Three-dimensional Light Confinement Research Articles

A High-Power Broadband Terahertz Source Enabled by Three-Dimensional Light Confinement in a Plasmonic Nanocavity

The scope and potential uses of time-domain terahertz imaging and spectroscopy are mainly limited by the low optical-to-terahertz conversion efficiency of photoconductive terahertz sources. State-of-the-art photoconductive sources utilize short-carrier-lifetime semiconductors to recombine carriers that cannot contribute to efficient terahertz generation and cause additional thermal dissipation. Here, we present a novel photoconductive terahertz source that offers a significantly higher efficiency compared with terahertz sources fabricated on short-carrier-lifetime substrates. The key innovative feature of this source is the tight three-dimensional confinement of the optical pump beam around the terahertz nanoantennas that are used as radiating elements. This is achieved by means of a nanocavity formed by plasmonic structures and a distributed Bragg reflector. Consequently, almost all of the photo-generated carriers can be routed to the terahertz nanoantennas within a sub-picosecond time-scale. This results in a very strong, ultrafast current that drives the nanoantennas to produce broadband terahertz radiation. We experimentally demonstrate that this terahertz source can generate 4 mW pulsed terahertz radiation under an optical pump power of 720 mW over the 0.1–4 THz frequency range. This is the highest reported power level for terahertz radiation from a photoconductive terahertz source, representing more than an order of magnitude of enhancement in the optical-to-terahertz conversion efficiency compared with state-of-the-art photoconductive terahertz sources fabricated on short-carrier-lifetime substrates.

Optofluidic Fabry-Pérot Micro-Cavities Comprising Curved Surfaces for Homogeneous Liquid Refractometry-Design, Simulation, and Experimental Performance Assessment.

In the scope of miniaturized optical sensors for liquid refractometry, this work details the design, numerical simulation, and experimental characterization of a Fabry-Pérot resonator consisting of two deeply-etched silicon cylindrical mirrors with a micro-tube in between holding the liquid analyte under study. The curved surfaces of the tube and the cylindrical mirrors provide three-dimensional light confinement and enable achieving stability for the cavity illuminated by a Gaussian beam input. The resonant optofluidic cavity attains a high-quality factor (Q)—over 2800—which is necessary for a sensitive refractometer, not only by providing a sharp interference spectrum peak that enables accurate tracing of the peak wavelengths shifts, but also by providing steep side peaks, which enables detection of refractive index changes by power level variations when operating at a fixed wavelength. The latter method can achieve refractometry without the need for spectroscopy tools, provided certain criteria explained in the details are met. By experimentally measuring mixtures of acetone-toluene with different ratios, refractive index variations of 0.0005 < Δn < 0.0022 could be detected, with sensitivity as high as 5500 μW/RIU.

Three-dimensional light confinement in a PT-symmetric nanocavity

We demonstrate a robust mechanism that can improve light confinement in nanostructures by breaking the parity–time (PT) symmetry in nanowire based nanocavities.

Improving the room-temperature confinement of light by miniaturizing mode sizes into a deep subwavelength scale using dielectric spheres in metal cavities

The confinement of light within nanometer-scale regions may result in the significant enhancement of light-matter interactions. However, light confinement to nanometers is hindered by the diffraction limit of a dielectric material. For a dielectric cavity, if the material loss is negligible, reducing the cavity size usually causes a significantly increase in radiation loss. Surface plasmons show great promise for potential subwavelength light confinement. However, in most circumstances, light confinement by dissipative metallic materials can cause ohmic losses at optical frequencies. In such cases, the realization of light confinement with deep subwavelength mode sizes results in great losses and thus has low quality factors. In the present study, a three-dimensional light confinement with deep subwavelength mode sizes is achieved using dielectric spheres in metal cavities. Contrary to other mechanisms for subwavelength light confinement that are based on the use of dielectric or metal cavities, the nanometer-scale regions ensure that most of the light energy is confined away from the metal-dielectric interfaces, thereby decreasing light absorption in the metal cavity. In turn, the metal cavity decreases the radiation loss of light. Thus, high quality factors ranging from 2×10(2) to 6×10(2) can be obtained at room temperature. An effective electrical mode volume ranging from 7×10(-5)λ(0)(3) to 2×10(-4)λ(0)(3) (where λ(0) is the resonant wavelength in a vacuum) can be achieved. Therefore, this method of three-dimensional light confinement with deep subwavelength mode sizes using dielectric spheres in metal cavities may have potential applications in the design of nanolasers, nanophoton detectors, nonlinear optical switches, and so on.

Superenhanced three-dimensional confinement of light by compound metal-dielectric microspheres

Dielectric microspheres are capable of confining light in a three dimensional region of sub-wavelength dimensions in appropriate illuminating conditions. A compound set of metal-dielectric microspheres permitting light confined in an effective volume as small as 0.095 (λ/n)3 is shown, together with a strong focusing effect when the spheres are illuminated by focused radially polarized beams. This strong confinement arises from the surface plasmon hotspots on the rear side of the metallic microsphere induced by the so called photonic nanojets of the dielectric microsphere, and the compound set has been optimized to achieve the best result. Full width at half maximum (FWHM) could be optimized to 73nm (~0.11λ) in axial direction and 146nm (~0.23λ) in transversal direction separately. The beam shaped in that way is suitable for applications requiring small effective volume and/or strong peak intensities.

Colloidal Quantum Dots in All-Dielectric High-Q Pillar Microcavities

We have fabricated all-dielectric high-Q optical pillar resonators with embedded colloidal CdSe/ZnS quantum dots or rods as light emitters by focused ion beam milling. Three-dimensional light confinement and distinct pillar microcavity modes are observed. Results from a waveguide model for the mode patterns and their spectral positions are in excellent agreement with the experimental data. Cavities with elliptical cross sections show higher quality factors in the short axis direction than do circular resonators of the same cross-sectional area.

Control of light-matter interaction in microresonators

The spontaneous emission of excitons confined in quantum dots that are embedded in microresonators with a three-dimensional confinement of light has been studied by time-resolved spectroscopy. The available decay channels have been investigated for different resonator types without and with lateral gold coating. For the latter type we were able to demonstrate both of the critical aspects of a control of spontaneous emission: an enhancement for on-resonance of the exciton transition with an optical cavity mode and a deenhancement for off-resonance.

Near-field mapping of the electromagnetic field in confined photon geometries

We study the optical modes in resonator structures obtained by assembling microcavities with a three-dimensional confinement of light. We used scanning confocal microscopy in order to measure light emission from these structures in the near optical field. Our studies give direct and detailed insight into the spatial distribution of the electromagnetic fields of confined modes in these photonic structures. We find similarities between the spatial modes of two coupled resonators and those of a diatomic molecule, and between modes of a chain of resonators and those of a periodic molecular crystal. In particular, we show that deviations of cavity size in the chain give rise to localized photonic modes which resemble impurity states in molecular crystals.

Parametric polariton scattering in microresonators with three-dimensional optical confinement

Parametric scattering in square shaped microcavities that exhibit a three-dimensional confinement of light has been observed under resonant continuous wave excitation. The three lowest lying modes in the optical mode spectrum of these resonators are equidistantly spaced in energy. When resonantly exciting the middle mode parametric scattering into the ground and the second excited mode is observed. This process conserves energy, but conservation of momentum is not required, due to the breakdown of the translation invariance.