The increasing demand for high-speed, large-capacity data processing necessitates the development of ultrafast, low-power optical switches to replace current power-hungry electronic circuits. All-optical switches, controlled by light, offer a potential solution by overcoming the speed limitations of electrical switches. While several approaches exist (photonic crystals, plasmonic metamaterials, etc.), epsilon-near-zero (ENZ) materials are promising due to their enhanced light-matter interaction, enabling efficient switching at low activation energies and potential for miniaturization. However, many ENZ-based switches lack CMOS compatibility, require high energy activation, and have limitations in switching speed and wavelength. This research aims to address these limitations by designing a Si-compatible ENZ metamaterial capable of ultrafast, two-color all-optical switching.

Existing ultrafast all-optical switches based on various materials like graphene, photonic crystals, plasmonic metamaterials, phase-change materials, and 2D materials have shown promise but often suffer from drawbacks such as CMOS incompatibility, high energy requirements, limited switching speed, or restricted operating wavelengths. ENZ materials offer a potential solution due to their unique optical properties near zero permittivity, enhancing light-matter interaction and leading to efficient switching. Previous research has explored ENZ materials like ITO and AZO, achieving ultrafast switching at telecom wavelengths, but these are limited by their intrinsic properties. Hyperbolic metamaterials (HMMs), multilayer subwavelength structures, are also considered because of their potential for high nonlinear effective susceptibility and ultrafast nonlinear response in the ENZ regime. However, challenges remain in tailoring ENZ response at desired wavelengths, using low-loss materials, and ensuring Si-compatibility for efficient integration with silicon photonics.

A multilayer metamaterial was designed using four bilayers of Si-compatible titanium nitride (TiN) and indium-tin-oxide (ITO). TiN films were deposited via magnetron sputtering, while ITO films were deposited using electron beam evaporation. Spectroscopic ellipsometry was used to determine the optical constants (refractive index and extinction coefficient) of the individual films. These data were used in Ansys Lumerical software to simulate and design the HMM structure, aiming for two ENZ resonances in the visible and near-infrared regions. The effective permittivity was calculated using effective media theory. Transient absorption spectroscopy (TAS) in transmission mode, using a pump-probe technique with sub-100 fs pulses, was employed to investigate the ultrafast optical response at the ENZ wavelengths. Intraband pumping was used to induce hot electrons, enabling lower energy consumption and sub-picosecond relaxation dynamics. The switching time responses were analyzed at various incident angles and pump wavelengths (700 nm, 1250 nm, 400 nm) to probe the dynamic behavior in both VIS-ENZ and NIR-ENZ regions. The absorption power per unit volume was calculated using the relation Pabs = 0.5ω|E|²imag(ε), where ω is the angular frequency, |E|² is the magnitude squared of the electric field, and imag(ε) is the imaginary part of the permittivity. Kinetic traces were fitted using the function S(t) = Aexp(-t/τ)exp(-(t-t₀)²/2(ln2)IRF²), where IRF is the instrument response function width, t₀ is time zero, A is amplitude, and τ is the decay time.

The fabricated TiN/ITO HMM exhibited two ENZ regions, one in the visible (VIS-ENZ, 649–810 nm) and one in the near-infrared (NIR-ENZ, 1238–1500 nm). Transient absorption spectroscopy revealed ultrafast all-optical switching in both regions. At 700 nm pump excitation, switching times as short as a few hundred femtoseconds were observed, with a rise time of 100 ± 13 fs and a fall time of 220 ± 17 fs at normal incidence and a slightly slower response at a 70° angle of incidence. Excitation at 1250 nm showed a rise time of 200 ± 16 fs and a fall time of 78 ± 6.4 ps in the NIR-ENZ region, indicating a fast initial response followed by a slower relaxation. Using 400 nm excitation, a fast response (110 ± 15 fs rise time, 330 ± 34 fs fall time) was observed in the VIS-ENZ region, whereas the NIR-ENZ region showed a faster rise (240 ± 61 fs) followed by a longer fall time (110 ± 1.7 ps). The absorption analysis indicated higher absorption in TiN at VIS-ENZ and higher ITO absorption at NIR-ENZ. The long tail observed in the NIR-ENZ at higher energy excitation was attributed to heat dissipation in the metadevice, with TiN potentially acting as a heat transducer. Adjusting the pump wavelength to 1250 nm (lower energy) allowed for complete recovery of the transient switching response in the NIR-ENZ region. Overall, the metadevice demonstrated a faster switching response than its homogeneous ENZ thin-film and metasurface counterparts.

The results demonstrate the successful fabrication and characterization of a Si-compatible two-color ultrafast all-optical switch based on a TiN/ITO HMM. The observed ultrafast switching times, achieved with minimal energy input via intraband pumping, highlight the potential of this approach for low-power, high-speed applications. The ability to tailor the ENZ regions and achieve switching at two distinct wavelengths enhances the functionality and versatility of the device. The influence of the incident angle and pump wavelength on the switching dynamics provides valuable insights into the underlying physical mechanisms, such as hot electron dynamics and thermal effects. The faster switching response in the HMM compared to its homogeneous counterparts underscores the advantages of utilizing metamaterial structures for enhanced nonlinear optical properties.

This study successfully demonstrated ultrafast all-optical switching in a Si-CMOS-compatible TiN/ITO HMM at two distinct wavelengths. The device exhibits switching times in the few hundred femtoseconds range with low energy consumption, significantly surpassing the performance of homogeneous ENZ thin-film and metasurface counterparts. This work opens up new avenues for developing tunable, reversible ultrafast all-optical switches suitable for integrated photonics. Future research could focus on optimizing the design for broader bandwidth, higher switching contrast, and integration with silicon photonic circuits for practical applications.

The study primarily focuses on the ultrafast switching behavior of the metamaterial. Further investigation is needed to fully characterize other parameters, such as the modulation depth and the impact of fabrication imperfections on device performance. The long relaxation times observed in certain conditions could potentially limit the achievable data rates, warranting further optimization of the metamaterial structure and material properties. The study utilizes specific pump fluences; the scalability and stability of the device under different operating conditions should also be evaluated.