Direct Measurement of the Local Density of Optical States in the Time Domain

One of the most fundamental and relevant properties of a photonic system is the local density of optical states (LDOS) as it defines the rate at which an excited emitter dissipates energy by coupling to its surrounding. However, the direct determination of the LDOS is challenging as it requires measurements of the complex electric field of a point dipole at its own position. We introduce here a near-field setup which can measure the terahertz electric field amplitude at the position of a point source in the time domain. From the measured amplitude, the frequency-dependent imaginary component of the electric field can be determined and the LDOS can be retrieved. As a proof of concept, this setup has been used to measure the partial LDOS (the LDOS for a defined dipole orientation) as a function of the distance to planar interfaces made of gold, InSb, and quartz. Furthermore, the spatially dependent partial LDOS of a resonant gold rod has been measured as well. These results have been compared with analytical results and simulations. The excellent agreement between measurements and theory demonstrates the applicability of this setup for the quantitative determination of the LDOS in complex photonic systems.</p

Direct Measurement of the THz Local Density of Optical States

We introduce a double near-field THz probe microscope using micro-structured photoconductive antennas that allows the local excitation and detection of THz transients. When the THz probes for emission and detection are at distances much shorter than THz wavelengths, this setup effectively detects the complex THz field at the position of the source. The imaginary component of this field corresponds to the partial local density of optical states (partial LDOS), which defines the strength of interaction of the local source with its surrounding photonic medium. We use this novel technique to perform the first direct measurement of the partial LDOS of a dipole source close to a planar interface, the so-called Drexhage configuration, achieving an excellent agreement with theory. Our direct determination of the partial LDOS by measuring the complex field at the position of the source illustrates the potential of THz near-field microscopy for the precise investigation of photonic media and can be easily applied to more complex resonant media.</p

Bound States in the Continuum Excited and Detected in the Near-Field

Bound states in the Continuum (BICs) represent a new paradigm for resonant photonics due to their infinite lifetimes associated with the full suppression of radiation losses. This property makes it impossible to directly investigate BICs with standard far-field spectroscopy. In this contribution, we demonstrate the local excitation and the direct measurement of the near-field of BICs in arrays of dimer resonators and report their extremely long lifetimes due to the out-of-phase field oscillations in the resonators

Direct Observation of THz Bound States in the Continuum

Bound states in the Continuum (BICs) represent a new paradigm for resonant photonics due to their infinite lifetime associated with the full suppression of radiation losses. This property makes it also impossible to directly measure BICs with standard far-field spectroscopic techniques. Here, we directly observe the temporal evolution of a BIC by using a near-field excitation and detection microscope

Thickness-dependent Auger scattering in a single WS2 microcrystal probed with time-resolved terahertz near-field microscopy

Time-resolved terahertz (THz) spectroscopy has been shown as a powerful technique to non-invasively determine the charge carrier properties in photoexcited semiconductors. However, the long wavelengths of terahertz radiation reduce the applicability of this technique to large samples. Using THz near-field microscopy, we show THz measurements of the lifetime of 2D single exfoliated microcrystals of transition metal dichalcogenides (WS2). The increased spatial resolution of THz near-field microscopy allows spatial mapping of the evolution of the carrier lifetime, revealing Auger assisted surface defect recombination as the dominant recombination channel. THz near-field microscopy allows for the non-invasive and high-resolution investigation of material properties of 2D semiconductors relevant for nanoelectronic and optoelectronic applications

Evolutionary Optimization of Nanophotonic Design for Optoelectronic Applications

Periodic nanophotonic structures provide a wide range of opportunities for applications in optoelectronic devices due to the lattice resonances that display strong electromagnetic field confinement, exciton-polaritons originating from strong light-matter coupling or bound-states in the continuum with infinite lifetimes and vanished radiation losses. In this contribution, we introduce an evolutionary optimization method to inverse design periodic arrays of nanoparticles for the optimization of the coupling strength in strongly coupled organic materials and the short-circuit current of organic solar cells.</p

Time-resolved THz time-domain near-field microscopy of exfoliated single flakes of WS2

Terahertz spectroscopy is a powerful and contactless technique that enables to measure charge carrier properties in metals and semiconductors. However, the relatively long wavelengths of THz radiation and the diffraction limit imposed by optical imaging systems, reduces the applicability of THz spectroscopy considerably. We have developed a time-resolved terahertz near-field microscope that allows measurements of the carrier dynamics with sub-diffraction resolution. This microscope is used to measure an exfoliated flake of a 2D transition metal dichalcogenide crystal with a few tens of microns' resolution. Mapping carrier dynamics of semiconductors, non-invasively, and on micron length scales, opens new possibilities for material characterization.</p