We report on our research in power scaling OPSL around 1 μm to exceed 100W per chip by combining a rigorous quantum design of an optimized MQW epitaxial structure, highly accurate and reproducible wafer growth and an efficient thermal management strategy. Recently we have utilized these state-of-the-art optimized OPSL chips to achieve a new record for a mode-locked OPSL with an intra-cavity SESAM. The average output power of the laser in the optimum operation point of mode-locked operation was 5.1W while being pumped with 25W of net pump power. This corresponds to a pulse energy of 3 nJ and a pulse peak power of 3.8 kW.
The microscopic theory for the nonequilibrium optical properties of VECSELs is summarized. Detailed experiments
of VECSELs under two-color operation conditons are performed utilizing streak camera measurements
of the laser output. A statistical analysis reveals the stability range of two-color emission and shows that this
operation mode is possible even in the presence of relatively large losses.
We present a handheld fiber-coupled terahertz spectrometer operating at a center wavelength of 1550 nm. The key
elements are a fs-fiber laser, a fiber stretcher delay line and fiber-coupled antennas, which contain novel InAlAs-InGaAs
multi layer chips. First experimental data obtained with this system demonstrates its great potential and robustness. In
addition, we investigate different hazardous and harmless liquids in reflection geometry. These experiments show that
liquids are in principle distinguishable by terahertz spectroscopy. Finally, first steps towards an algorithm that allows for
an extraction of the liquids dielectric properties are discussed. The algorithm works for the analysis of reflection data
even if the liquid is located inside a container.
We demonstrate concepts for compact and cost effective THz technology based on semiconductor diode lasers. In detail,
we analyze diode laser based THz sources and detectors. Continuous wave THz radiation is generated by two color diode
lasers either with external photomixers or direct difference frequency generation in the diode laser. For time domain THz
sampling applications we present a suitable mode-locked diode laser system. Further we present a method to detect THz
radiation with diode lasers at room temperature: A THz signal coupled into the active region of a diode laser results in a
variation of the voltage across the p-n-junction.
We demonstrate the detection of metallic and nonmetallic foreign bodies in chocolate using pulsed terahertz imaging. Investigating the shape of the temporal waveform allows for the discrimination between wanted ingredients like nuts on one hand and foreign bodies like stone, glass, or plastic particles on the other hand. Yet, the intensity image alone does not provide enough information to evaluate the quality of the chocolate bar. To achieve a low false-alarm rate it is important to measure the height profile of the sample and to include the measured results in the image-processing step. Our results show that terahertz imaging can be used for the detection of contaminations in chocolate bars. Furthermore, other kinds of dry food can be investigated with our technique.
The worldwide production volume of polymers is still rising exponentially and the number of applications for plastic
components steadily increases. Yet, many branches within the polymer industry are hardly supported by non-destructive
testing techniques. We demonstrate that terahertz (THz) spectroscopy could be the method of choice to ensure high-quality
polymer products. Applications range from the in-line monitoring of extrusion processes and the quality control
of commodities in a mass production up to a total inspection of high-tech safety relevant products. Furthermore, we
present an extension to THz time-domain spectroscopy in the form of a new data extraction algorithm, which derives the
absorption coefficient, the refractive index and the thickness of a sample with very high precision in a single pass.
Apart from that, we discuss the ability of THz systems for quality control of polymeric compounds. Here, it is essential
to monitor the additive content as well as additive inhomogeneities within the mixture. Recently, we built a fiber-coupled
THz spectrometer for in-line monitoring of compounding processes. Additionally, we demonstrate the potential of THz
systems for the non-destructive and contactless testing of structural components. THz imaging is capable of analyzing
material thicknesses, superstructures, the quality of plastic weld joints, and of detecting flaws in components.
Plastics and THz form a very fruitful symbiosis. In return, plastics industry can provide THz systems with custom-tailored
components, which have very attractive properties and extremely low costs. Examples of this development are
photonic crystals or polymeric Bragg filters, which have recently been demonstrated.
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.
Polymers cover the whole range from commodities to high-tech applications. Plastic products have also gained in
importance for construction purposes. This draws the attention to joining techniques like welding. Common evaluation of
the weld quality is mostly mechanical and destructive. Existing non-destructive techniques are mostly not entirely
reliable or economically inefficient. Here, we demonstrate the potential of terahertz time-domain spectroscopy imaging
as a non-destructive testing tool for the inspection of plastic weld joints. High-density polyethylene sheets welded in a
lap joint with varying quality serve as samples for terahertz transmission measurements. Imperfections within the weld
contact area can clearly be detected by displaying the transmitted intensity in a limited frequency range. Contaminations
such as metal or sand are identified since they differ significantly from the polymer in the terahertz image. Furthermore,
this new and promising technique is capable of detecting the boundaries of a weld contact area. Aside from revealing a
contrast between a proper weld joint and no material connection, the size of an air gap between two plastic sheets can be
determined by considering the characteristic frequency-dependent transmission through the structure: The spectral
positions of the maxima and minima allow for the calculation of the air layer thickness.
We present two-dimensional photonic-crystal waveguides for fluid-sensing applications in the sub-terahertz range. The
structures are produced using a standard machining processes and are characterized in the frequency range from 67 to
110 GHz using a vector network analyzer. The photonic crystal consists of an air-hole array drilled into a high-density
polyethylene block. A waveguide is introduced by reducing the diameter of the holes in one row. The holes can be
loaded with liquid samples. For all structures we observe photonic band gaps between 97 and 109 GHz. While the pure
photonic crystal shows the deepest stop band (28 dB), its depth is reduced by 5 dB when inserting a waveguiding
structure. The depth of the photonic band gap is further reduced by several decibels depending on the refractive index of
the liquid that is inserted. With this type of fluid sensor we can clearly distinguish between cyclohexane and
tetrachloromethane with refractive indices of 1.42 and 1.51, respectively. The results are in good agreement with
theoretical calculations based on the 2D finite-difference time-domain (FDTD) method.
To evaluate the potential of THz imaging systems for mail and luggage inspection we study a set of letters containing different hazardous items. The samples are investigated with three different THz systems available in our group: A microwave based system working around 100 GHz, a THz time-domain system and a THz gas laser. We provide a comparative discussion on our results and the advantages and disadvantages of each system.
Dielectric mirrors are widely used in optical setups for spectral regions such as UV, visible, as well as IR. Yet, for the rapidly growing field of terahertz spectroscopy dielectric multilayer optics are sparsely utilized. But with low-loss materials high quality THz optics can be obtained. We present two approaches for the realization of highly effective dielectric THz mirrors. First, four thin slices of high-resistivity silicon and five common polypropylene (PP) foils were alternately stacked together to obtain a broad reflection band. This stop-band blueshifts with increasing angles of incidence. But due to the high index step between Si and PP a band from 0.32 to 0.375 THz always remains the stopband for all incidence angles and both the s- and p-polarization. The measurement data obtained in reflection and transmission geometry are reproduced well by numerical simulations. With a minor change of the layer sequence a microresonator is obtained which reveals a sharp transmission peak at around 0.3 THz within the reflection band. The second material system consists of ceramic laminates of alumina (A) and alumina-zirconia (AZ). Measurements on 12.5 pairs of A/AZ layers yield a strong stop-band from 0.3 to 0.37 THz at normal incidence, which again match numerical simulations. The big advantage of the ceramic mirror is the rugged, quasimonolithic design of the sintered multilayer structure.
In this paper we show that the real emission frequency of a photoconductive dipole antenna operating in
conjunction with photomixers is not its natural resonant frequency, but a frequency where the antenna's input resistance
is the highest and the corresponding mismatch with the source is the smallest. We also introduce a new kind of antenna
that offers a much higher input resistance to the photomixer and hence enhances the efficiency of the continuous-wave
We demonstrate a semi-confocal THz imaging system based on a THz gas laser operating at 2.52 THz (118 μm) with power up to tens of milliwatts. Our ultimate goal is the development of a fully confocal THz microscope. We discuss the stability of the system and present THz images obtained on different objects to illustrate the potential of the system.
Short range wireless communication systems are expanding at rapid rate, finding application in offices, congested urban areas and homes. Development of wireless local area networks is accompanied by a steady increase in the demand for ever higher data rates. This in turn entails the necessity to develop communication systems which operate at higher frequencies. Currently WLAN works at a few GHz, while systems operating at several ten GHz appear already feasible. It can be expected that wireless short-range communication networks will soon push towards the THz frequency range and that systems which handle high-density information and support wider bandwidth communications will be developed in a few years time. Since THz radiation is strongly absorbed by the atmosphere, working distances may be short and individual THz pico-cells may cover only single rooms or at most one building. For an indoor system of practical importance it must be robust against shadowing. Recently, flexible all-plastic mirrors, supporting specular reflections in the THz range have been demonstrated. They are cheap and easy to produce and can be used as frequency selective wall-paper to enhance the reflectivity of walls and hence facilitate non-line-of-sight communication in a THz cell. For this case the spatial and temporal characteristics of the indoor THz propagation channel in a room with randomly placed objects and moving people are derived with ray-tracing methodology and Monte Carlo simulations. Our simulations show that high-gain antennas will be needed for the realization of THz communication in indoor environments. Furthermore, indirect transmission paths between transmitter and receiver, supported by dielectric mirrors make the communication channel much more robust against shadowing.
Polymers are often mixed with other additives or fillers to yield compounds with modified physical properties. In most cases a homogeneous mixture is desired. Yet, it often remains difficult to verify the degree of homogeneity of the resulting compound especially for nano-scaled fillers. We present initial experiments to evaluate the potential of terahertz (THz) spectroscopy for the quality control of polymeric compounds. We study low-density polyethylene (LDPE) samples which contain titanium dioxide nanospheres with a typical diameter of 270 nm which themselves are coated with even smaller silver nanoparticles with a typical diameter of 20 nm. Images obtained with a standard terahertz time-domain spectrometer show significant inhomogeneities in the compound on a millimeter scale. The imaging results indicate imperfectly mixed material regions. On the other hand, we show that also fluctuations in the sample thickness can lead to inhomogeneous terahertz images. A final conclusion, if the inhomogeneities observed in our LDPE/Ag-TiO2 samples result from variations in the compound composition or from thickness fluctuations, cannot be drawn at this point.
We investigate small peptides using standard terahertz (THz) time-domain spectroscopy. As a test set we examine the tripeptides glutathione, gly-gly-gly and enalapril maleate at room temperature. While earlier investigations of short-chain polypeptides with a conventional FTIR spectrometer were performed at higher THz frequencies, we present first measurements between 150 GHz and 2 THz and compare our measurements to density functional theory (DFT) calculations in order to assign the measured resonances to distinct molecular motions. DFT calculations obtained for a single molecule (glutathione), dimers (gly-gly-gly) and ion pairs (enalapril maleate) coupled via H-bonds, reproduce correctly the number of resonances observed in the experiment.
We have used device structures based on the well-established technology for producing high electron mobility transistors to modulate the amplitude and the phase of broadband terahertz pulses. The application of an external gate voltage allowed us to deplete the two-dimensional electron gas, which in turn increased the transmission of the device to THz radiation. In differential transmission experiments at room-temperature we achieved a typical amplitude change of 2-3 % for THz pulses passing through the device. In this paper we present the results of a detailed investigation of the device behavior including I-V characteristics and capacitance measurements that reflect the depletion of the two-dimensional electron gas. In a first demonstration experiment we were also able to use the modulator for audio signal transmission over a THz communication channel. To do this we modified a standard THz time-domain spectrometer to transmit signals up to 25 kHz imprinted onto a 75 MHz train of broadband THz pulses with frequencies between
100 GHz and 3 THz.
We observe ultrafast polarization dynamics in strongly internally biased InGaN/GaN multiple quantum wells during intense femtosecond optical excitation by means of time-resolved detection of THz emission, correlated with time-integrated photoluminescence measurements. We demonstrate that in the case of strong enough excitation the built-in bias field (on the order of MV/cm) can be completely screened by the carriers excited into spatially separated states. This ultrafast screening of the initial bias field across the quantum well leads to dynamical modification of the band structure of the sample, and consequently to dynamical modification of the optical absorption coefficient within the duration of the excitation pulse. We show that such an optically induced dynamical screening of the biased quantum well can be described in terms of discharging of a nano-scale capacitor with a femtosecond laser pulse. The electrostatic energy stored in the capacitor is released via THz emission. A realistic quantum-mechanical model of the temporal evolution of the polarization inside the quantum wells shows that due to its nonlinearity such a process may lead to emission of a THz pulse with bandwidth significantly exceeding that of the excitation pulse.
We present and analyze a new concept for a semiconductor laser with a Fourier-transform external cavity that allows for purely electronically operated wavelength tuning, simultaneous control of multiple gain media within one shared external cavity, and simultaneous multi-wavelength emission from one laser diode only. We investigate the new setup with particular focus on simultaneous two or even multi-wavelength operation from the same diode. The simultaneity of the two modes is unambiguously proven by sum-frequency generation. In addition, we discuss applications of the simultaneous two-wavelength operation with a particular focus on Terahertz difference frequency generation in combination with a photomixer.
We present a first investigation of liquid crystals in the frequency range around 1 THz using THz time-domain spectroscopy. As a model system we use 4-(trans-4'- pentylcyclohexyl)-benzonitril (PCH-5). The THz refractive indices for the nematic state are comparable to that for visible frequencies. Yet, the THz birefringence of PCH-5 is smaller than its optical birefringence. Furthermore we find that PCH-5 has a rather small absorption.
We investigate the potential of THz imaging for the examination of histo-pathological samples. Data obtained on a pig larynx and on a human liver containing cancerous tissue are presented. Different types of tissue are clearly resolved due to their distinct spectral absorption characteristics or due to a density dependent THz transmission.
In the beginning of this article we briefly outline the working principle of terahertz imaging. This relatively new technique is based on THz time-domain spectroscopy and has the potential to lead to the first portable far-IR imaging spectrometer. For such a spectrometer many applications can be foreseen in the fields of biology, medicine, chemistry and material science. Here we present two biological applications. First we show that THz-imaging is an ideal tool for dendrochronology as it allows us to obtain density profiles of wood specimen. Secondly, we monitor water take up in plants after plant water stress.
We present an electrical pump optical probe experiment to investigate the temporal and spectral gain dynamics of an actively modelocked diode laser. The temporal behavior of the gain is studied with picosecond time resolution by measuring the transmission of a modelocked Ti:sapphire laser synchronized to the semiconductor laser through the active region of the diode. Our results show that the gain transients exhibit a strong temporal asymmetry which leads to incomplete modelocking.