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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 12 — Jun. 4, 2012
  • pp: 13215–13225
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Photothermal optical modulation of ultra-compact hybrid Si-VO2 ring resonators

Judson D. Ryckman, V. Diez-Blanco, Joyeeta Nag, Robert E. Marvel, B. K. Choi, Richard F. Haglund, Jr., and Sharon M. Weiss  »View Author Affiliations


Optics Express, Vol. 20, Issue 12, pp. 13215-13225 (2012)
http://dx.doi.org/10.1364/OE.20.013215


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Abstract

We demonstrate photothermally induced optical switching of ultra-compact hybrid Si-VO2 ring resonators. The devices consist of a sub-micron length ~70nm thick patch of phase-changing VO2 integrated onto silicon ring resonators as small as 1.5μm in radius. The semiconductor-to-metal transition (SMT) of VO2 is triggered using a 532nm pump laser, while optical transmission is probed using a tunable cw laser near 1550nm. We observe optical modulation greater than 10dB from modest quality-factor (~103) resonances, as well as a large –1.26nm change in resonant wavelength Δλ, resulting from the large change in the dielectric function of VO2 in the insulator-to-metal transition achieved by optical pumping.

© 2012 OSA

1. Introduction

Silicon-based photonics has rapidly emerged as the leading candidate platform from which to revolutionize modern computing and communications. By replacing electrical components, such as the copper interconnects, with optical devices and architectures, dramatic improvements to a variety of metrics including bandwidth, speed, loss, cross-talk, and power consumption can be achieved [1

1. G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics 4(8), 518–526 (2010). [CrossRef]

]. For the realization of integrated silicon photonics, it is vital to develop compact silicon-based optical modulators that exceed the performance metrics of comparable electronic devices. Micro-ring resonant cavities have already been used to realize compact optical modulators, capable of operating at ~GHz speeds, based on the electro-optic effect in silicon [2

2. Q. F. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005). [CrossRef] [PubMed]

]. The micro-ring platform has also been used to demonstrate compact all-optical switching and logic operations on Si [3

3. V. R. Almeida, C. A. Barrios, R. R. Panepucci, and M. Lipson, “All-optical control of light on a silicon chip,” Nature 431(7012), 1081–1084 (2004). [CrossRef] [PubMed]

, 4

4. Q. F. Xu and M. Lipson, “All-optical logic based on silicon micro-ring resonators,” Opt. Express 15(3), 924–929 (2007). [CrossRef] [PubMed]

], and offers a convenient architecture for performing wavelength division multiplexing (WDM) [5

5. Q. F. Xu, B. Schmidt, J. Shakya, and M. Lipson, “Cascaded silicon micro-ring modulators for WDM optical interconnection,” Opt. Express 14(20), 9431–9435 (2006). [CrossRef] [PubMed]

]. While the resonator geometry offers substantial reductions in size and energy requirements for optical switching compared to millimeter-scale interferometer approaches [6

6. A. S. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427(6975), 615–618 (2004). [CrossRef] [PubMed]

], such compact all-Si optical modulators still face a number of limitations with respect to speed and performance. The small plasma dispersion effect in silicon requires very narrow band, high Q-factor devices to be utilized. But high Q-factor devices are difficult to implement, both in terms of required fabrication tolerances and in terms of sensitivity to ambient conditions such as temperature fluctuations [1

1. G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics 4(8), 518–526 (2010). [CrossRef]

]. Indeed, high Q-factor silicon ring resonators are known to be highly temperature sensitive (30-80 pm K–1) [7

7. J. Teng, P. Dumon, W. Bogaerts, H. B. Zhang, X. G. Jian, X. Y. Han, M. S. Zhao, G. Morthier, and R. Baets, “Athermal Silicon-on-insulator ring resonators by overlaying a polymer cladding on narrowed waveguides,” Opt. Express 17(17), 14627–14633 (2009). [CrossRef] [PubMed]

, 8

8. W. N. Ye, J. Michel, and L. C. Kimerling, “Athermal high-index-contrast waveguide design,” IEEE Photon. Technol. Lett. 20(11), 885–887 (2008). [CrossRef]

], therefore requiring power hungry temperature compensation schemes to maintain temperature tolerances to less than ± 1 °C [9

9. S. Manipatruni, R. K. Dokania, B. Schmidt, N. Sherwood-Droz, C. B. Poitras, A. B. Apsel, and M. Lipson, “Wide temperature range operation of micrometer-scale silicon electro-optic modulators,” Opt. Lett. 33(19), 2185–2187 (2008). [CrossRef] [PubMed]

]. Additionally, the switching times of silicon based optical modulators are typically limited by carrier lifetimes for injection based devices (e.g. ~0.35 ns) [2

2. Q. F. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005). [CrossRef] [PubMed]

], and RC constants in accumulation or depletion based devices (e.g. ~0.014 ns) [1

1. G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics 4(8), 518–526 (2010). [CrossRef]

, 10

10. F. Y. Gardes, G. T. Reed, N. G. Emerson, and C. E. Png, “A sub-micron depletion-type photonic modulator in Silicon On Insulator,” Opt. Express 13(22), 8845–8854 (2005). [CrossRef] [PubMed]

].

Phase-changing VO2 is a particularly attractive candidate hybrid material that is well known for its reversible semiconductor-to-metal transition (SMT) occurring near 67°C, accompanied by a structural change from monoclinic to tetragonal crystal structure [17

17. V. Eyert, “The metal-insulator transitions of VO2: a band theoretical approach,” Ann. Phys. (Leipzig) 9, 650–704 (2002).

]. The SMT results in large changes to resistivity, near-infrared transmission, and refractive index (~1.96 to 3.25) [18

18. F. J. Morin, “Oxides which show a metal-to-insulator transition at the Neel temperature,” Phys. Rev. Lett. 3(1), 34–36 (1959). [CrossRef]

]. Importantly, the phase transition in VO2 can be triggered by a variety of stimuli aside from temperature including: strain [19

19. J. Cao, E. Ertekin, V. Srinivasan, W. Fan, S. Huang, H. Zheng, J. W. L. Yim, D. R. Khanal, D. F. Ogletree, J. C. Grossman, and J. Wu, “Strain engineering and one-dimensional organization of metal-insulator domains in single-crystal vanadium dioxide beams,” Nat. Nanotechnol. 4(11), 732–737 (2009). [CrossRef] [PubMed]

], electric field [20

20. G. Stefanovich, A. Pergament, and D. Stefanovich, “Electrical switching and Mott transition in VO2,” J. Phys. Condens. Matter 12(41), 8837–8845 (2000). [CrossRef]

, 21

21. D. Ruzmetov, G. Gopalakrishnan, J. D. Deng, V. Narayanamurti, and S. Ramanathan, “Electrical triggering of metal-insulator transition in nanoscale vanadium oxide junctions,” J. Appl. Phys. 106(8), 083702 (2009). [CrossRef]

], or optical excitation [22

22. A. Cavalleri, C. Tóth, C. W. Siders, J. A. Squier, F. Ráksi, P. Forget, and J. C. Kieffer, “Femtosecond structural dynamics in VO2 during an ultrafast solid-solid phase transition,” Phys. Rev. Lett. 87(23), 237401 (2001). [CrossRef] [PubMed]

]. In the case of pulsed optical excitation, the SMT has been shown to occur on time-scales less than 100 fs [23

23. R. Lopez, R. F. Haglund, L. C. Feldman, L. A. Boatner, and T. E. Haynes, “Optical nonlinearities in VO2 nanoparticles and thin films,” Appl. Phys. Lett. 85(22), 5191–5193 (2004). [CrossRef]

, 24

24. A. Pashkin, C. Kubler, H. Ehrke, R. Lopez, A. Halabica, R. F. Haglund, R. Huber, and A. Leitenstorfer, “Ultrafast insulator-metal phase transition in VO(2) studied by multiterahertz spectroscopy,” Phys. Rev. B 83(19), 195120 (2011). [CrossRef]

], perhaps offering the possibility for achieving optical modulation at THz speeds.

Here we report on a silicon-based hybrid optical modulator incorporating VO2 as the optical switching element on an ultra-compact silicon micro-ring resonator. By combining a very small, ~0.28μm2, active area of VO2 on a low-mode volume, ~1μm3, resonator, large changes to resonant wavelength, and thus optical transmission, can be induced by triggering the SMT. This hybrid Si-VO2 resonator lays the foundation for a new class of electro-optic or all-optical modulators utilizing VO2 on Si.

2. Device fabrication

The Si-VO2 hybrid micro-ring resonator structure was fabricated on a silicon-on-insulator (SOI) substrate with a 220 nm p-type, 14–22 Ω cm resistivity, Si(100) layer and 1 μm buried oxide layer (SOITEC). Electron-beam lithography (JEOL JBX-9300–100kV) was performed using ZEP 520A e-beam resist spun at 6,000 rpm (~300nm thick). After pattern exposure and development in xylenes for 30s followed by an IPA rinse and N2 drying, anisotropic reactive-ion etching was performed (Oxford PlasmaLab 100) using C4F8/SF6/Ar process gases to completely etch the exposed portion of the 220nm Si layer.

A second stage of electron-beam lithography (Raith eLine) was performed to open windows for VO2 deposition, using ZEP 520A spun at 2,000 rpm (~500nm thick) to better facilitate VO2 lift-off. Amorphous VOx was then deposited by electron-beam vaporization of VO2 powder (100 mesh, 99.5% purity) in an Ångstrom Engineering deposition system. After deposition, lift-off in acetone under ultra-sonication was performed, leaving behind amorphous VOx patches across the silicon rings. Samples were then annealed in a vacuum chamber with 250 mTorr of oxygen at 450°C for five minutes.

3. Experiment and analysis

3.1 Measurement set-up and passive spectral measurements

In preparation for optical measurements, samples were cleaved across each end of the bus waveguide, several millimeters away from the central device, and mounted on an XY positioning stage. Piezo-controled XYZ stages were used to position and couple light to/from polarization maintaining lensed fibers (OZ Optics Ltd.) as shown in Fig. 2(a)
Fig. 2 (a) Schematic of the optical measurement set-up. (b) Passive spectral measurements of optical transmission on 1.5μm radius micro-ring resonators with (bottom) and without (top) an integrated VO2 patch. For clarity, the top all-Si curve has been offset by + 20dBm.
. A tunable cw laser (Santec TSL-510) was used to perform passive transmission measurements, utilizing quasi-TE polarization, over the wavelength range 1500–1630nm.

3.2 Photothermally induced optical modulation

To characterize the optical response of the hybrid Si-VO2 micro-ring resonator in both states of the SMT, a 532nm cw pump laser (New Focus 3951-20) is focused onto the device with a 20x objective as shown in Fig. 2(a). An infrared (IR) camera was used for alignment purposes. IR imaging at maximum exposure and contrast settings was used to determine an upper bound for the Gaussian beam size, w0 ≈90μm. Given that nearly 100% of the total power (Ptotal) is contained within the radius 2w0, we estimate the average intensity to be approximately 15 W/cm2. The peak on-axis intensity is therefore ~30 W/cm2. In our experiments we found that precise positioning of the pump beam, to within approximately 10 microns, was required to trigger the SMT, suggesting that the peak on-axis intensity may be a more reasonable indicator of the laser intensity near the Si-VO2 micro-ring. We note that photothermal VO2 switching has previously been demonstrated with threshold intensities below 10 W/cm2 for thin-films on glass substrates while under continuous optical pumping at 532nm [29

29. T. Ben-Messaoud, G. Landry, J. P. Gariepy, B. Ramamoorthy, P. V. Ashrit, and A. Hache, “High contrast optical switching in vanadium dioxide thin films,” Opt. Commun. 281(24), 6024–6027 (2008). [CrossRef]

]. However, the threshold intensity and switching dynamics depend strongly on the VO2 film thickness, and will further vary strongly with the properties of the substrate (e.g., thermal conductivity and diffusivity) and the particular geometry employed.

In Fig. 3
Fig. 3 Optical transmission of the 1.5μm radius hybrid Si-VO2 ring resonator as a function of wavelength, before and after triggering the SMT with a 532nm pump laser. The lines are Lorentzian fits. Inset: IR camera images revealing vertical radiation at a fixed probe wavelength, λ = 1568.78nm (dashed line).
, we present the optical transmission of the 1.5μm radius hybrid Si-VO2 ring resonator as measured before and after triggering the SMT with the 532nm pump laser. After unblocking the 532nm laser, the system is given a few minutes to reach thermal equilibrium before again measuring the optical transmission with the tunable laser. These measurements reveal a sizeable shift Δλ = –1.26nm in the resonance wavelength, coinciding with an optical modulation greater than 10dB at the initial resonance position, λ = 1568.78nm. Because VO2 exhibits a dramatically reduced refractive index in the metallic state, a blue-shift in resonance frequency is naturally expected to arise from triggering the SMT. However, additional effects are also expected to be present during this experiment, including dependence on: (1) the thermo-optic (TO) effect in silicon, Δn/ΔT = + 1.86 × 10−4/K [30

30. G. Cocorullo, F. G. Della Corte, I. Rendina, and P. M. Sarro, “Thermo-optic effect exploitation in silicon microstructures,” Sens. Actuators A Phys. 71(1-2), 19–26 (1998). [CrossRef]

], and (2) the free-carrier index (FCI) [6

6. A. S. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427(6975), 615–618 (2004). [CrossRef] [PubMed]

]. These two effects are weaker than the much larger optical response of the VO2 and, in this experiment, also carry opposite signs. Thus, the photothermal approach for triggering VO2’s SMT enables silicon’s TO and FCI refractive index contributions to be used against each other so that the optical signature of VO2 can be more readily distinguished. We verified that the dominant contribution to this resonance shift was indeed coming from the VO2 by performing a control experiment (not shown) on an all-Si micro-ring with the same dimensions, revealing a Δλ = + 0.938nm net red-shift in resonance wavelength. This indicates that in the absence of TO or FCI effects, the achievable resonance blue-shift arising solely from VO2’s SMT is even larger than the Δλ = –1.26nm value reported from this experiment. A further analysis of the interplay between these effects can be found in Section 3.2.

Figure 4
Fig. 4 Optical transmission as a function of time at λ = 1568.78nm, in the 1.5μm radius hybrid Si-VO2 device, when turning the 532nm pump laser (a) ON and (b) OFF. (c) Single-frame images from the IR camera movie Media 1, highlighting the delayed probe response observed in (b).
illustrates the time-dependent optical response at the fixed probe wavelength λ = 1568.78nm. When VO2 is in the semiconducting state, this wavelength corresponds to being ‘on-resonance’, and thus the initial optical transmission is very low. Illuminating the device with the 532nm pump laser results in an immediate increase in the optical transmission followed by a ~15s decay toward low transmission and then an increase toward high transmission lasting about 3 min (Fig. 4(a)). This time-dependent optical response can be explained entirely in the context of laser-induced heating. The initial spike to high transmission results from rapid heating of the silicon ring and a thermo-optic dominated shift to longer resonance wavelength; this was confirmed by repeating the experiment at slightly red-shifted probe wavelength (not shown). However, once the SMT threshold temperature is approached, the resonance immediately begins to blue-shift back toward its original position and beyond, effectively sweeping across the resonance and producing a dip and rise in transmission. Here we emphasize that the photothermal approach for triggering the SMT, taken in this experiment (Fig. 2(a)), can be tuned to reduce response time by over three orders of magnitude by increasing the pump intensity [29

29. T. Ben-Messaoud, G. Landry, J. P. Gariepy, B. Ramamoorthy, P. V. Ashrit, and A. Hache, “High contrast optical switching in vanadium dioxide thin films,” Opt. Commun. 281(24), 6024–6027 (2008). [CrossRef]

]. Further, the photothermal technique is very robust, especially when compared to substrate heating methods. Substrate heating/cooling is relatively slow and also leads to unwanted thermal expansion or contraction of the substrate and heating stage, making it very difficult to maintain consistent coupling on- and off-chip. Localized photothermal excitation overcomes these problems and further exhibits an optical configuration in which future nanosecond and even ultrafast all-optical switching measurements could be carried out.

In Figs. 4(b) and 4(c) we examine the optical response, again at λ = 1568.78nm, after turning off (i.e. blocking) the 532nm pump laser. When this occurs, the device immediately begins cooling off, ultimately resulting in a return to the VO2 semiconducting state and an ‘on-resonance’ level of low transmission. However, an immediate drop in transmission is not observed; rather, turning off the laser coincides with a very small (~0.5dB) increase in transmission followed by a ~2s delay before a dramatic, ~2-5dB, drop in transmission. The initial increase, which is repeatably observed during multiple experiments, might possibly be attributed to either a small TO shift from the cooling silicon ring waveguide or to the recombination of photo-generated carriers in the silicon, which would eliminate free-carrier absorption effects. Most importantly however, we attribute the ~2s delay to device cooling at temperatures above the threshold SMT temperature. Once the threshold temperature is reached, the transition between metallic and semiconducting states is triggered. The ~2s delay between the pump shut-off and large probe response is similarly observed using the IR camera (Fig. 4(c), Media 1), and is strong evidence that a complete SMT indeed takes place prior to blocking the pump laser beam. After crossing the metal-to-semiconductor threshold temperature, the device requires several more minutes to cool completely back to room temperature. We note that this cooling time-scale is not fundamental to the phase-transition, but is rather a function of our experimental configuration and the absence of well-designed heat dissipation components in this proof-of-concept experiment.

3.3 Influence of ring-size and VO2 fractional coverage

Reducing the fractional coverage of VO2 on the ring (e.g., by increasing R or decreasing LVO2) diminishes the effect of the SMT on the overall effective index change. This results in a corresponding reduction in resonant response, as observed in Fig. 5(a). This sort of behavior would be present even in the absence of TO or FCI effects. Further, the observed change of sign in the resonant response (from blue-shift to red-shift) for the largest ring radii, is an expected result, occurring when the TO effect in Si dominates the overall effective index change. When adapting this device structure for operation under different conditions (e.g., triggering the SMT by electrical or all-optical excitation) it is important to recognize that the achievable resonance shift can be precisely tuned by varying the fractional VO2 coverage on the resonator and probably by the volume of the VO2 patch. Furthermore, the resonant response could be adjusted by employing alternate waveguide geometries, such as the slot waveguide or pinch waveguide [31

31. J. T. Robinson, L. Chen, and M. Lipson, “On-chip gas detection in silicon optical microcavities,” Opt. Express 16(6), 4296–4301 (2008). [CrossRef] [PubMed]

, 32

32. J. D. Ryckman and S. M. Weiss, “Localized field enhancements in guided and defect modes of a periodic slot waveguide,” IEEE Photon. J 3(6), 986–995 (2011). [CrossRef]

], which could simultaneously maximize ΔNSMT and minimize ΓSi. In considering alternate waveguide geometries, however, it will be important to take account of the trade-off between increased resonant response, Δλ/λ, and the potential for increased losses, or reduced Q-factor.

4. Outlook

From prior estimates on the energy density required to thermally switch thin-film VO2, ~102 J/cm3 [16

16. R. M. Briggs, I. M. Pryce, and H. A. Atwater, “Compact silicon photonic waveguide modulator based on the vanadium dioxide metal-insulator phase transition,” Opt. Express 18(11), 11192–11201 (2010). [CrossRef] [PubMed]

], the minimum energy required to thermally switch our hybrid device is expected to be ~3 × 10−12 J. For faster and lower threshold switching, either electric-field assisted switching or all-optical excitation should be employed. Given the measured threshold laser fluence of ~0.25 mJ/cm2 for nanoscale VO2 pumped at 1550 nm, and an active area ~0.3μm2 of VO2 [33

33. M. Rini, A. Cavalleri, R. W. Schoenlein, R. López, L. C. Feldman, R. F. Haglund Jr, L. A. Boatner, and T. E. Haynes, “Photoinduced phase transition in VO2 nanocrystals: ultrafast control of surface-plasmon resonance,” Opt. Lett. 30(5), 558–560 (2005). [CrossRef] [PubMed]

], the minimum energy required for ultra-fast all-optical switching in the hybrid Si-VO2 ring resonator is predicted to be of order ~190 fJ/bit (~750 fJ × 1/4; accounting for the likelihood of a 0-1 transition in a random signal) [34

34. S. Manipatruni, K. Preston, L. Chen, and M. Lipson, “Ultra-low voltage, ultra-small mode volume silicon microring modulator,” Opt. Express 18(17), 18235–18242 (2010). [CrossRef] [PubMed]

]. Similar switching energies are expected for electrically driven devices, although the SMT would occur on slower timescales, on the order of 10−8 s compared to 10−13 s for all-optical excitation [16

16. R. M. Briggs, I. M. Pryce, and H. A. Atwater, “Compact silicon photonic waveguide modulator based on the vanadium dioxide metal-insulator phase transition,” Opt. Express 18(11), 11192–11201 (2010). [CrossRef] [PubMed]

]. For comparison, forward biased electro-optic ring-resonator modulators with switching times below 10−9 s have been shown to use ~300 fJ/bit after accounting for temperature stabilization [1

1. G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics 4(8), 518–526 (2010). [CrossRef]

, 35

35. C. Batten, A. Joshi, J. Orcutt, A. Khilo, B. Moss, C. Holzwarth, M. Popovic, L. Hanqing, H. Smith, J. Hoyt, F. Kartner, R. Ram, V. Stojanovic, and K. Asanovic, “Building manycore processor-to-DRAM networks with monolithic silicon photonics,” in 16th IEEE Symposium on High Performance Interconnects,2008. HOTI ‘08., 2008), pp. 21–30.

]. Notably, the low mode-volume resonator geometry employed in this work enables >10dB optical modulation to be achieved with approximately 1/10th the active area of VO2 that would be required to achieve similar modulation depths in a single-pass broadband absorption modulator [16

16. R. M. Briggs, I. M. Pryce, and H. A. Atwater, “Compact silicon photonic waveguide modulator based on the vanadium dioxide metal-insulator phase transition,” Opt. Express 18(11), 11192–11201 (2010). [CrossRef] [PubMed]

], thereby promoting reduced power requirements as well as a compact device footprint.

Unlike the athermal and ultrafast ~10−13 s optically driven SMT of VO2, the structural phase transition (SPT) — from the tetragonal, metallic crystal structure back into the monoclinic, semiconducting state — relies on carrier-phonon interactions taking place on longer ~10−9 s timescales. Improving the relaxation time is currently a topic of significant research interest. A number of factors are known or expected to influence this relaxation time, including the active VO2 volume, crystallite domain size, doping, and excitation fluence. For example, measurements of the THz signal show that up to a threshold of about 3 mJ/cm2 in a thin VO2 film pumped by a femtosecond 800 nm pulse, the recovery from the metallic state takes only about 1 ps [24

24. A. Pashkin, C. Kubler, H. Ehrke, R. Lopez, A. Halabica, R. F. Haglund, R. Huber, and A. Leitenstorfer, “Ultrafast insulator-metal phase transition in VO(2) studied by multiterahertz spectroscopy,” Phys. Rev. B 83(19), 195120 (2011). [CrossRef]

]. For the nanoscale VO2 patches needed for the ring resonator, the fluence needed to reach the threshold should be substantially lower, particularly for near band-edge pump wavelengths. It may also be possible to speed up the relaxation time by incorporating an electrical bias or diode configuration to rapidly sweep away excess carriers. Critically important to resolving this issue are measurements of the time-dependent dielectric function as a function of fluence, and measurements of the SMT threshold and SPT relaxation for near band-edge pump wavelengths.

While there are still a number of challenges and opportunities associated with the details of the SMT of VO2, these experiments demonstrate that the hybrid Si-VO2 resonator offers an attractive platform for robust optical modulation and reconfigurable optical routing. Further, this platform may form the basis for future electro-optic or all-optical modulators utilizing a hybrid Si-VO2 geometry.

5. Summary

We have demonstrated photothermally induced optical switching of hybrid Si-VO2 micro-ring resonators. Triggering the SMT in VO2 results in a large reduction in refractive index and a correspondingly large blue-shift in resonant frequency. Optical modulation greater than 10dB from a low mode volume (~1μm3) silicon-based micro-ring device is found to require only a very small (~0.28μm2) active area of VO2. Combined with a large FSR, modest Q-factor, short cavity lifetime, and the potential for ultrafast operation, the hybrid Si-VO2 micro-ring platform presents a robust framework for next-generation optical switching.

Acknowledgments

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M. Hochberg, T. Baehr-Jones, G. X. Wang, M. Shearn, K. Harvard, J. D. Luo, B. Q. Chen, Z. W. Shi, R. Lawson, P. Sullivan, A. K. Y. Jen, L. Dalton, and A. Scherer, “Terahertz all-optical modulation in a silicon-polymer hybrid system,” Nat. Mater. 5(9), 703–709 (2006). [CrossRef] [PubMed]

15.

M. Liu, X. B. Yin, E. Ulin-Avila, B. S. Geng, T. Zentgraf, L. Ju, F. Wang, and X. Zhang, “A graphene-based broadband optical modulator,” Nature 474(7349), 64–67 (2011). [CrossRef] [PubMed]

16.

R. M. Briggs, I. M. Pryce, and H. A. Atwater, “Compact silicon photonic waveguide modulator based on the vanadium dioxide metal-insulator phase transition,” Opt. Express 18(11), 11192–11201 (2010). [CrossRef] [PubMed]

17.

V. Eyert, “The metal-insulator transitions of VO2: a band theoretical approach,” Ann. Phys. (Leipzig) 9, 650–704 (2002).

18.

F. J. Morin, “Oxides which show a metal-to-insulator transition at the Neel temperature,” Phys. Rev. Lett. 3(1), 34–36 (1959). [CrossRef]

19.

J. Cao, E. Ertekin, V. Srinivasan, W. Fan, S. Huang, H. Zheng, J. W. L. Yim, D. R. Khanal, D. F. Ogletree, J. C. Grossman, and J. Wu, “Strain engineering and one-dimensional organization of metal-insulator domains in single-crystal vanadium dioxide beams,” Nat. Nanotechnol. 4(11), 732–737 (2009). [CrossRef] [PubMed]

20.

G. Stefanovich, A. Pergament, and D. Stefanovich, “Electrical switching and Mott transition in VO2,” J. Phys. Condens. Matter 12(41), 8837–8845 (2000). [CrossRef]

21.

D. Ruzmetov, G. Gopalakrishnan, J. D. Deng, V. Narayanamurti, and S. Ramanathan, “Electrical triggering of metal-insulator transition in nanoscale vanadium oxide junctions,” J. Appl. Phys. 106(8), 083702 (2009). [CrossRef]

22.

A. Cavalleri, C. Tóth, C. W. Siders, J. A. Squier, F. Ráksi, P. Forget, and J. C. Kieffer, “Femtosecond structural dynamics in VO2 during an ultrafast solid-solid phase transition,” Phys. Rev. Lett. 87(23), 237401 (2001). [CrossRef] [PubMed]

23.

R. Lopez, R. F. Haglund, L. C. Feldman, L. A. Boatner, and T. E. Haynes, “Optical nonlinearities in VO2 nanoparticles and thin films,” Appl. Phys. Lett. 85(22), 5191–5193 (2004). [CrossRef]

24.

A. Pashkin, C. Kubler, H. Ehrke, R. Lopez, A. Halabica, R. F. Haglund, R. Huber, and A. Leitenstorfer, “Ultrafast insulator-metal phase transition in VO(2) studied by multiterahertz spectroscopy,” Phys. Rev. B 83(19), 195120 (2011). [CrossRef]

25.

Q. F. Xu, D. Fattal, and R. G. Beausoleil, “Silicon microring resonators with 1.5-microm radius,” Opt. Express 16(6), 4309–4315 (2008). [CrossRef] [PubMed]

26.

J. Nag, E. A. Payzant, K. L. More, and R. F. Haglund, “Enhanced performance of room-temperature-grown epitaxial thin films of vanadium dioxide,” Appl. Phys. Lett. 98(25), 251916 (2011). [CrossRef]

27.

J. Y. Suh, R. Lopez, L. C. Feldman, and J. R. F. Haglund, “Semiconductor to metal phase transition in the nucleation and growth of VO[sub 2] nanoparticles and thin films,” J. Appl. Phys. 96(2), 1209–1213 (2004). [CrossRef]

28.

M. S. Nawrocka, T. Liu, X. Wang, and R. R. Panepucci, “Tunable silicon microring resonator with wide free spectral range,” Appl. Phys. Lett. 89(7), 071110 (2006). [CrossRef]

29.

T. Ben-Messaoud, G. Landry, J. P. Gariepy, B. Ramamoorthy, P. V. Ashrit, and A. Hache, “High contrast optical switching in vanadium dioxide thin films,” Opt. Commun. 281(24), 6024–6027 (2008). [CrossRef]

30.

G. Cocorullo, F. G. Della Corte, I. Rendina, and P. M. Sarro, “Thermo-optic effect exploitation in silicon microstructures,” Sens. Actuators A Phys. 71(1-2), 19–26 (1998). [CrossRef]

31.

J. T. Robinson, L. Chen, and M. Lipson, “On-chip gas detection in silicon optical microcavities,” Opt. Express 16(6), 4296–4301 (2008). [CrossRef] [PubMed]

32.

J. D. Ryckman and S. M. Weiss, “Localized field enhancements in guided and defect modes of a periodic slot waveguide,” IEEE Photon. J 3(6), 986–995 (2011). [CrossRef]

33.

M. Rini, A. Cavalleri, R. W. Schoenlein, R. López, L. C. Feldman, R. F. Haglund Jr, L. A. Boatner, and T. E. Haynes, “Photoinduced phase transition in VO2 nanocrystals: ultrafast control of surface-plasmon resonance,” Opt. Lett. 30(5), 558–560 (2005). [CrossRef] [PubMed]

34.

S. Manipatruni, K. Preston, L. Chen, and M. Lipson, “Ultra-low voltage, ultra-small mode volume silicon microring modulator,” Opt. Express 18(17), 18235–18242 (2010). [CrossRef] [PubMed]

35.

C. Batten, A. Joshi, J. Orcutt, A. Khilo, B. Moss, C. Holzwarth, M. Popovic, L. Hanqing, H. Smith, J. Hoyt, F. Kartner, R. Ram, V. Stojanovic, and K. Asanovic, “Building manycore processor-to-DRAM networks with monolithic silicon photonics,” in 16th IEEE Symposium on High Performance Interconnects,2008. HOTI ‘08., 2008), pp. 21–30.

OCIS Codes
(160.6840) Materials : Thermo-optical materials
(230.3120) Optical devices : Integrated optics devices
(230.4110) Optical devices : Modulators
(230.5750) Optical devices : Resonators
(130.4815) Integrated optics : Optical switching devices

ToC Category:
Integrated Optics

History
Original Manuscript: February 27, 2012
Revised Manuscript: May 19, 2012
Manuscript Accepted: May 24, 2012
Published: May 29, 2012

Citation
Judson D. Ryckman, V. Diez-Blanco, Joyeeta Nag, Robert E. Marvel, B. K. Choi, Richard F. Haglund, and Sharon M. Weiss, "Photothermal optical modulation of ultra-compact hybrid Si-VO2 ring resonators," Opt. Express 20, 13215-13225 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-12-13215


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References

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  11. Y. H. Kuo, Y. K. Lee, Y. S. Ge, S. Ren, J. E. Roth, T. I. Kamins, D. A. B. Miller, and J. S. Harris, “Strong quantum-confined Stark effect in germanium quantum-well structures on silicon,” Nature437(7063), 1334–1336 (2005). [CrossRef] [PubMed]
  12. L. Liu, J. Van Campenhout, G. Roelkens, R. A. Soref, D. Van Thourhout, P. Rojo-Romeo, P. Regreny, C. Seassal, J. M. Fédéli, and R. Baets, “Carrier-injection-based electro-optic modulator on silicon-on-insulator with a heterogeneously integrated III-V microdisk cavity,” Opt. Lett.33(21), 2518–2520 (2008). [CrossRef] [PubMed]
  13. H. W. Chen, Y. H. Kuo, and J. E. Bowers, “25Gb/s hybrid silicon switch using a capacitively loaded traveling wave electrode,” Opt. Express18(2), 1070–1075 (2010). [CrossRef] [PubMed]
  14. M. Hochberg, T. Baehr-Jones, G. X. Wang, M. Shearn, K. Harvard, J. D. Luo, B. Q. Chen, Z. W. Shi, R. Lawson, P. Sullivan, A. K. Y. Jen, L. Dalton, and A. Scherer, “Terahertz all-optical modulation in a silicon-polymer hybrid system,” Nat. Mater.5(9), 703–709 (2006). [CrossRef] [PubMed]
  15. M. Liu, X. B. Yin, E. Ulin-Avila, B. S. Geng, T. Zentgraf, L. Ju, F. Wang, and X. Zhang, “A graphene-based broadband optical modulator,” Nature474(7349), 64–67 (2011). [CrossRef] [PubMed]
  16. R. M. Briggs, I. M. Pryce, and H. A. Atwater, “Compact silicon photonic waveguide modulator based on the vanadium dioxide metal-insulator phase transition,” Opt. Express18(11), 11192–11201 (2010). [CrossRef] [PubMed]
  17. V. Eyert, “The metal-insulator transitions of VO2: a band theoretical approach,” Ann. Phys. (Leipzig)9, 650–704 (2002).
  18. F. J. Morin, “Oxides which show a metal-to-insulator transition at the Neel temperature,” Phys. Rev. Lett.3(1), 34–36 (1959). [CrossRef]
  19. J. Cao, E. Ertekin, V. Srinivasan, W. Fan, S. Huang, H. Zheng, J. W. L. Yim, D. R. Khanal, D. F. Ogletree, J. C. Grossman, and J. Wu, “Strain engineering and one-dimensional organization of metal-insulator domains in single-crystal vanadium dioxide beams,” Nat. Nanotechnol.4(11), 732–737 (2009). [CrossRef] [PubMed]
  20. G. Stefanovich, A. Pergament, and D. Stefanovich, “Electrical switching and Mott transition in VO2,” J. Phys. Condens. Matter12(41), 8837–8845 (2000). [CrossRef]
  21. D. Ruzmetov, G. Gopalakrishnan, J. D. Deng, V. Narayanamurti, and S. Ramanathan, “Electrical triggering of metal-insulator transition in nanoscale vanadium oxide junctions,” J. Appl. Phys.106(8), 083702 (2009). [CrossRef]
  22. A. Cavalleri, C. Tóth, C. W. Siders, J. A. Squier, F. Ráksi, P. Forget, and J. C. Kieffer, “Femtosecond structural dynamics in VO2 during an ultrafast solid-solid phase transition,” Phys. Rev. Lett.87(23), 237401 (2001). [CrossRef] [PubMed]
  23. R. Lopez, R. F. Haglund, L. C. Feldman, L. A. Boatner, and T. E. Haynes, “Optical nonlinearities in VO2 nanoparticles and thin films,” Appl. Phys. Lett.85(22), 5191–5193 (2004). [CrossRef]
  24. A. Pashkin, C. Kubler, H. Ehrke, R. Lopez, A. Halabica, R. F. Haglund, R. Huber, and A. Leitenstorfer, “Ultrafast insulator-metal phase transition in VO(2) studied by multiterahertz spectroscopy,” Phys. Rev. B83(19), 195120 (2011). [CrossRef]
  25. Q. F. Xu, D. Fattal, and R. G. Beausoleil, “Silicon microring resonators with 1.5-microm radius,” Opt. Express16(6), 4309–4315 (2008). [CrossRef] [PubMed]
  26. J. Nag, E. A. Payzant, K. L. More, and R. F. Haglund, “Enhanced performance of room-temperature-grown epitaxial thin films of vanadium dioxide,” Appl. Phys. Lett.98(25), 251916 (2011). [CrossRef]
  27. J. Y. Suh, R. Lopez, L. C. Feldman, and J. R. F. Haglund, “Semiconductor to metal phase transition in the nucleation and growth of VO[sub 2] nanoparticles and thin films,” J. Appl. Phys.96(2), 1209–1213 (2004). [CrossRef]
  28. M. S. Nawrocka, T. Liu, X. Wang, and R. R. Panepucci, “Tunable silicon microring resonator with wide free spectral range,” Appl. Phys. Lett.89(7), 071110 (2006). [CrossRef]
  29. T. Ben-Messaoud, G. Landry, J. P. Gariepy, B. Ramamoorthy, P. V. Ashrit, and A. Hache, “High contrast optical switching in vanadium dioxide thin films,” Opt. Commun.281(24), 6024–6027 (2008). [CrossRef]
  30. G. Cocorullo, F. G. Della Corte, I. Rendina, and P. M. Sarro, “Thermo-optic effect exploitation in silicon microstructures,” Sens. Actuators A Phys.71(1-2), 19–26 (1998). [CrossRef]
  31. J. T. Robinson, L. Chen, and M. Lipson, “On-chip gas detection in silicon optical microcavities,” Opt. Express16(6), 4296–4301 (2008). [CrossRef] [PubMed]
  32. J. D. Ryckman and S. M. Weiss, “Localized field enhancements in guided and defect modes of a periodic slot waveguide,” IEEE Photon. J3(6), 986–995 (2011). [CrossRef]
  33. M. Rini, A. Cavalleri, R. W. Schoenlein, R. López, L. C. Feldman, R. F. Haglund, L. A. Boatner, and T. E. Haynes, “Photoinduced phase transition in VO2 nanocrystals: ultrafast control of surface-plasmon resonance,” Opt. Lett.30(5), 558–560 (2005). [CrossRef] [PubMed]
  34. S. Manipatruni, K. Preston, L. Chen, and M. Lipson, “Ultra-low voltage, ultra-small mode volume silicon microring modulator,” Opt. Express18(17), 18235–18242 (2010). [CrossRef] [PubMed]
  35. C. Batten, A. Joshi, J. Orcutt, A. Khilo, B. Moss, C. Holzwarth, M. Popovic, L. Hanqing, H. Smith, J. Hoyt, F. Kartner, R. Ram, V. Stojanovic, and K. Asanovic, “Building manycore processor-to-DRAM networks with monolithic silicon photonics,” in 16th IEEE Symposium on High Performance Interconnects,2008. HOTI ‘08., 2008), pp. 21–30.

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