Spontaneous symmetry breaking in plasmon lattice lasers.

Spontaneous symmetry breaking (SSB) is key for our understanding of phase transitions and the spontaneous emergence of order. In this work, we report that, for a two-dimensional (2D) periodic metasurface with gain, SSB occurs in the lasing transition. We study diffractive hexagonal plasmon nanoparticle lattices, where the K-points in momentum space provide two modes that are degenerate in frequency and identically distributed in space. Using femtosecond pulses to energize the gain medium, we simultaneously capture single-shot real-space and Fourier-space images of laser emission. By combining Fourier and real space, we resolve the two order parameters for which symmetry breaking simultaneously occurs: spatial parity and U(1) (rotational) symmetry breaking, evident respectively as random relative mode amplitude and phase. The methodology reported in this work is generally applicable to 2D plasmonic and dielectric metasurfaces and opens numerous opportunities for the study of SSB and the emergence of spatial coherence in metaphotonics.

Temporal Dynamics of Collective Resonances in Periodic Metasurfaces.

Temporal dynamics of confined optical fields can provide valuable insights into light-matter interactions in complex optical systems, going beyond their frequency-domain description. Here, we present a new experimental approach based on interferometric autocorrelation (IAC) that reveals the dynamics of optical near-fields enhanced by collective resonances in periodic metasurfaces. We focus on probing the resonances known as waveguide-plasmon polaritons, which are supported by plasmonic nanoparticle arrays coupled to a slab waveguide. To probe the resonant near-field enhancement, our IAC measurements make use of enhanced two-photon excited luminescence (TPEL) from semiconductor quantum dots deposited on the nanoparticle arrays. Thanks to the incoherent character of TPEL, the measurements are only sensitive to the fundamental optical fields and therefore can reveal clear signatures of their coherent temporal dynamics. In particular, we show that the excitation of a high-Q collective resonance gives rise to interference fringes at time delays as large as 500 fs, much greater than the incident pulse duration (150 fs). Based on these signatures, the basic characteristics of the resonances can be determined, including their Q factors, which are found to exceed 200. Furthermore, the measurements also reveal temporal beating between two different resonances, providing information on their frequencies and their relative contribution to the field enhancement. Finally, we present an approach to enhance the visibility of the resonances hidden in the IAC curves by converting them into spectrograms, which greatly facilitates the analysis and interpretation of the results. Our findings open up new perspectives on time-resolved studies of collective resonances in metasurfaces and other multiresonant systems.

Extreme-Ultraviolet Shaping and Imaging by High-Harmonic Generation from Nanostructured Silica.

Coherent extreme-ultraviolet pulses from high-harmonic generation have ample applications in attosecond science, lensless imaging, and industrial metrology. However, tailoring complex spatial amplitude, phase, and polarization properties of extreme-ultraviolet pulses is made nontrivial by the lack of efficient optical elements. Here, we have overcome this limitation through nanoengineered solid samples, which enable direct control over amplitude and phase patterns of nonlinearly generated extreme-ultraviolet pulses. We demonstrate experimental configurations and emitting structures that yield spatially patterned beam profiles, increased conversion efficiencies, and tailored polarization states. Furthermore, we use the emitted patterns to reconstruct height profiles, probe the near-field confinement in nanostructures below the diffraction limit of the fundamental radiation, and to image complex structures through coherent diffractive emission from these structures. Our results pave the way for introducing sub-fundamental-wavelength resolution imaging, direct manipulation of beams through nanoengineered samples, and metrology of nanostructures into the extreme-ultraviolet spectral range.

Monitoring the gold nanoshell growth mechanism: stabilizing and destabilizing effects of PEG-SH molecules.

Plasmonic nanoshells have attracted significant interest due to their resonant optical properties providing excellent spectral tunability, promising for various biophotonic applications. In this work we discuss our experimental and theoretical results related to the synthesis and optical characterization of surface-modified gold nanoshells. The nanoshell growth mechanism is monitored by IR spectroscopy, and the effects of modification of the gold nanoshell surface by PEG-SH ((11-mercaptoundecyl)tetra(ethylene glycol)) molecules are studied using TEM and optical methods. A red shift of localized surface plasmon resonance is observed upon formation of a layer of PEG-SH molecules on the completed gold nanoshells. Uncompleted gold shells show tendency to detach from the spherical silica cores, and the underlying destabilizing mechanism is discussed. The experimentally measured optical extinction properties are in good agreement with the results of numerical simulations, which additionally shed light on the localized plasmon modes contributing to the extinction, as well as on the effects of nanoshell surface nonuniformity on the resonant plasmonic properties and local field enhancements.

Second harmonic generation from gold meta-molecules with three-fold symmetry.

The unique optical properties of arrays of metallic nanoparticles are of great interest for many applications such as in optical data storage, sensing applications, optoelectronic devices or as platforms to increase the detection limit in spectroscopic measurements. Nonlinear optical phenomena can also be altered by metallic nanostructures opening new possible applications. In this work, arrays composed of non-centrosymmetric individual structures with three fold axial symmetry made of gold are designed and fabricated using electron beam lithography. The nonlinear optical properties of these structures are investigated using second-harmonic generation microscopy (SHGM) with a femtosecond excitation source set near the plasmon resonance frequency. Modeling of the electromagnetic field distribution around the metallic structures is performed using the Finite Difference Time Domain (FDTD) method, highlighting the confinement of the SHG signal and its polarization dependence. Polarization-resolved measurements are conducted to correlate the SHG signal with the structure and symmetry of the individual nanostructures. Since both two-photon induced photoluminescence (TPPL) and SHG signals are produced upon excitation of these structures, lifetime measurements are performed to further evaluate the magnitude of these two effects.

Spectral dependence of nonlinear optical properties of symmetrical octatetraynes with p-substituted phenyl end-groups.

Organic compounds containing conjugated carbon chains have been extensively investigated due to their interesting properties including nonlinear optical response. Polyynes are a group of compounds where the conjugation can be modified by appropriate selection of end-groups, enabling "control" and improvement of nonlinear effects. In this work, we investigate three newly synthesized aryl end-capped octatetraynes, exhibiting strong nonlinear absorption and nonlinear refraction properties, which can be attributed to the presence of aryl end-groups with electron-withdrawing functional groups suitable for further extending the conjugation. Nonlinear optical measurements were performed using a f-scan (modified Z-scan) technique with a femtosecond laser system at various wavelengths in the visible and near-infrared range (540-1600 nm), revealing strong negative third-order nonlinear refraction (3NR) and two-photon absorption (2PA) below 600 nm and a noticeable three-photon absorption (3PA) at longer wavelengths. The results of spectroscopic characterization and the crystallographic data for the investigated compounds are also presented.

Effects of surface plasmon polariton-mediated interactions on second harmonic generation from assemblies of pyramidal metallic nano-cavities.

We use polarization-resolved two-photon microscopy to investigate second harmonic generation (SHG) from individual assemblies of site-controlled nano-pyramidal recess templates covered with silver films. We demonstrate the effect of the surface plasmon polaritons (SPPs) at fundamental and second-harmonic frequencies on the effective second order susceptibility tensor as a function of pyramid arrangement and inter-pyramid distance. These results open new perspectives for the application of SHG microscopy as a sensitive probe of coherently excited SPPs, as well as for the design of new plasmonic nanostructure assemblies with tailored nonlinear optical properties.

Nanoengineering the second order susceptibility in semiconductor quantum dot heterostructures.

We study second-harmonic generation from single CdTe/CdS core/shell rod-on-dot nanocrystals with different geometrical parameters, which allow to fine tune the nonlinear properties of the nanostructure. These hybrid semiconductor-semiconductor nanoparticles exhibit extremely strong and stable second-harmonic emission, although the size of CdTe core is still within the strong quantum confinement regime. The orientation sensitive polarization response is analyzed by means of a pointwise additive model of the third-order tensors associated to the nanoparticle components. These findings prove that engineering of semiconducting complex heterostructures at the single nanoparticle scale can lead to extremely bright nanometric nonlinear light sources.