Sub-wavelength GHz-fast broadband ITO Mach–Zehnder modulator on silicon photonics

Abstract

Here, we demonstrate a spectrally broadband, GHz-fast Mach-Zehnder interferometeric modulator, exhibiting a miniscule VpL of 95 V•µm, deploying a sub-wavelength short electrostatically tunable plasmonic phase-shifter, based on heterogeneously integrated ITO thin films into silicon photonics.Indium tin oxide (ITO), belonging to the class of transparent conductive oxides, is a material extensively adopted in high-tech industry such as in touchscreen displays of smartphones or contacts for solar cells. Recently, ITO has been explored for electro-optic modulation using its free-carrier dispersive effect enabling unity-strong index modulation [1][2][3][4]. However, GHz-fast modulation capability using ITO is yet to be demonstrated -a feature we show herein. Given the ubiquitous usage of phase-shifter technologies, such as in data communication, optical phased arrays, analog and RF photonics, sensing etc.; here we focus on a Mach-Zehnder interferometer (MZI) based modulator to demonstrate a comprehensive platform of heterogeneous integration of ITO-based opto-electronics into silicon photonic integrated circuits (PIC). Since for phaseshifters only the real-part of the optical refractive index (n) is of interest, in previous studies we have shown the interplay between a selected optical mode (e.g. photonic bulk vs. plasmonic) and the material's figure of merit (Dn/Da), where a is the optical loss, directly resultant form Kramers-Kronig relations [5]. Additionally, ITO can be selectively prepared (via process conditions [6]) for operating in either an n-dominant or adominated region [5]. Using this approach, we recently showed a photonic-mode ITO-oxide-Si MZI on silicon photonics characterized by a VpL = 0.52 V•mm [3], and a plasmonic version deploying a lateral gate exhibiting a VpL = 0.063 V•mm [7]. Indeed, a plasmonic mode enables a strong light-matter-interaction (e.g. extrinsic slow-light effect), which, when superimposed with ITO's intrinsic slow-light effect, proximal epsilon-near-zero (ENZ) effects [8], enables realization of just 1-5 µm short phase-shifters [5]. The device-advantage of such micrometer-compact opto-electronics are small capacitances, in the order of ~fF, enabling low power

Heterogeneously integrated ITO plasmonic Mach–Zehnder interferometric modulator on SOI

“…The effective index change with modulation can be expressed by a linear approximation with applied voltage as ∼ 7.98 × 10 −3 V −1 ~ 1%V -1 . The material index change shows meager nonlinearity for negative biasing as we operate in the ITO material’s n -dominant region sufficiently close to ENZ beneficially extracting proximal ENZ effects while staying away from the high-loss ENZ point to minimize IL concerns 37 . Note, the change in both the indices (material and effective) resemble a decrease in the corresponding indices as modulation assimilates to blue-shifts in device resonance, however, is barely resolvable in our single pass MZ configuration (Fig.…”

“…While many of these schemes essentially offer acceptable performance, they are mostly difficult to integrate in the mature Si process. Our choice of ITO as the active modulation material can avail ease of fabrication and potential CMOS integration 37 .…”

Integrated photonic FFT for photonic tensor operations towards efficient and high-speed neural networks

“…We selected a minimum waveguide bending radius of 50 μm to keep the radiative bending losses low, resulting in a delay-line spiral area of 3.9 × 10 −3 mm 2 for T. Sampling is required to obtain the frequency components of the transfer function of the FFT, realized with back-end electrooptic modulators in silicon photonics e.g. Ref [16] or alternatively with emerging modulator-concepts featuring heterogeneous integration of strong-index changes materials such as transparent conductive oxides featuring strong light matter interaction near epsilon near zero (ENZ) operating points [54,55], and micrometer compact MZI ITObased modulators [21][22][23], see also work from the Wong and Brongersma groups (see Figure 4 for nanophotonics- Figure 2: Two options of implementing an optical temporal FFT. Photonic CNN paradigm utilizing Fourier-optics based on integrated photonics.…”

“…channel crosstalk (Figure 3f). Physically this range corresponds to a small (ΔT = 0.54 K) temperature change that the waveguide index must tolerate to keep within the selected attenuation [21][22][23] or electro-absorption modulators [19,20], and also to <10 μm-short 2 × 2 directional couplers [56] allowing for improved N-scaling of OFFT-based convolutions. GPU processing times obtained by fitting runtime.…”

“…In the latter a digitalto-analog converter (DAC) may be needed front-end, and an ADC back-end. Next, the dot-product multiplication to be performed as part of core of the tensor processor can be achieved in a multitude of options such as via electro-optic weighting by controlling the amplitude such as via an electro-absorption modulator [19,20], and phase-shifter using an Mach Zehnder interferometer (MZI) [16,[21][22][23], or via photonic nonvolatile memory such as enabled by phasechange-material weighted states [24], or all-optically [25]. Each of these options can be performed non-resonantly (i.e.…”