SUPPLEMENTARY INFORMATION

Transcription

1 Supplementary Information High-Speed Plasmonic Phase Modulators A. Melikyan 1, L. Alloatti 1, A. Muslija 2, D. Hillerkuss 3, P. C. Schindler 1, J. Li 1, R. Palmer 1, D. Korn 1, S. Muehlbrandt 1, D. Van Thourhout 4, B. Chen 5, R. Dinu 5, M. Sommer 1, C. Koos 1, M. Kohl 1, W. Freude 1, J. Leuthold 1,3 1: Institutes IPQ & IMT, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany, 2: Karlsruhe Nano Micro Facility (KNMF), a Helmholtz Research Infrastructure at KIT, Germany 3: ETH Zurich, Zurich, Switzerland 4: Photonics Research Group, Ghent University IMEC, Gent, Belgium 5: GigOptix Inc., Switzerland and GigOptix Bothell, Washington, USA Authors addresses: Argishti.Melikyan@kit.edu, Wolfgang.Freude@kit.edu, JuergLeuthold@ethz.ch Contents 1 Experimental details First order perturbation theory Comparison of the PPM with other state-of-the art devices Integration density RF bandwidth Operation across a large optical spectral range Power consumption Comparison with the state-of-the-art References... 9 NATURE PHOTONICS 1

2 1 Experimental details Fabrication: Si nanowire waveguides with a height of 220 nm and a width of 450 nm are fabricated on a silicon on insulator (SOI) wafer with a silicon dioxide (SiO 2) thickness of 2 µm. A 193 nm DUV lithography is used followed by Si dry etching. Silicon tapers with a taper angle of 15 and a tip size of 120 nm are structured in the final etching process, see Figure S1 [1]. The metallic slot waveguides are fabricated by a standard lift-off process with PMMA e-beam resist. A 150 nm thick gold layer is evaporated onto the samples by an electron beam evaporation system. After completing the lift-off process the samples are coated with the commercially available polymer M3 having a maximum electro-optic coefficient of r 33 = 70 pm / V [2]. The electro-optic property of the polymer is then activated by a poling procedure, where a static electric field aligns the randomly oriented dipole moments of the chromophores at an elevated temperature [3]. After rapid cooling to room temperature, the ordering of the dipole moments pertains even after removal of the electric field. Figure S1. Scanning electron microscope image of the fabricated plasmonic phase modulator. The photograph is taken before coating with an electrooptic polymer. Black bars are one micrometre. Optical characterization: Standard diffraction grating couplers and silicon nanowires are used for coupling the light from the tunable laser source (TLS) into the modulator, and for collecting the modulated light from the phase modulator. The optical spectrum at the output of the chip is recorded with an optical spectrum analyser (OSA). The transmission spectrum of a plain silicon nanowire without the phase modulator serves as a reference. 2 NATURE PHOTONICS

3 SUPPLEMENTARY INFORMATION RF characterization: We used an impedance standard calibration substrate to define the RF measurement reference plane to the probe tip. The 1stG modulator is contacted with a ground signal ground (GSG) RF probe and the complex reflection parameter S 11 is measured with an Anritsu 37397C Vector Network Analyzer (VNA). Electro-optical characterization: The optical carrier with frequency f c = ω c / (2π) is phase modulated with a frequency f m and a phase modulation index η U m, resulting in an optical signal cos ω t ηsinω t. The carrier has a relative amplitude in proportion to the 0 th -order Bessel function c m J0 η, and the first sideband has an amplitude in proportion to the 1 st -order Bessel function J η. 2 2 From the ratio of the respective line heights in the power spectrum, J η J η, one therefore can extract the modulation index η The RF frequency response of the device is studied by driving the modulator with a sinusoidal signal having a frequency in the range f m = (1 65) GHz and an amplitude of U m = 0.1 V. We measure the phase modulation index η for each frequency. Due to the capacitive termination, the resulting voltage across the device is doubled when comparing with the voltage across a matched 50 Ohm terminating resistor. This amplitude is kept constant during the RF frequency sweep by calibrating the electrical power absorbed in a matched load, before the RF probe was connected. Data modulation experiments: Infrared light with a wavelength of nm is first amplified to a power level of 15dBm, and then launched into the chip (measured in the fiber before coupling the signal into the chip). The phase of the surface plasmon polariton (SPP) is encoded with a PRBS signal at the voltage swing of U pp measured across a 50 Ω resistor. The resulting binary phase shift keyed (BPSK) signal is amplified in an EDFA which acts as an optical pre-amplifier as part of the receiver constituted by an Agilent N4391A Optical Modulation Analyser (OMA). In case of the 2ndG modulator, the resulting BPSK signal was subsequently converted into an intensity modulated signal by means of a delay interferometer (DI) with a free spectral range (FSR) of 40 GHz. The signal is then directly detected with a single photodiode. To compensate the losses of the in-house built DI, an EDFA was used. The eye diagram and the bit error ratio are measured with an Infiniium DCA-J NATURE PHOTONICS 3

4 Agilent 86100C digital communication analyser (DCA) and an Anritsu MP1776A bit error ratio tester (BERT), respectively. 2 First order perturbation theory The change in the propagation constant (eigenvalue β) due to the perturbation of the refractive index n can be estimated using first order perturbation theory. Here, we derive the change of the complex propagation constant β making use of the field vectors E +, H + of the forward propagating mode and E -, H - of the backward propagating mode resulting from the adjoint form of the eigenvalue problem [4]. Defining k 0 = 2π / λ c as the wave vector of light in vacuum at a carrier wavelength λ c, and Z 0 as the vacuum field impedance, the accumulated phase shift can be expressed as 2n nk0l δφ = β L = Z 0 Active Area + EEdd x y E H E H + + x x, dd x y where E = E x E y+ E z, H = H x+ H y H z. z x y z x y z (1) Here, the vectors x, y, z represent the basis of the right-handed Cartesian coordinate system, n is the refractive index of the nonlinear polymer, n is the refractive index change of the polymer due to the Pockels effect, 1 3 U n= nr33 (2) 2 w gap Because of the interrelation of the fields E +, H + and E -, H -, the only unknown quantities in Eq. (1) are either of the E +, H + and E -, H - field pairs. The fields E +, H + are computed with a finite element method. The integral in the numerator of Eq. (1) is calculated only in the slot region, to which the modulating electric field is mostly confined. Simulation: The eigenmodes of the metallic slot waveguides are simulated with a finite element method (FEM) [5]. The resulting field distributions for E and H according to Eq. (1) are further processed with Matlab to estimate the change of the propagation constant β. 4 NATURE PHOTONICS

5 SUPPLEMENTARY INFORMATION Geometrical parameters of the fabricated modulators are used as inputs for the FEM simulations. Metallic slots with a width of 200 nm and 140 nm are defined for simulating the gap plasmon in the 1stG and 2ndG modulators, respectively. In both cases, the thickness of the gold electrodes is 150 nm. The following material properties apply: n Si = 3.48, n SiO2 = 1.44, n NL polymer = The optical properties of gold are extracted from a Drude model [6] with plasma frequency ω p = s -1 and collision frequency γ = s -1. NATURE PHOTONICS 5

6 3 Comparison of the PPM with other state-of-the art devices 3.1 Integration density Plasmonics offers a unique opportunity to bring down the size of optical components. This is because the large permittivity contrast at the metal/dielectric interface allows the SPPs to be confined to the sub-wavelength areas. Using gap SPPs, for example, it is possible to design passive and active components with ultra-compact dimensions in both transvers and longitudinal directions. As we show in our paper, a plasmonic modulator with a footprint of 29 µm 2 is feasible by making use of the gap SPPs. Thus, plasmonics can be considered as an alternative path towards a new generation of more densely integrated photonic circuits with dimensions that would not be possible by any other conventional photonic technology. 3.2 RF bandwidth Several speed limiting factors are common in silicon optical modulators: RC time constant due to the finite conductivity of the silicon contacts and due to a large device capacitance Walk-off between electrical and optical signals Photon lifetime in resonator based devices Carrier recombination lifetimes in forward biased injection-type of modulators The above factors limit the RF bandwidth of the silicon modulators. Still, so far RF bandwidths of up to 30 GHz have been reported [7]. None of the above speed limiting factors plays a role in our proposed plasmonic modulator. We show that by combining an instantaneous Pockels effect with a metallic slot structure, an RF bandwidth of 65 GHz and beyond is doable 6 NATURE PHOTONICS

7 SUPPLEMENTARY INFORMATION 3.3 Operation across a large optical spectral range Our plasmonic device with a gap SPP in a metal-insulator-metal waveguide allows operation of the device across a large optical spectral range of 120 nm and so far is only limited by our ability to test the device at high speed beyond this wavelength range. This is different to other high-speed integrated device technologies, where normally a compact size is only obtained by making the device resonant such as a photonic crystals or a ring resonator, where operation typically is limited to e.g. a 12 nm window[8] or a small 1 nm spectral range [9][10]. 3.4 Power consumption Recent nonlinear organic materials offer electro-optic coefficients of 180 pm / V [11]. The usage of such organic materials in our device will allow us to decrease the driving voltage U significantly. Further reduction in the driving voltage can be achieved by decreasing the metallic slot width down to 70 nm, for instance. We therefore anticipate a decrease of U by at least a factor of 20 in the near future. For a device with a length of 21 µm and a slot width of 70 nm which is covered with recently developed electro-optic materials we predict a peak-to-peak phase modulation of π / 2 with a driving voltage swing of U = 3 V. With such a driving voltage swing and a device capacitance of 4 ff (based on the capacitance measured for the 1stG device) an average energy per bit of 9 fj bit -1 can be anticipated. A π / 2 phase shift would also be needed to operate a MZM for generating advanced modulation formats. For an MZM the energy requirements then would be in the order of 18 fj bit -1. The optical loss will decrease while making the device shorter by using smaller slots and better electro-optic polymers. The optical losses can be further reduced to 3 5dB by replacing the gold with silver, which provides much lower optical losses for the SPP. NATURE PHOTONICS 7

8 3.5 Comparison with the state-of-the-art In this subsection, we compare the performance of our plasmonic modulator with state-of-the-art silicon modulators, all of which have recently demonstrated operation at 40 Gbit s -1 and above. From the Table it can be seen that the plasmonic phase modulator (PPM) is not only offering the largest RF bandwidth, but also the smallest footprint with a very low electrical power consumption. The PPM only falls short in the optical losses. However, as commented above, there is room for improvement for our plasmonic technology and the total loss is not too far off from what has been reported for other devices. Photonic crystal [8] Ring resonator [9] Ring resonator [10] Si-WG MZM [7] Our PPM Phase modulation possible impossible impossible possible possible V π L [V mm] N. A. 20 Vmm 1.3 Vmm f 3dB [GHz] N. A. 1GHz N. A GHz > 65 GHz Bit-rate achieved 40 Gbit/s 50 Gbit/s 40 Gbit/s 60 Gbit/s 40 Gbit/s Pre-emphasis sig. without only with without without without Operating range λ [nm] 12nm < 1nm < 1nm N. A. 120 nm Footprint [µm 2 ] ~540 µm 2 ~150 µm 2 ~150 µm 2 > 10 4 µm 2 ~ 29 µm 2 Energy consumption N. A. N. A. 32 fj / bit ~ 3.5 pj / bit 60 fj / bit at 40Gbits -1 + FCS Insertion Loss [db] 6.2dB 5.2dB 10.5 db 6.5 db 12dB Energy consumption of the feedback control system that is needed to stabilize the device temperature 8 NATURE PHOTONICS

9 SUPPLEMENTARY INFORMATION 4 References [1] D. F. P. Pile and D. K. Gramotnev, Adiabatic and nonadiabatic nanofocusing of plasmons by tapered gap plasmon waveguides, Appl. Phys. Lett., vol. 89, no. 4, pp , [2] GigOptix, retrieved [3] L. Alloatti, D. Korn, R. Palmer, D. Hillerkuss, J. Li, A. Barklund, R. Dinu, M. Fournier, J. Fedeli, H. Yu, W. Bogaerts, P. Dumon, and R. Baets, 42.7 Gbit s -1 electro-optic modulator in silicon technology, Opt. Express, vol. 19, no. 12, pp , [4] P. Y. Chen, R. C. McPhedran, C. M. de Sterke, C. G. Poulton, A. A. Asatryan, L. C. Botten, and M. J. Steel, Group velocity in lossy periodic structured media, Phys. Rev. A, vol. 82, no. 5, pp , [5] Comsol Multiphysics, retrieved [6] P. Johnson and R. Christy, Optical constants of the noble metals, Phys. Rev. B, vol. 6, no. 12, pp , [7] X. Xiao, H. Xu, X. Li, Z. Li, T. Chu, Y. Yu, and J. Yu, High-speed, low-loss silicon Mach Zehnder modulators with doping optimization, Opt. Express, vol. 21, no. 4, pp , Feb [8] H. C. Nguyen, S. Hashimoto, M. Shinkawa, and T. Baba, Compact and fast photonic crystal silicon optical modulators., Opt. Express, vol. 20, no. 20, pp , [9] T. Baba, S. Akiyama, M. Imai, N. Hirayama, H. Takahashi, Y. Noguchi, T. Horikawa, and T. Usuki, 50-Gb / s ring-resonator-based silicon modulator, Opt. Epxress, vol. 21, no. 10, pp , [10] D. J. Thomson, F. Y. Gardes, D. C. Cox, J.-M. Fedeli, G. Z. Mashanovich, and G. T. Reed, Self-aligned silicon ring resonator optical modulator with focused ion beam error correction, J. Opt. Soc. Am. B, vol. 30, no. 2, pp. 445, [11] Palmer, R. et al. High-speed silicon-organic hybrid (SOH) modulator with 1.6 fj bit -1 and 180 pm V -1 in-device nonlinearity, in ECOC London 2013, paper We3B3. NATURE PHOTONICS 9