High-speed asynchronous optical sampling ͑ASOPS͒ is a novel technique for ultrafast time-domain spectroscopy ͑TDS͒. It employs two mode-locked femtosecond oscillators operating at a fixed repetition frequency difference as sources of pump and probe pulses. We present a system where the 1 GHz pulse repetition frequencies of two Ti:sapphire oscillators are linked at an offset of ⌬f R = 10 kHz. As a result, their relative time delay is repetitively ramped from zero to 1 ns within a scan time of 100 s. Mechanical delay scanners common to conventional TDS systems are eliminated, thus systematic errors due to beam pointing instabilities and spot size variations are avoided when long time delays are scanned. Owing to the multikilohertz scan-rate, high-speed ASOPS permits data acquisition speeds impossible with conventional schemes. Within only 1 s of data acquisition time, a signal resolution of 6 ϫ 10 −7 is achieved for optical pump-probe spectroscopy over a time-delay window of 1 ns. When applied to terahertz TDS, the same acquisition time yields high-resolution terahertz spectra with 37 dB signal-to-noise ratio under nitrogen purging of the spectrometer. Spectra with 57 dB are obtained within 2 min. A new approach to perform the offset lock between the two femtosecond oscillators in a master-slave configuration using a frequency shifter at the third harmonic of the pulse repetition frequency is employed. This approach permits an unprecedented time-delay resolution of better than 160 fs. High-speed ASOPS provides the functionality of an all-optical oscilloscope with a bandwidth in excess of 3000 GHz and with 1 GHz frequency resolution.
Abstract:We demonstrate a rapid scanning high-resolution THz spectrometer capable of acquiring THz field transients with 1 ns duration without mechanical delay line. The THz spectrometer is based on two 1-GHz Ti:sapphire femtosecond lasers which are linked with a fixed repetition rate difference in order to perform high-speed asynchronous optical sampling. One laser drives a high-efficiency large-area GaAs based THz emitter, the other laser is used for electro-optic detection of the emitted THz-field. At a scan rate of 9 kHz a time resolution of 230 fs is accomplished. High-resolution spectra from 50 GHz up to 3 THz are obtained and water absorption lines with a width of 11 GHz are observed. The use of femtosecond lasers with 1 GHz repetition rate is essential to obtain rapid scanning and high time-resolution at the same time.
The B to A conformational transition of highly oriented DNA films due to a hydration change is observed with time-domain terahertz spectroscopy. Wet-spun films of calf thymus and salmon DNA are investigated for different film thicknesses and for different polarizations of the terahertz radiation relative to the DNA orientation. A clear polarization dependence is observed. Asynchronous optical sampling allows recording of terahertz absorption and background spectra in a few 10 s, permitting the tracking of the dehydration dynamics on a time scale of minutes. The observation of a phase transition is corroborated by Raman spectroscopy. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2743401͔The conformation of DNA is directly linked to important biological processes such as DNA replication and DNA repair. While DNA in aqueous solution is predominantly in the B form, it is also frequently seen to deviate from this conformation. Dehydration of B-type DNA to below 30% water content by weight converts it into A-type DNA. This conformational transition and its origins in terms of free energy difference between A-and B-type conformations have been intensively studied theoretically 1 and experimentally, e.g., by x-ray diffraction. 2-4Low-frequency vibrational excitations are expected to play an important role in conformational dynamics of DNA, as well as in the hydration shell, and might influence the dynamics of drug binding. 5 In particular, Raman and Brillouin scattering 6,7 as well as thermal neutron scattering 8 have been used to observe low-frequency excitation spectra. Far-infrared absorption spectroscopy in the terahertz frequency range provides another access to the low-frequency vibrational excitations of DNA. Besides Fourier-transform spectroscopy 9-11 and absorption spectroscopy using tunable terahertz light sources, 12,13 time-domain terahertz spectroscopy became available recently to study optical properties of biomolecules and DNA. [14][15][16][17] In this letter, we use a recently developed terahertz spectrometer based on high-speed asynchronous optical sampling ͑ASOPS͒ to measure the terahertz transmission characteristics of highly oriented wet-spun DNA films. We describe the dependences of the terahertz absorption in the range of 100 GHz-2.0 THz on the degree of hydration. Here, we take advantage of the fast sampling possibilities that are inherent to the ASOPS technique. The results from the terahertz experiments are corroborated by x-ray diffraction and Raman scattering experiments.Highly oriented films are produced from DNA sodium salt ͑Sigma-Aldrich Co.͒ using the wet-spinning method developed by Rupprecht. 18 A highly concentrated DNA solution is pumped through a spinneret and precipitated in 77% ethanol solution containing 0.3M NaCl at a temperature of 5°C such that thin DNA fibers are generated and spooled onto a rotating cylinder. Multilayered DNA films are taken off the cylinder and are dried at 5°C and are rehydrated at 75% relative humidity ͑r.h.͒ at room temperature for at least three d...
Abstract:We demonstrate a rapid scanning high-resolution THz spectrometer capable of acquiring THz field transients with 1 ns duration without mechanical delay line. The THz spectrometer is based on two 1-GHz Ti:sapphire femtosecond lasers which are linked with a fixed repetition rate difference in order to perform high-speed asynchronous optical sampling. One laser drives a high-efficiency large-area GaAs based THz emitter, the other laser is used for electro-optic detection of the emitted THz-field. At a scan rate of 9 kHz a time resolution of 230 fs is accomplished. High-resolution spectra from 50 GHz up to 3 THz are obtained and water absorption lines with a width of 11 GHz are observed. The use of femtosecond lasers with 1 GHz repetition rate is essential to obtain rapid scanning and high time-resolution at the same time.
Submonolayer homoepitaxy on Ir͑111͒ is studied by a refined kinetic lattice Monte Carlo ͑KLMC͒ model and compared to results obtained from scanning tunneling microscopy experiments. The KLMC model not only considers individual atomic jumps on regular and stacking-fault sites, but also describes the cooperative motion of small adatom clusters, which determines the temperature-dependent probability of stacking-fault island formation. A complete catalog of diffusion processes at island edges is included that allows one to model the variations of island shapes with temperature. By taking input parameters for cluster and edge diffusion from experiments, calculated island densities as well as the probability of stacking-fault formation agree very well with experimental results for different temperatures. The comparison of simulated and experimental island shapes, however, reveals obvious differences. After systematic modifications of the event database for edge diffusion processes, all features of island shape evolution are well reproduced.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.