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Enhanced resonance tuning of photonic crystal nanocavities by integration of optimized near-field multitip nanoprobes

[+] Author Affiliations
Xiongyeu Chew, Guangya Zhou, Fook Siong Chau

National University of Singapore, Department of Mechanical Engineering, 9 Engineering Drive 1, Singapore 117576

Jie Deng

Institute of Materials Research and Engineering (IMRE), 3 Research Link, Singapore 117602

J. Nanophoton. 5(1), 059503 (April 21, 2011). doi:10.1117/1.3582145
History: Received February 10, 2011; Revised March 27, 2011; Accepted March 28, 2011; Published April 21, 2011; Online April 21, 2011
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A compact and low power control of photonic crystal nanocavity resonance was devised, simulated, and experimentally validated utilizing a hybrid integration of a microelectromechanical systems driven nanoprobe. The experimental results demonstrated a reversible resonance tuning up to 5.4 nm with minimal Q-factor degradation.

Figures in this Article

One of the most promising applications of the photonic crystals (PhCs) is micro/nanocavities,1 which demonstrate good light confinement capabilities and can be used for many applications including add/drop multiplexers2 and sensing.3 Integration of mechanically-driven dielectric structures with photonic micro/nanocavities offers a simple and flexible approach to achieve low-loss and precise control of photonic resonances.46 Recently, we have proposed a tunable approach utilizing submicron microelectromechanical systems (MEMS)7 technology to achieve a mechanically-controlled optical resonance based on heuristically optimized lunate nanotips8 tuning in the near-field regimes of a one-dimensional (1D) PhC nanocavity. We have also demonstrated wavelength tunability utilizing coupled nanocavities9 to achieve large resonance splitting. Although the coupled-nanocavity approach offers larger spectral shifting than perturbative nanotip tuning mechanisms, resonance splitting will generate two resonances (even and odd modes) which may be unfavorable in certain applications, for example in wavelength division multiplexing communication systems that normally require adding or dropping of a single channel instead of two. A perturbative nanotip tuning mechanism offers an alternative approach by shifting a single resonance in a broad photonic bandgap and is worthy of further investigation. In this work, we extend the part of work in near-field nanoprobe tuning by systematically investigating various multitip nanoprobe configurations for even larger spectral tuning capability.

1D PhC Nanocavity Design

As the design of a PhC nanocavity utilizes the forbidden photonic bandgap to achieve light confinement, intentionally structuring a defect cavity within the PhC will enable a resonance mode to be formed within the defect region. We use a 3D finite-different time-domain (3D-FDTD) numerical tool to design and investigate the properties of the 1D PhC nanocavity. The PhC structure is designed based on a periodicity of a = 370 nm with periodic holes of radius r = 84 nm for a sufficiently wide bandgap centered at communication wavelength region. The width and thickness of the air-suspended waveguide is 500 and 340 nm, respectively. A launch impulse Gaussian source is placed in an arbitrary position near the defect. The electric field amplitude is then measured by an arbitrarily placed time monitor. The time-dependent field amplitude is then post-processed with (fast Fourier transformation) to obtain the spectrum, where we find that the defect length ad = 440 nm is corresponding to a central resonance at 1596 nm. A five-hole linear aperiodic taper design is used to achieve gradual confinement of the resonance mode. In combination with a three periodic Bragg hole, we were able to obtain a reasonably good waveguide loaded Q of 6.6 × 103.

Multitip Nanoprobe Design

As previously reported, a single rectangular nanotip causes strong scattering losses and will drastically reduce the transmission power and Q of the 1D PhC nanocavity.8 Reducing the width of the tip effectively reduces the scattering losses, however this is a trade-off which will in return achieve less resonance perturbation. We first introduce the approach of a periodic multitip nanoprobe that intuitively will enhance larger resonance tuning with multiple smaller tip width.10 The proposed design of the periodic multitip consists of a total of eight nanotips which are designed to perturb into the 1D PhC resonance mode. The layout of the periodic nanotips, as annotated in Figs. 1, possesses a tip-pitch ag varying from 350 to 600 nm in this investigation. For simplicity, the width wg and depth d of the nanoprobe are fixed at 250 nm. The thickness of the perturbation structure is equivalent to the thickness of the photonic waveguide structure. The center of the multitip nanoprobe is aligned directly to the center of the 1D PhC nanocavity. For comparison, we positioned the multitip nanoprobe to an offset gap of 70 nm from the nanocavity. Such an offset gap can be easily and stably controlled through a MEMS actuator. As shown in Fig. 2, the resonance shift and Q of the nanocavity are shown as functions of the pitch of the multitips. At a pitch of 350 nm, we notice that a large resonance shift of 5.8 nm is achieved, while Q deteriorates significantly to 290. The deterioration of Q can be quantitatively observed in the mode field distribution as shown in Fig. 1 where significant energy leakage into the multitip nanoprobe is obvious. As the pitch is increased, more tips displace away from the center of the 1D PhC nanocavity resonance evanescent field, effectively reducing the perturbation strength. When the pitch is increased to 500 nm, the resonance is tuned 3.78 nm while maintaining a reasonably high Q. As illustrated in Fig. 1, the perturbed electric field distribution of the nanocavity demonstrates a lower loss as the nanotips are located in the edge of the evanescent field. As both mode field distributions are normalized equally, we qualitatively observe a lower leaky energy coupling into the tip with a pitch of 500 nm, thus explaining for the higher Q at 1.9 × 103 for that probe. Intuitively, the main energy loss mechanism can be understood simply by energy coupling from evanescent fields of the resonance modes into the dielectric nanotips.

Graphic Jump LocationF1 :

Schematic of the 1D PhC nanocavity with a periodic multitip nanoprobe having a pitch of (a) 350 nm (λ = 1602 nm), (b) 500 nm (λ = 1600 nm), and (c) aperiodic multitip nanoprobe (λ = 1601 nm) with a tip width of 250 nm. All probes are at a near-field gap distance of 70 nm. E-field distributions are normalized equally.

Next, we attempt to minimize the energy leakage into the dielectric multitips by reducing the overlap integral of the evanescent field and the waveguide mode of the dielectric tip. We introduce aperiodic multitip probe and fix the nanotip width wg and depth d both at 250 nm for good comparison with the periodic case. It is important to note that at such dimensions, the nanotip may not support a propagating quasi-TE and quasi-TM fundamental order mode. However, we think that the energy loss mechanism here may not strictly require a propagating mode within the nanotip as the length of the nanotip is small. We first study the resonant mode field distributions of the 1D PhC nanocavity. Due to the aperiodic linear tapering of the Bragg-holes of the 1D PhC nanocavity, the fields distributions are nonperiodic. A total of nine positions of minima field energies are identified for perturbation. The nanotips are then positioned at the vicinity of the identified points of minimal mode field energies, which ensures minimum overlap integral of the cavity evanescent field and fundamental mode field (may be leaky in this case) in each nanotip, thus effectively minimizing the energy leak through the nanotips. The distance between each tip is defined to be a0 = 300 nm, a1 = 332 nm, a2 = 333 nm, a3 = 335 nm, from the center to the edge as illustrated in Fig. 1. In this investigation, we also varied the tip width from 100 to 250 nm. The results of resonance wavelength shift and Q of the perturbed cavity as functions of the tip width are shown in Fig. 2. At a tip-cavity offset gap of 70 nm and a tip width of 100 nm, a resonance shift of only 1.31 nm is observed due to the smallness of the perturbing dielectric volume. The perturbed Q factor is, however, as high as 5.5 × 103 which means that very minimal energy is lost into the nanotip. Further increasing the tip width, we observe a monotonic increase in the resonance shifting. At a tip width of 250 nm, we observe a resonance shift of 5 nm and yet maintain Q reasonably high at 2.3 × 103. The calculated field distributions in Fig. 1 show minimal energy leakage into the nanoprobe. Through the 3D-FDTD simulation, we concluded that such aperiodic multitip design is capable of achieving large resonance tuning (up to 8 nm at an offset gap of 50 nm) with minimal Q degradation.

Graphic Jump LocationF2 :

Simulated effects on nanocavity resonance tuning of (a) periodic multitip and (b) aperiodic multitip nanoprobes. The offset distance of the probe from the nanocavity is fixed at 70 nm.

We fabricated our device on an SOI wafer with a device layer thickness of 340 nm and a buried oxide of 1 μm thick. First, an electron beam (E-beam) resist is spin-coated and patterned using a 100 keV E-beam lithography system. To transfer the patterns to the device layer, we utilize an inductively coupled plasma reactive ion etching system. Finally, the photonic and nanomechanical structures are released using a HF vapor dry etching system. Figure 3 illustrates the fabricated 1D PhC on a suspended waveguide. The multitip nanoprobe is driven by a submicron MEMS actuator. We first calibrate the MEMS actuator (voltage to offset gap relationship) by electrically actuating it under a scanning electron microscope (SEM). Figure 3 shows a close-up view of the probe at an offset gap of 100 nm. We then proceed to characterize our device with a tunable laser (ANDO AQ4321D) synchronized with an optical spectrum analyzer (ANDO AQ6317C). The fiber polarization controller and a polarizing-maintaining single-mode fiber are used to control the launch polarization state into the silicon waveguides through conventional end-firing. The input waveguides are then adiabatically tapered down to a waveguide width of 500 nm.

Graphic Jump LocationF3 :

(a) SEM diagram of the fabricated MEMS-driven multitip nanoprobe in the vicinity of an air-suspended 1D PhC nanocavity. (b) SEM of the nanoprobe displaced to 100 nm gap.

The experimental results for the periodic multitip device having a periodicity of 470 nm are shown in Fig. 4. Various transmission peaks clearly demonstrate the resonant wavelength shift at different probe perturbing distances. The fabricated 1D PhC nanocavity possesses a resonance at 1608 nm with an experimental loaded Q of 1.2 × 103 before perturbation. We observe that as the perturbing multitip is actuated at 14.2 V, which corresponds to a 160 nm offset gap, the resonance is tuned about 0.92 nm. However, Q is simply degraded to 5.8 × 102 due to energy leakage as discussed in Sec. 2. We also noticed that the transmission power of the resonance dropped by more than half. Further actuating the structure at 15 V, the tuning is observed to shift 2.38 nm. However, we observed that Q has degraded significantly to 2.8 × 102 together with a drastic drop in transmission. Further perturbing the cavity would cause the resonance peak to be indistinguishable from the background noise. Figure 4 summarizes the experimental results for the periodic multitip probe tuning, which are in good agreement with the numerical results previously obtained. The fabricated 1D PhC nanocavity for the aperiodic multitip tuning possesses a resonance at 1610 nm with an experimental loaded Q of 1.7 × 103 before perturbation. The multitip nanoprobe possesses a tip width of 255 nm. The layout of the aperiodic design is indicated in Sec. 2. As shown in Fig. 4, as the perturbing tip is actuated at 14.5 V which corresponds to a 135 nm offset gap, the resonance is tuned by 1.8 nm. The Q of the corresponding resonance degraded slightly to 8 × 102. Further actuation at 15 V which corresponds to an 88 nm offset gap, found that the resonance further tuned to 5.4 nm. The Q, however, did not degrade much but was maintained at a value of 1.5 × 103. The slight degradation in Q is expected due to some incurred scattering losses from perturbing multitips. Figure 4 further summarizes the results of the aperiodic multitip. These experimental results again are in good agreement with the theoretical predictions of the aperiodic multitip design.

Graphic Jump LocationF4 :

Experimental results and Lorentz-fitted peaks of resonant spectra for (a) periodic multitip nanoprobe (ag = 470 nm) and (c) aperiodic multitip nanoprobe (measured wg ≈ 255 nm). Resonance tuning results are summarized in (b) and (d) for periodic and aperiodic nanoprobes, respectively.

We have numerically investigated and experimentally validated a new design for PhC nanocavity resonance tuning through periodic and aperiodic multitip probe perturbative approaches. By integrating with submicron MEMS, we demonstrated a resonance tuning up to 5.4 nm with minimal Q and transmission degradation. The response time of such device is dependent on the natural frequency of the nanomechanical structure. Since the mechanical design here is similar to our previous work,8 we estimate the rise time of the system to be close to approximately 2 μs. Such precise, low loss, and large resonance control may be utilized to compensate present fabrication process imperfections or even utilized as sensitive displacement sensors or tunable filters.

This work was supported by Ministry of Education (MOE) of Singapore (Research Grant No. UNSPECIFIED R-265-000-306-112 ).

Akahane  Y., , Asano  T., , Bong-Shik  S., , and Noda  S., “ High-Q photonic nanocavity in a two-dimensional photonic crystal. ,” Nature (London). 425, , 944–947  ((2003)).
Takahashi  K., , Kanamori  Y., , Kokubun  Y., , and Hane  K., “ A wavelength-selective add-drop switch using silicon microring resonator with a submicron-comb electrostatic actuator. ,” Opt. Express. 16, , 14421–14428  ((2008)).
Lee  M. R., and Fauchet  P. M., “ Nanoscale microcavity sensor for single particle detection. ,” Opt. Lett.. 32, (22 ), 3284–3286  ((2007)).
Wong  C. W., , Rakich  P. T., , Johnson  S. G., , Qi  M., , Smith  H. I., , Ippen  E. P., , Kimerling  L. C., , Jeon  Y., , Barbastathis  G., , and Kim  S.-G., “ Strain-tunable silicon photonic band gap microcavities in optical waveguides. ,” Appl. Phys. Lett.. 84, (8 ), 1242–1244  ((2004)).
Koenderink  A. F., , Kafesaki  M., , Buchler  B. C., , and Sandoghdar  V., “ Controlling the resonance of a photonic crystal microcavity by a near-field probe. ,” Phys. Rev. Lett.. 95, (15 ), 153904  ((2005)).
Frank  I. W., , Deotare  P. B., , McCutcheon  M. W., , and Loncar  M., “ Programmable photonic crystal nanobeam cavities. ,” Opt. Express. 18, , 8705–8712  ((2010)).
Zhou  G., and Dowd  P., “ Tilted folded-beam suspension for extending the stable travel range of comb-drive actuators. ,” J. Micromech. Microeng.. 13, , 178–183  ((2003)).
Chew  X., , Zhou  G., , Yu  H., , Chau  F. S., , Deng  J., , Loke  Y. C., , and Tang  X., “ An in-plane nano-mechanics approach to achieve reversible resonance control of photonic crystal nanocavities. ,” Opt. Express. 18, (21 ), 22232–22244  ((2010)).
Chew  X., , Zhou  G., , Chau  F. S., , Deng  J., , Tang  X. S., , and Loke  Y. C., “ Dynamic tuning of an optical resonator through MEMS-driven coupled photonic crystal nanocavities. ,” Opt. Lett.. 35, (15 ), 2517–2519  ((2010)).
Chew  X., , Zhou  G., , and Chau  F. S., “ Integration of near-field probes and photonic crystal nanocavities for precise and low-loss resonance control. ,” Proc. SPIE. 7930, , 79300T  ((2011)).
© 2011 Society of Photo-Optical Instrumentation Engineers (SPIE)

Citation

Xiongyeu Chew ; Guangya Zhou ; Fook Siong Chau and Jie Deng
"Enhanced resonance tuning of photonic crystal nanocavities by integration of optimized near-field multitip nanoprobes", J. Nanophoton. 5(1), 059503 (April 21, 2011). ; http://dx.doi.org/10.1117/1.3582145


Figures

Graphic Jump LocationF1 :

Schematic of the 1D PhC nanocavity with a periodic multitip nanoprobe having a pitch of (a) 350 nm (λ = 1602 nm), (b) 500 nm (λ = 1600 nm), and (c) aperiodic multitip nanoprobe (λ = 1601 nm) with a tip width of 250 nm. All probes are at a near-field gap distance of 70 nm. E-field distributions are normalized equally.

Graphic Jump LocationF2 :

Simulated effects on nanocavity resonance tuning of (a) periodic multitip and (b) aperiodic multitip nanoprobes. The offset distance of the probe from the nanocavity is fixed at 70 nm.

Graphic Jump LocationF3 :

(a) SEM diagram of the fabricated MEMS-driven multitip nanoprobe in the vicinity of an air-suspended 1D PhC nanocavity. (b) SEM of the nanoprobe displaced to 100 nm gap.

Graphic Jump LocationF4 :

Experimental results and Lorentz-fitted peaks of resonant spectra for (a) periodic multitip nanoprobe (ag = 470 nm) and (c) aperiodic multitip nanoprobe (measured wg ≈ 255 nm). Resonance tuning results are summarized in (b) and (d) for periodic and aperiodic nanoprobes, respectively.

Tables

References

Akahane  Y., , Asano  T., , Bong-Shik  S., , and Noda  S., “ High-Q photonic nanocavity in a two-dimensional photonic crystal. ,” Nature (London). 425, , 944–947  ((2003)).
Takahashi  K., , Kanamori  Y., , Kokubun  Y., , and Hane  K., “ A wavelength-selective add-drop switch using silicon microring resonator with a submicron-comb electrostatic actuator. ,” Opt. Express. 16, , 14421–14428  ((2008)).
Lee  M. R., and Fauchet  P. M., “ Nanoscale microcavity sensor for single particle detection. ,” Opt. Lett.. 32, (22 ), 3284–3286  ((2007)).
Wong  C. W., , Rakich  P. T., , Johnson  S. G., , Qi  M., , Smith  H. I., , Ippen  E. P., , Kimerling  L. C., , Jeon  Y., , Barbastathis  G., , and Kim  S.-G., “ Strain-tunable silicon photonic band gap microcavities in optical waveguides. ,” Appl. Phys. Lett.. 84, (8 ), 1242–1244  ((2004)).
Koenderink  A. F., , Kafesaki  M., , Buchler  B. C., , and Sandoghdar  V., “ Controlling the resonance of a photonic crystal microcavity by a near-field probe. ,” Phys. Rev. Lett.. 95, (15 ), 153904  ((2005)).
Frank  I. W., , Deotare  P. B., , McCutcheon  M. W., , and Loncar  M., “ Programmable photonic crystal nanobeam cavities. ,” Opt. Express. 18, , 8705–8712  ((2010)).
Zhou  G., and Dowd  P., “ Tilted folded-beam suspension for extending the stable travel range of comb-drive actuators. ,” J. Micromech. Microeng.. 13, , 178–183  ((2003)).
Chew  X., , Zhou  G., , Yu  H., , Chau  F. S., , Deng  J., , Loke  Y. C., , and Tang  X., “ An in-plane nano-mechanics approach to achieve reversible resonance control of photonic crystal nanocavities. ,” Opt. Express. 18, (21 ), 22232–22244  ((2010)).
Chew  X., , Zhou  G., , Chau  F. S., , Deng  J., , Tang  X. S., , and Loke  Y. C., “ Dynamic tuning of an optical resonator through MEMS-driven coupled photonic crystal nanocavities. ,” Opt. Lett.. 35, (15 ), 2517–2519  ((2010)).
Chew  X., , Zhou  G., , and Chau  F. S., “ Integration of near-field probes and photonic crystal nanocavities for precise and low-loss resonance control. ,” Proc. SPIE. 7930, , 79300T  ((2011)).

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