Tunable structures and modulators for THz light

In this work we present a technique for optically modulating a terahertz pulse inside a parallel plate waveguide. A novel semiconductor filled waveguide is formed by coating both sides of a thin, high resistivity silicon slab with a transparent conducting oxide. While the waveguide is intrinsically lossy due to the low conductivity of the oxides, it permits photoexcitation through the plates, generating free carriers within the silicon that modulates the terahertz pulse transmission. We demonstrate this modulation by observing the Drude response of photoexcited carriers within the silicon in a narrow strip inside the waveguide.

Optical modulation of terahertz pulses in a parallel plate waveguide.

Ultrafast and low-power terahertz wave modulator based on organic photonic crystal

This thesis is devoted to an experimental study of dielectric properties of incipient ferroelectrics. The terahertz time-domain spectroscopy was used to investigate the complex permittivity spectra in both single crystals and various strained thin film structures versus temperature and applied electric bias. Namely, it allowed characterizing the ferroelectric soft-mode dynamics and its coupling to a central mode. An electric-field tunability of bulk single crystals of SrTiO3 was determined up to room temperature for the first time. The phenomenon is governed by soft-mode stiffening under applied field. As an application we proposed and characterized a tunable one-dimensional photonic structure with a thin SrTiO3 plate inserted as a defect layer. The importance of the soft-mode dynamics was stressed also in the study of bulk high density KTaO3 ceramics. A systematic study was performed of a KTaO3 thin film and especially of a set of strained multilayers consisting of SrTiO3/DyScO3 bilayers grown on DyScO3 substrate. Strain-induced ferroelectric transition was observed in these films governed by the soft mode coupled to a lower-frequency relaxation. A general model was developed describing the whole family of the studied samples. Effect of the composition stoichiometry of SrTiO3 films grown on DyScO3 was also investigated.

Tunable materials and structures for terahertz spectral range

THz optics has experienced a tremendous increase in interest among the scientific community as
better THz sources and detection schemes are discovered. With recent studies in THz modulation
experiment using optically excited Si opens a new possibility in constructing THz optics using an
optically controlled THz SLM. Thus, various patterns, such as zone lenses, could be optically
constructed and tuned in real time for used in THz beam correction. For example, optically
constructed Fresnel zone lenses on high-resistivity Si can be actively tuned for focal length and
chromatic aberration, which are just some possible applications of this methodology. In this paper,
we will present results for an optically controlled single pixeled THz semiconductor SLM for use as
a modulator and discuss extensions to applications in zone lenses, diffraction gratings, and other
optical components.

Semiconductor based optically controlled thz optics

High-Efficiency Silicon THz Modulator Using Optically Injected Carriers

Optically controlled modulation of broadband THz radiation with a comparably uniform spatial distribution is demonstrated in a Si-based semiconductor structure with moderate doping. Using THz Time-Domain Spectroscopy a maximum intensity modulation of more than 99% was demonstrated for a spectrum ranging from 50GHz to 3.5THz with 3dB attenuation already for optical excitation as low as only 5mW. The uniformity of the modulation was measured and compared to the THz beam profile.

Highly efficient THz modulation using optically excited silicon

The THz transmittance of a silicon sample could be optically controlled, allowing broadband THz attenuation to less than 1% measured in a standard THz time domain spectroscopy setup.

Canan Karaalioglu

Papers

Semiconductor based optically controlled thz optics

High-Efficiency Silicon THz Modulator Using Optically Injected Carriers

Highly efficient THz modulation using optically excited silicon

Broadband THz Attenuation by Optically Excited Charge Carriers in Silicon