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.

We present time domain measurements of surface-plasmon polaritons ͑SPP's͒ at terahertz ͑THz͒ frequencies. SPP's are generated by coupling THz radiation into dielectric films deposited on flat gold surfaces. In this way we are able to perform very sensitive and broadband THz measurements of thin dielectric films, demonstrating the capabilities of SPP's for Thz thin film spectroscopy. By varying the thickness of these films we observe drastic changes in the field distribution of the SPP's.Surface plasmon polaritons ͑SPP's͒ at metal-dielectric interfaces have proven to be a reliable technique for the investigation of thin films, allowing to derive properties such as optical and dielectric constants, film thickness, and inhomogeneities at interfaces with high precision ͑for an overview see Ref. 1͒. Additionally, it has been discovered recently that SPP's play a crucial role in the extraordinary high transmission of light through arrays of sub-wavelength holes. 2,3 Thanks to SPP's light can be concentrated and controlled over length scales much smaller than the wavelength. As a result, a new and promising research field known as plasmonics has emerged, one of its primary goals being the fabrication of nanoscale photonic circuits. 4 With the development of short-pulse lasers terahertz ͑THz͒ spectroscopy has opened up an interesting but hardly accessible spectral window where a large variety of gases, liquids, and solids show specific resonances. THz applications range from studies of coherent excitations in semiconductor heterostructures to medical diagnostics and threedimensional imaging systems for monitoring industrial processes. 5,6 Key biological constituents such as proteins, ribonucleic acids, and deoxyribonucleic acids have resonances at THz frequencies, 7-10 which makes THz radiation of keen interest for direct and simple biosensing. The latter often involves the analysis of typically very thin biomolecular films. Surface plasmon polariton ͑SPP͒ spectroscopy at THz frequencies has a large potential for biosensing applications, providing high electromagnetic field strengths in the films together with long interaction lengths. 1 In spite of these important advantages almost no work has been done so far on SPP's in the THz frequency range. 11 In this paper we present the first time-domain THz study of SPP's on metal surfaces and metals covered with dielectric films. The broadband character of our THz setup allows us to obtain information over a wide range of frequencies between 0.2 and 2 THz in a single measurement. As we will show, small changes on the film thickness relative to the THz wavelength lead to drastic modifications on the SPP's field distribution and spectrum. These modifications demonstrate the capabilities of terahertz SPP's for sensitive spectroscopy of thin films. Figure 1͑a͒ illustrates the field geometry used for exciting SPP's. A semi-infinite dielectric and a semi-infinite metal define an interface at zϭ0. A p-polarized electromagnetic wave incident from the (y,z) plane is coupled into a surf...

Two femtosecond Ti:sapphire lasers with slightly different repetition rates near 1 GHz are coupled to implement high-speed asynchronous optical sampling. The application of this technique is successfully demonstrated in the field of terahertz time-domain spectroscopy (TDS). A time delay of 1 ns is scanned at a frequency of 5 kHz without moving mechanical parts. Compared with that of conventional TDS schemes based on lock-in detection and moving mirrors, the readout time of integrated resonant THz sensors is reduced by a factor of 20, opening the way for high-throughput THz sensing in marker-free DNA analysis.

We report an ultrafast time-domain spectroscopy system based on high-speed asynchronous optical sampling operating without mechanical scanner. The system uses two 1 GHz femtosecond oscillators that are offset-stabilized using high-bandwidth feedback electronics operating at the tenth repetition rate harmonics. Definition of the offset frequency, i.e. the time-delay scan rate, in the range of a few kilohertz is accomplished using direct-digital-synthesis electronics for the first time. The time-resolution of the system over the full available 1 ns time-delay window is determined by the laser pulse duration and is 45 fs. This represents a three-fold improvement compared to previous approaches where timing jitter was the limiting factor. Two showcase experiments are presented to verify the high time-resolution and sensitivity of the system.

We report a terahertz time-domain spectrometer with more than 6 THz spectral coverage and 1 GHz resolution based on high-speed asynchronous optical sampling. It operates at 2 kHz scan rate without mechanical delay stage. The frequency error of the system at 60 s acquisition time is determined by comparing a measured water vapor absorption spectrum to data reported in the HITRAN database. The mean error of 87 evaluated absorption lines is 142 MHz.