Photothermally tunable silicon-microring-based optical add-drop filter through integrated light absorber

An optically pumped thermo-optic (TO) silicon ring add-drop filter with fast thermal response is experimentally demonstrated. We propose that metal-insulator-metal (MIM) light absorber can be integrated into silicon TO devices, acting as a localized heat source which can be activated remotely by a pump beam. The MIM absorber design introduces less thermal capacity to the device, compared to conventional electrically-driven approaches. Experimentally, the absorber-integrated add-drop filter shows an optical response time of 13.7 μs following the 10%-90% rule (equivalent to a exponential time constant of 5 μs) and a wavelength shift over pump power of 60 pm/mW. The photothermally tunable add-drop filter may provide new perspectives for all-optical routing and switching in integrated Si 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 links 1. 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 lowloss 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. 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Introduction
Silicon (Si) photonics is the rising technology in integrated photonic circuits, for realizing high speed data interconnect with minimized size and low power consumption.As one key element of Si photonics, Si microring resonators [1] are featured by the properties of high-quality-factor resonance, compact size, and tailorable free spectral range, which show various functionality in integrated photonic devices, e. g. spectrum filter [2], radio frequency signal processing [3,4], optical delay-line [5,6], add-drop filter [7][8][9], modulator [10], and nonreciprocal optical diode [11].
Using the nonlinearity of the optical property of Si, researchers have been able to tune Si ring resonators by electric field, optical pump, or heat.Electrically, a Si ring modulator based on charge carriers effect was first demonstrated by Xu et al. [10] and later improved to a modulation speed over 10 Gbit/s by Dong et al. [12] All-optical control of light in Si ring with relaxation 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].Thermal tuning of Si rings using an electrically driven heater has also been reported [15][16][17], motivated by 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][19][20] is on the order of tens of microsecond.The state-of-art electrically-driven thermo-optic switch based on Si ring resonator was reported by Watts et al. [21], where a section of the Si ring was doped and directly heated by the injecting current from the suspended feed-line.
The performance of the TO ring switches can be judged by their tuning sensitivity (pumppower derivative of wavelength-shift) and switching time (rise/fall response time).The two indicators are intrinsically governed by the effective heat capacity of the device and the thermal conduction to heat sink.Therefore, localized heat generation and minimized redundant heat capacity are critical for achieving high-speed TO ring switches.
Driven by an optical pump, metal-insulator-metal (MIM) light absorber was proven as a compact-size and efficient heat generator [22][23][24].Using standard thin-film deposition technique, MIM absorber can be integrated in Si photonic devices for non-contact localized heating, free of any feed line or contact.The absorption resonant peak of the MIM absorber can be tailored to specific wavelength in visible or infrared spectrum range.Therefore, one can design the MIM structure to absorb light, for instance at wavelength of 1064 nm, which is not strongly absorbed by Si material and far from telecommunication wavelength in the ring resonator.Benefited from these advantages, the MIM absorber-based Si ring resonators show better switching time, compared to standard electrically-heated ring resonators.In this paper, the design, fabrication and characterization of the all-optical tuning of absorber-based Si micro-ring add-drop filter (ADF) are presented.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 absorber with a diameter of 10 µm.The MIM absorber has Au/Al 2 O 3 /Au three layers with thicknesses of 20/245/60 nm, respectively.The absorption peak of MIM absorber is designed around 1064 nm in wavelength.The vertically incident pump light at 1064 nm in wavelength will excite a Fabry-Pérot resonance between the two Au plates in the MIM absorber [23].The measured and simulated absorbance spectra of the MIM absorber are shown in Fig. 1(b).The MIM absorber shows a measured peak-absorbance of 0.9 at 1050 nm in wavelength, with a full width at half maximum (FWHM) of 210 nm.The fabricated silicon strip waveguide structures have a 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 free spectral range (FSR) of the Si ring is 10.4 nm and the FWHM of the resonance is 144 pm.The horizontally traveling probe light with wavelength around 1550 nm is coupled from the input strip waveguide to the ring resonator.At the whispering-gallery (WG) mode with azimuthal mode number of m and wavelength of λ m , the light in the ring is constructively self-interfering and building up in energy.As the energy of the light is building up in the ring, certain light power 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 the resonance wavelengths of λ m .According to coupled mode theory [1], the transfer function at through port (T-port) is

Design and fabrication
where the Q e (Q d ) is the coupling Q factor between the input (output) waveguide and the resonator, Q i is the intrinsic Q factor of the resonator, λ m is the resonance wavelength.The normalized transmittance (T thro ) at T-port is defined as |t thro | 2 .If the input and output coupling Q factors are symmetric, the normalized transmittance at D-port [1] is . (2) The measured transmittance at T-port and D-port are fitted in Fig. 1(d), using Eq. 1 and 2. We found that Q i of the ring is 5.8 × 10 4 and Q e between the strip waveguide and the ring is 2.6 × 10 4 .The scanning electron microscope (SEM) image of the Si ring add-drop filter (ADF) is shown in 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 waveguide is 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 light path of the telecommunication wavelength in the add-drop filter is shown in Fig. 2(b)., the pump power derivative of the temperature increase ( ∆T ∆P 0 ) in the Si ring is 0.80 K/mW.

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 the 5.0-mW-laser-pumped Si ring is shown in Fig. 4(a).The ∆T of the MIM absorber is 57 K, while the ∆T of Si ring is only 4.66 K. Therefore, according to heat conduction analysis, ∆T ∆P 0 in Si ring is 0.93 K/mW.The heat conduction between the heat source (MIM disk) and the Si lightguiding region (Si ring) is via the SiO 2 layer, which has relatively low thermal conductivity 1.4 W/(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, where the Si layer could be partially etched and the residual Si layer could facilitate the heat conduction.
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 marked line is the calculated optical transmittance at T-port according to Eq. 1.The time variation of the resonance wavelength is written as The blue dashed line is the fitting curve to the transmittance, using time-domain functions in form of 1 − exp(−t/τ r ) as rise edge and exp(t/τ f ) as fall edge.The time constant at rise edge(τ r ) and fall edge (τ f ) of the temperature increase of the Si ring is 7.5 µs and 8.0 µs, respectively.The τ r and τ f of the calculated optical transmittance of the Si ring at T-port is 3.0 µs and 4.0 µs, respectively.

Experiment: dynamic photothermal switching
The measured transient optical transmittance of the Si ring is shown in Fig. 5, which is pumped by a modulated laser beam with duty cycle of 50% and time period ranging from 100 µs to 10 µs.It shows that the modulated optical signals at T-port and D-port are complimentary to each other in time domain.Fitted by exponential decay function mentioned in previous section, the exponential time constant at rise edge (τ r ) and fall edge (τ f ) of the measured optical transmittance of Si ring are 4.5 µs and 5.0 µs, respectively, which is slightly larger than the simulated time constants discussed previously.In analog signal processing, the rise/fall time is normally 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 shown in Fig. 5(a), the horizontal solid lines indicate the 10% and 90% step height of the measured optical transmission signal at T-port.Following the 10%-90% rule, the rise/fall time at T-port is 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 and one 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 absorberintegrated Si ring has shorter response time than most of the electrically driven TO Si rings and the optically driven TO PLSPP ring.The only faster electrically driven TO-Si-ring is reported by Watts et al. [21], where a section of the suspended Si ring is doped and directly heated by injecting a current from a vertically coupled feed-line.The delicate and complex design by Watts et 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 of MIM absorber heated ring structure improves the optical response time without introducing fabrication complexity.Moreover, the performance of the TO add-drop filter could be even boosted by merging the technique of optically driven MIM absorber and suspended Si structure in the device.It should be mentioned that optically-driven TO photonic switches with fast thermal response are emerging, e.g.MIM nanostrip integrated Si waveguide [24], polymer-loaded SPP 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 works listed in Table 1 is not consistent.For instance, in Wang et al.'s work [18], the response time is defined as the time taken for signal to change from 0% to 50% of the step height (labelled with b ); in Watts et al.'s work [21], the response time is defined as exponential time constant (labelled with c ).For fair evaluation, the response time value defined by other methods are converted to the values following the 10%-90% rule (labelled with a ), which is commonly adopted in the majority of the reported works [19,20,25].

Thermal stability
The thermal stability of the SOI die under continuous photothermal switching is another issue needed to be addressed.In this work, the SOI die under test is mounted on a thermal stage, to maintain the temperature of the substrate bottom (T d ) at 300 K.Under such configuration, the long-term thermal fluctuation of the device (i.e. the thermal drifting of the resonant wavelength of the Si ring) is minimized.In an ideal case, i. e. T d is fixed at 300 K, the input heat power during continuous photothermal switching is balanced to the heat transfer from the die to the surrounding, including heat conduction from the die to the mount (mainly), heat convection from 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, T d will be raised by the continuous switching.In such a case, we assume that the heat transfer coefficient at the substrate bottom side has a moderate value of 100 W/(m 2 •K).The size of the die is 5 mm in diameter, the substrate thickness is 0.5 mm and the pump laser beam has a total optical power of 5 mW with an on-off duty cycle of 50%.Numerical heat analysis based on FEM is conducted and the resulted temperature increase of the die (∆T d ) at the substrate bottom is 0.9 K and that the rise time constant for heating the whole die is 16 s.In case of ∆T d = 0.9 K, the WG-mode resonant of the Si ring at both "ON" and "OFF" states are simultaneously redshifted by 70 pm.Therefore, the photothermal switching of the transmitted light through the Si ring is still operational, but the suitable working wavelength of the device is slightly red-shifted.

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

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

Fig. 2 .
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.

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

Fig. 4 .
Fig. 4. The simulation results of (a) the temperature distribution and heat flux in the laser pumped absorber-integrated Si ring, with pump power of 5 mW.(b) The simulated transient thermal response of the absorber-integrated Si ring, with pump laser square-wave modulated.The black solid line is the normalized temperature increase of the ring.The magenta marked line is the calculated optical transmittance at T-port.The blue dashed line is the fitting curve to the transmittance.

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

Table 1 .
Experimental performances of recently reported TO microring devices, together with present work.
a Defined by the 10%-90% rule.b Defined by the 0%-50% rule.c Defined by the exponential decay constant.