Step-scan time-domain terahertz magneto-spectroscopy.

We present a novel approach for terahertz time-domain spectroscopy of magneto-optic phenomena. The setup used in this work combines a tabletop pulsed magnet and a standard terahertz time-domain spectroscopy system. The approach is based on repetitive operation of the pulsed magnet and step-wise increment of the delay time of the time-domain spectroscopy system. The method is demonstrated by plotting the magneto-transmission spectra of linearly polarized THz pulses through the hole gas of a Ge sample and the electron gas of GaAs, InSb and InAs samples. Cyclotron resonance spectra are displayed in the frequency range from 200 GHz to 2 THz and for a magnetic field up to 6 T. The GaAs spectra are analyzed in more detail using simulations based on the Drude model.

Combless broadband terahertz generation with conventional laser diodes.

We present a novel technique to generate a continuous, combless broadband Terahertz spectrum with conventional low-cost laser diodes. A standard time-domain spectroscopy system using photoconductive antennas is pumped by the output of two tunable diode lasers. Using fine tuning for one laser and fine and coarse tuning for the second laser, difference frequency generation results in a continuous broadband THz spectrum. Fast coarse-tuning is achieved by a simple spatial light modulator introduced in an external cavity. The results are compared to multi-mode operation for THz generation.

High-speed terahertz time-domain spectroscopy of cyclotron resonance in pulsed magnetic field.

We present time-resolved cyclotron resonance spectra of holes in p-Ge measured during single magnetic field pulses by using a rapid-scanning, fiber-coupled terahertz time-domain spectroscopy system. The key component of the system is a rotating monolithic delay line featuring four helicoid mirror surfaces. It allows measurements of THz spectra at up to 250 Hz repetition rate. Here we show results taken at 150 Hz. In a single 900 ms measurement 135 cyclotron resonance spectra were recorded that fully agree with what is expected from literature.

Coherent electro-optical detection of terahertz radiation from an optical parametric oscillator.

We report the realization of coherent electro-optical detection of nanosecond terahertz (THz) pulses from an optical parametric oscillator, which is pumped by a Q-switched nanosecond Nd:YVO4 laser at 1064 nm and emits at approximately 1.5 THz. The beam profile and wavefront of the THz beam at focus are electro-optically characterized toward the realization of a real-time THz camera. A peak dynamic range of approximately 37 dB/radical Hz is achieved with single-pixel detection.

Sensitivity increase for coating thickness determination using THz waveguides.

We report on layer thickness determination down to a thickness of 2.5 microns using terahertz waveguide spectroscopy. Compared to typical single-pass transmission measurements in the time domain, the effective THz pulse delay is considerably increased for a given layer thickness by using the high filling factor of the THz waveguide. This corresponds to a sensitivity increase up to a factor of 50 for the measured delay, allowing the direct measurement of layer thicknesses down to below hundredths of a THz wavelength.

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.

Nanosecond terahertz optical parametric oscillator with a novel quasi phase matching scheme in lithium niobate.

We present an optical parametric oscillator pumped by a single mode Q-switched nanosecond Nd:YVO(4) laser for terahertz generation in periodically poled lithium niobate with a new phase matching scheme. This new method leads to an emission of terahertz radiation close to the Cherenkov angle and to a parallel propagation of the pump and signal wave. The emission frequency of this novel source is chosen by the poling period to 1.5 THz. For spectral narrowing the signal wave of the OPO is injection seeded. In the optical spectrum also cascaded processes are observed demonstrating a powerful generation of terahertz waves.

Generation of Narrowband Tunable THz-Radiation via Optical Rectification in Periodically Poled Materials.

The generation of tunableTHz-radiation via optical rectification offs-pulses in periodically poled nonlinearmaterials is reported and its applicationfor biomedical sensing using THz-senorsbased on planar wave guides is discussed indetail.

Generation of tunable narrow-band surface-emitted terahertz radiation in periodically poled lithium niobate.

Generation of tunable narrow-band terahertz (THz) radiation perpendicular to the surface of periodically poled lithium niobate by optical rectification of femtosecond pulses is reported. The generated THz radiation can be tuned by use of different poling periods and different observation angles, limited only by the available bandwidth of the pump pulse. Typical bandwidths were 50-100 GHz, depending on the collection angle and the number of periods involved.

Origin of magnetic field enhancement in the generation of terahertz radiation from semiconductor surfaces.

We present a theory of the magnetic field enhancement of terahertz (THz) emission from photogenerated carriers in the surface depletion region of a semiconductor. A combination of the Drude-Lorentz model for the carrier dynamics with an appropriate solution of the radiation problem is sufficient to explain the strong B -field enhancement in THz radiation that has been observed experimentally. The effect arises primarily from the increased radiation efficiency of transient currents flowing in the plane of the surface. The model provides quantitative agreement with experiment for the pronounced angular dependence of the enhancement and predicts the correct trend for the enhancement in a variety of materials.

Tuning characteristics of narrowband THz radiation generated via optical rectification in periodically poled lithium niobate.

The tuning properties of pulsed narrowband THz radiation generated via optical rectification in periodically poled lithium niobate have been investigated. Using a disk-shaped periodically poled crystal tuning was easily accomplished by rotating the crystal around its axis and observing the generated THz radiation in forward direction. In this way no beam deflection during tuning was observed. The total tuning range extended from 180 GHz up to 830 GHz and was limited by the poling period of 127 microm which determines the maximum THz frequency in forward direction.

Two wavelength femtosecond laser induced DNA-protein crosslinking.

Nucleic acid-protein interactions are essential for storage, reproduction and expression of genetic information. Biochemical methods, such as dimethyl sulfate genomic footprinting, have been developed to study stable protein-DNA interactions in vivo and chemical crosslinking has been used for less stable interactions, but the chemical agents are slow, damage cells and perturb native equilibria. To avoid these perturbations, UV laser crosslinking offers an alternative, although the energies required for significant crosslinking cause extensive DNA damage. We find that a combination of femtosecond laser pulses at two different wavelengths, in the UV and the visible range, increases the crosslinking efficiency while minimizing DNA damage. This technique also allowed us to directly measure the singlet S1lifetime of native DNA (tauS1 = 3.2 +/- 0.2 ps), which is mainly determined by the lifetime of thymine [tauS1 = 2.8 +/- 0.4 ps for (dT)16], the photochemically most reactive base. Our results suggest that two wavelength femtosecond laser pulses are well suited for the identification of transcription factors interacting with defined sequences and for studying the kinetics of protein-nucleic acid interactions in intact cells.

Crosslinking of progesterone receptor to DNA using tuneable nanosecond, picosecond and femtosecond UV laser pulses.

UV laser crosslinking is a potentially powerful tool to investigate transient DNA-protein interactions and binding kinetics in intact cells. As the processes underlying UV laser crosslinking are not fully understood, we have performed a study of the influence of laser pulses with different physical parameters on crosslinking of the progesterone receptor to an oligonucleotide containing a hormone-responsive element. We also studied the influence of the various parameters on the amount of laser-irradiated DNA that can be correctly primer extended as an operational measurement of DNA integrity. A strong influence of pulse intensity and pulse length on the crosslink yield was found, likely due to a change in the 'two photon' processes responsible for crosslinking. The highest efficiency of protein crosslinking to DNA was achieved with femtosecond pulses and should be sufficient to enable use of this technique for in vivo studies.

External frequency conversion of cw mode-locked Ti:Al(2)O(3) laser radiation.

Efficient frequency conversion of cw mode-locked Ti:A1(2)O(3) laser radiation using lithium triborate, beta-barium borate, and lithium iodate crystals as the nonlinear material is demonstrated. Second-, third-, and fourth harmonic generation results in tunable blue and ultraviolet radiation down to 205 nm with transform-limited pulses and pulse lengths below 1 ps. Maximum average powers (repetition rate 82 MHz) of 700 mW at 400 nm, 120 mW at 272 nm, and 10 mW at 210 nm were obtained. The effects of group-velocity dispersion have to be taken into account to optimize the different frequency-conversion processes.

Atomic Rydberg-state spectroscopy of Ca I using pulsed colorcenter lasers.

The utility of using color-center lasers for atomic spectroscopy is demonstrated. Pulsed, color-center lasers are used in conjunction with pulsed dye lasers for multiphoton ionization spectroscopy studies of Ca I. The 4s6d(1)D(2)-4snf(1)F( masculine)(3) spectrum for n = 11-16 is observed. Two other series of lines are identified as 4s5f(1)F( masculine)(3)-4sng(1)G(4) and X-4sng(1)G(4) transitions.