Si-CMOS compatible epsilon-near-zero metamaterial for two-color ultrafast all-optical switching

The study addresses the need for ultralow-energy, ultrafast, and compact all-optical switches to overcome the power and speed limitations of electronic interconnects in modern information and communications technology. All-optical switches, based on third-order nonlinearities, can surpass the speed limits of electronic devices and enable high bit rate, low-power optical communications, optical computing, and integrated photonics. Epsilon-near-zero (ENZ) materials are promising for ultrafast switching due to enhanced light–matter interaction near ENZ, reduced phase mismatch, mitigation of group velocity dispersion, and potential for miniaturization and CMOS-compatible integration. The research question is how to realize a Si-CMOS compatible ENZ metamaterial that offers low-energy activation and ultrafast switching at practically relevant wavelengths, and whether a multilayer hyperbolic metamaterial approach can provide dual-wavelength (visible and near-IR) effective ENZ operation with sub-picosecond switching dynamics.

Prior work has demonstrated ultrafast all-optical switching using diverse platforms including photonic crystals, plasmonic metamaterials, phase-change materials, 2D materials, and perovskites, but challenges remain in CMOS compatibility, loss, energy requirements, and wavelength targeting. Homogeneous ENZ transparent conducting oxides (TCOs) such as ITO and AZO are CMOS-compatible and operate near telecom wavelengths, showing large nonlinearities and ultrafast response, yet are limited by intrinsic optical properties and fixed ENZ wavelengths. Multilayer hyperbolic metamaterials (HMMs) enable engineered effective ENZ responses, even into the visible, and can enhance effective third-order susceptibility. HMMs based on metal/dielectric bilayers (e.g., Ag/SiO2, Au/Al2O3) have shown strong nonlinearities and picosecond-scale responses, including high-contrast switching and broadband coherent perfect absorption. However, many prior HMMs utilize noble metals incompatible with CMOS and may exhibit higher losses. Thus, a need exists for Si-compatible, low-loss multilayers with tailored ENZ bands (broadband or multiple bands) at target wavelengths and with ultrafast, low-energy operation.

Design and simulations: TiN and ITO single layers were first deposited on float glass, and their optical constants (n, k) were retrieved by spectroscopic ellipsometry. These measured permittivities were used in Ansys Lumerical to design a four-bilayer TiN/ITO hyperbolic metamaterial (HMM) targeting two effective ENZ regions. The final stack alternates TiN (11 nm) and ITO (32 nm) layers for four bilayers. Effective medium theory was used to compute the effective parallel (ε||) and perpendicular (ε⊥) permittivities from measured εTiN, εITO and layer thicknesses. Angle- and polarization-dependent reflection/transmission were modeled via transfer-matrix method (TMM) and finite-difference time-domain (FDTD), and compared with experiments. Fabrication: TiN was grown by reactive magnetron sputtering (Plasmalab System 400) using Ar/N2 with parameters optimized for plasmonic figure of merit; base pressure <5×10⁻⁷ Torr; room-temperature deposition; 11 s to reach 11 nm. ITO was deposited by electron-beam evaporation (custom chamber) with Telemark 264 e-gun, quartz thickness monitor, substrate heating to 200 °C, and oxygen partial pressure 7×10⁻⁴ mbar; target composition 10 wt% SnO₂, 90 wt% In₂O₃. Linear optics: Broadband (Energetiq EQ-99XFC LDLS) reflectance and transmittance measured using a 20× objective (NA 0.4) and Ocean Optics Flame UV–VIS–NIR spectrometer. Angle-dependent p- and s-polarized responses characterized; simulations validated experimentally. Ultrafast transient absorption spectroscopy (TAS): An amplified Ti:sapphire laser with OPA produced <100 fs pulses at 1 kHz. 90% output fed to OPA for tunable pump (UV–VIS–NIR); 10% formed a broadband probe via white-light generation, sent through a delay line. Pump was chopper-modulated; measurements in transmission mode with an OMA and de-chirping algorithm. Pump repetition 500 Hz; probe 1 kHz. Pump wavelengths used included 400 nm (3.1 eV), 700 nm (1.7 eV), and 1250 nm (0.99 eV). Fluences were kept low (typically 1.16–1.40 mJ cm⁻²). Temporal IRF-limited resolution used; kinetics fitted with a Gaussian-convolved exponential model S(t) per Methods. Absorption distribution: Absorbed power density Pabs = 0.5 ω |E|² Im(ε) computed to identify which layers absorb at pump wavelengths (TiN dominant at 700 nm; ITO dominant at 1250 nm).

- Dual effective ENZ bands in the TiN/ITO HMM: - VIS-ENZ (ε||≈0): 649–810 nm; Im(ε||) ≈ 1.4. - NIR-ENZ (ε⊥≈0): 1238–1500 nm; Im(ε⊥) ≈ 0.4. - Angle dependence (p-polarized): VIS-ENZ persists up to ~70° incidence; NIR-ENZ emerges above ~40° and maximizes near 70°. - Linear optics: Good agreement between measured and simulated (FDTD/TMM) reflection and transmission at normal incidence and at 70°; NIR-ENZ signatures evident at 70°. - Ultrafast switching at VIS-ENZ under 700 nm pump (fluence 1.40 mJ cm⁻²): - Probe at 650 nm: normal incidence switching time τ ≈ 320 fs (rise 100±13 fs; fall 220±17 fs). At 70° incidence, τ ≈ 380 fs (fall 280±26 fs). Oblique incidence yields stronger modulation amplitude but slightly slower kinetics. - Cross-ENZ response (VIS pump to NIR-ENZ): - Probe at 1240 nm (NIR-ENZ) under 700 nm pump: rise ≈ 200±16 fs followed by a slow relaxation tail with lattice heat release; extended relaxation time ≈ 78±6.4 ps (phonon-phonon dominated) when probed to 200 ps. - Ultrafast switching with NIR intraband pump (1250 nm, 1.16 mJ cm⁻², 70°): - Probe at 783 nm (VIS-ENZ): rise 160±26 fs; fall 230±37 fs; overall switching window ≈ 390 fs. - Probe at 1325 nm (NIR-ENZ): rise 60±6.6 fs; fall 480±43 fs; overall switching window ≈ 540 fs; full recovery achievable at low fluence. - Higher-energy pump (400 nm, 3.1 eV, 1.40 mJ cm⁻², 70°): - Probe at 650 nm (VIS-ENZ): rise 110±15 fs; fall 330±34 fs; overall ≈ 440 fs. - Probe at 1240 nm (NIR-ENZ): rise 240±61 fs; fall ≈ 110±1.7 ps, reflecting stronger lattice heating and heat dissipation dominated dynamics. - Absorption partitioning: TiN layers absorb more strongly at 700 nm; ITO layers absorb more at 1250 nm, enabling selective intraband excitation of different constituents. - Comparative performance: The HMM exhibits faster effective-ENZ switching than homogeneous ENZ thin films and metasurfaces; single-layer TiN (11 nm) and ITO (32 nm) show slower dynamics (per Supplementary data). Low pump fluences (<~1.5 mJ cm⁻²) suffice for sub-ps switching.

The multilayer TiN/ITO HMM meets the objectives of achieving CMOS-compatible, dual-wavelength ENZ operation with low-energy, ultrafast all-optical switching. Engineering the effective anisotropic permittivity yields two ENZ bands (visible ε||≈0 and near-IR ε⊥≈0) that support strong nonlinear modulation via enhanced light–matter interaction and reduced phase mismatch. Intraband excitation produces hot-electron distributions that thermalize via electron–electron scattering within tens to hundreds of femtoseconds, enabling sub-100–300 fs rise times. Recovery dynamics are governed by electron–phonon coupling and, at higher pump energies, by phonon–phonon interactions and lattice heating, leading to picosecond to sub-nanosecond tails. Angle dependence is significant for NIR-ENZ operation, with maximum response near 70°, consistent with anisotropic effective permittivity and p-polarized coupling. The device demonstrates faster kinetics and lower activation energy than comparable homogeneous ENZ films due to multilayer-enabled field distribution and absorption partitioning (TiN-dominated at 700 nm; ITO-dominated at 1250 nm). Controlling pump wavelength and fluence tunes the balance between ultrafast electronic and slower thermal pathways, enabling reversible, low-energy switching particularly in the NIR-ENZ at 1250 nm. These results validate HMMs as a route toward integrated, dual-color ultrafast modulators and switches for silicon photonics.

A Si-CMOS compatible TiN/ITO hyperbolic metamaterial with two effective ENZ bands (visible and near-IR) was demonstrated. The device achieves ultrafast all-optical switching with sub-picosecond windows and femtosecond rise times at low pump fluences by leveraging intraband excitation and hot-electron dynamics. Switching times include ~320–440 fs in the VIS-ENZ and ~540 fs in the NIR-ENZ under appropriate pump conditions, with picosecond-scale recovery tails at higher pump energies. The multilayer HMM outperforms homogeneous ENZ thin films in effective-ENZ switching speed and energy efficiency, and its CMOS compatibility enables potential integration with silicon photonics. Future work could focus on on-chip integration and waveguide coupling, dispersion and loss optimization, angular and polarization engineering to relax oblique-incidence requirements, broadband/multi-ENZ tailoring, and thermal management to minimize slow recovery components.

- Angular dependence: Strong NIR-ENZ response requires oblique incidence (>40°, optimized at ~70°), which may complicate normal-incidence device integration. - Thermal effects: At higher pump photon energies (e.g., 400 nm), significant lattice heating leads to long relaxation tails (tens to ~110 ps), potentially limiting high-repetition-rate operation without thermal management. - Material loss and imaginary permittivity: Non-negligible Im(ε) in ENZ regions (Im(ε||)≈1.4, Im(ε⊥)≈0.4) may limit modulation depth and efficiency. - Fluence sensitivity: Nitrides show weak sensitivity to pump fluence, and balancing fast and slow components requires careful control of pump wavelength and intensity. - Geometry specificity: Results are demonstrated for a specific four-bilayer TiN(11 nm)/ITO(32 nm) stack on glass; generalizability to other substrates and on-chip architectures is not experimentally shown here.