Thin film lithium niobate electro-optic modulator with terahertz operating bandwidth

We present a thin film crystal ion sliced (CIS) LiNbO3 phase modulator that demonstrates an unprecedented measured electro-optic (EO) response up to 500 GHz. Shallow rib waveguides are utilized for guiding a single transverse electric (TE) optical mode, and Au coplanar waveguides (CPWs) support the modulating radio frequency (RF) mode. Precise index matching between the co-propagating RF and optical modes is responsible for the device’s broadband response, which is estimated to extend even beyond 500 GHz. Matching the velocities of these co-propagating RF and optical modes is realized by cladding the modulator’s interaction region in a thin UV15 polymer layer, which increases the RF modal index. The fabricated modulator possesses a tightly confined optical mode, which lends itself to a strong interaction between the modulating RF field and the guided optical carrier; resulting in a measured DC half-wave voltage of 3.8 V·cm. 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Introduction
Despite its ubiquity in fiber-optic telecommunications and attractive nonlinear properties, the evolution of LiNbO 3 integrated optics can be considered sluggish relative to its Si and III-V counterparts.Discrete devices fabricated in bulk single crystalline LiNbO 3 generally rely on low index contrast optical waveguides with large bend radii [1], and specialized micromachining processes for sustaining broadband operation [2], which inhibits dense integration.Although the first instance of CIS LiNbO 3 was reported in 1998 [3], the recent widespread availability of full 75 mm wafers of CIS thin film LiNbO 3 from a number of distributors: NanoLN (China), Partow Industries (Florida), and SRICO (Ohio), has provided a fertile environment for LiNbO 3 device research and innovation [4,5].
Notable devices that take advantage of the high index contrast provided by a thin LiNbO 3 substrate are tunable ring resonators [6], Mach-Zehnder interferometers [7], switches [8], and standalone phase modulators [7,[9][10][11].Developed in parallel to these are various hybrid devices, that rely on either Si [12][13][14][15][16][17] or Si 3 N 4 [16,18,19] for loading and guiding of an optical mode.A common theme among all devices mentioned herein is that they possess a reduced mode size.The reduced mode size leads to vastly improved EO activity over their bulk predecessors, most notably resulting in reduced half-wave voltages.Reduced half-wave voltage length products coupled with the ability to bend and fold the high index contrast optical waveguides leads to a substantially decreased device footprint ideal for future integrated photonic systems.
Up to this point however, the other major advantage of thin-film LiNbO 3 , the significantly lower permittivity of the material system, has yet to be convincingly exploited [7,8,11,16].To this end we present the first LiNbO 3 -based EO modulator that is engineered to perform continuously from DC to THz frequencies.It is a device that can be used to optically upconvert RF signals directly at a system's RF front-end sensor, or antenna element.In so doing, the received RF signal becomes a sideband on an optical carrier that can be subsequently processed and, or routed using low loss conventional off-the-shelf optical components.A broad range of applications in the THz regime, including sensing [20], imaging [21,22], and high data rate communications [23], are currently limited by the inherent difficulties in routing THz signals electronically.Given the results presented in this work, we propose that optical routing of THz signals can be enabled by an EO up-converting modulator to provide both a simple and effective frontend alternative.

Device design and fabrication
A schematic of the broadband phase modulator's interaction region can be seen in Fig. 1(c), where the devices begin with commercially available CIS thin film LiNbO 3 on insulator procured from NanoLN.The substrate consists of a 700 nm thick x-cut LiNbO 3 device layer, affixed to a 500 µm thick quartz handle wafer via a 2 µm thick plasma enhanced chemical vapor deposited SiO 2 intermediate bonding layer.A single mode rib waveguide sustains the ypropagating TE polarized light to be modulated.The rib is 1.1 µm wide at the top and 1.8 µm wide at its base; the etch depth is 160 nm resulting in a sidewall angle of 24.57degrees.Lumerical FDTD Mode Solver is used to simulate the waveguide structure and provides an effective optical group index (n opt ) of 2.2608 for the fundamental TE mode at 1550 nm.The discrepancy between bulk LiNbO 3 's optical indices (n extraordinary = 2.14 and n ordinary = 2.21) at a wavelength of 1550 nm and the simulated group index stems from structure dependent waveguide dispersion and LiNbO 3 material dispersion.
To form the optical waveguide an 80 nm thick chromium blanket layer is first sputtered onto the substrate.A soft-mask is patterned on top of the Cr layer with NR9-1500P photoresist from Futurrex.The soft-mask pattern is transferred into the Cr hard-mask with a time multiplexed Cl based inductively coupled plasma (ICP) dry etch.After pattern transfer, any residual resist is removed in an O 2 plasma ash.The waveguide pattern is finally transferred into the LiNbO 3 with a directional, highly anisotropic LiNbO 3 etch, obtained using an ICP CF 4 (6 sccm)/N 2 (28 sccm)/O 2 (0.5 sccm) etch.The etch is time multiplexed to prevent overheating of the sample.The number of cycles determines etch depth and each cycle consists of 1 minute etching in a 600 W plasma under 400 W bias.The etch rate of x-cut LiNbO 3 is ~27 nm per minute and the selectivity between LiNbO 3 and Cr is ~5.4:1.Any remaining Cr is stripped in a chemically selective wet etch.   of frequency.M wn in Fig. 3 Fig. 1 Cross modu of 15 norma across The modu configuration standard gold photolithograp deposited via resulting in 3 done with NR hard-baked at survive the el followed by a layer.Au is th photoresist an deionized wa CPW electrod optical waveg CPW.Electro width, and ga interaction re ~30 Ω up to 5 taper into the possess a sign the launch is 1. (a,b) SEM imag -sectional schema lating RF electric 550 nm are overl alized to its maxim s the GSG electrod ulating electric , which are d d electroplating phy, electropl a electron bea 300 nm total t R9-3000P phot t 120 C for 30 ectroplating ba a brief Ti wet e hen built up to nd metal seed l ater, and a KI des patterned guide situated ode dimension ap, are 1.8 µm gion is 0.92 cm 500 GHz.At th e interaction re nal width of 53 ~50 Ω up to 5 ges of a fabricated atic of the modula field at 110 GHz laid onto the illu mum value, while des.c field is appli efined directly g process and ating, and see am evaporation thickness.Nex toresist from F 0 minutes to d ath.Between li etch in a hydro o the desired el layers are strip based Au etch directly on th in the gap bet ns in the modu m, 9.5 µm, and m and the sim he input and ou egion over ano 3 µm and gap o 500 GHz to ma Fig. 3 500 G Plotted in values for hal following rela