1. Introduction

Advances in electric field sensors are important for a host of applications including electromagnetic compatibility (EMC) measurements, high-frequency electronic circuit diagnostics, medical equipment field monitoring, radio-frequency reception, and high power microwave detection [1–5]. In the case of electric field sensors based on the linear electro-optic (Pockels) effect, a high-frequency (DC to THz) electric field modifies the indices of refraction of an electro-optic medium, resulting in high-speed modulation of an optical carrier signal. Electric field sensors based on optical technology are advantageous, compared to electronic technology, because they can be metal free, compact, and broadband [6]. The use of metal free sensors minimizes invasiveness. Furthermore, optical technology is amenable to realizing high spatial resolution sensor arrays and signal routing can utilize fiber optics.

Submicrometer thin films of lithium niobate (LiNbO₃) are attractive for miniaturized electric field sensors because of the high optical confinement, large electro-optic coefficients, fast response time, and transparency at telecommunications wavelengths [7,8]. Structuring of the thin film into planar waveguide devices yields compact and high density integrated optics. The bend radius, however, is limited by the refractive index contrast between core and cladding. By comparison, the larger refractive index contrast in the silicon-on-insulator (SOI) material system enables waveguide bends with micrometer scale radii of curvature [9]. Unstrained crystalline silicon is, however, centrosymmetric. Therefore, it does not exhibit a linear electro-optic effect. A hybrid material system consisting of both SOI and LiNbO₃ enables compact integrated optics with functionality provided by second order susceptibility. Recent efforts in this direction include demonstrations of direct bonding of LiNbO₃ to SOI to realize compact high speed optical modulators and fast tuning of filters [10]. Generally, direct bonding of two materials involves the concatenation of two smooth and clean material interfaces without the use of an intermediate layer. Direct bonding typically requires very flat surfaces, demanding process technology, and specialized equipment. An alternative to direct bonding is indirect bonding. Indirect bonding of two materials involves the use of an intermediate layer such as polymer, spin-on-glass, or metal [11]. It is a robust and low temperature technique that is relatively insensitive to surface topography [12].

In this paper, compact and metal-free electric field sensors based on the indirect bonding of submicrometer thin films of LiNbO₃ to silicon microring resonators are presented. An electric field sensor is designed and fabricated by utilizing a 600 nm thick z-cut LiNbO₃ ion-sliced thin film as the top cladding of a 20 μm radius silicon microring resonator. The optical quasi transverse magnetic (quasi-TM) mode is used to access the r₃₃ electro-optic coefficient in the LiNbO₃ (r₃₃ = 31 pm V⁻¹, r₃₁ = 8 pm V⁻¹ in bulk LiNbO₃) [13,14]. The intermediate bonding layer is benzocyclobutene (BCB). Operation of the device as an electric field sensor is demonstrated by detecting the fringing fields from a microwave frequency microstrip circuit operating at 1.86 GHz.

The paper is organized as follows. Section two describes the design of the electric field sensor. Section three conveys the fabrication details of the hybrid material system using indirect bonding. Section four describes the experimental setup and characterization results of the electric field sensor detecting the fringing fields from a microwave frequency microstrip circuit. Finally, concluding remarks are given in section five.

2. Design

A schematic of the hybrid Si/LiNbO₃ electric field sensor is shown in Fig. 1a . The sensor consists of a bus coupled SOI strip waveguide ring resonator and a thin film of LiNbO₃ which serves as a portion of the top cladding. The silicon waveguide core width is 450 nm and the height is 250 nm. The LiNbO₃ thin film is 600 nm thick and is bonded to the silicon resonator via BCB. The whole sensor is covered by one micrometer thick plasma enhanced chemical vapor deposition (PECVD) SiO₂. Figure 1b shows a scanning electron micrograph (SEM) of the cross-section of the sensor structure. A ring radius of 20 μm is chosen to avoid large bending losses.

 

Fig. 1 (a) Schematic of an electric field sensor based on the indirect bonding of a lithium niobate thin film to a silicon microring resonator. For clarity, a PECVD SiO₂ top-cladding layer is not shown. (b) SEM of the cross-section of the sensor structure.

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An optical carrier signal propagating as a guided wave in the SOI bus waveguide is evanescently coupled into the ring resonator via a 375 nm wide coupling gap. A portion of the guided-mode is within the LiNbO₃ thin film. For optical wavelengths near the resonance wavelengths of the ring resonator, the optical transmission is sensitive to modulations in the effective index of the guided-mode in the ring resonator. The use of a high-Q resonator is desirable because the sensitivity depends on the slope of the optical transmission versus wavelength. When the sensor is immersed within a high-frequency electric field, the electric field can be detected because it modifies the refractive indices in the LiNbO₃ thin film via the electro-optic effect. Consequently, the effective index of the guided-mode in the ring resonator is modulated, resulting in an intensity modulation of the optical carrier.

The optical polarization of the guided mode is chosen to maximize the fraction of the optical mode that overlaps with the LiNbO₃. Maximum overlap optimizes the change in the effective index of the optical mode that occurs for a change in the refractive indices of the LiNbO₃. Figure 2 shows the cross-section electric field distributions, for the quasi-TE and quasi-TM modes at 1550 nm optical wavelength, calculated using the beam propagation method (BPM). The effective index calculates to 2.33 for TM mode and 2.58 for TE mode. Since the TM mode is less confined, a larger fraction of the optical mode is in the LiNbO₃ for the TM mode than the TE mode. In addition, the TM mode accesses the r₃₃ electro-optic coefficient of the LiNbO₃, whereas the TE mode accesses the r₁₃ electro-optic coefficient. Therefore, the optical TM mode is chosen in the design. Compared to fabricated devices, the simulations shown in Fig. 2 neglect several tens of nanometers of BCB that resides on the top surface of the silicon core. The presence of BCB between the top of the silicon core and the bottom of the LiNbO₃ thin film results in a reduction of the TM mode electric field inside the LiNbO₃ due to the electromagnetic boundary conditions. Therefore, in order to maximize the fraction of the optical mode in the LiNbO₃, it is important to minimize the BCB thin film thickness that is directly on top of the silicon core.

 

Fig. 2 Optical electric field distributions in the hybrid Si/LiNbO₃ sensor for the quasi-TM (E_y component) and quasi-TE (E_x component) modes at 1550 nm wavelength. Material boundaries are indicated by the white dashed lines and the material regions are indicated in the inset.

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3. Fabrication

The fabrication process is shown in Fig. 3 . The process begins with a SOI wafer. The thickness of the silicon device layer is 250 nm, the thickness of the buried oxide (BOX) is 1 μm, and the thickness of the silicon substrate is thinned to 210 μm. Silicon strip waveguides forming a silicon microring and a bus waveguide with inverse width tapers are defined in HSQ resist using electron-beam lithography and inductively coupled plasma reactive ion etching (ICP-RIE) using Cl₂/O₂ chemistry. The cross-sectional width and height of the silicon strip waveguides are 450 nm and 250 nm, respectively. The radius of the ring resonator is 20 μm and the coupling gap is 375 nm.

 

Fig. 3 Fabrication process of electric field sensor: (a) Silicon strip waveguide ring resonator patterned on SOI wafer using electron beam lithography and plasma etch, (b) spin-coat of BCB, (c) indirect bonding of LiNbO₃ thin film, (d) plasma etch of BCB, (e) deposition of PECVD SiO₂, (f) fabrication of cantilever couplers.

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Thin films of LiNbO₃ are obtained from a bulk single crystal z-cut LiNbO₃ wafer using helium ion implantation and thermal treatment [10]. The implantation energy is 195 keV and the fluence is 4×10¹⁶ ions cm⁻². After implantation, the wafer is annealed using rapid thermal annealing (RTA) and then etched in hydrofluoric acid (HF) solution [15]. Non-uniform stress produces 700 nm thick LiNbO₃ thin films in the areal shape of strips and triangles whose edges are formed along crystal planes. The edge length ranges from several tens of micrometers to several hundred micrometers. The surface roughness of the implanted side of the LiNbO₃ thin film is reduced to 4 nm by ICP etching with Ar chemistry resulting in a final film thickness of 600 nm.

Next, BCB is spin-coated from solution onto the silicon strip waveguides. LiNbO₃ thin films are transferred to the top of the SOI ring resonators using a fiber tip on a probe station. After transfer, the sample is annealed to 300 °C to cure the BCB and to partially recover the r₃₃ coefficient [7], resulting in a BCB layer approximately 20 nm thick between the LiNbO₃ and the silicon microring. The LiNbO₃ thin film does not crack after annealing despite the large mismatch in thermal expansion coefficients between silicon and LiNbO₃. BCB that is not covered by the LiNbO₃ thin film is etched using CF₄/O₂ ICP-RIE. A 1 μm thick SiO₂ top cladding is deposited by PECVD. Finally, cantilever couplers are fabricated for fiber coupling to the bus waveguide [16]. The bus waveguide, including the cantilever couplers, is 640 μm long.

4. Measurements

The electric field sensor is demonstrated in the laboratory by detecting fringing electric fields from a radio-frequency (RF) microstrip circuit. At a constant RF power, the RF electric field varies spatially above the circuit. By positioning the sensor at various points above the circuit, a field map of the electric field distribution can be obtained. The microstrip circuit is an RF coupled line resonator operating on resonance at 1.86 GHz with an RF return loss of 7.4 dB. The RF frequency of the circuit is well within the RF bandwidth of the sensor which is limited by the photon lifetime of the ring resonator.

The experimental setup is shown in Fig. 4 along with a top-view optical micrograph of the fabricated electric field sensor. In the demonstration, the chip is placed on the surface of the RF microstrip circuit in a region where the fringing electric field is a maximum. Port 1 of a calibrated microwave vector network analyzer (VNA) drives the RF microstrip circuit with 1 mW of RF power. An infrared continuous-wave laser source is connected to a polarization controller which outputs linearly polarized quasi-TM light with cross-polarization rejection ratio of more than 17 dB. Tapered optical fibers with tip diameter approximately equal to 2 μm are butt-coupled to the input and output cantilever couplers of the electric field sensor.

 

Fig. 4 (a) Measurement setup; (b) Top-view optical micrograph of fabricated electric field sensor.

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Fringing electric fields from the microstrip circuit modify the effective index of the optical mode in the electric field sensor, producing an intensity modulation on the optical beam. The modulated lightwave is out-coupled via optical fiber which is terminated in a high-speed photodiode with responsivity equal to 0.9 A/W and transimpedance gain equal to 1000 V/A, resulting in a photoreceiver conversion gain of 900 V/W. The demodulated RF signal is fed into port 2 of the VNA. The RF power entering port 2 is proportional to the square of the magnitude of the vertical component of the RF electric field in the LiNbO₃ portion of the sensor, weighted by the optical mode intensity distribution. Furthermore, the RF power entering port 2 is proportional to the square of the optical power and the square of the slope of the optical transmission versus wavelength. The microwave VNA is set to measure the port 1 to port 2 S₂₁ scattering parameter. The square of the magnitude of S₂₁, |S₂₁|², is the RF power delivered to port 2 normalized by the RF power available from port 1. The VNA operates with an intermediate frequency (IF) bandwidth of 10 Hz and 20 averages.

The measured optical transmission of the electric field sensor is shown in Fig. 5a for 1.3 dBm of input optical power near an optical resonance at 1526.274 nm. The full width half maximum (FWHM) is 118 pm, the free spectral range (FSR) is 5.05 nm, and the extinction ratio is 13.7 dB. The total optical insertion loss is 5.3 dB. The insertion loss is from 4 dB of fiber-to-waveguide coupling loss and 1.3 dB of waveguide transmission loss. The loaded quality factor is 13,000 and the finesse is 43. The measured optical resonance has a slight asymmetric red shift due to the relatively large thermo-optic nonlinearity in silicon [17]. Figures 5b and 5c show the corresponding values of the microwave VNA S₂₁ scattering parameter versus CW laser wavelength. The magnitude of S₂₁, denoted |S₂₁|, peaks at approximately –104 dB on the red and blue sides of the optical resonance. Since the thermal nonlinearity is a slow effect compared to the electro-optic effect, the peak RF S₂₁ magnitude remains equal on both sides of the optical resonance. At the resonance wavelength, the |S₂₁| is more than 10 dB less than the peak values. The |S₂₁| data is consistent with the steep slopes of the optical transmission near the optical resonance. The phase of S₂₁ exhibits a phase change of approximately 180 degrees as the optical wavelength is swept through the resonance, consistent with the sign change in the slope of the optical transmission.

 

Fig. 5 (a) Measured optical transmission of the electric field sensor; (b) Magnitude of measured microwave VNA S₂₁ RF scattering parameter versus CW laser wavelength; (c) Corresponding phase of RF S₂₁ versus CW laser wavelength.

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The VNA noise, shown as the dashed red curve in Fig. 5b, is obtained by terminating port 1 of the VNA with a matched 50 Ω load and also connecting the laser directly into the photodiode. As the laser wavelength is swept in time, the VNA noise is recorded. A noise floor of −115 dB is calculated by fitting a Gaussian distribution to the linear magnitude of the noise data. The noise floor is defined as the sum of the mean value of the linear magnitude of the noise and three times its standard deviation.

As shown in Fig. 5b, there is an 11 dB difference between the peak |S₂₁| and the noise floor. Numerical modeling of the electric field sensor together with the RF microwave circuit is conducted, using the finite element method, to estimate the RF electric field sensitivity. Since the microwave circuit and the optical thin films composing the sensor differ in spatial dimensions by 4 to 5 orders of magnitude, the GHz varying electromagnetic field in the optical thin films are considered to be quasi-static. Therefore, an approximate two-step modeling procedure is utilized. First, a three-dimensional time-harmonic RF simulation is conducted involving the RF microstrip circuit and the silicon substrate without the optical thin films (ie. without the BOX, BCB, patterned silicon waveguide core, LiNbO₃, and PECVD oxide). The RF resonance frequency from the simulation is 1.86 GHz and the RF return loss is 6.8 dB, in agreement with the measurements of the RF circuit. The maximum vertical component of the electric field over the top surface of the silicon substrate is then used as an approximate input source field in a static two-dimensional simulation involving only the optical thin films. The small (<3%) effect of the optical thin films on the value of the input source field is neglected. From the simulation, the vertical component of the electric field in the LiNbO₃ portion of the sensor, weighted by the optical mode intensity distribution, is found to be 50.8 V/m for an RF input power of 1 mW. From Fig. 5b, reduction of the RF input power by 11 dB produces a signal-to-noise ratio of one. The corresponding RF electric field scales from 50.8 V/m to 14.3 V/m. Based on our system bandwidth of 10 Hz, the demonstrated sensitivity to electric fields is 4.5 V m^-1 Hz^-1/2. The sensitivity depends on several factors specific to our experiments. Most notably, the RF signal power entering port 2 of the VNA scales quadratically with input optical power, slope of the resonator optical transmission versus wavelength, photoreceiver conversion gain, and r₃₃ in the electro-optic medium.

5. Conclusion

A compact and metal-free hybrid silicon and lithium niobate microring electric field sensor is designed, fabricated, and characterized. The hybrid device combines an ion-sliced lithium niobate thin film with a silicon-on-insulator strip waveguide ring resonator using BCB as an intermediate bonding layer. The sensor has a ring radius of 20 μm and measured loaded quality factor of 13,000 at infrared wavelengths for the quasi-TM mode. The sensor is demonstrated by detecting the fringing electric fields from a microwave circuit operating at 1.86 GHz using a test and measurement setup that incorporates a VNA. The dependence of the magnitude and phase of VNA S₂₁ scattering parameter on the optical wavelength is consistent with electro-optic modulation from the lithium niobate portion of the electric field sensor. Future work involves optimizing the radius of the ring resonator to reduce the footprint of the sensor. Reduction in footprint enables dense integration of multiple electric field sensors for high spatial resolution field mapping of free-space electromagnetic fields.