In recent years, silicon nitride has drawn attention for the realisation of integrated photonic devices due to its fabrication flexibility and advantageous intrinsic properties that can be tailored to fulfill the requirements of different linear and non-linear photonic applications. This paper focuses on our progress in the demonstration of enhanced functionalities in the near infrared wavelength regime with our low temperature (<350°) SiN platform. It discusses (de)multiplexing devices, nonlinear all optical conversion, photonic crystal structures, the integration with novel phase change materials, and introduces applications in the 2μm wavelength range.

In this review we present some of the recent advances in the field of silicon nitride photonic integrated circuits. The review focuses on the material deposition techniques currently available, illustrating the capabilities of each technique. The review then expands on the functionalisation of the platform to achieve nonlinear processing, optical modulation, nonvolatile optical memories and integration with III-V materials to obtain lasing or gain capabilities.

We propose a reconfigurable and non-volatile Bragg grating in the telecommunication C-band based on the combination of novel low-loss phase-change materials (specifically Ge₂Sb₂Se₄Te₁ and Sb₂S₃) with a silicon nitride platform. The Bragg grating is formed by arrayed cells of phase-change material, whose crystallisation fraction modifies the Bragg wavelength and extinction ratio. These devices could be used in integrated photonic circuits for optical communications applications in smart filters and Bragg mirrors and could also find use in tuneable ring resonators, Mach–Zehnder interferometers or frequency selectors for future laser on chip applications. In the case of Ge₂Sb₂Se₄Te₁, crystallisation produces a Bragg resonance shift up to ∼ 15 nm, accompanied with a large amplitude modulation (insertion loss of 22 dB). Using Sb₂S₃, low losses are presented in both states of the phase change material, obtaining a ∼ 7 nm red-shift in the Bragg wavelength. The gratings are evaluated for two period numbers, 100 and 200 periods. The number of periods determines the bandwidth and extinction ratio of the filters. Increasing the number of periods increases the extinction ratio and reflected power, also narrowing the bandwidth. This results in a trade-off between device size and performance. Finally, we combine both phase-change materials in a single Bragg grating to provide both frequency and amplitude modulation. A defect is introduced in the Sb₂S₃ Bragg grating, producing a high quality factor resonance (Q ∼ 10⁴) which can be shifted by 7 nm via crystallisation. A GSST cell is then placed in the defect which can modulate the transmission amplitude from low loss to below -16 dB.

Photonic integrated circuits currently use platform intrinsic thermo-optic and electro-optic effects to implement dynamic functions such as switching, modulation and other processing. Currently, there is a drive to implement field programmable photonic circuits, a need which is only magnified by new neuromorphic and quantum computing applications. The most promising non-volatile photonic components employ phase change materials such as GST and GSST, which had their origin in electronic memory. However, in the optical domain, these compounds introduce significant losses potentially preventing a large number of applications. Here, we evaluate the use of two newly introduced low loss phase change materials, Sb₂S₃ and Sb₂Se₃, on a silicon nitride photonic platform for future implementation in neuromorphic computing. We focus the study on Mach–Zehnder interferometers that operate at the O and C bands to demonstrate the performance of the system. Our measurements show an insertion loss below 0.04 dB μm⁻¹ for Sb₂S₃ and lower than 0.09 dB μm⁻¹ for Sb₂Se₃ cladded devices for both amorphous and crystalline phases. The effective refractive index contrast for Sb₂S₃ on SiNx was measured to be 0.05 at 1310 nm and 0.02 at 1550 nm, whereas for Sb₂Se₃, it was 0.03 at 1310 nm and 0.05 at 1550 nm highlighting the performance of the integrated device.

We present the design and experimentally demonstrate a dual-level grating coupler with subdecibel efficiency for a 220 nm thick silicon photonics waveguide which was fabricated starting from a 340 nm silicon-on-insulator wafer. The proposed device consists of two grating levels designed with two different linear apodizations, with opposite chirping signs, and whose period is varied for each scattering unit. A coupling efficiency of −0.8 dB at 1550 nm is experimentally demonstrated, which represents the highest efficiency ever reported in the telecommunications C-band in a single-layer silicon grating structure without the use of any backreflector or index-matching material between the fiber and the grating.

We have experimentally demonstrated an O-band Mach–Zehnder interferometer (MZI) based on an N-rich silicon nitride platform combined with ${\text{Ge}}_2{\text{Sb}}_2{\text{Te}}_5$ for future optical communication applications. The device operation relies on controlling the waveguide's losses using a phase change material cell, which can be changed from amorphous (low-loss) to crystalline (high-loss). An extinction ratio as high as 11 dB was obtained between the amorphous (ON) and the crystalline (OFF) states of the MZI optical building block. The insertion loss of the MZI structure per cell unit length was measured to be as high as 0.87 dB/μm in the OFF state and as low as 0.064 dB/μm in the ON state for TM polarization.