Achieving Reversible and Non-Volatile Optical Phase Shifts in Silicon Nitride Waveguides

Introduction

Silicon nitride (SiN) is a promising material for integrated photonic circuits due to its transparency across the visible and infrared spectrum and its compatibility with CMOS fabrication processes. A key challenge in developing large-scale photonic integrated circuits (PICs) for applications like neural network computing and quantum computing is reducing power consumption from the microheaters used to control the phase in interferometers. Non-volatile interferometers that can maintain their phase state without continuous power could provide a solution.

In this work, researchers from the National Institute of Advanced Industrial Science and Technology (AIST) in Japan demonstrated a reversible and non-volatile optical phase shift in SiN waveguides by alternating thermal annealing and ultraviolet (UV) irradiation processes. Their approach takes advantage of the reversible charge state transitions of K-center dangling bonds, which are major silicon dangling bonds present in SiN films deposited by chemical vapor deposition (CVD).

Device Structure

The researchers fabricated asymmetric Mach-Zehnder interferometers (AMZIs) with SiN waveguide cores to study the phase shift effect. The AMZIs had an arm length difference of 50 μm and three variations were made - "one arm UV shaded", "both arms unshaded", and "both arms shaded" - to evaluate the effect of UV irradiation.

Each AMZI consisted of two 1x2 multimode interference (MMI) couplers and two waveguide arms with a 1000 nm wide, 500 nm thick SiN core on a silicon substrate with 2 μm thermal oxide under-cladding and 2.3 μm SiO2 over-cladding, as shown in Figures 1 and 1(f).

The "shaded" arms had 50 μm wide, 400 nm thick aluminum films deposited on the over-cladding to block UV exposure to those waveguide sections during the UV irradiation process.

Fig. 1 Schematic illustration of AMZIs with (a) one arm shaded, (b) both arms unshaded and (c) both arms shaded. Schematic cross-section of phase shifter (d) without and (e) with UV shading structure. (f) Cross-sectional STEM image of SiN waveguide.

Experimental Process

The researchers performed an alternating process of thermal annealing at 250°C for 30 minutes and UV irradiation for 30 minutes on the AMZI devices. Both steps were carried out in an argon atmosphere with less than 0.1% oxygen.

The transmission spectra of the AMZIs were measured after each annealing and UV irradiation step using an amplified spontaneous emission light source coupled to the SiN waveguide. The polarization was set to the TE mode.

Results

The transmission spectra in Figures 2(a)-2(c) show that only the "one arm UV shaded" AMZI exhibited a reversible wavelength shift in response to the alternating annealing and UV irradiation processes. The "both arms unshaded" and "both arms shaded" AMZIs showed no significant wavelength shifts.

Fig. 2 Transmission spectra of AMZI with (a) one arm shaded, (b) both arms unshaded and (c) both arms shaded. “A0” represents just after the pretreatment, and “U1–U3” and “A1–A3” represent a process sequence of UV irradiation and annealing in the experiments.

The history of the wavelength shift for the interference dip in the "one arm UV shaded" AMZI is plotted in Figure 3. A clear reversible shift with an amplitude of 3.16 nm was observed.

Fig. 3 History of wavelength of interferometer dip in Fig. 2(a).

From the 3.16 nm wavelength shift and the 23.6 nm free spectral range of the AMZI, the researchers calculated an optical phase shift of 0.268π rad or about 15% of the full 2π range.

By measuring AMZIs with different phase shifter lengths of 200 μm and 1000 μm, they confirmed the phase shift scaled linearly with the length of the UV-irradiated waveguide section. The effective refractive index change Δneff of the waveguide was estimated to be 4.50 x 10^-4.

Taking into account the 68.6% optical confinement within the SiN core, the refractive index change of the SiN film itself was calculated to be 6.57 x 10^-4. This is about half the refractive index change measured in the researchers' previous work on SiN films, likely due to improved deposition conditions affecting the density and photochemical response of dangling bonds.

Significance

This work demonstrates the first reversible and non-volatile optical phase shifter in a SiN waveguide that could enable low-power, large-scale photonic integrated circuits. By using an alternating thermal annealing and UV irradiation process, the charge state of K-center dangling bonds in the SiN film can be reversibly switched, inducing a refractive index change.

The researchers achieved a phase shift up to 0.268π or 15% of the full 2π range in a 400 μm long phase shifter section. Further optimization of the SiN deposition conditions could potentially increase the maximum phase shift.

This non-volatile phase shifting capability could significantly reduce power consumption compared to current thermal microheater approaches. It opens up new possibilities for energy-efficient, large-scale interferometer arrays for applications like optical neural networks and quantum photonic circuits.

Reference

[1] Y. Maegami, G. Cong, R. Kou, N. Yamamoto, T. Narushima, T. Tsuchizawa, H. Kawashima, and K. Yamada, "Reversible and Non-Volatile Optical Phase Shift in Silicon Nitride Waveguide," National Institute of Advanced Industrial Science and Technology (AIST), Onogawa 16-1, Tsukuba, Ibaraki 305-8569, Japan, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.