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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 6 — Mar. 15, 2010
  • pp: 5420–5425
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Pillar-array based optical sensor

Tao Xu, Ning Zhu, Michelle Y.-C. Xu, Lech Wosinski, J. Stewart Aitchison, and H. E. Ruda  »View Author Affiliations


Optics Express, Vol. 18, Issue 6, pp. 5420-5425 (2010)
http://dx.doi.org/10.1364/OE.18.005420


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Abstract

An optical microcavity based on pillar arrays has been fabricated in Si/SiO2 material system. Transmission measurement was taken and a quality factor as high as 27,600 was observed. This cavity was tested for sensing applications by immersing into optical fluids with accurate refractive indices. For refractive index change of 0.01, a resonance peak wavelength shift of 3.5 nm was measured. We also compare cavities consisting of pillars with different aspect ratios.

© 2010 Optical Society of America

1. Introduction

Compared with hole-array based PhCs, pillar arrays have several advantages for sensing applications. Firstly, a pillar array has a much larger percentage of void space. In this paper, a cavity with more than 80% of air space was fabricated and tested. The big open space inside the cavity makes it much easier for the infiltration of the analytes. Secondly, as opposite to the hole arrays, in a pillar array cavity, the void space is connected, giving a flowing channel for the fluid carrying the analytes. Thirdly, due to the large and connect void space, the electric field of the confined mode tends to locates more out of the pillars, which will increase the overlapping with the analytes and then enhance the sensitivity.

In the following, we first describe the fabrication and measurement of a heterogenous pillar array based microcavity in Si/SiO2 material system. Optical fluids with accurate refractive indices were applied to the cavity. The peak wavelength shift was recorded and used to characterize the sensitivity of the cavity to the ambient refractive index change. In the last section, we analyze the dependence of the quality factor and the sensitivity of the cavity on the aspect ratio of the pillars.

2. Fabrication and theoretical calculation

Although seldom used as a microcavity, pillar-array based 2D PhCs have been successfully fabricated with top-down microfabrication process including advanced lithography and dry etching and used as optical interconnection components [8

8. M. Tokushima, H. Yamada, and Y. Arakawa, “1.5-μm-wavelength light guiding in waveguides in square-lattice-of-rod photonic crystal slab,” Appl. Phys. Lett. 84, 4298 (2004). [CrossRef]

,9

9. X. Ao, L. Liu, L. Wosinski, and S. He, “Polarization beam splitter based on a two-dimensional photonic crystal of pillar type,” Appl. Phys. Lett. 89, 171115 (2006). [CrossRef]

,10

10. A. Kok, E. J. Geluk, B. Docter, J. van der Tol, R. Nözel, M. Smit, and R. Baets, “Transmission of pillar-based photonic crystal waveguides in InP technology,” Appl. Phys. Lett. 91, 201109 (2007). [CrossRef]

,11

11. S. H. G. Teo, A. Q. Liu, J. B. Zhang, M. H. Hong, J. Singh, M. B. Yu, N. Singh, and G. Q. Lo, “Photonic bandgap crystal resonator enhanced, laser controlled modulations of optical interconnects for photonic integrated circuits,” Opt. Express 16, 7842 (2008). [CrossRef] [PubMed]

]. It is worth noting that high aspect ratio pillar array can also be grown with bottom-up methods such as chemical vapor deposition method [12

12. U. Philipose, T. Xu, S. Yang, P. Sun, H. E. Ruda, Y. Q. Wang, and K. L. Kavanagh, “Enhancement of band edge luminescence in ZnSe nanowires,” J. Appl. Phys. 100, 084316 (2006). [CrossRef]

] or metal-organic vapor phase epitaxy [13

13. T. Måtensson, M. Borgström, W. Seifert, B. J. Ohlsson, and L. Samuelson, “Fabrication of individually seeded nanowire arrays by vapour liquid solid growth,” Nanotechnology 14, 1255–1258 (2003). [CrossRef]

], catalyzed by metal particles . In this letter, we adopted top-down method with e-beam lithography and reactive ion etching (RIE).

The SEM images of the fabricated structure are shown in the Fig. 1. A buffer SiO2 layer (2 μm thick) and then a hydrogenated amorphous silicon (α-Si:H) guiding layer (730 nm thick) are deposited with plasma enhanced chemical vapor deposition on the crystalline Si substrate. The input/output waveguides and pillar array were defined by e-beam machine in a negative tone e-beam resist (MaN 2403) mask. The patterns were then transferred to the Si:H layer with inductively coupled plasma (ICP) RIE. SF6/C4F8 combined chemistry was used in the etching process and optimized to obtain smooth and vertical side walls for the pillars. After the etching, the e-beam resist mask was removed. The final device includes an array of finite-height (730 nm) pillars with diameter of about 242 nm, and two tapered coupling waveguides, which are tapered up from 2 μm to 4.5 μm to match the mode of the cavity.

Fig. 1. SEM images of a heterogeneous pillar-array microcavity. (a) top view; (b) an image taken at a slanting angle.

Fig. 2. (a) A schematic plot of vertical cross section of the pillar array cavity. The vertical (b) and horizontal (c) cross sections of the fundamental mode’s electric field component Ez. The vertical cross section is taken at the position shown as the dash line in the horizontal cross section image and vice versa.

3. Optical measurement

Optical transmission measurement was taken to characterize the fabricated microcavity. As shown in Fig. 3(a), tunable laser sources (JDS Uniphase: SWS15101 from 1460 to 1600 nm and SWS16101 from 1510 to 1650 nm) were used. The light was coupled into the input waveguide by a lens fiber. A polarization controller was used to maximize the TM component of the incident light. The transmitted light was collected by an 60× objective lens and filtered by a polarization beam splitter before entering a photodetector. The transmission spectrum through the cavity shown in Fig. 1 is displayed in the figure. The measured Q is 27,600, about 2.4 times smaller than the theoretical value. The discrepancy may come from two sources, the fabrication defects and the material loss of the amorphous Si:H [7

7. T. Xu, N. Zhu, M. Y.-C. Xu, L. Wosinski, J. S. Aitchison, and H. E. Ruda, “A pillar-array based two-dimensional photonic crystal microcavity,” Appl. Phys. Lett. 94, 241110 (2009). [CrossRef]

].

Fig. 3. (a) A schematic of the transmission measurement setup and a transmission spectrum measured. (b) The transmission spectra taken as the cavity was immersed into optical fluids of different refractive indices. (c) The peak positions of the transmission spectra in (b) is shown. The experimental data is compared to the 3D FDTD simulation. To ease the comparison, we assume both data have the same starting point at n=1.392.

4. Dependence on aspect ratio of pillars

When other parameters keep fixed and the aspect ratio of pillars increases, the electric field of the mode tends to locate more inside the pillars and the normalized frequency of the mode becomes smaller and further below the light line of the substrate. As a result, the vertical leakage will decrease and the quality factor will increase. However, as the quality factor increases, the cavity will become less sensitive. It is because that the overlap between the electric field of the mode and the dielectric materials introduced close to the pillars becomes smaller.

As an example, we can look at the cavities presented in Ref. [7

7. T. Xu, N. Zhu, M. Y.-C. Xu, L. Wosinski, J. S. Aitchison, and H. E. Ruda, “A pillar-array based two-dimensional photonic crystal microcavity,” Appl. Phys. Lett. 94, 241110 (2009). [CrossRef]

] and above in Section 2. When normalized over the period of the lattice, these two cavities have similar parameters except for the aspect ratio of the pillars. As the aspect ratio increases from 1.7 to 3.0. The percentage of the electric field energy in pillars increases from 64.1% to 79.8%. We used 3D FDTD calculation to obtain the mode field pattern and the following formula to calculate the percentage of the electric field energy in pillars.

Fig. 4. The dependence of quality factor and sensitivity of the cavity on the aspect ratio of the pillars. To characterize the sensitivity, we used the peak wavelength shift when the cavity is immersed into an optical fluid with refractive index of 1.392. In the plot, the filled symbols stand for the simulation data, while the unfilled stand for experimental data.
F=Vpillar1/2×εr(r)E(r)2dVV1/2×εr(r)E(r)2dV,
(1)

where the Vpillar refers to the space inside the pillars and V refers to whole mode volume. εr(r) and E(r) are the real part of the dielectric constant and electric field at spatial position r respectively. The measured quality factor increase from 7,350 to 27,600. When both of the cavities were immersed into an optical fluid with refractive index of 1.392, the shift of the peak wavelength was measured to be 167 nm and 130 nm for the aspect ratio of 1.7 and 3.0. It is clearly seen from this example that the quality factor and the sensitivity goes in opposite directions when aspect ratio changes. This trend can be further illustrated in Fig. 4 with aspect ratio increasing to 10.

5. Conclusion

We have demonstrated an ultra-sensitive optical sensor that is based on silicon pillar arrays. A Q as high as 2.7 × 104 was measured. The cavity has 80% connected void space and thus is suitable for sensing applications. The confined mode has a large percent of electric field energy locating in the void space and is very sensitive to the analyte introduced there. Based on the experiments, the cavity is capable to resolve a refractive index difference of 3 × 10-4 Increasing the height of the pillars can increase Q factor, while reducing the sensitivity of the cavity. For certain application, it is needed to find the balancing point between them.

Acknowledgments

We gratefully acknowledge financial support from NSERC, CIPI, OCE, CSA, AFOSR and ECTI.

References and links

1.

Y. Akahane, T. Asano, B.-S. Song, and S. Noda, “High-Q photonic nanocavity in a two-dimensional photonic crystal,” Nature 425, 944–947 (2003). [CrossRef] [PubMed]

2.

K. Srinivasan, P. E. Barclay, O. Painter, J. Chen, A. Y. Cho, and C. Gmachl, “Experimental demonstration of a high quality factor photonic crystal microcavity,” Appl. Phys. Lett. 83, 1915–1917 (2003). [CrossRef]

3.

T. Tanabe, M. Notomi, E. Kuramochi, A. Shinya, and H. Taniyama, “Trapping and delaying photons for one nanosecond in an untrasmall high-Q photonic-crystal nanocavity,” Nat. Photonics 1, 49–52 (2007). [CrossRef]

4.

A. R. M. Zain, N. P. Johnson, M. Sorel, and R. M. De La Rue, “Ultra high quality factor one dimensional photonic crystal/photonic wire micro-cavities in silicon-on-insulator (SOI),” Opt. Express 16, 12084–12089 (2008). [CrossRef] [PubMed]

5.

E. Chow, A. Grot, L.W. Mirkarimi, M. Sigalas, and G. Girolami, “Ultracompact biochemical sensor built with two-dimensional photonic crystal microcavity,” Opt. Lett. 29, 1093–1095 (2004). [CrossRef] [PubMed]

6.

M. Lee and P. M. Fauchet, “Two-dimensional silicon photonic crystal based biosensing platform for protein detection,” Opt. Express 15, 4530–4535 (2007). [CrossRef] [PubMed]

7.

T. Xu, N. Zhu, M. Y.-C. Xu, L. Wosinski, J. S. Aitchison, and H. E. Ruda, “A pillar-array based two-dimensional photonic crystal microcavity,” Appl. Phys. Lett. 94, 241110 (2009). [CrossRef]

8.

M. Tokushima, H. Yamada, and Y. Arakawa, “1.5-μm-wavelength light guiding in waveguides in square-lattice-of-rod photonic crystal slab,” Appl. Phys. Lett. 84, 4298 (2004). [CrossRef]

9.

X. Ao, L. Liu, L. Wosinski, and S. He, “Polarization beam splitter based on a two-dimensional photonic crystal of pillar type,” Appl. Phys. Lett. 89, 171115 (2006). [CrossRef]

10.

A. Kok, E. J. Geluk, B. Docter, J. van der Tol, R. Nözel, M. Smit, and R. Baets, “Transmission of pillar-based photonic crystal waveguides in InP technology,” Appl. Phys. Lett. 91, 201109 (2007). [CrossRef]

11.

S. H. G. Teo, A. Q. Liu, J. B. Zhang, M. H. Hong, J. Singh, M. B. Yu, N. Singh, and G. Q. Lo, “Photonic bandgap crystal resonator enhanced, laser controlled modulations of optical interconnects for photonic integrated circuits,” Opt. Express 16, 7842 (2008). [CrossRef] [PubMed]

12.

U. Philipose, T. Xu, S. Yang, P. Sun, H. E. Ruda, Y. Q. Wang, and K. L. Kavanagh, “Enhancement of band edge luminescence in ZnSe nanowires,” J. Appl. Phys. 100, 084316 (2006). [CrossRef]

13.

T. Måtensson, M. Borgström, W. Seifert, B. J. Ohlsson, and L. Samuelson, “Fabrication of individually seeded nanowire arrays by vapour liquid solid growth,” Nanotechnology 14, 1255–1258 (2003). [CrossRef]

14.

T. Xu, M. S. Wheeler, S. V. Nair, H. E. Ruda, M. Mojahedi, and J. S. Aitchison, “Highly confined mode above the light line in a two-dimensional photonic crystal slab,” Appl. Phys. Lett. 93, 241105 (2008). [CrossRef]

15.

T. Xu, S. Yang, S. V. Nair, and H. E. Ruda, “Nanowire-array-based photonic crystal cavity by finite-difference time-domain calculations,” Phys. Rev. B 75, 125104 (2007). [CrossRef]

OCIS Codes
(350.4238) Other areas of optics : Nanophotonics and photonic crystals

ToC Category:
Sensors

History
Original Manuscript: September 8, 2009
Manuscript Accepted: November 7, 2009
Published: March 2, 2010

Citation
Tao Xu, Ning Zhu, Michelle Y.-C. Xu, Lech Wosinski, J. Stewart Aitchison, and H. E. Ruda, "Pillar-array based optical sensor," Opt. Express 18, 5420-5425 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-6-5420


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References

  1. Y. Akahane, T. Asano, B.-S. Song, and S. Noda, "High-Q photonic nanocavity in a two-dimensional photonic crystal," Nature 425,944-947 (2003). [CrossRef] [PubMed]
  2. K. Srinivasan, P. E. Barclay, O. Painter, J. Chen, A. Y. Cho, and C. Gmachl, "Experimental demonstration of a high quality factor photonic crystal microcavity," Appl. Phys. Lett. 83,1915-1917 (2003). [CrossRef]
  3. T. Tanabe, M. Notomi, E. Kuramochi, A. Shinya, and H. Taniyama, "Trapping and delaying photons for one nanosecond in an untrasmall high-Q photonic-crystal nanocavity," Nat. Photonics 1,49-52 (2007). [CrossRef]
  4. A. R.M. Zain, N. P. Johnson, M. Sorel, and R. M. De La Rue, "Ultra high quality factor one dimensional photonic crystal/photonic wire micro-cavities in silicon-on-insulator (SOI)," Opt. Express 16, 12084-12089 (2008). [CrossRef] [PubMed]
  5. E. Chow, A. Grot, L. W. Mirkarimi, M. Sigalas, and G. Girolami, "Ultracompact biochemical sensor built with twodimensional photonic crystal microcavity," Opt. Lett. 29, 1093-1095 (2004). [CrossRef] [PubMed]
  6. M. Lee and P. M. Fauchet, "Two-dimensional silicon photonic crystal based biosensing platform for protein detection," Opt. Express 15, 4530-4535 (2007). [CrossRef] [PubMed]
  7. T. Xu, N. Zhu, M. Y.-C. Xu, L. Wosinski, J. S. Aitchison, and H. E. Ruda, "A pillar-array based two-dimensional photonic crystal microcavity," Appl. Phys. Lett. 94, 241110 (2009). [CrossRef]
  8. M. Tokushima, H. Yamada, and Y. Arakawa,"1.5-?m-wavelength light guiding in waveguides in square-latticeof-rod photonic crystal slab," Appl. Phys. Lett. 84, 4298 (2004). [CrossRef]
  9. X. Ao, L. Liu, L. Wosinski, and S. He, "Polarization beam splitter based on a two-dimensional photonic crystal of pillar type," Appl. Phys. Lett. 89, 171115 (2006). [CrossRef]
  10. A. Kok, E. J. Geluk, B. Docter, J. van der Tol, R. Nozel, M. Smit, and R. Baets, "Transmission of pillar-based photonic crystal waveguides in InP technology," Appl. Phys. Lett. 91, 201109 (2007). [CrossRef]
  11. S. H. G. Teo, A. Q. Liu, J. B. Zhang, M. H. Hong, J. Singh, M. B. Yu, N. Singh, and G. Q. Lo, "Photonic bandgap crystal resonator enhanced, laser controlled modulations of optical interconnects for photonic integrated circuits," Opt. Express 16, 7842 (2008). [CrossRef] [PubMed]
  12. U. Philipose, T. Xu, S. Yang, P. Sun, and H. E. Ruda, Y. Q. Wang, and K. L. Kavanagh, "Enhancement of band edge luminescence in ZnSe nanowires," J. Appl. Phys. 100, 084316 (2006). [CrossRef]
  13. T. Matensson, M. Borgstrom, W. Seifert, B. J. Ohlsson and L. Samuelson, "Fabrication of individually seeded nanowire arrays by vapour liquid solid growth," Nanotechnology 14, 1255-1258 (2003). [CrossRef]
  14. T. Xu, M. S. Wheeler, S. V. Nair, H. E. Ruda, M. Mojahedi, and J. S. Aitchison, "Highly confined mode above the light line in a two-dimensional photonic crystal slab," Appl. Phys. Lett. 93, 241105 (2008). [CrossRef]
  15. T. Xu, S. Yang, S. V. Nair, and H. E. Ruda, "Nanowire-array-based photonic crystal cavity by finite-difference time-domain calculations," Phys. Rev. B 75, 125104 (2007). [CrossRef]

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