Ultrafast all-optical switching enabled by epsilon-near-zero modes in metal-insulator nanocavities

Ultrafast control of light-matter interactions constitutes a crucial feature in view of new technological frontiers of information processing. However, conventional optical elements are either static or feature switching speeds that are extremely low with respect to the timescales at which it is possible to control light. Here, we exploit high-quality-factor engineered epsilon-near-zero (ENZ) modes of a metal-insulator-metal nanocavity to realize an all-optical ultrafast modulation of the reflectance of light at a tailored wavelength. Our approach is based on the presence of the two, spectrally separated, ENZ absorption resonances of the cavity. Optical pumping the system at its high-energy ENZ mode leads to a strong red-shift of the low energy mode because of the transient increase of the local dielectric function, which leads to a sub-3-ps control of the reflectance at a specific wavelength with a modulation depth approaching 120%.

Overcoming the fundamental limits of classic electronics, such as band-width, clock-time/frequency and heating of the device, is the main promise of photonics [1]. Many recent advancements in this direction rely on the use of light as information carrier, paving the way towards light-based technologies, which will have a huge impact in terms of reduced energy consumption and performance efficiency. Moreover, the possibility of controlling electronics at optical frequencies has recently become possible, thus introducing a new paradigm towards attosecond opto-electronics [2]. In this framework, it is fundamental to develop new, affordable, and energy-efficient strategies to reach a fast (>100 GHz) and fully tailorable control of optical states at scales which are well below the diffraction limit of electromagnetic radiation. Unfortunately, conventional optical elements, such as mirrors, polarizers, optical isolators and waveplates, used to process the information (intensity, polarization, etc.) carried by light, are either passive or possess very low switching speeds and efficiencies in terms of signal modulation. Therefore, all-optical switching [3] has attracted great attention because it can potentially overcome the speed and heat dissipation limitation imposed by electrical switching or passive optical devices [4] [5]. Examples of low-loss and high-speed optical devices based on photonic [6] and plasmonic [7] crystals, semiconducting nanostructures [8], microring resonators [9], and single nanoantennas [10] have already been proposed. Recently, Grinblat and co-workers [11] showed that Gallium Phosphide can generate sub-30-fs transmission modulation of up to ~70% in the visible (VIS) and near-infrared (NIR) range through the excitation of optical non-linearities. Another interesting approach is represented by the use of natural epsilon-near-zero (ENZ) materials [12]. Yang et al. [13] have demonstrated that, through intra-band optical pumping in an In-doped CdO-based plasmonic perfect absorber, it is possible to switch the polarization state of the incident light with an extinction ratio of 91 within 800 fs. Furthermore, Boltasseva et al. [14] showed that yttrium doped cadmium oxide (CdO) films can enable a light intensity modulation up to 135% in the mid-infrared (mid-IR) region close to the ENZ point with a relaxation time of 45.6 ps for a pump fluence of 1.3 mJ/cm 2 . However, all the aforementioned switching schemers still rely on either demanding architectures, or on the use of strong optical non-linearities (i.e., high pumping power). In the case of ENZ switching, very challenging material processing techniques are required to tailor the ENZ wavelength.
Recently, it has been found that resonances occurring in metal-insulator-metal (MIM) nanocavities can be described as effective ENZ resonances [15]. Their spectral position can be easily engineered by acting on the refractive index and thickness of the embedded dielectric, while their quality(Q)-factor (Δλ/λ0) can be optimized by adopting non-symmetric geometries, yielding near-zero reflectance R at the ENZ modes (superabsorbers) [16] [17]. Therefore, these systems constitute a promising and flexible alternative to natural ENZ materials.
Here, we propose a novel approach for ultrafast all-optical switching. The basic idea consists in the linear perturbation of an ENZ symmetric mode in the NIR through optical pumping of an anti-symmetric one in the UV in a MIM nanocavity. The technological core of the architecture is depicted in Figure 1, and consists in a tailorable high-Q-factor ENZ-type superabsorber in the visible/near-infrared (VIS/NIR) spectral range. At the steady state, both the high energy (HE) and the low energy (LE) ENZ modes enable almost complete absorption of the incoming light at the resonances (top panel). The high-Q coupled cavity is then used for alloptical switching upon photoexcitation. By optical pumping the HE ENZ mode, the LE resonance strongly redshifts because of the transient increase of the dielectric function upon excitation of charge carriers in the metallic layers [18] [19] [20], which leads to an induced modulation of the reflectance R at the wavelength of the LE mode (bottom panel). It is worth mentioning here that this concept is general, since the cavity resonances can be designed at will in a broad range of wavelengths (from the VIS to mid-IR), and it relies on a simple fabrication process (more details are reported in Methods). Moreover, differently from the previously reported approaches, the one we propose here allows to pump at one ENZ mode to induce a strong modulation of another ENZ mode. To prove the ENZ nature of our system, we first characterized the steady state spectral response in terms of absolute R as a function of the angle of incidence  (see also Supplementary Figure S1). In Figure 2a  All-optical modulation of the superabsorber reflectance has been proved by performing wavelength-and timeresolved pump-probe experiments. The MIM nanocavity was pumped at the HE ENZ mode, while the temporal dynamics were investigated by probing in the 700 nm -800 nm spectral range (Figure 3a), where the LE ENZ mode is located (more details on the pump and probe signals, as well as on the experimental setup [24], can be found in Methods and Supplementary Figures S2 and S3). Upon resonant pumping, electrons are photoexcited in the metallic layers, and quickly thermalize via electron-electron scattering, leading to an elevated electronic temperature and thus a transient change of the local dielectric function [19]. This introduces a redshift of the LE ENZ resonance due to the local increase of permittivity, and thus a pump-induced change ΔR/R close to the LE resonance. For a pump fluence of ~4.4 mJ/cm 2 , we indeed observe a positive change of the reflectance R around 8 % at the wavelength of the LE ENZ mode, corresponding to a modulation R/R of about 100%, and a negative R/R of about 50% at longer wavelengths (above 735 nm) which both decay within 5 ps, as it can be seen in Figure 3b and Figure 4a. The induced reflectance modulations, either positive or negative, are strongly localized at specific wavelengths, and do not shift throughout the relaxation process, at least within the first 5 ps (for time-dependent R/R at longer delays see Supplementary Figure S4). The absorption of incident light by the metallic layers is drastically enhanced by the presence of the HE mode. For pump wavelengths that are detuned from the HE ENZ mode, a very low R/R (<0.1%) is observed (see Figure   4a, grey circles), thus confirming that the mutual presence of two ENZ modes is fundamental to achieve such large modulations, when at the same time keeping pump and signal pulses spectrally separated.   However, as can be seen in Figure 4b, the decay time remains approximately 3 ps in the range of investigated fluences. For a pump fluence of 5.2 mJ/cm 2 , we find a decay time of (2.5 ± 0.3) ps corresponding to an alloptical switching bandwidth of about 400 GHz with a modulation depth around 120 %. The decay time can be further reduced to below 1 ps through optimizing the nanocavity quality factor Q. This can be easily done by either increasing the thickness of the metal, by working at higher order modes, or even by embedding the MIM nanocavity in a specifically designed Bragg microcavity, thus enabling modulation of the signal at the THz frequencies [25]. Our superabsorber can be engineered to work at a desired wavelength, from UV to mid-IR, with practically no limitation regarding metals and dielectrics. Moreover, the free spectral range (FSR) between the ENZ resonances can be easily designed by exploiting multiple cavity geometries [17]. Finally, since our approach is based on the use of two ENZ modes, the induced red-shift on resonant wavelengths allows versatile all-optical data processing, including data wavelength conversion and data format following/inversion/gating, on the data patterns transferred to the probe, thus enabling a nanocavity based alloptical data format follower and inverter [26].
In conclusion, we have demonstrated all-optical, ultrafast (sub-3-ps) modulation of the reflectance of a metal-insulator-metal multilayer approaching 120% in the VIS-NIR spectral range. Our approach is based on the near-perfect absorbance of the nanocavities' ENZ modes, whose spectral position can easily be tailored at will from UV to mid-IR frequencies, thus lifting from demanding fabrication processes to tailor the spectral position of the ENZ resonance. Via pumping of one ENZ mode, we achieve a modulation of reflectance at wavelengths close to the other mode. Without the need of high-power excitation to drive higher order effects for ultrafast switching, our system is based on linear absorption, providing large modulation exceeding 100 % and switching bandwidths of few hundred GHz at moderate excitation fluence, due to the high Q-factor of the ENZ modes. Combining all-optical ultrafast modulation with two spectrally separated super-absorbing ENZ modes, the proposed architecture represents a promising approach in the framework of future and emerging all-light-driven technologies. Woollam spectroscopic ellipsometer. Spectroscopic ellipsometry supplied with p-and s-polarized transmittance and reflectance in the spectral range between 300 and 1300 nm was performed to measure the ellipsometric angles ψ and Δ, whose fitting led to the effective dielectric permittivity of the superabsorber. Pand s-polarization R and transmittance measurements were performed in the angular range from 30° to 80°.
Pump-probe experiments. Transient reflection measurements are carried out with a home built spectroscopy system based on a commercial Yb:KGW regenerative amplifier system at a laser repetition rate of 50 kHz [24].
A non-collinear optical parametric amplifier working in the VIS/NIR spectral range initially delivers bandwidth-limited pulses at 790 nm that are frequency-doubled using a BBO crystal yielding the final pump pulses with ~2 nm spectral bandwidth (FWHM) centred at 395 nm (Fourier limit of pulses ~250 fs). Residual spectral components at lower energy are suppressed with a dielectric short-pass filter (Thorlabs, FESH500).
The pump induced change of reflection is probed by a white-light supercontinuum between 500 nm and 900 nm which is temporally compressed by custom designed dielectric chirped mirrors (DCMs). The probe pulse energy is then adjusted to ensure a 1:20 energy ratio compared to the pump. Using an off-axis parabolic (OAP) mirror with 50.8 mm focal length, focal diameters of 20 µm and 25 µm are achieved for probe and pump pulses respectively. Pump and probe pulses are focused onto the sample non-collinearly, in order to spatially filter the probe pulse after sample interaction. Residual scattered pump radiation is further spectrally suppressed with a dielectric long-pass filter (Thorlabs, FELH600). Spectrally resolved detection of the probe pulse after sample interaction is achieved by using a spectrograph (Acton) in combination with a high-speed charge coupled device (CCD) camera operating at 50 kHz. Finally, a Pockels cell modulates the pump pulse train at half the repetition rate of the laser system, allowing the calculation of R/R on a 25 kHz basis.
Two-step fit model. To extract the decay time of the modulation, the following model 1. was used.