Giant ultrafast dichroism and birefringence with active nonlocal metasurfaces

Many physical, chemical, and biological light–matter interactions are polarization dependent, making polarization a key degree of freedom for applications and fundamental studies. Active, ultrafast polarization control would enable high-speed data encoding and processing for free-space optical links at GHz rates in the visible/near-IR, and benefit quantum information processing and pseudospin-based quantum devices. It also promises THz-speed tuning or switching of material processes (e.g., lattice excitations, molecular dynamics) and advances in probing and controlling chiral systems and chiroptical effects relevant to chemistry and biology. To overcome speed limits of electro-optical approaches, an all-optical paradigm is pursued: an ultrashort control pulse triggers third-order nonlinearity to transiently modulate permittivity experienced by a probe beam, allowing light-by-light control. While active tailoring of amplitude, phase, and polarization has been shown in thin films, plasmonic metamaterials, and epsilon-near-zero architectures, practical polarization control requires flexible, compact, near-normal-incidence devices with high modulation efficiency at fluences well below damage thresholds. Photonic metasurfaces—quasi-2D arrays of resonant nanostructures—offer strong field enhancement and rich design flexibility. High-index semiconductors such as AlGaAs can support high-Q resonances (Mie, quasi-BIC) with low loss and strong nonlinearity from photogenerated carriers near the band edge (via Drude response, band filling, bandgap renormalization). Prior work indicated band filling dominates near the bandgap, producing real permittivity changes below bandgap and reduced losses above bandgap. However, the impact of this mechanism on macroscopic observables near the band edge in dielectric metasurfaces remained unexplored. This work demonstrates giant ultrafast all-optical modulation of dichroism and birefringence in a tailored AlGaAs metasurface by combining band-edge carrier dynamics with a polarization-selective nonlocal high-Q resonance at the bandgap.

The study builds on all-optical control frameworks that use ultrashort pulses to induce third-order nonlinearities for amplitude/phase/polarization modulation in platforms including semi-metal thin films, plasmonic metamaterials, and epsilon-near-zero structures. Flat optics and metasurfaces enable compact, flexible devices with strong resonant field enhancement. High-index semiconductor metasurfaces (e.g., AlGaAs) support magnetic Mie-type resonances and quasi-BICs with reduced absorption losses compared to plasmonics. In semiconductors, photocarrier dynamics (Drude plasma, band filling, bandgap renormalization) modulate permittivity; for direct bandgap materials like AlGaAs, band filling near the band edge can dominate, yielding large real permittivity shifts and reduced imaginary parts above bandgap (transient reduction of loss, potential gain). Previous work on AlGaAs nanoantennas highlighted the efficiency of band filling near the bandgap; however, the effect on macroscopic reflectance/reflectivity near the band edge in nonlocal metasurfaces had not been fully explored.

Device design and fabrication: A metasurface consisting of an array of Al0.18Ga0.82As nanowires was fabricated atop a ~900 nm Al2O3 buffer on a GaAs substrate. Nominal unit cell parameters: wire width W ≈ 165 nm, height H ≈ 400 nm, period P ≈ 450 nm. The structure supports polarization-selective resonances near the AlGaAs bandgap (≈750 nm for this Al composition), including a TM-polarized nonlocal extended-state resonance at ~758 nm and a TE-polarized quasi-BIC at ~770 nm. Static reflectance spectra confirmed strong dichroism. Experimental setup: Polarization-resolved ultrafast pump–probe spectroscopy was performed. Pump: 400 nm center wavelength, ~100 fs FWHM pulses; fluences used included 70 µJ cm⁻² (dichroism) and higher for birefringence measurements. Probe: broadband spanning 700–800 nm. Incidence geometry: probe incident at ~9°; polarization set to either TM (perpendicular to wires) or TE (parallel to wires) for dichroism; for birefringence, the incident probe was linearly polarized at 135° in the TM–TE basis. Detection: For dichroism, reflected probe passed through a polarization analyzer (Pol.1 and Pol.2 at 0°/90° along TM/TE axes) and a spectrometer. For birefringence, a quarter-wave plate was inserted before the analyzer; Pol.1 and Pol.2 at 135°, and the waveplate fast axis angle β was varied to reconstruct the transient polarization ellipse via ultrafast polarimetry. Measurement metric: Differential reflectance ΔR/R = (R' − R)/R was recorded versus probe wavelength and pump–probe delay for TM and TE components; transient phase retardance between TM/TE components was extracted from polarimetry for birefringence. Modeling: A multistep semiclassical–electromagnetic framework was used. Carrier–lattice dynamics were described by a three-temperature/rate-equation model with variables: n1(t) (carrier density in the pump-absorbing hot-spot near the surface), n2(t) (carrier density in the wire bulk increasing via diffusion from the skin depth region and decreasing via recombination), and θ(t) (lattice temperature). Pump absorption at 400 nm creates electron–hole pairs highly localized within ~16 nm skin depth; subsequent diffusion occurs over first few picoseconds. Dominant recombination is trap-assisted (surface states) due to high surface-to-volume ratio; Auger recombination is negligible under these conditions. Permittivity modulation mechanisms included: Drude intraband response, band filling (Pauli blocking) affecting interband transitions (negative ΔIm(ε) above bandgap and Kramers–Kronig-linked ΔRe(ε) across the band edge), and a small thermo-optic contribution from lattice heating. Time-dependent complex permittivity ε(t,λ) = ε⁰(λ) + Δε(t,λ) was computed and used in full-wave electromagnetic simulations of the metasurface to obtain transient reflectance spectra for TE and TM as functions of delay and wavelength. Comparison with experiment validated dynamics and spectral features.

• Giant ultrafast enhancement of dichroism: Under pump fluence F = 70 µJ cm⁻², TM-polarized ΔR/R peaked at ~470% near λ ≈ 748–750 nm, with reflectance increasing from ~10% (static) to nearly 60% transiently. TE modulation was weaker, up to ~70% with broadband features and ~−10% near 748 nm, highlighting strong transient dichroism. The dichroic modulation peak occurs around 2 ps after pump arrival. - Birefringence control: Up to a π/2 transient phase shift (quarter-wave retardance) between TM and TE components was achieved with a control fluence of 180 µJ cm⁻². - Temporal dynamics: The maximum modulation is delayed (~2 ps), attributed to carrier diffusion from surface hot-spots into the nanowire bulk. - Spectral behavior: The TM resonance undergoes an ultrafast blueshift (~10 nm) and reshaping; peak reflectivity increases markedly at 2 ps. The TE quasi-BIC feature shows smaller, more complex broadband changes. - Mechanism: Modeling shows band filling dominates the permittivity modulation near the band edge, exceeding Drude contributions by up to ~1 order of magnitude in Re(Δε) at bandgap and nearly 2 orders in Im(Δε) on the high-energy side. Thermo-optic effects are negligible on the few-ps timescale. - Transient gain: The imaginary part of the total permittivity becomes negative in a narrow spectral window (≈738–750 nm) within the first picosecond and broadens with delay, indicating population inversion and stimulated emission that, combined with the high-Q TM resonance, drives the large reflectivity increase. - Simulations vs experiment: Full-wave simulations coupled with the carrier model reproduce the amplitude, spectral features, and delayed temporal peak of ΔR/R for both polarizations, with minor quantitative discrepancies (e.g., slightly narrower simulated spectral features, a red-shifted simulated dip relative to experiment). - Efficiency and safety: The record-high ΔR/R is achieved at fluences at least an order of magnitude below the AlGaAs damage threshold (~1 mJ cm⁻²) and up to 5× higher modulation than prior reports, indicating high efficiency at moderate excitation levels.

The work addresses the need for ultrafast, efficient, and compact polarization control by leveraging an AlGaAs metasurface engineered to host a polarization-selective, high-Q nonlocal resonance at the material band edge where photocarrier-induced nonlinearities are strongest. The observed giant transient dichroism and controllable birefringence demonstrate that sub-picosecond all-optical polarization manipulation is achievable at low fluences. The delayed peak in modulation (≈2 ps) links device performance to carrier diffusion dynamics, suggesting that tailoring carrier transport (geometry, surface passivation) can further optimize response speed and magnitude. The synergy between band filling—which strongly modifies both real and imaginary parts of permittivity near the band edge—and the metasurface resonance leads to transient gain in a narrow spectral window, amplifying reflectance and enabling exceptional modulation depth. These findings validate the design strategy of aligning high-Q resonances with band-edge nonlinearities in direct-gap semiconductors. The results are pertinent for ultrafast free-space optical communication, time-resolved spectroscopy of chiral and complex systems, and on-chip photonic signal processing where polarization control at THz bandwidths is desirable.

This study demonstrates record-level ultrafast all-optical modulation of dichroism and birefringence in an AlGaAs metasurface by combining a polarization-selective nonlocal high-Q resonance with band-edge carrier dynamics dominated by band filling. Experimentally, ΔR/R up to ~470% (TM) at 70 µJ cm⁻² and transient π/2 phase retardance at 180 µJ cm⁻² were achieved, with dynamics peaking around 2 ps. A validated semiclassical carrier model coupled to full-wave simulations clarifies the mechanisms, including transient gain arising from population inversion near 740–750 nm. The work establishes active dielectric metasurfaces as powerful platforms for ultrafast polarization control at safe fluences, outperforming prior approaches. Future directions include optimizing nanowire geometry and surface passivation to tailor diffusion and recombination, engineering resonance linewidth and spectral placement for broader bandwidth or targeted wavelengths, extending operation closer to normal incidence and across different spectral regions, and exploring device integration for practical ultrafast polarimetric modulators and sensors.

• The detailed affiliations of authors and full fabrication specifics are not provided in the excerpt. - Experimental–simulation discrepancies remain: simulations yield narrower spectral features and a red-shifted dip compared to measurements. - The strongest effects are narrowband around the band edge; broadband polarization control may require additional design optimization. - Operation was demonstrated at a small oblique incidence (~9°); performance at strictly normal incidence and angular tolerance are not detailed here. - The birefringence results are summarized without full spectral–temporal maps in the excerpt; quantitative phase dynamics versus wavelength and delay are not provided. - The transient response peaks at ~2 ps due to diffusion, implying a finite response time that may limit ultimate switching speed if sub-ps operation is required without amplitude trade-offs. - The study focuses on reflection-mode modulation; transmission-mode performance and insertion losses are not discussed in the provided text.