]

11.

J. S. Foresi, P. R. Villeneuve, J. Ferrera, E. R. Thoen, G. Steinmeyer, S. Fan, J. D. Joannopoulos, L. C. Kimerling, Henry I. Smith, and E. P. Ippen, “Photonic-bandgap microcavities in optical waveguides,” Nature 390, 143 (1997) [CrossRef]

12.

D. Peyrade, E. Silberstein, Ph. Lalanne, A. Talneau, and Y. Chen, “Short Bragg mirrors with adiabatic modal conversion,” Appl. Phys. Lett. 81, 829 (2002) [CrossRef]

13.

P. Velha, J. C. Rodier, P. Lalanne, J. P. Hugonin, D. Peyrade, E. Picard, T. Charvolin, and E. Hadji, “Ultracompact silicon-on-insulator ridge-waveguide mirrors with high reflectance,” Appl. Phys. Lett. 84, 171121 (2006) [CrossRef]

14.

C. Sauvan, G. Lecamp, P. Lalanne, and J.P. Hugonin, “Modal-reflectivity enhancement by geometry tuning in Photonic Crystal microcavities,” Opt. Express.13, 245 (2005), http://www.opticsinfobase.org/abstract.cfm?URI=oe-13-1-245 [CrossRef] [PubMed]

15.

P Velha, J C Rodier, P Lalanne, J P Hugonin, D Peyrade, E Picard, T Charvolin, and E Hadji, “Ultra-high-reflectivity photonic-bandgap mirrors in a ridge SOI waveguide,” New Journal of Physics 8, 204 (2006) [CrossRef]

16.

P. Yeh, “Optical Waves in Layered Media,” Wiley-Interscience (1988)

17.

E. Silberstein, P. Lalanne, J. P. Hugonin, and Q. Cao, “Use of grating theories in integrated optics,” J. Opt. Soc. Am. A 18, 2865–2875 (2001) [CrossRef]

18.

G. Lecamp, J.P. Hugonin, and P. Lalanne, “Theoretical and computational concepts for periodic optical waveguides,” Opt. Express 15, 11042–60 (2007). [CrossRef] [PubMed]

19.

H. Kim, I.M. Lee, and B. Lee, “Extended scattering-matrix method for efficient full parallel implementation of rigorous coupled-wave analysis,” J. Opt. Soc. Am. A 24, 2313–2327 (2007). [CrossRef]

OCIS Codes
(130.0130) Integrated optics : Integrated optics
(250.5300) Optoelectronics : Photonic integrated circuits
(230.5298) Optical devices : Photonic crystals
(130.7408) Integrated optics : Wavelength filtering devices

ToC Category:
Integrated Optics

History
Original Manuscript: August 8, 2007
Revised Manuscript: October 11, 2007
Manuscript Accepted: October 14, 2007
Published: November 20, 2007

Citation
P. Velha, E. Picard, T. Charvolin, E. Hadji, J.C. Rodier, P. Lalanne, and D. Peyrade, "Ultra-High Q/V Fabry-Perot microcavity on SOI substrate," Opt. Express 15, 16090-16096 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-24-16090


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References

  1. J. Poon, L. Zhu, G. DeRose, and A. Yariv, "Transmission and group delay of microring coupled-resonator optical waveguides," Opt. Lett. 31, 456 (2006) [CrossRef] [PubMed]
  2. A. Melloni and M. Martinelli, "Synthesis of Direct-Coupled-Resonators Bandpass Filters for WDM Systems," J. Lightwave Technol. 20, 296 (2002) [CrossRef]
  3. B. Schmidt, Q. Xu, J. Shakya, S. Manipatruni, and M. Lipson, "Compact electro-optic modulator on silicon-on-insulator substrates using cavities with ultra-small modal volumes," Opt. Express 15, 3140 (2007). [CrossRef] [PubMed]
  4. K. Sakoda, Optical Properties of Photonic Crystals (Springer 2004).
  5. O. Painter, R. K. Lee, A. Scherer, A. Yariv, J. D. O. Brien, P. D. Dapkus, I. Kim, "Two-Dimensional Photonic Band-Gap Defect Mode Laser," Science 84, 1819 (1999). [CrossRef]
  6. D. K. Armani, T. J. Kippenberg, S. M. Spillane and K. J. Vahala, "Ultralow-threshold microcavity Raman laser on a microelectronic chip," Nature 421, 925 (2003). [CrossRef] [PubMed]
  7. T. Tanabe, M. Notomi, E. Kuramochi, A. Shinya and H. Taniyama, "Trapping and delaying photons for one nanosecond in an ultrasmall high-Q photonic-crystal nanocavity," Nat. Photonics 1, 49 (2007). [CrossRef]
  8. S. Noda, M. Fujita and T. Asano, "Spontaneous-emission control by photonic crystals and nanocavities" Nat. Photonics 1, 449 (2007). [CrossRef]
  9. M. W. Pruessner, T. H. Stievater, and W. S. Rabinovich," Integrated waveguide Fabry-Perot microcavities with silicon/air Bragg mirrors," Opt. Lett. 32, 533 (2007). [CrossRef] [PubMed]
  10. Y. Tanaka, T. Asano, R. Hatsuta, and S. Noda, "Investigation of point-defect cavity formed in two-dimensional photonic crystal slab with one-sided dielectric cladding," Appl. Phys. Lett. 88, 011112 (2006). [CrossRef]
  11. J. S. Foresi, P. R. Villeneuve, J. Ferrera, E. R. Thoen, G. Steinmeyer, S. Fan, J. D. Joannopoulos, L. C. Kimerling, HenryI. Smith and E. P. Ippen, "Photonic-bandgap microcavities in optical waveguides," Nature 390, 143 (1997). [CrossRef]
  12. D. Peyrade, E. Silberstein, Ph. Lalanne, A. Talneau, and Y. Chen, "Short Bragg mirrors with adiabatic modal conversion," Appl. Phys. Lett. 81, 829 (2002). [CrossRef]
  13. P. Velha, J. C. Rodier, P. Lalanne, J. P. Hugonin, D. Peyrade, E. Picard, T. Charvolin, E. Hadji, "Ultracompact silicon-on-insulator ridge-waveguide mirrors with high reflectance," Appl. Phys. Lett. 84, 171121 (2006). [CrossRef]
  14. C. Sauvan, G. Lecamp, P. Lalanne and J. P. Hugonin, "Modal-reflectivity enhancement by geometry tuning in Photonic Crystal microcavities," Opt. Express. 13, 245 (2005). [CrossRef] [PubMed]
  15. P. Velha, J. C. Rodier, P. Lalanne, J. P. Hugonin, D. Peyrade, E. Picard, T. Charvolin and E. Hadji, "Ultra-high-reflectivity photonic-bandgap mirrors in a ridge SOI waveguide," New J. Phys. 8, 204 (2006). [CrossRef]
  16. P. Yeh, Optical Waves in Layered Media (Wiley-Interscience 1988).
  17. E. Silberstein, P. Lalanne, J. P. Hugonin, and Q. Cao, "Use of grating theories in integrated optics," J. Opt. Soc. Am. A 18, 2865-2875 (2001). [CrossRef]
  18. G. Lecamp, J. P. Hugonin and P. Lalanne, "Theoretical and computational concepts for periodic optical waveguides," Opt. Express 15, 11042-60 (2007). [CrossRef] [PubMed]
  19. H. Kim, I. M. Lee and B. Lee, "Extended scattering-matrix method for efficient full parallel implementation of rigorous coupled-wave analysis," J. Opt. Soc. Am. A 24, 2313-2327 (2007). [CrossRef]

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