Benjamin Reinhard

Spectrally Wide-Band Terahertz Wave Modulator Based on Optically Tuned Graphene

New applications in the realms of terahertz (THz) technology require versatile adaptive optics and powerful modulation techniques. Semiconductors have proven to provide fast all-optical terahertz wave modulation over a wide frequency band. We show that the attenuation and modulation depth in optically driven silicon modulators can be significantly enhanced by deposition of graphene on silicon (GOS). We observed a wide-band tunability of the THz transmission in a frequency range from 0.2 to 2 THz and a maximum modulation depth of 99%. The maximum difference between the transmission through silicon and GOS is Δt = 0.18 at a low photodoping power of 40 mW. At higher modulation power, the enhancement decreased due to charge carrier saturation. We developed a semianalytical band structure model of the graphene-silicon interface to describe the observed attenuation and modulation depth in GOS.

Negative index bulk metamaterial at terahertz frequencies

We present a bulk metamaterial with negative refractive index in the terahertz frequency range. The structure is composed of pairs of metallic crosses embedded in Benzocyclobutene (BCB). The design is specifically chosen to provide a low-loss, free-standing material which operates under normal incidence and independently of the polarization of the incident radiation. These qualities allow the fabrication of 3D structures by mechanical stacking of multiple thin films.

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.

Gradient index metamaterial based on slot elements

We present a gradient-index (GRIN) metamaterial based on an array of annular slots. The structure allows a large variation in the effective refractive index under normal-to-plane incidence and thus enables the construction of GRIN devices consisting of only a small number of functional layers. Using full-wave simulations, we demonstrate the annular slot concept by means of a three-unit-cell thin GRIN lens for the terahertz (THz) range. In the presented realizations, we achieved an index contrast of Δn=1.5 resulting in a highly refractive flat lens which is suitable for focusing THz radiation to a spot size smaller than the wavelength.

Experimental and numerical studies of terahertz surface waves on a thin metamaterial film

We present experimental and numerical studies of localized terahertz surface waves on a subwavelength-thick metamaterial film consisting of in-plane split-ring resonators. A simple and intuitive model is derived that describes the propagation of surface waves as guided modes in a waveguide filled with a Lorentz-like medium. The effective medium model allows us to deduce the dispersion relation of the surface waves, in excellent agreement with the numerical data obtained from 3D full-wave calculations. Both the accuracy of the analytical model and the numerical calculations are confirmed by spectroscopic terahertz time-domain measurements.

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.

Metamaterial-Based Photonic Devices for Terahertz Technology

We review recent research on metamaterial-based devices for the terahertz frequency range. Possible applications for terahertz metamaterials include high-speed modulators, highly sensitive devices for refractometry or thin film sensing, efficient polarization optics, terahertz-absorbing materials, and high-performance gradient index optics. Due to the high flexibility in metamaterial design, these devices have the potential to outperform existing devices based on conventional materials.

Band structure of terahertz metallic photonic crystals with high metal filling factor

The band structure of two-dimensional photonic crystals consisting of metallic cylinders is investigated both experimentally and by numerical simulations. The crystals show large photonic band gaps in the terahertz spectral range.

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.