ABSTRACT
The operation of photonic devices often relies on modulation of their refractive index. While the sub-bandgap index change through bound-electron optical nonlinearity offers a faster response than utilizing free carriers with an overbandgap pump, optical switching often suffers from inefficiency. Here, we use a recently observed metasurface based on mirror-induced optical bound states in the continuum, to enable superior modulation characteristics. We achieve a pulsewidth-limited switching time of 100 fs, reflectance change of 22%, remarkably low energy consumption of 255 µJ/cm2, and an enhancement of modulation contrast by a factor of 440 compared to unpatterned silicon. Additionally, the narrow photonic resonance facilitates the detection of the dispersive nondegenerate two-photon nonlinearity, allowing tunable pump and probe excitation. These findings are explained by a two-band theoretical model for the dispersive nonlinear index. The demonstrated efficient and rapid switching holds immense potential for applications, including quantum photonics, sensing, and metrology.
ABSTRACT
Dielectric metasurfaces made of high refractive index and low optical loss materials have emerged as promising platforms to achieve high-quality factor modes enabling strong light-matter interaction. Bound states in the continuum have shown potential to demonstrate narrow spectral resonances but often require asymmetric geometry and typically feature strong polarization dependence, complicating fabrication and limiting practical applications. We introduce a novel approach for designing high-quality bound states in the continuum using magnetic dipole resonances coupled to a mirror. The resulting metasurface has simple geometric parameters requiring no broken symmetry. To demonstrate the unique features of our photonic platform we show a record-breaking third harmonic generation efficiency from the metasurface benefiting from the strongly enhanced electric field at high-quality resonances. Our approach mitigates the shortcomings of previous platforms with simple geometry enabling facile and large-area fabrication of metasurfaces paving the way for applications in optical sensing, detection, quantum photonics, and nonlinear devices.
ABSTRACT
Plasmonic structures can precisely localize electromagnetic energy to deep subwavelength regions resulting in significant field enhancement useful for efficient on-chip nonlinear generation. However, the origin of large nonlinear enhancements observed in plasmonic nanogap structures consisting of both dielectrics and metals is not fully understood. For the first time, here we probe the third harmonic generation (THG) from a variety of dielectric materials embedded in a nanogap plasmonic cavity. From comprehensive spectral analysis of the THG signal, we conclude that the nonlinear response results primarily from the dielectric spacer layer itself as opposed to the surrounding metal. We achieved a maximum enhancement factor of more than six orders of magnitude compared to a bare gold film, which represents a nonlinear conversion efficiency of 8.78 × 10-4%. We expect this new insight into the nonlinear response in ultrathin gaps between metals to be promising for on-chip nonlinear devices such as ultrafast optical switching and entangled photon sources.
ABSTRACT
Accurate determination of the concentrations, including the mass concentration (MC) and number concentration (NC), of metal nanoparticle (NP) colloid is highly demanded in the synthesis, metrology, and application of NPs. The commonly used inductively coupled plasma mass spectrometry (ICP-MS) can only measure the MC of NPs, which is destructive to the NPs and requires advanced operation skills. Here, we present a simple approach based on an improved optical extinction-scattering spectroscopic (OESS) method to fast determining the MC and NC of metal nanorod colloids simultaneously. Unlike most existing spectroscopic methods that can only deal with low-concentration NP colloids, the improved OESS method can accurately solve the inverse scattering problem of NP colloids with higher concentrations, so that a two-dimensional joint probability density function of both the width and aspect ratio of nanorods can be retrieved, which makes the basis for the accurate determination of the MC and NC of the colloids in a large range of concentration. The reliability and accuracy of the method are validated by measuring several typical nanorod colloids with different concentrations and comparing the results with those obtained by the standard ICP-MS method. It is shown that the improved OESS method can cover a broad MC measurement range of at least 10-50 µg/mL and a NC measurement range of 10(9)-10(11)/mL. The uncertainty and sources of error in the measurement are also analyzed. Since the improved OESS method is fast, cost-effective, non-destructive, and easy to implement, it provides a simple way to determine the concentrations of metal NPs and has the potential to be extended to other metal NPs.
