Jens Neu

Metamaterial-based gradient index lens with strong focusing in the THz frequency range

The development of innovative terahertz (THz) imaging systems has recently moved in the focus of scientific efforts due to the ability to screen substances through textiles or plastics. The invention of THz imaging systems with high spatial resolution is of increasing interest for applications in the realms of quality control, spectroscopy in dusty environment and security inspections. To realize compact THz imaging systems with high spatial resolution it is necessary to develop lenses of minimized thickness that still allow one to focus THz radiation to small spot diameters with low optical aberrations. In addition, it would be desirable if the lenses offered adaptive control of their optical properties to optimize the performance of the imaging systems in the context of different applications. Here we present the design, fabrication and the measurement of the optical properties of spectrally broadband metamaterial-based gradient index (GRIN) lenses that allow one to focus THz radiation to a spot diameter of approximately one wavelength. Due to the subwavelength thickness and the high focusing strength the presented GRIN lenses are an important step towards compact THz imaging systems with high spatial resolution. Furthermore, the results open the path to a new class of adaptive THz optics by extension of the concept to tunable metamaterials.

Metamaterial near-field sensor for deep-subwavelength thickness measurements and sensitive refractometry in the terahertz frequency range

We present a metamaterial-based terahertz (THz) sensor for thickness measurements of subwavelength-thin materials and refractometry of liquids and liquid mixtures. The sensor operates in reflection geometry and exploits the frequency shift of a sharp Fano resonance minimum in the presence of dielectric materials. We obtained a minimum thickness resolution of 12.5 nm (1/16 000 times the wavelength of the THz radiation) and a refractive index sensitivity of 0.43 THz per refractive index unit. We support the experimental results by an analytical model that describes the dependence of the resonance frequency on the sample material thickness and the refractive index.

Metamaterial-based gradient index beam steerers for terahertz radiation

We designed, fabricated, and optically characterized single and double layer metamaterial-based gradient index beam steerers for terahertz radiation. We measured a maximal deflection angle of 6°. The operation bandwidth of the beam steerers was 300 GHz around a center frequency of 1.3 THz. Within this bandwidth, the amplitude transmission was higher than 50%. Due to a thickness of only 100 μm or below, the implemented beam steerers are ideally suited for integration in compact terahertz measurement systems.

In-plane focusing of terahertz surface waves on a gradient index metamaterial film.

We designed and implemented a gradient index metasurface for in-plane focusing of confined terahertz (THz) surface waves. We measured the spatial propagation of the surface waves by two-dimensional mapping of the complex electric field using a THz near-field spectroscope. The surface waves were focused to a diameter of 500 μm after a focal length of approximately 2 mm. In the focus, we measured a field amplitude enhancement of a factor of 3.

Terahertz time domain spectroscopy for carrier lifetime mapping in the picosecond to microsecond regime.

We demonstrate a terahertz time-domain spectroscope for spatially resolved pump-probe experiments which are based on terahertz probe pulse generation with a photo-conductive switch synchronized to the pump pulse generation of a nanosecond laser. The accessible pump-probe delay ranges from 600 ps up to 200 μs. The spatial resolution of the spectroscope is better than 50 μm. We use the measurement system for spatially resolved lifetime mapping of photo-induced carriers in thin silicon in dependence on the photoexcitation intensity of the pump laser. At an optical pump intensity of 70mW/cm2, we measured a carrier lifetime of 15.7 μs for silicon with 200 μm thickness and a much shorter carrier lifetime of 233 ns for 30 μm thick silicon.

Optical tuning of ultra-thin, silicon-based flexible metamaterial membranes in the terahertz regime

We present a fabrication method for flexible optically tunable terahertz metamaterial membranes based on thinning and embedding of commercially available silicon wafers in the metamaterial structure. The resulting membrane thickness of less than 25 em allows for quasi etalon-effect free devices which can be designed to show impedance matching to the surrounding air. We fabricated a thin film spectral bandpass filter with a maximal transmission of 85% and a modulation depth upon optical tuning of 98% at an operating frequency of 0.65THz. Further, we discussed the charge carrier dynamics and the requirements for optical tuning.

Gas-phase reactivity of Cp* group IX metal complexes bearing aromatic N,N′-chelating ligands

A set of cationic group IX metal complexes of the type [(η5-Cp*)M(Cl)(N,N′)]+ bearing N,N′-coordinating ligands structurally related to 2,2′-bipyridine were synthesized and characterized by means of elemental analysis, spectroscopy and single crystal X-ray diffraction. These complexes can undergo two different modes of CH activation in the gas phase, which were examined by CID ESI-MS spectrometry and by DFT calculations. On one hand, a roll-over cyclometallation can occur at one of the aromatic rings of the N,N′-coordinated ligands. On the other hand, a deprotonation of one of the Cp* methyl groups can take place, leading to complexes in the oxidation state +I with a tetramethylfulvene ligand coordinated. In both cases, the chlorido ligand acts as an internal base in the ESI-MS CID experiment.

Bound terahertz waves on meta-surfaces and active metamaterials

We present a numerical and experimental study of the dispersion relation and propagation properties of bound surface waves on a meta-surface consisting of a single layer array of split ring resonators. Furthermore, we introduce an analytic model that allows one to determine the influence of nonlinear effects on the temporal dynamics of a coupled system composed of a split ring resonator metamaterial and a two-level atomic gain medium.

Metamaterial-based optical components for THz radiation

We present different metamaterial-based optical components that open exciting new ways to deliberately manipulate the spatial or spectral properties of terahertz (THz) radiation. The central scope of the paper is the design, fabrication and optical characterization of a 3-layer gradient index (GRIN) lens that allows one to strongly focus THz radiation to a spot diameter in the order of one wavelength. The GRIN lens is based on a specific metamaterial structure that is fully embedded in a polymer background matrix and thus provides a free-standing optics with high transmission and minimal Fresnel reflection. THz lenses with such strong focusing capabilities are especially intriguing with respect to the resolution enhancement of THz imaging systems.

Metamaterial-based, gradient index beam steerer for terahertz radiation

We designed and fabricated a metamaterial-based beam steerer that deflects an incident terahertz beam by 4.2° due to an implemented gradient index structure. We characterized it using THz-TDS with three-dimensional subwavelength spatial resolution.