Photothermally tunable silicon-microring-based optical add ...€¦ · Photothermally tunable...

Photothermally tunablesilicon-microring-based optical add-dropfilter through integrated light absorber

Xi Chen,1,4 Yuechun Shi,2,1,4 Fei Lou,1 Yiting Chen,1 Min Yan,1Lech Wosinski,1 and Min Qiu3,1,∗

1Optics and Photonics, School of Information and Communication Technology, KTH RoyalInstitute of Technology, Electrum 229, 164 40 Kista, Sweden

2National Laboratory of Solid State Microstructures, College of Engineering and AppliedSciences, Nanjing University, 210093 Nanjing, China

3State Key Laboratory of Modern Optical Instrumentation, Department of OpticalEngineering, Zhejiang University, 310027 Hangzhou, China.

4These authors contributed equally to this work.∗[email protected]

Abstract: An optically pumped thermo-optic (TO) silicon ring add-dropfilter with fast thermal response is experimentally demonstrated. We pro-pose that metal-insulator-metal (MIM) light absorber can be integrated intosilicon TO devices, acting as a localized heat source which can be activatedremotely by a pump beam. The MIM absorber design introduces lessthermal capacity to the device, compared to conventional electrically-drivenapproaches. Experimentally, the absorber-integrated add-drop filter showsan optical response time of 13.7 µs following the 10%-90% rule (equivalentto a exponential time constant of 5 µs) and a wavelength shift over pumppower of 60 pm/mW. The photothermally tunable add-drop filter mayprovide new perspectives for all-optical routing and switching in integratedSi photonic circuits.

© 2014 Optical Society of America

OCIS codes: (190.4870) Photothermal effects; (230.1150) All-optical devices; (310.3915)Metallic, opaque, and absorbing coatings; (130.4815) Optical switching devices.

References and links1. B. Little, S. Chu, H. Haus, J. Foresi, and J. P. Laine, “Microring resonator channel dropping filters,” J. Lightw.

Technol. 15, 998–1005 (1997).2. M. S. Rasras, K.-Y. Tu, D. M. Gill, Y.-K. Chen, A. White, S. Patel, A. Pomerene, D. Carothers, J. Beattie,

M. Beals, J. Michel, and L. Kimerling, “Demonstration of a tunable microwave-photonic notch filter using low-loss silicon ring resonators,” J. Lightw. Technol. 27, 2105–2110 (2009).

3. M. H. Khan, H. Shen, Y. Xuan, L. Zhao, S. Xiao, D. E. Leaird, A. M. Weiner, and M. Qi, “Ultrabroad-bandwidtharbitrary radiofrequency waveform generation with a silicon photonic chip-based spectral shaper,” Nature Pho-ton. 4, 117–122 (2010).

4. Q. Li, Z. Zhang, F. Liu, M. Qiu, and Y. Su, “Dense wavelength conversion and multicasting in a resonance-splitsilicon microring,” Appl. Phys. Lett. 93, 081113 (2008).

5. F. Xia, L. Sekaric, and Y. Vlasov, “Ultracompact optical buffers on a silicon chip,” Nature Photon. 1, 65–71(2007).

6. Q. Li, F. Liu, Z. Zhang, M. Qiu, and Y. Su, “System performances of on-chip silicon microring delay line for RZ,CSRZ, RZ-DB and RZ-AMI signals,” J. Lightw. Technol. 26, 3744–3751 (2008).

7. B. Little, J. Foresi, G. Steinmeyer, E. Thoen, S. Chu, H. Haus, E. Ippen, L. Kimerling, and W. Greene, “Ultra-compact Si-SiO2 microring resonator optical channel dropping filters,” IEEE Photon. Technol. Lett. 10, 549–551(1998).

#215265 - $15.00 USD Received 3 Jul 2014; revised 16 Sep 2014; accepted 27 Sep 2014; published 8 Oct 2014(C) 2014 OSA 20 October 2014 | Vol. 22, No. 21 | DOI:10.1364/OE.22.025233 | OPTICS EXPRESS 25233

8. S. Xiao, M. H. Khan, H. Shen, and M. Qi, “A highly compact third-order silicon microring add-drop filter with avery large free spectral range, a flat passband and a low delay dispersion,” Opt. Express 15, 14765–14771 (2007).

9. A. Prabhu, A. Tsay, Z. Han, and V. Van, “Ultracompact soi microring adddrop filter with wide bandwidth andwide fsr,” IEEE Photon. Technol. Lett. 21, 651–653 (2009).

10. Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435,325–327 (2005).

11. L. Fan, J. Wang, L. T. Varghese, H. Shen, B. Niu, Y. Xuan, A. M. Weiner, and M. Qi, “An all-silicon passiveoptical diode,” Science 335, 447–450 (2012).

12. P. Dong, S. Liao, D. Feng, H. Liang, D. Zheng, R. Shafiiha, C.-C. Kung, W. Qian, G. Li, X. Zheng, A. V. Krish-namoorthy, and M. Asghari, “Low Vpp, ultralow-energy, compact, high-speed silicon electro-optic modulator,”Opt. Express 17, 22484–22490 (2009).

13. V. Almeida, C. Barrios, R. Panepucci, and M. Lipson, “All-optical control of light on a silicon chip,” Nature 431,1081–1084 (2004).

14. Y. Vlasov, W. M. J. Green, and F. Xia, “High-throughput silicon nanophotonic wavelength-insensitive switch foron-chip optical networks,” Nature Photon. 2, 242–246 (2008).

15. M. S. Nawrocka, T. Liu, X. Wang, and R. R. Panepucci, “Tunable silicon microring resonator with wide freespectral range,” Appl. Phys. Lett. 89, 071110 (2006).

16. F. Gan, T. Barwicz, M. Popovic, M. Dahlem, C. Holzwarth, P. Rakich, H. Smith, E. Ippen, and F. Kartner,“Maximizing the thermo-optic tuning range of silicon photonic structures,” in “Photonics in Switching,” (2007),67–68.

17. M. S. Dahlem, C. W. Holzwarth, A. Khilo, F. X. Kartner, H. I. Smith, and E. P. Ippen, “Reconfigurable multi-channel second-order silicon microring-resonator filterbanks for on-chip WDM systems,” Opt. Express 19, 306–316 (2011).

18. X. Wang, J. A. Martinez, M. Nawrocka, and R. Panepucci, “Compact thermally tunable silicon wavelengthswitch: Modeling and characterization,” IEEE Photon. Technol. Lett. 20, 936–938 (2008).

19. P. Dong, W. Qian, H. Liang, R. Shafiiha, D. Feng, G. Li, J. E. Cunningham, A. V. Krishnamoorthy, and M. As-ghari, “Thermally tunable silicon racetrack resonators with ultralow tuning power,” Opt. Express 18, 20298–20304 (2010).

20. W. S. Fegadolli, L. Feng, M. M.-U. Rahman, J. E. B. Oliveira, V. R. Almeida, and A. Scherer, “Experimentaldemonstration of a reconfigurable silicon thermo-optical device based on spectral tuning of ring resonators foroptical signal processing,” Opt. Express 22, 3425–3431 (2014).

21. M. R. Watts, W. A. Zortman, D. C. Trotter, G. N. Nielson, D. L. Luck, and R. W. Young, “Adiabatic resonant mi-crorings (arms) with directly integrated thermal microphotonics,” in “Conference on Lasers and Electro-Optics,”(2009), CPDB10.

22. J. Hao, J. Wang, X. Liu, W. J. Padilla, L. Zhou, and M. Qiu, “High performance optical absorber based on aplasmonic metamaterial,” Appl. Phys. Lett. 96, 251104 (2010).

23. M. Yan, “Metal-insulator-metal light absorber: a continuous structure,” J. Opt. 15, 025006 (2013).24. X. Chen, Y. Chen, Y. Shi, M. Yan, and M. Qiu, “Photothermal switching of SOI waveguide-based Mach-Zehnder

interferometer with integrated plasmonic nanoheater,” Plasmonics 9, 1197–1205 (2014).25. J.-C. Weeber, T. Bernardin, M. G. Nielsen, K. Hassan, S. Kaya, J. Fatome, C. Finot, A. Dereux, and N. Pleros,

“Nanosecond thermo-optical dynamics of polymer loaded plasmonic waveguides,” Opt. Express 21, 27291–27305 (2013).

26. M. G. Nielsen, T. Bernardin, K. Hassan, E. E. Kriezis, and J.-C. Weeber, “Silicon-loaded surface plasmon polari-ton waveguides for nanosecond thermo-optical switching,” Opt. Lett. 39, 2282–2285 (2014).

1. Introduction

Silicon (Si) photonics is the rising technology in integrated photonic circuits, for realizing highspeed data interconnect with minimized size and low power consumption. As one key elementof Si photonics, Si microring resonators [1] are featured by the properties of high-quality-factorresonance, compact size, and tailorable free spectral range, which show various functionalityin integrated photonic devices, e. g. spectrum filter [2], radio frequency signal processing [3,4], optical delay-line [5, 6], add-drop filter [7–9], modulator [10], and nonreciprocal opticaldiode [11].

Using the nonlinearity of the optical property of Si, researchers have been able to tune Siring resonators by electric field, optical pump, or heat. Electrically, a Si ring modulator basedon charge carriers effect was first demonstrated by Xu et al. [10] and later improved to a mod-ulation speed over 10 Gbit/s by Dong et al. [12] All-optical control of light in Si ring withrelaxation time of 1-ns timescale was achieved by injecting charge carriers through a pump


light, side-coupled to the ring [13] or vertically focused on the ring from the top [14]. Thermaltuning of Si rings using an electrically driven heater has also been reported [15–17], motivatedby the large thermo-optic (TO) effect of Si. Limited by the heat dissipation rate of the devices,the rise/fall time of the thermally tuned Si rings [18–20] is on the order of tens of microsecond.The state-of-art electrically-driven thermo-optic switch based on Si ring resonator was reportedby Watts et al. [21], where a section of the Si ring was doped and directly heated by the injectingcurrent from the suspended feed-line.

The performance of the TO ring switches can be judged by their tuning sensitivity (pump-power derivative of wavelength-shift) and switching time (rise/fall response time). The twoindicators are intrinsically governed by the effective heat capacity of the device and the thermalconduction to heat sink. Therefore, localized heat generation and minimized redundant heatcapacity are critical for achieving high-speed TO ring switches.

Driven by an optical pump, metal-insulator-metal (MIM) light absorber was proven as acompact-size and efficient heat generator [22–24]. Using standard thin-film deposition tech-nique, MIM absorber can be integrated in Si photonic devices for non-contact localized heat-ing, free of any feed line or contact. The absorption resonant peak of the MIM absorber can betailored to specific wavelength in visible or infrared spectrum range. Therefore, one can designthe MIM structure to absorb light, for instance at wavelength of 1064 nm, which is not stronglyabsorbed by Si material and far from telecommunication wavelength in the ring resonator. Ben-efited from these advantages, the MIM absorber-based Si ring resonators show better switchingtime, compared to standard electrically-heated ring resonators. In this paper, the design, fabri-cation and characterization of the all-optical tuning of absorber-based Si micro-ring add-dropfilter (ADF) are presented.

2. Design and fabrication

Fig. 1. (a) The schematic diagram of the Si micro-ring add-drop filter with a metal-insulator-metal (MIM) absorber disk in the center of the ring. (b) The measured and simu-lated absorbance spectra of the MIM absorber with a 10-µm-diameter disk shape. (c) Themeasured transmittance spectra of the add-drop filter at through port and drop port. (d) Themeasured transmittance spectra of through port and drop port around 1552.0 nm, fitted bya damped oscillation model.


The schematic diagram of the proposed absorber-integrated Si micro-ring is shown in Fig. 1(a).The 18-µm-diameter silicon-on-insulator (SOI) ring resonator is integrated with a MIM ab-sorber with a diameter of 10 µm. The MIM absorber has Au/Al2O3/Au three layers with thick-nesses of 20/245/60 nm, respectively. The absorption peak of MIM absorber is designed around1064 nm in wavelength. The vertically incident pump light at 1064 nm in wavelength will excitea Fabry-Perot resonance between the two Au plates in the MIM absorber [23]. The measuredand simulated absorbance spectra of the MIM absorber are shown in Fig. 1(b). The MIM ab-sorber shows a measured peak-absorbance of 0.9 at 1050 nm in wavelength, with a full widthat half maximum (FWHM) of 210 nm. The fabricated silicon strip waveguide structures havea width of 450 nm and a height of 250 nm. The transmittance spectra of through port (T-port)and drop port (D-port) of the add-drop filter (ADF) are shown together in Fig. 1(c). The freespectral range (FSR) of the Si ring is 10.4 nm and the FWHM of the resonance is 144 pm. Thehorizontally traveling probe light with wavelength around 1550 nm is coupled from the inputstrip waveguide to the ring resonator. At the whispering-gallery (WG) mode with azimuthalmode number of m and wavelength of λm, the light in the ring is constructively self-interferingand building up in energy. As the energy of the light is building up in the ring, certain lightpower couples to the output strip waveguide, and eventually reaches the D-port. Therefore,the transmission ditches of the T-port and the transmission peaks of the D-port coincide at theresonance wavelengths of λm. According to coupled mode theory [1], the transfer function atthrough port (T-port) is

tthro = 1−1

Qe

i(λm−λ

λ)+ 1

2Qi+ 1

2Qe+ 1

2Qd

, (1)

where the Qe (Qd) is the coupling Q factor between the input (output) waveguide and theresonator, Qi is the intrinsic Q factor of the resonator, λm is the resonance wavelength. Thenormalized transmittance (Tthro) at T-port is defined as |tthro|2. If the input and output couplingQ factors are symmetric, the normalized transmittance at D-port [1] is

Tdrop =( 1

Qe)2

(λm−λ

λ)2 +( 1

2Qi+ 2

2Qe)2. (2)

The measured transmittance at T-port and D-port are fitted in Fig. 1(d), using Eq. 1 and 2.We found that Qi of the ring is 5.8× 104 and Qe between the strip waveguide and the ring is2.6×104.

Fig. 2. (a) The SEM image of the MIM-absorber-based Si ring add-drop filter (ADF). (b)The optical microscopic bright-field image of Si ring ADF.

The scanning electron microscope (SEM) image of the Si ring add-drop filter (ADF) is shownin Fig. 2(a), where the diameter of the fabricated Si ring is 18 µm and the diameter of the MIM


absorber in the center is 10 µm, and gap between the ring resonator and the strip waveguideis around 150 nm. The optical bright-field image of the ring ADF is also shown in Fig. 2(b),where the MIM absorber shows a pink color due to its absorption in visible spectrum. The lightpath of the telecommunication wavelength in the add-drop filter is shown in Fig. 2(b).

3. Experiment: steady-state photothermal tuning

Fig. 3. The measured transmission spectra of the Si ring at (a) through port and (b) dropport, tuned by a thermal stage from 20◦C to 35◦C. The measured transmission spectra ofthe Si ring at (c) through port and (d) drop port, when the Si ring is optically tuned by aCW laser beam with pump power ranging from 0 mW to 4.16 mW.

The measured transmission spectra of the Si ring at T-port and D-port thermally tuned by athermal stage are shown in Figs. 3(a) and 3(b). By linear fitting, temperature derivative ofwavelength shift ( ∆λm

∆T ) of the Si ring is 75.3 pm/K. The absorber-integrated Si ring is thenoptically pumped by a CW laser with wavelength of 1064 nm and Gaussian beam waist around4.6 µm. The measured transmission spectra at at T-port and D-port with optical pump powerranging from 0.00 mW to 4.16 mW is shown in Figs. 3(c) and 3(d). By linear fitting, thepump power derivative of the wavelength shift ( ∆λm

∆P0) of the Si ring is 60 pm/mW. According to

dTdP0

= dλmdP0

(dλmdT

)−1, the pump power derivative of the temperature increase ( ∆T

∆P0) in the Si ring

is 0.80 K/mW.

4. Numerical heat analysis

The heat conduction analysis of the laser pumped absorber-integrated Si ring is implemented,using finite element method (FEM). The distribution of the temperature increase (∆T ) in the5.0-mW-laser-pumped Si ring is shown in Fig. 4(a). The ∆T of the MIM absorber is 57 K, whilethe ∆T of Si ring is only 4.66 K. Therefore, according to heat conduction analysis, ∆T

∆P0in Si

ring is 0.93 K/mW. The heat conduction between the heat source (MIM disk) and the Si light-guiding region (Si ring) is via the SiO2 layer, which has relatively low thermal conductivity 1.4W/(m·K). It leads to the large temperature difference between the MIM absorber and the ring.To further improve the heat conduction, we suggested rib-waveguide structure can be adapted,


Fig. 4. The simulation results of (a) the temperature distribution and heat flux in the laserpumped absorber-integrated Si ring, with pump power of 5 mW. (b) The simulated transientthermal response of the absorber-integrated Si ring, with pump laser square-wave modu-lated. The black solid line is the normalized temperature increase of the ring. The magentamarked line is the calculated optical transmittance at T-port. The blue dashed line is thefitting curve to the transmittance.

where the Si layer could be partially etched and the residual Si layer could facilitate the heatconduction.

The simulation results of transient thermal response of the Si ring are shown in Fig. 4(b).The black solid line is the normalized temperature increase of the ring. The magenta markedline is the calculated optical transmittance at T-port according to Eq. 1. The time variation ofthe resonance wavelength is written as

λm(t) = λm(0)+dλm

dT∆T (t). (3)

The blue dashed line is the fitting curve to the transmittance, using time-domain functionsin form of 1− exp(−t/τr) as rise edge and exp(t/τ f ) as fall edge. The time constant at riseedge(τr) and fall edge (τ f ) of the temperature increase of the Si ring is 7.5 µs and 8.0 µs,respectively. The τr and τ f of the calculated optical transmittance of the Si ring at T-port is 3.0µs and 4.0 µs, respectively.

5. Experiment: dynamic photothermal switching

The measured transient optical transmittance of the Si ring is shown in Fig. 5, which is pumpedby a modulated laser beam with duty cycle of 50% and time period ranging from 100 µs to10 µs. It shows that the modulated optical signals at T-port and D-port are complimentary toeach other in time domain. Fitted by exponential decay function mentioned in previous sec-tion, the exponential time constant at rise edge (τr) and fall edge (τ f ) of the measured opticaltransmittance of Si ring are 4.5 µs and 5.0 µs, respectively, which is slightly larger than thesimulated time constants discussed previously. In analog signal processing, the rise/fall time isnormally defined as the time taken for a signal to change from 10% to 90% of the step height.Theoretically, the 10%-90% rise/fall time is 2.2 times the exponential time constant. As shownin Fig. 5(a), the horizontal solid lines indicate the 10% and 90% step height of the measuredoptical transmission signal at T-port. Following the 10%-90% rule, the rise/fall time at T-portis 10.0 µs /13.7 µs and the rise/fall time at D-port is 12.0 µs /10.3 µs.

The performances of four previously reported electrically driven TO Si ring switches andone optically driven TO polymer loaded surface plasmon polariton (PLSPP) ring switch [25]are listed in Table 1, together with the present work. It shows that our optically driven absorber-integrated Si ring has shorter response time than most of the electrically driven TO Si rings and


Fig. 5. The measured transient optical response of the absorber-integrated Si ring, withpump laser square-wave modulated, duty cycle of 50%, and period of (a) 100 µs, (b) 40µs, (c) 20 µs, and (d) 10 µs. The magenta solid line is the optical transient transmissionsignal of the Si ring at T-port. The black dash line is the transient transmission signal atD-port. The blue solid line is transient synchronization signal of the pump light. The greenhorizontal solid line indicates the 10% and 90% step height of the optical transmissionsignal at T-port, which is used to extract the optical rise/fall time of the device.

the optically driven TO PLSPP ring. The only faster electrically driven TO-Si-ring is reportedby Watts et al. [21], where a section of the suspended Si ring is doped and directly heated by in-jecting a current from a vertically coupled feed-line. The delicate and complex design by Wattset al. gives the fastest thermo-optic Si device, at the expense of multiple processes of doping,etching and deposition. Comparing to standard electric-heated TO Si devices, our design ofMIM absorber heated ring structure improves the optical response time without introducingfabrication complexity. Moreover, the performance of the TO add-drop filter could be evenboosted by merging the technique of optically driven MIM absorber and suspended Si structurein the device. It should be mentioned that optically-driven TO photonic switches with fast ther-mal response are emerging, e.g. MIM nanostrip integrated Si waveguide [24], polymer-loadedSPP waveguide [25] and Si loaded SPP waveguide [26].

It is worth noting that the definition of response time of the TO devices in the reported workslisted in Table 1 is not consistent. For instance, in Wang et al.’s work [18], the response time isdefined as the time taken for signal to change from 0% to 50% of the step height (labelled withb); in Watts et al.’s work [21], the response time is defined as exponential time constant (labelledwith c). For fair evaluation, the response time value defined by other methods are converted tothe values following the 10%-90% rule (labelled with a), which is commonly adopted in themajority of the reported works [19, 20, 25].

6. Thermal stability

The thermal stability of the SOI die under continuous photothermal switching is another issueneeded to be addressed. In this work, the SOI die under test is mounted on a thermal stage, tomaintain the temperature of the substrate bottom (Td) at 300 K. Under such configuration, thelong-term thermal fluctuation of the device (i. e. the thermal drifting of the resonant wavelength


Table 1. Experimental performances of recently reported TO microring devices, togetherwith present work.

Group Year Description Responsetime [µs]

Sensitivity[nm/mW]

Electrically driven TO Si ringWang et al. [18] 2008 10-µm-diameter ring 44a or 14b 0.06Watts et al. [21] 2009 Doped Si ring directly heated,

suspended structure2.2a or 1c 1.8

Dong et al. [19] 2010 Suspended structure 170a 4.79Fegadolli et al. [20] 2014 Signal processing application 15a 0.25

Optically driven TO ringWeeber et al. [25] 2013 Polymer loaded SPP ring on Au

film, pump@533 nm18a N.A.

Present work 2014 MIM absorber heated Si ring,pump@1064 nm

13.7a or 5c 0.06

a Defined by the 10%-90% rule.b Defined by the 0%-50% rule.c Defined by the exponential decay constant.

of the Si ring) is minimized. In an ideal case, i. e. Td is fixed at 300 K, the input heat powerduring continuous photothermal switching is balanced to the heat transfer from the die to thesurrounding, including heat conduction from the die to the mount (mainly), heat convectionfrom the die surface to the air and even heat radiation from the die to the surrounding.

In another scenario, supposed the die is mounted on a stage without thermal controlling unit,Td will be raised by the continuous switching. In such a case, we assume that the heat transfercoefficient at the substrate bottom side has a moderate value of 100 W/(m2·K). The size of thedie is 5 mm in diameter, the substrate thickness is 0.5 mm and the pump laser beam has a totaloptical power of 5 mW with an on-off duty cycle of 50%. Numerical heat analysis based onFEM is conducted and the resulted temperature increase of the die (∆Td) at the substrate bottomis 0.9 K and that the rise time constant for heating the whole die is 16 s. In case of ∆Td = 0.9 K,the WG-mode resonant of the Si ring at both ”ON” and ”OFF” states are simultaneously red-shifted by 70 pm. Therefore, the photothermal switching of the transmitted light through the Siring is still operational, but the suitable working wavelength of the device is slightly red-shifted.

7. Conclusion

In conclusion, a photothermally tunable Si ring based on MIM absorber is proposed and ex-perimentally investigated. The concept of design, steady-state tuning and dynamic switchingmeasurements are described. The MIM absorber heated Si ring can be optically controlled bya vertically incident laser beam and used as an all-optical router with a rise/fall time of 13.7µs following the 10%-90% rule and an equivalent exponential time constant of 5 µs. Thermalstability analysis showed that the die temperature will be raised by 0.9 K under continuousswitching with a pump power of 5 mW and a duty cycle of 50%, when active thermal manage-ment is not presented. Due to the efficient light-to-heat conversion and compact-size of MIMabsorber, the proposed photothermal device can be an elementary component in integrated pho-tonic circuit for all-optical routing.


Acknowledgment

We thank Walter Margulis (Acreo AB, Sweden), Yu Xiang and Urban Westergren (KTH) fortheir help with the characterization setup. This work is supported by the Swedish Foundationfor Strategic Research (SSF), the Swedish Research Council (VR), VR’s Linnaeus center inAdvanced Optics and Photonics (ADOPT). M.Q. acknowledges the support by the NationalScience Foundation of China (Grants Nos. 61275030, 61205030, and 61235007).


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