Rafal Wilk

Highly accurate optical material parameter determination with THz time-domain spectroscopy

We improve the existing data extraction algorithms for THz time-domain spectroscopy (THz TDS) in two aspects. On the one hand, we merge the up-to-date knowledge of THz TDS signal processing into a single powerful optical material parameter extraction algorithm. On the other hand, we introduce a novel iterative algorithm that further enhances the accuracy of the parameter extraction. In contrast to most of the published experiments, we are able to reliably investigate samples with thicknesses as small as 100microm, samples with low indexes of refraction, i.e. close to 1, as well as samples with sharp peaks in the material parameter curves.

Terahertz time-domain spectroscopy and imaging of artificial RNA

We use terahertz time-domain spectroscopy (THz-TDS) to measure the far-infrared dielectric function of two artificial RNA single strands, composed of polyadenylic acid (poly-A) and polycytidylic acid (poly-C). We find a significant difference in the absorption between the two types of RNA strands, and we show that we can use this difference to record images of spot arrays of the RNA strands. Under controlled conditions it is possible to use the THz image to distinguish between the two RNA strands. We discuss the requirements to sample preparation imposed by the lack of sharp spectral features in the absorption spectra.

Liquid crystal based electrically switchable Bragg structure for THz waves.

We demonstrate the first electronically switchable Bragg structure for THz frequencies. The structure works as stop-band filter and as mirror. It exhibits the 60 GHz broad stop-band around 300 GHz which can be removed by reorienting liquid crystal molecules in an external electric field. Our first proof-of-principle experiments agree very well with transfer matrix calculations.

Optical sampling by laser cavity tuning

Most time-resolved optical experiments rely either on external mechanical delay lines or on two synchronized femtosecond lasers to achieve a defined temporal delay between two optical pulses. Here, we present a new method which does not require any external delay lines and uses only a single femtosecond laser. It is based on the cross-correlation of an optical pulse with a subsequent pulse from the same laser. Temporal delay between these two pulses is achieved by varying the repetition rate of the laser. We validate the new scheme by a comparison with a cross-correlation measurement carried out with a conventional mechanical delay line.

Continuous wave terahertz spectrometer as a noncontact thickness measuring device

We present a low cost terahertz (THz) spectrometer with coherent detection based on two simple and robust dipole antennas driven by two laser diodes. The spectrometer covers frequencies up to 1 THz, with a peak signal-to-noise ratio exceeding 40 dB for a lock-in integration time of 30 ms. We demonstrate that the thickness profile of a sample can be reconstructed from an acquired THz image.

High-Power Monolithic Two-Mode DFB Laser Diodes for the Generation of THz Radiation

We have devolved 1064 nm high-power monolithic distributed feedback lasers which operate simultaneously on two longitudinal modes. These modes correspond to the fundamental and first-order lateral mode and arise from a 7.5 mum width ridge waveguide supporting both of them. They are further stabilized by a first-order grating built into an InGaP/GaAs/InGaP multilayer structure. The threshold current of the laser is 66 mA, the slope efficiency is 0.5 W/A, and an output power of ~500 mW is reached. Detailed investigations of the intensity distribution of lateral and vertical far fields and the spectral behavior are shown. The longitudinal mode spacing at 260 mW is 0.56 nm corresponding to approximately 150 GHz. THz generation is demonstrated by mixing the two-line laser emission in a LT-GaAsSb photomixer.

Highly Accurate THz Time-Domain Spectroscopy of Multilayer Structures

We extract the optical material parameters of a layer within a multilayer sample from terahertz (THz) time-domain spectroscopy data. To this aim, we derive a suited transfer function for multilayer structures in general and use the recently introduced spatially variant moving average filter to improve the accuracy of the algorithm. We demonstrate two applications of our method experimentally: the determination of the thickness and optical material parameters of an unknown layer on a substrate and the investigation of liquids in a known cuvette.

Impact of the contact metallization on the performance of photoconductive THz antennas

Both AuGe based alloys and Ti/Au metal layer stacks are widely used as ohmic metal contacts for photoconductive THz antennas made of low temperature grown GaAs. Here, we present the first systematic comparison between these two metallization types. A series of antennas of both kinds is excited by femtosecond laser pulses and by the emission from two diode lasers, i.e. we test the structures as pulsed THz emitters and as photomixers. In both cases, coherent and incoherent detection schemes are employed. We find that the power emitted from the antennas with AuGe metallization is 50% higher than that of antennas with a Ti/Au metal layer. From a comparison with a photomixer model we conclude that the higher output power results from a lower contact resistance of the AuGe contacts leading to an increased current flow. However, Ti/Au contacts have a higher thermal stability which might be advantageous if high system stability is called for.

Fiber-coupled THz spectroscopy for monitoring polymeric compounding processes

We present a compact, robust, and transportable fiber-coupled THz system for inline monitoring of polymeric compounding processes in an industrial environment. The system is built on a 90cm x 90cm large shock absorbing optical bench. A sealed metal box protects the system against dust and mechanical disturbances. A closed loop controller unit is used to ensure optimum coupling of the laser beam into the fiber. In order to build efficient and stable fiber-coupled antennas we glue the fibers directly onto photoconductive switches. Thus, the antenna performance is very stable and it is secured from dust or misalignment by vibrations. We discuss fabrication details and antenna performance. First spectroscopic data obtained with this system is presented.

Probing Noncovalent Interactions in Biomolecular Crystals with Terahertz Spectroscopy

The far-infrared vibrational spectra of molecular crystals are dominated by intramolecular (internal) modes, which are also present in the isolated molecule, and noncovalent intermolecular modes, which arise from the interaction of the nearest neighbours (external modes). Conceptually this has long been understood and early experiments using Fourier transform infrared (FTIR) spectroscopy confirmed the existence of a rich vibrational spectrum in polypeptides in the low-energy region. Yet, the assignment of the experimentally observed peaks in the low-energy region is often unsatisfying. Recently, the advent of terahertz time-domain spectroscopy (THz TDS) has revitalized the field. Using this convenient room-temperature technique, noncovalent interactions between several smalland medium-sized molecules have been investigated, including nucleobases and nucleosides, short-chain polypeptides, cystine and glutathione, retinal, and saccharides. Chen et al. and Nagai et al. could clearly distinguish between intramolecular and intermolecular modes by comparing crystalline structures and solvated molecules. Furthermore, it was shown that different isomers as well as diverse crystalline forms show distinctively different terahertz spectra. Most of these experiments have benefited from a comparison with quantum-mechanical calculations of the normal modes, which are nowadays feasible due to advances both in computer technology and software development. Herein, we study two types of molecular crystals—one with weak [dimethyluracil (DMU), see Figure 1] and one with strong hydrogen bonds [thymine (THY), see Figure 2]. In order to gain insight into the chemical nature of the THz signals by assigning individual low-frequency modes, we compare our data to a