[Paper Review] Two-color all-optical switching in Si-compatible epsilon-near-zero hyperbolic metamaterials

Alessandro Pianelli, Rakesh Dhama+3 more|arXiv (Cornell University)|May 11, 2023

Metamaterials and Metasurfaces Applications4 citations

TL;DR

This paper proposes a Si-compatible hyperbolic metamaterial (HMM) based on titanium nitride (TiN) and indium-tin-oxide (ITO) multilayers that exhibits two distinct epsilon-near-zero (ENZ) wavelengths in the visible and near-infrared (telecom O- to S-band) regions. Using ultrafast pump-probe transient absorption spectroscopy, the authors demonstrate all-optical switching with response times down to a few hundred femtoseconds, enabling simultaneous multi-wavelength modulation for ultrafast photonic integrated circuits.

ABSTRACT

All-optical ultrafast switches enabled by artificial materials are considered at the forefront of the next generation of photonic communications and data processing. During the last two decades, the photonic applications, impact, and interest have tremendously increased in the framework of epsilon-near-zero (ENZ) photonics. Here, we experimentally propose a novel multilayered metamaterial utilizing Si-compatible titanium nitride and indium-tin-oxide materials. The device exhibits two effective ENZ wavelengths in the visible and near-infrared spectrum, with switching times down to a few hundred femtoseconds at the corresponding ENZ regions. This novel approach will bring ENZ metamaterials towards new hybrid integrated CMOS photonic circuit components for ultrafast all-optical terahertz modulation.

Motivation & Objective

• To develop a Si-compatible, low-loss hyperbolic metamaterial for ultrafast all-optical switching in integrated photonic circuits.
• To achieve dual-color ENZ operation at visible and near-infrared wavelengths for multi-wavelength modulation.
• To enable sub-picosecond switching speeds via engineered nonlinearities in multilayered HMMs.
• To demonstrate experimentally that hot electron dynamics in the ENZ regime enable faster switching than in homogeneous TiN or ITO films.
• To provide a CMOS-compatible platform for terahertz-rate all-optical modulators in hybrid photonic integrated systems.

Proposed method

• Fabrication of TiN and ITO thin films via magnetron sputtering and electron beam evaporation, respectively, on glass substrates.
• Use of spectroscopic ellipsometry to retrieve optical constants (n and k) of individual TiN and ITO layers.
• Design of a multilayered HMM with alternating TiN and ITO bilayers to achieve two effective ENZ wavelengths in the visible and near-IR spectrum.
• Employment of broadband reflectance and transmittance measurements using a 20X objective and Ocean Optics Flame UV-VIS spectrometer to characterize spectral response.
• Application of ultrafast pump-probe transient absorption spectroscopy with 100 fs pulses at 800 nm center wavelength and tunable OPA-generated pump pulses to probe nonlinear dynamics.
• Use of a delay line to control time delay between pump and probe pulses, enabling time-resolved absorption measurements at 500 Hz pump and 1 kHz probe repetition rates.

Experimental results

Research questions

• RQ1 Can a Si-compatible hyperbolic metamaterial be engineered to exhibit two distinct epsilon-near-zero (ENZ) wavelengths in the visible and near-infrared spectrum?
• RQ2 What is the ultrafast all-optical switching response time of such a multilayered HMM at its effective ENZ regions?
• RQ3 How do hot electron dynamics in the HMM compare to those in individual TiN or ITO films at ENZ wavelengths?
• RQ4 Can this HMM support simultaneous all-optical modulation at multiple wavelengths with sub-100 fs response times?
• RQ5 To what extent can this HMM enable high-speed, low-loss, and CMOS-integrated photonic switching for terahertz-rate applications?

Key findings

• The HMM exhibits two effective ENZ wavelengths—one in the visible range and one in the near-infrared (telecom O- to S-band) spectrum—enabling dual-color all-optical switching.
• Ultrafast transient absorption spectroscopy reveals switching response times as fast as a few hundred femtoseconds at both ENZ wavelengths.
• Hot electron dynamics in the HMM are significantly faster than in individual TiN or ITO thin films, indicating enhanced nonlinear response in the ENZ regime.
• The device supports all-optical modulation in the 1.8–3.1 THz frequency range, demonstrating potential for terahertz-rate photonic switching.
• The use of Si-compatible materials (TiN and ITO) with low optical losses enables compatibility with CMOS photonic platforms and hybrid integration.
• The HMM design achieves broadband nonlinear enhancement through hyperbolic dispersion, enabling strong light-matter interaction at the ENZ condition.