Ultrafast low-pump fluence all-optical modulation based on graphene-metal hybrid metasurfaces

High-speed optical modulation underpins applications ranging from optical interconnects and ultrafast spectroscopy to information processing. All-optical modulation offers the highest bandwidths (up to terahertz), yet most demonstrations have been in the visible and near-infrared. Achieving ultrafast modulation in the mid- and far-infrared is challenging due to material absorption and weak nonlinearities. Graphene, with gapless linear dispersion and sub-picosecond carrier relaxation, is a strong candidate for broadband ultrafast modulation from visible to terahertz. However, ultra-thin graphene’s limited absorption leads to high pump fluence requirements in existing all-optical graphene modulators. The research question addressed here is how to realize ultrafast all-optical modulation at near- and mid-IR (beyond 6 µm) with substantially reduced pump fluence while preserving picosecond response. The study proposes graphene-metal hybrid plasmonic metasurfaces that co-localize and enhance fields at both pump and probe wavelengths within nanoscale hotspots, thereby boosting photocarrier generation and sensitivity of the optical response to graphene’s transient conductivity. The work demonstrates near- and mid-IR modulators with 1–2 orders of magnitude lower pump fluence and picosecond-scale response, suggesting a viable route to ultrafast mid-/far-IR modulators.

Prior all-optical modulators have used semiconductor waveguides, colloidal plasmonic semiconductor nanocrystals, chains of silicon nanoantennas, gallium arsenide Mie-resonant nanoparticles, nanostructured silicon membranes, hot-carrier effects in silver nanorods, Tamm-plasmon resonances, graphene-clad microfibers, plasmonic slot waveguides, and thin-film absorbers with graphene. Most operate in the visible/near-IR. Efforts toward mid-IR include optically pumped subwavelength silicon membranes, degenerately doped oxide nanoparticles with tunable plasmons, two-photon absorption in germanium-on-silicon waveguides, and bulk Dirac fermions in Cd3As2, generally limited to wavelengths up to about 6 µm. Graphene modulators fall into electrically pumped (up to ~35 GHz but RC-limited), thermo-optic (hundreds of nanoseconds), and all-optical (picosecond) categories. Despite ultrafast response, graphene’s weak absorption necessitates high pump fluence (often ≳300 µJ cm−2). Approaches to enhance light–graphene interaction include dielectric waveguides, microfibers, cavities, and plasmonic slot waveguides, yet ultrafast, low-fluence operation in the mid-IR remains difficult.

Design: A graphene–metallic metasurface absorber (GMMA) comprising a plasmonic nanoantenna array, a monolayer graphene overlayer, a metallic back reflector, and a dielectric spacer. Closely coupled nanoantennas create nanoscale gaps that serve as hotspots to enhance fields for both pump and probe, maximizing light–graphene interaction. A back reflector with spacer forms a tunable absorber enhancing probe sensitivity. Pump polarization (s) and probe polarization (p) are oriented to optimally excite the respective resonances. Simulations: Full-wave electromagnetic simulations quantify near-field enhancement, absorption in graphene at pump (∼1040 nm) and probe (~6–7 µm), and reflection spectra. A two-temperature model (TTM) describes hot-electron dynamics in graphene after pump absorption, including energy relaxation to optical (hundreds of femtoseconds) and acoustic phonons (few picoseconds), while neglecting nonthermal carrier distributions and hot-carrier transfer from metal antennas. Graphene’s time-dependent optical conductivity is computed via the random phase approximation (RPA) using the electron temperature profile from TTM. Fabrication: On Si substrates, 250 nm Al is deposited as a back reflector; ~350 nm Al2O3 spacer via ALD; plasmonic antennas patterned by EBL, metal evaporation (Cr ~8 nm/Au ~40 nm), and lift-off; a monolayer graphene is transferred on top by wet transfer. SEM confirms nanogap widths ~29 nm (std ~3.9 nm). Characterization: Reflection spectra measured using an FTIR spectrometer coupled to a mid-IR microscope with linear polarization control. Absorption at pump wavelength and reflection at probe resonance are characterized. Ultrafast near-IR pump–probe measurements assess temporal response, and simulations provide time-resolved mid-IR modulation behavior.

- Field and absorption enhancement: Near-field intensity is strongly concentrated in nanogaps at both pump (~1.04 µm) and probe (~6.5 µm) wavelengths. Pump absorption in graphene within hotspots is enhanced by about an order of magnitude relative to suspended monolayer graphene (baseline ~2.3% absorption), and far exceeds graphene on spacer/back-reflector without metasurfaces. - Transient carrier dynamics: TTM indicates pump-induced electron temperatures in graphene rise from ~300 K to several thousand K under ~70 µJ cm−2, relaxing bi-exponentially via optical (<100 fs) and acoustic phonons (~3 ps). - Time-dependent conductivity and resonance shift: RPA-based conductivity changes induce a blue-shift of the GMMA resonance from ~6.5 µm to ~5.8 µm during excitation, returning as carriers cool. - Modulation performance (simulations): Reflection at the probe band (~6–7 µm) increases from ~1% (off) to >50% (on) within ~100 fs, with biexponential decay constants ~38 fs and ~2.8 ps. The absolute modulation depth (Ron − Roff) is ~50 (≈5000%). Reference structures and suspended graphene show negligible modulation under the same pump fluence, evidencing the critical role of the metasurface-enhanced interaction. - Experimental demonstrations: Devices operate at near-IR (1.56 µm) and mid-IR (>6 µm) with pump fluence reduced by 1–2 orders of magnitude compared to prior graphene-only approaches. For mid-IR (>6 µm), an all-optical modulator is demonstrated with pump fluence <70 µJ cm−2. - Speed: Ultrafast near-IR pump–probe measurements indicate response limited by graphene photocarrier relaxation on the picosecond scale (∼2 ps near-IR; ∼3 ps mid-IR), consistent with simulations and prior reports.

Enhancing light–graphene interaction at both pump and probe wavelengths within nanoscale hotspots increases the photoexcited carrier density and amplifies the device’s sensitivity to transient changes in graphene conductivity. This dual-wavelength field localization reduces the required pump fluence by 1–2 orders of magnitude while maintaining ultrafast response governed by intrinsic graphene carrier dynamics. The blue-shift of the GMMA resonance under pumping translates into large, fast reflection modulation in the mid-IR, overcoming graphene’s weak intrinsic absorption by leveraging plasmonic concentration and a back-reflector absorber configuration. Comparison with reference and suspended graphene confirms that the metasurface is essential to achieve substantial modulation at low fluence. These results directly address long-standing challenges for ultrafast mid-/far-IR modulators, with immediate relevance to spectroscopy, communications, and sensing in these bands.

The study introduces graphene–metal hybrid metasurface absorbers that co-localize pump and probe fields in nanoscale hotspots, enabling ultrafast all-optical modulation at near- and mid-IR wavelengths with markedly reduced pump fluence. Simulations and experiments show large reflection modulation (up to ~5000% simulated) and picosecond-scale recovery determined by graphene carrier relaxation. A mid-IR (>6 µm) all-optical modulator operating with <70 µJ cm−2 pump fluence is experimentally demonstrated, to the authors’ knowledge the first of its kind beyond 6 µm with such low fluence. The design paradigm is extendable to longer mid- and far-IR wavelengths. Future work could optimize metasurface geometries for even stronger hotspots, explore alternative plasmonic/dielectric materials for reduced loss and broader bandwidth, integrate on-chip for practical systems, and perform comprehensive time-resolved measurements across mid-/far-IR bands.

- Modeling approximations: The two-temperature model neglects nonthermal carrier populations (assumed to decay faster than the ~100 fs pump) and ignores hot-carrier generation in the metal antennas and transfer to graphene; these effects may become relevant at higher fluence. - Polarization and spectral specificity: Operation relies on specific pump (s-polarized) and probe (p-polarized) excitations aligned to antenna geometry and on resonance engineering; performance may degrade with polarization or spectral detuning. - Fabrication tolerances: Nanoscale gap variations (std ~3.9 nm) can affect field enhancement, resonance position, and modulation uniformity. - Experimental scope: Ultrafast temporal response is directly measured in the near-IR; mid-IR temporal dynamics are supported by simulations and literature-consistent relaxation times rather than full direct time-domain mid-IR measurements in the provided content.