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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 18 — Aug. 29, 2011
  • pp: 17813–17819
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Experimental demonstration of coupled-resonator-induced-transparency in silicon-on-insulator based ring-bus-ring geometry

S. Darmawan, L. Y. M. Tobing, and D. H. Zhang  »View Author Affiliations


Optics Express, Vol. 19, Issue 18, pp. 17813-17819 (2011)
http://dx.doi.org/10.1364/OE.19.017813


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Abstract

We experimentally demonstrate coupled-resonator-induced-transparency (CRIT) phenomenon in ring-bus-ring (RBR) geometry synergistically integrated with Mach-Zehnder interferometer (MZI). The RBR consists of two detuned resonators indirectly coupled through a center bus waveguide. The transparency is obtained by increasing the light intercavity interaction through tailoring the RBR phase response while ensuring balanced MZI operation. In this work, a CRIT resonance with a quality factor of ~18,000 is demonstrated with cavity size detuning of ~0.035% and power coupling of ~60%, which are in good agreement with the theory.

© 2011 OSA

1. Introduction

The outline of this paper is given as follows: A brief theory is presented in Section 2, the experimental results and some discussions are presented in Section 3. Lastly, the conclusion is given in Section 4.

2. Theory

3. Experimental results and discussions

Figure 1
Fig. 1 (a) The proposal of using RBRMZI to excite CRIT spectrum. (b) The fabricated RBRMZI devices on silicon-on-insulator platform: DUT 01 to 06. The RBR racetrack coupler length is 6μm with the first ring radius of 5μm. The second ring radius is varied from 5.35, 5 and 4.65μm to excite different cavity detuning of γ~1.05, 1, and 0.95, respectively. The DUT 01 to 03 have no passivation layer (air cladding) whereas the DUT 04 to 06 are coated with i-line resist.
shows the fabricated RBRMZI devices where the dimensions of each components is given as follows. The 3dB couplers in the MZI section are based on multi-mode interferometers (MMI) with a nominal width of 3.5μm and a length of 11.5μm with I/O tapers to reduce insertion loss. The first ring has a radius of 5µm with a racetrack coupler length of LC~6µm, giving a total circumference of ~43.42μm. It should be noted that the coupling strength is changed by adjusting the gap separation between the rings and the center waveguides. This is done to prevent the change of resonance wavelength as a result of changing the coupler length. The measurement methodologies are briefly outlined as follows. To facilitate fiber input/output (I/O) coupling, vertical grating couplers are fabricated at both ends of each device. The fibers are butt-coupled to the grating couplers at 10° off vertical. The device transmission is then measured with an amplified spontaneous emission (ASE) broadband light source (1.41 to 1.62 μm) and an optical spectrum analyzer (OSA).In this experiment, we explored the change of gap separation (g) and cavity size detuning (γ). Here three values of γ is chosen, namely 0.95, 1.0, and 1.05, while the gap separation is varied to 150 nm and 200 nm, totaling to 6 devices. Note that the above values are nominal and experimentally the values may be slightly shifted due to fabrication imperfection. The devices are grouped in two groups: the first group (DUT01-03) is when the devices are cladded with air, while the second group (DUT04-06) is when the devices are cladded with i-Line resist. The curve-fitting strategy is described as follows. First, the initial guess of the group index is deduced from the free-spectral range (FSR), resonant wavelength, and the cavity length, i.e. λ2/(FSR × L CAV), which then is readjusted to match the closest resonance order. It is found that the resonance order is 129 and 119, for the first and second group, respectively. Second, the coupling strength (r 1 = r 2) is obtained from independent fitting of the drop transmission of one-ring-two-bus (1R2B) devices on the same chip (using the same gap separation and coupler length). Third, the initial guess of the MZI phase imbalance is deduced from fitting the transmission of RBRMZI with symmetric rings (γ~1), which makes it easier since the resonance is characterized only by a single-ring. Fourth, by matching the relative separation between the two resonances over three free spectral ranges (FSR), it is possible to find a combination of group index and cavity size detuning that will fit the experimental results. The third and fourth step can be repeated for fine-adjustments until the fitting converges. The measurements of gap separations and obtained fittings are summarized in Table 1

Table 1. The Device under Test (DUT) and the Fitting Parameters*

table-icon
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. The cavity power cavity loss is ~1% which is responsible to the slight reduced transmission in the transparency peak. The reduction in transmission peak becomes more prominent in sharper and narrower transparency resonances, due to the dispersive nature at the transparency condition.

Lastly, the advantage of the synergistic integration of RBR with MZI can also be seen from DUT 02 where a CRIT resonance with finesse value of ~70 (2.8 × higher) is achieved using relatively lower finesse rings ~25. For other cases like DUT 01, 03, 04, 06, the ring resonances are detuned further away form each other, resulting in weaker cavity interaction between the two resonators. Based on the fitting of the asymmetric Fano resonance, we estimate a lumped MZI arm phase imbalance of +0.35π for DUT 02 and −0.8π for DUT 05. The MZI phase imbalance of the asymmetric RBR counterparts is not far off from the symmetric ones. The general spectral responses in Fig. 2(b)
Fig. 2 The measured (bold black) and the fitted (dashed red) RBRMZI transmission: (a) DUT 01 to 03 (bare silicon) and (b) DUT 04 to 06 (i-line resist coating). The measurement is based on transverse electric (TE) polarization.
(DUT 01 to 03) are characteristically an inverted version from those in Fig. 2(a) (DUT 04 to 06), due to ~π phase difference in the MZI phase imbalance between them (see Table 1). Apart from the measurement results shown in Fig. 2, we have done more device measurements and collected the Q-factors of the CRIT resonance in Fig. 3
Fig. 3 The demonstrated Q-factors of RBRMZI devices as a function of the operating wavelength, which also indicates the location of the CRIT resonances.
. Based on the results, it is clear that the highest Q-factor is achieved when the designated γ is almost unity. This is consistent with our earlier point which states that the coupling induces phase shift is almost canceled in the symmetric RBR and a very slight deviation of cavity size detuning can give rise to very sharp CRIT resonance. Based on the gathered results, we estimate that ~0.035% cavity size deviation (with ~60% power coupling and ~1% cavity power loss) is sufficient for generating CRIT resonance with Q-factors more than 18,000.

To obtain better understanding of the design parameter variations, Fig. 4(a)
Fig. 4 (a) The contour plot showing the finesse [log10(F)] (solid) and the background envelope linewidth Δδ1/2/(2π) outside transparency band (dashed) as a function of r 1,2 and cavity size detuning ΔγDEV=(γ1.051)×100%. (b) The RBRMZI transmission (upper) and RBR phase (lower) for r 0 = ± 0.75 (A,C) and 0 (B), assuming lossless case and balanced MZI.
shows the theoretical contour plot of the calculated finesse and the calculated background envelope linewidth outside the transparency band, as a function of coupling strength and cavity size detuning, assuming lossless case and balanced MZI. Figure 4(b) shows the corresponding MZI transmission and the RBR phase response for three different cases (A to C). It is evident that by increasing the coupling strength, the CRIT resonance becomes more distinct as shown by the increased transparency finesse as the background envelope is suppressed. Case (A) in Fig. 4 shows the simulated case with r0 = +0.75 where the RBR phase jump of (2 × 2π) is derived mainly from the resonant phase contribution of the two rings which in turn results in stronger background envelope (Δδ1/2/2π~0.1). This is to be contrasted to case (C) where r0 is negative (−0.75) and the finesse is increased by one order of magnitude (39 ×) [two order of magnitude means >100 × ]. Here, the RBR phase jump is dominated by a sharper non-resonant tri-coupler phase jump (4 × π/2) and is more localized towards the resonant order δ1/2π = 121 [6

6. Y. Zhang, S. Darmawan, L. Y. M. Tobing, T. Mei, and D. H. Zhang, “Coupled resonator-induced transparency in ring-bus-ring Mach-Zehnder interferometer,” J. Opt. Soc. Am. B 28(1), 28–36 (2011). [CrossRef]

] whereas the background linewidth outside the transparency is increased by 9 times (Δδ1/2/2π~0.9). For completeness, we show an intermediate case (B) where r 0~0 brings out a moderately high finesse value of ~274. The background envelope (Δδ1/2/2π~0.5) is the most sinusoidal in comparison to other r0 values. Based on the trend, it is more desirable to operate with strong coupling condition (low finesse resonators) for obtaining the most optimized CRIT condition while ensuring balanced MZI.

4. Conclusion

We have demonstrated RBRMZI capable of generating narrow CRIT resonance by means of synergistic integration of the RBR geometry with the MZI device on SOI material platform. In contrast to other existing CRIT schemes, the CRIT in RBRMZI is generated through phase engineering facilitated by inter-pathway interference in RBR and MZI structure. This leads to CRIT or absorption using low-finesse resonators, which is qualitatively different from other existing schemes that require high-finesse resonators. We demonstrated a CRIT resonance with a Q-factor of ~18,000 using cavity size deviation of ~0.035% and power coupling of ~60%. It is interesting to note that similar transparency effect was also found in a plasmonic system with relatively similar geometry [10

10. Z. Han and S. I. Bozhevolnyi, “Plasmon-induced transparency with detuned ultracompact Fabry-Perot resonators in integrated plasmonic devices,” Opt. Express 19(4), 3251–3257 (2011), http://www.opticsinfobase.org/abstract.cfm?URI=oe-19-4-3251. [CrossRef] [PubMed]

].

Acknowledgments

This work is supported by Ministry of Education (ARC 16/07), National Research Foundation (NRF-G-CRP 2007-01), Singapore and Asian Office of Aerospace Research. The authors would like to thank Dr. Pieter Dumon for consolidating the SOI device fabrication and Mr. Zhang Yanbing for his kind assistance in device measurements.

References and links

1.

D. D. Smith, H. Chang, K. A. Fuller, A. T. Rosenberger, and R. W. Boyd, “Coupled-resonator-induced transparency,” Phys. Rev. A 69(6), 063804 (2004). [CrossRef]

2.

Q. Xu, S. Sandhu, M. L. Povinelli, J. Shakya, S. Fan, and M. Lipson, “Experimental realization of an on-chip all-optical analogue to electromagnetically induced transparency,” Phys. Rev. Lett. 96(12), 123901 (2006). [CrossRef] [PubMed]

3.

A. Naweed, G. Farca, S. I. Shopova, and A. T. Rosenberger, “Induced transparency and absorption in coupled whispering gallery microresonators,” Phys. Rev. A 71(4), 043804 (2005). [CrossRef]

4.

K. Totsuka, N. Kobayashi, and M. Tomita, “Slow light in coupled-resonator-induced transparency,” Phys. Rev. Lett. 98(21), 213904 (2007). [CrossRef] [PubMed]

5.

M. D. Lukin and A. Imamoğlu, “Controlling photons using electromagnetically induced transparency,” Nature 413(6853), 273–276 (2001). [CrossRef] [PubMed]

6.

Y. Zhang, S. Darmawan, L. Y. M. Tobing, T. Mei, and D. H. Zhang, “Coupled resonator-induced transparency in ring-bus-ring Mach-Zehnder interferometer,” J. Opt. Soc. Am. B 28(1), 28–36 (2011). [CrossRef]

7.

http://www.epixfab.eu.

8.

S. Darmawan, L. Y. Tobing, and T. Mei, “Coupling-induced phase shift in a microring-coupled Mach-Zehnder interferometer,” Opt. Lett. 35(2), 238–240 (2010). [CrossRef] [PubMed]

9.

S. K. Selvaraja, W. Bogaerts, P. Dumon, D. Van Thourhout, and R. Baets, “Subnanometer linewidth uniformity in silicon-nanophotonic waveguide devices using CMOS fabrication technology,” IEEE J. Sel. Top. Quantum Electron. 16(1), 316–324 (2010). [CrossRef]

10.

Z. Han and S. I. Bozhevolnyi, “Plasmon-induced transparency with detuned ultracompact Fabry-Perot resonators in integrated plasmonic devices,” Opt. Express 19(4), 3251–3257 (2011), http://www.opticsinfobase.org/abstract.cfm?URI=oe-19-4-3251. [CrossRef] [PubMed]

OCIS Codes
(130.0130) Integrated optics : Integrated optics
(130.2790) Integrated optics : Guided waves
(230.0230) Optical devices : Optical devices
(230.3990) Optical devices : Micro-optical devices
(230.5750) Optical devices : Resonators
(130.3990) Integrated optics : Micro-optical devices

ToC Category:
Integrated Optics

History
Original Manuscript: June 18, 2011
Revised Manuscript: August 8, 2011
Manuscript Accepted: August 8, 2011
Published: August 25, 2011

Citation
S. Darmawan, L. Y. M. Tobing, and D. H. Zhang, "Experimental demonstration of coupled-resonator-induced-transparency in silicon-on-insulator based ring-bus-ring geometry," Opt. Express 19, 17813-17819 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-18-17813


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References

  1. D. D. Smith, H. Chang, K. A. Fuller, A. T. Rosenberger, and R. W. Boyd, “Coupled-resonator-induced transparency,” Phys. Rev. A69(6), 063804 (2004). [CrossRef]
  2. Q. Xu, S. Sandhu, M. L. Povinelli, J. Shakya, S. Fan, and M. Lipson, “Experimental realization of an on-chip all-optical analogue to electromagnetically induced transparency,” Phys. Rev. Lett.96(12), 123901 (2006). [CrossRef] [PubMed]
  3. A. Naweed, G. Farca, S. I. Shopova, and A. T. Rosenberger, “Induced transparency and absorption in coupled whispering gallery microresonators,” Phys. Rev. A71(4), 043804 (2005). [CrossRef]
  4. K. Totsuka, N. Kobayashi, and M. Tomita, “Slow light in coupled-resonator-induced transparency,” Phys. Rev. Lett.98(21), 213904 (2007). [CrossRef] [PubMed]
  5. M. D. Lukin and A. Imamoğlu, “Controlling photons using electromagnetically induced transparency,” Nature413(6853), 273–276 (2001). [CrossRef] [PubMed]
  6. Y. Zhang, S. Darmawan, L. Y. M. Tobing, T. Mei, and D. H. Zhang, “Coupled resonator-induced transparency in ring-bus-ring Mach-Zehnder interferometer,” J. Opt. Soc. Am. B28(1), 28–36 (2011). [CrossRef]
  7. http://www.epixfab.eu .
  8. S. Darmawan, L. Y. Tobing, and T. Mei, “Coupling-induced phase shift in a microring-coupled Mach-Zehnder interferometer,” Opt. Lett.35(2), 238–240 (2010). [CrossRef] [PubMed]
  9. S. K. Selvaraja, W. Bogaerts, P. Dumon, D. Van Thourhout, and R. Baets, “Subnanometer linewidth uniformity in silicon-nanophotonic waveguide devices using CMOS fabrication technology,” IEEE J. Sel. Top. Quantum Electron.16(1), 316–324 (2010). [CrossRef]
  10. Z. Han and S. I. Bozhevolnyi, “Plasmon-induced transparency with detuned ultracompact Fabry-Perot resonators in integrated plasmonic devices,” Opt. Express19(4), 3251–3257 (2011), http://www.opticsinfobase.org/abstract.cfm?URI=oe-19-4-3251 . [CrossRef] [PubMed]

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