Room-Temperature and Selective Triggering of Supramolecular DNA Assembly/Disassembly by Nonionizing Radiation.

Recent observations have suggested that nonionizing radiation in the microwave and terahertz (THz; far-infrared) regimes could have an effect on double-stranded DNA (dsDNA). These observations are of significance owing to the omnipresence of microwave emitters in our daily lives (e.g., food preparation, telecommunication, and wireless Internet) and the increasing prevalence of THz emitters for imaging (e.g., concealed weapon detection in airports, skin cancer screenings) and communication technologies. By examining multiple DNA nanostructures as well as two plasmid DNAs, microwaves were shown to promote the repair and assembly of DNA nanostructures and single-stranded regions of plasmid DNA, while intense THz pulses had the opposite effect (in particular, for short dsDNA). Both effects occurred at room temperature within minutes, showed a DNA length dependence, and did not affect the chemical integrity of the DNA. Intriguingly, the function of six proteins (enzymes and antibodies) was not affected by exposure to either form of radiation under the conditions examined. This particular detail was exploited to assemble a fully functional hybrid DNA-protein nanostructure in a bottom-up manner. This study therefore provides entirely new perspectives for the effects, on the molecular level, of nonionizing radiation on biomolecules. Moreover, the proposed structure-activity relationships could be exploited in the field of DNA nanotechnology, which paves the way for designing a new range of functional DNA nanomaterials that are currently inaccessible to state-of-the-art assembly protocols.

Ultra-high dynamic range electro-optic sampling for detecting millimeter and sub-millimeter radiation.

Time-domain spectroscopy using coherent millimeter and sub-millimeter radiation (also known as terahertz radiation) is rapidly expanding its application, owing greatly to the remarkable advances in generating and detecting such radiation. However, many current techniques for coherent terahertz detection have limited dynamic range, thus making it difficult to perform some basic experiments that need to directly compare strong and weak terahertz signals. Here, we propose and demonstrate a novel technique based on cross-polarized spectral-domain interferometry to achieve ultra-high dynamic range electro-optic sampling measurement of coherent millimeter and sub-millimeter radiation. In our scheme, we exploit the birefringence in a single-mode polarization maintaining fiber in order to measure the phase change induced by the electric field of terahertz radiation in the detection crystal. With our new technique, we have achieved a dynamic range of 7 × 10(6), which is 4 orders of magnitude higher than conventional electro-optic sampling techniques, while maintaining comparable signal-to-noise ratio. The present technique is foreseen to have great impact on experiments such as linear terahertz spectroscopy of optically thick materials (such as aqueous samples) and nonlinear terahertz spectroscopy, where the higher dynamic range is crucial for proper interpretation of experimentally obtained results.

Self-referenced spectral domain interferometry for improved signal-to-noise measurement of terahertz radiation.

In this work, we demonstrate self-referenced spectral domain interferometry for the electro-optic sampling of terahertz (THz) electric fields. This technique allows reduction of the phase noise of the measurement, thus increasing the signal-to-noise ratio (SNR). Using the proposed technique, we achieve a more than sixfold improvement in the SNR of the detected THz electric field.

Wavelength scaling of terahertz generation by gas ionization.

Low-frequency currents induced by ultrashort laser-driven ionization can emit extremely broadband, single-cycle terahertz pulses. We present a model that predicts a strong wavelength dependence of the THz emission in good agreement with our experimental study. This reveals that the combined effects of plasma currents rising proportionally to the square of the pump wavelength and wavelength-dependent focusing conditions lead to 30 times higher THz emission at 1800 nm compared to an 800 nm wavelength. Unrivaled single-cycle electric field strengths of 4.4 MV/cm are achieved with this compact table-top setup.

Laser ablation threshold dependence on pulse duration for fused silica and corneal tissues: experiments and modeling.

The surface ablation threshold fluence of fused silica and two porcine cornea layers, the epithelium and the stroma, is characterized as a function of the laser pulse duration in the range of 100 fs-5 ps for a wavelength of 800 nm (Ti:sapphire laser system). The plateaulike region observed between 100 fs and 1 ps for the corneal layers indicates that for use in laser surgery, laser pulse durations chosen within this range should be practically equivalent. Our model predicts that the ablation threshold will decrease rapidly for pulse durations in the low end of the femtosecond regime.