Macroscopically Aligned Carbon Nanotubes as a Refractory Platform for Hyperbolic Thermal Emitters

Nanophotonic thermal emitters with large photonic density of states (PDOS) have the potential to significantly enhance the efficiency of radiative cooling and waste heat recovery. Because of their nearly infinite PDOS, refractory hyperbolic materials make a promising material platform for thermal emitters. However, it is challenging to achieve a prominent PDOS in existing refractory hyperbolic materials, especially in a broad bandwidth. Here, we demonstrate macroscopically aligned carbon nanotubes as an excellent refractory material platform for hyperbolic nanophotonic devices. Aligned carbon nanotubes are thermally stable up to 1600 °C and exhibit extreme anisotropy: metallic in one direction and insulating in the other two directions. Such extreme anisotropy results in an exceptionally large PDOS over a broadband spectrum range (longer than 4.3 μm) in the mid-infrared, manifesting as strong resonances in deeply subwavelength-sized cavities. We demonstrate polarized, spectrally selective, thermal emission from aligned carbon nanotube films and indefinite cavities of volume as small as ∼λ3/700 operating at 700 °C. These experiments suggest that aligned carbon nanotubes enhance PDOS and hence also thermal photon density by over 2 orders of magnitude, making them a promising refractory nanophotonics platform

Cascaded optical nonlinearities in dielectric metasurfaces

Since the discovery of the laser, optical parametric nonlinearities have been at the core of efficient light conversion sources. Typically, thick transparent crystals or quasi-phase matched waveguides, are utilized in conjunction with phase-matching techniques to select a single parametric process. In recent years, due to the rapid developments in artificially structured materials, optical frequency mixing has been achieved at the nanoscale in subwavelength resonators arrayed as metasurfaces. Phase matching becomes relaxed for these wavelength-scale structures, and all allowed nonlinear processes can, in principle, occur on an equal footing. This could promote harmonic generation via a cascaded (consisting of several frequency mixing steps) process. However, so far, all reported work on dielectric metasurfaces have assumed frequency mixing from a direct (single step) nonlinear process. In this work, we prove the existence of cascaded second-order optical nonlinearities by analyzing the second and third wave mixing from a highly nonlinear metasurface in conjunction with polarization selection rules and crystal symmetries. We find that the third wave mixing signal from a cascaded process can be of comparable strength to that from conventional third harmonic generation, and that surface nonlinearities are the dominant mechanism that contributes to cascaded second order nonlinearitie

Cascaded Optical Nonlinearities in Dielectric Metasurfaces

Since the discovery of the laser, optical nonlinearities have been at the core of efficient light conversion sources. Typically, thick transparent crystals or quasi-phase matched waveguides are utilized in conjunction with phase-matching techniques to select a single parametric process. In recent years, due to the rapid developments in artificially structured materials, optical frequency mixing has been achieved at the nanoscale in subwavelength resonators arrayed as metasurfaces. Phase matching becomes relaxed for these wavelength-scale structures, and all allowed nonlinear processes can, in principle, occur on an equal footing. This could promote harmonic generation via a cascaded (consisting of several frequency mixing steps) process. However, so far, all reported work on dielectric metasurfaces have assumed frequency mixing from a direct (single step) nonlinear process. In this work, we prove the existence of cascaded second-order optical nonlinearities by analyzing the second- and third-wave mixing from a highly nonlinear metasurface in conjunction with polarization selection rules and crystal symmetries. We find that the third-wave mixing signal from a cascaded process can be of comparable strength to that from conventional third-harmonic generation and that surface nonlinearities are the dominant mechanism that contributes to cascaded second-order nonlinearities in our metasurface

Near-field Imaging of Optical Resonance Modes in Silicon Metasurfaces Using Photoelectron Microscopy

Precise control of light-matter interactions at the nanoscale lies at the heart of nanophotonics. Yet their experimental examination is challenging since the corresponding electromagnetic near-field is often confined within volumes below the resolution of conventional optical microscopy. In this work, we demonstrate that photoelectron emission microscopy (PEEM) can be used to image near-field optical fields of nanophotonic structures with a sub-monolayer potassium surface layer to lower the work function and enable two-photon photoemission using near-infrared illumination. We present concurrent spectroscopy and imaging of the near-field distribution of the resonance modes supported by broken-symmetry silicon metasurfaces. We find that the photoemission signal reveals the wavelength dependent, electric near-field distribution of the optical modes. In addition, the large field of view of PEEM allows us to examine the evolution of the collective modes by monitoring individual resonators near the boundary of the metasurface and deduce that coupling between eight resonators establishes the collective excitations. All told, the high-spatial resolution of this hyperspectral imaging approach, combined with an extended field of view, is valuable for the metrology of collective, non-local optical resonances in nanophotonic systems

Terahertz pulse generation with binary phase control in non-linear InAs metasurface

The effect of terahertz (THz) pulse generation has revolutionized broadband coherent spectroscopy and imaging at THz frequencies. However, THz pulses typically lack spatial structure, whereas structured beams are becoming essential for advanced spectroscopy applications. Non-linear optical metasurfaces with nanoscale THz emitters can provide a solution by defining the beam structure at the generation stage. We develop a non-linear InAs metasurface consisting of nanoscale optical resonators for simultaneous generation and structuring of THz beams. We find that THz pulse generation in the resonators is governed by optical rectification. It is more efficient than in ZnTe crystals, and it allows us to control the pulse polarity and amplitude, offering a platform for realizing binary-phase THz metasurfaces. To illustrate this capability, we demonstrate an InAs metalens, which simultaneously generates and focuses THz pulses. The control of spatiotemporal structure using nanoscale emitters opens doors for THz beam engineering and advanced spectroscopy and imaging applications

Terahertz Pulse Generation with Binary Phase Control in Nonlinear InAs Metasurface

The effect of terahertz (THz) pulse generation has revolutionized broadband coherent spectroscopy and imaging at THz frequencies. However, THz pulses typically lack spatial structure, whereas structured beams are becoming essential for advanced spectroscopy applications. Nonlinear optical metasurfaces with nanoscale THz emitters can provide a solution by defining the beam structure at the generation stage. We develop a nonlinear InAs metasurface consisting of nanoscale optical resonators for simultaneous generation and structuring of THz beams. We find that THz pulse generation in the resonators is governed by optical rectification. It is more efficient than in ZnTe crystals, and it allows us to control the pulse polarity and amplitude, offering a platform for realizing binary-phase THz metasurfaces. To illustrate this capability, we demonstrate an InAs metalens, which simultaneously generates and focuses THz pulses. The control of spatiotemporal structure using nanoscale emitters opens doors for THz beam engineering and advanced spectroscopy and imaging applications