Ultrafast excitation of Bloch plasmon polaritons in hyperbolic metamaterials with an extreme ultra-violet transient grating

Abstract: Manipulating materials properties with light drives advances in materials science and photonics. Hyperbolic metamaterials are promising candidates as next-generation quantum optical media. They support Bloch plasmon polaritons, which are characterized by potentially infinite wave-vectors and long lifetimes, but cannot be excited through direct light illumination due to momentum mismatch. Here, we experimentally show that a transient grating, formed via interference of fully coherent seeded free-electron laser pulses in a thin insulator film, enables the excitation of Bloch plasmon polaritons in an underlying hyperbolic metamaterial. Finite element simulations confirm the role of the transient grating in facilitating phase-matching and mode excitation. Our findings demonstrate a route to spatiotemporally excite Bloch plasmon polaritons modes, offering an alternative to permanently nanostructured gratings and potentially enabling ultrafast control of optical modes excitation.

• The paper demonstrates that EUV-induced transient gratings enable momentum matching for ultrafast Bloch plasmon polariton excitation in hyperbolic metamaterials.
• Key time-resolved reflectance measurements and simulations reveal a spectral dip at 1230 nm, confirming a sub-picosecond transient response.
• The approach offers reconfigurable, non-destructive optical control that can enhance applications in quantum photonics and dynamic nanophotonic devices.

Ultrafast Excitation of Bloch Plasmon Polaritons in Hyperbolic Metamaterials via Extreme Ultraviolet Transient Grating

Introduction

This work demonstrates the spatiotemporal excitation of Bloch plasmon polariton (BPP) modes in hyperbolic metamaterials (HMMs) using an optically induced transient grating (TG) generated by coherent extreme ultraviolet (EUV) pulses. The study addresses the phase-matching barrier for BPP excitation, presenting an all-optical, reconfigurable alternative to static nanostructured gratings. The approach leverages permittivity modulation in an Al2O3 thin film atop a multilayer Au/Al2O3 HMM, producing in situ coupling conditions on sub-picosecond timescales.

Background and Motivation

HMMs, constructed from alternately stacked metal-dielectric layers, exhibit unique dispersions with hyperbolic iso-frequency surfaces. Such systems are central to nanophotonics, featuring negative refraction, strong nonreciprocity, and promising capacities for quantum optics, nonlinear photonics, biosensing, and super-resolution imaging. BPPs supported by HMMs are collective optical modes with large (in principle, unlimited) in-plane wavevectors and extended lifetimes relative to conventional surface-plasmon or photonic modes. However, their excitation fundamentally requires momentum beyond that of free-space photons, necessitating advanced phase-matching schemes.

Traditionally, permanent surface patterning or embedding gratings has been employed to provide the required in-plane momentum. This work extends the paradigm by inducing ultrafast, optically tunable momentum-matched conditions via transient electronic modulation, circumventing lithographic constraints and enabling dynamic mode control.

Experimental Approach

A well-defined [Au (15 nm)/Al2O3 (30 nm)]×8 multilayer HMM stack was fabricated and capped with an additional Al2O3 (30 nm) overlayer. The TG was formed by interfering femtosecond EUV pulses ($\lambda_{\mathrm{EUV}} = 22.7$ nm, crossing angle $3.4^\circ$) from a seeded free-electron laser. The resulting permittivity grating in the Al2O3 layer had a spatial period of 383 nm.

The BPP excitation was probed by time-resolved reflectance measurements using a delayed near-infrared (NIR) pulse ($1150$–$1550$ nm, $45^\circ$ incidence). Control experiments included: (1) single-beam EUV excitation delivering equivalent energy without spatial modulation, and (2) a reference Al2O3/SiO2 substrate lacking the HMM stack.

Fundamental Mechanism and Analysis

The permittivity modulation via the EUV TG provides an additional in-plane momentum component, enabling phase-matching between the incident NIR probe and the high-k BPP modes of the HMM. This is evidenced by the emergence of a reflectance dip at $1230$ nm ($\Delta R/R < 0$ at $0.1$ ps delay) that disappears at $2$ ps, concurrent with the relaxation/diffusion of non-equilibrium carriers and loss of grating contrast. The phenomenon cannot be reproduced with single-beam (unmodulated) excitation or in the absence of the underlying HMM structure, unambiguously confirming the necessity of the transient grating and the multilayer system for BPP coupling.

Finite element method (FEM) and transfer-matrix method (TMM) simulations substantiate the experimental observations, reproducing the spectral dip and revealing near-field distributions of the excited BPP consistent with eigenmode calculations for the unpatterned HMM stack. The TG-activated field strongly overlaps with the BPP eigenmode, confirming the excitation mechanism is not due to defect or surface roughness, but to coherent grating-assisted phase-matching.

Numerical Results and Robustness

• Reflectance dip at $1230$ nm in the pump-probe spectrum under TG excitation, in agreement with FEM predictions.
• Decay of the TG effect within $3.4^\circ$0 ps, confirming a temporal window defined by carrier relaxation ($3.4^\circ$1 ps), which sets the practical limits for ultrafast mode programming.
• No BPP signature is observed in controls (single-beam excitation or bare Al2O3 reference).
• Damage assessment shows that transient exposure remains non-destructive under optimized parameters, supporting the robustness and repeatability of the experiment.

Implications and Prospects

This work demonstrates a reconfigurable, ultrafast coupling scheme for activating otherwise inaccessible BPP modes in HMMs. The approach removes the need for permanent nanoscale patterning, allowing sub-picosecond programmability of coupling conditions via optically imposed transient gratings. This opens pathways for dynamic polaritonic devices, ultrafast optical switching, and manipulation of strong light-matter interactions in quantum photonics.

Future developments may include:

• Optimization of TG depth, contrast, and spatial profile to maximize coupling strength and BPP quality factor.
• Engineering multilayer HMMs to reduce optical losses and further extend BPP propagation lengths.
• Exploration of materials with more favorable ultrafast index modulation characteristics for higher modulation depth and longer TG lifetimes.
• Integration of single-shot or low-duty-cycle excitations to avoid long-term sample degradation.

Theoretically, this scheme offers a platform to investigate ultrafast polaritonic kinetics, non-equilibrium dynamics, and coherent control protocols in complex photonic structures.

Conclusion

The study establishes that EUV-induced transient gratings in dielectric films on HMMs provide a non-invasive, reconfigurable route for ultrafast excitation of Bloch plasmon polaritons. The demonstrated approach matches multiphoton phase requirements, is robust against surface degradation under proper operational regimes, and is validated by both experimental data and electromagnetic field simulations. This technique significantly extends the toolbox for time-dependent manipulation of polaritonic and photonic states in advanced metamaterials, with practical relevance for programmable nanophotonic and quantum devices.