Climate variability in the Indian Ocean region seems to be, in some aspects, independent of forcing by external phenomena such as the El Nino/Southern Oscillation1,2,3,4 But the extent to which, and how, internal coupled ocean–atmosphere dynamics determine the state of the Indian Ocean system have not been resolved Here we present a detailed analysis of the strong seasonal anomalies in sea surface temperatures, sea surface heights, precipitation and winds that occurred in the Indian Ocean region in 1997–98, and compare the results with the record of Indian Ocean climate variability over the past 40 years We conclude that the 1997–98 anomalies—in spite of the coincidence with the strong El Nino/Southern Oscillation event—may primarily be an expression of internal dynamics, rather than a direct response to external influences We propose a mechanism of ocean–atmosphere interaction governing the 1997–98 event that may represent a characteristic internal mode of the Indian Ocean climate system In the Pacific Ocean, the identification of such a mode has led to successful predictions of El Nino5; if the proposed Indian Ocean internal mode proves to be robust, there may be a similar potential for predictability of climate in the Indian Ocean region

Coupled ocean–atmosphere dynamics in the Indian Ocean during 1997–98

We present what is to our knowledge the first imaging system based on optoelectronic terahertz time-domain spectroscopy Terahertz time-domain waveforms are downconverted from the terahertz to the kilohertz frequency range, and the waveform for each pixel is frequency analyzed in real time with a digital signal processor to extract compositional information at that point We demonstrate applications to package inspection and chemical content mapping in biological objects

Imaging with terahertz waves

The use of terahertz pulses for imaging has opened new possibilities for scientific and industrial applications in the terahertz frequency range. In this article, we describe the technology necessary to perform terahertz "T-ray" imaging, novel imaging techniques, and commercial applications of T-ray imaging.

T-ray imaging

We propose a generally applicable velocity matching method for THz-pulse generation by optical rectification in the range below the phonon frequency of the nonlinear material. Velocity matching is based on pulse front tilting of the ultrashort excitation pulse and is able to produce a large-area THz beam. Tuning of the THz radiation by changing the tilt angle is experimentally demonstrated for a narrow line in the range between 0.8-0.97 times the phonon frequency. According to model calculations broadband THz radiation can be generated at lower frequencies. Advantages of the new velocity matching technique in comparison to the electro-optic Cherenkov effect and non-collinear beam mixing are discussed.

Velocity matching by pulse front tilting for large area THz-pulse generation.

Recent studies of the electromagnetic response of high-Tc superconductors using terahertz, infrared, and optical spectroscopies are reviewed. In combination these experimental techniques provide a comprehensive picture of the low-energy excitations and charge dynamics in this class of materials. These results are discussed with an emphasis on conceptual issues, including evolution of the electronic spectral weight in doped Mott-Hubbard insulators, the d-wave superconducting energy gap and the normal-state pseudogap, anisotropic superfluid response, electronic phase segregation, emergence of coherent electronic state as a function of both temperature and doping, the vortex state, and the energetics of the superconducting transition. Because the theoretical understanding of these issues is still evolving the review is focused on the analysis of the universal trends that are emerging out of a large body of work carried on by many research teams. Where possible data generated by infrared/optical techniques are compared with the data from other spectroscopic and transport methods.

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Electrodynamics of high- T c superconductors

We measure directly both the real and imaginary parts of the sub-band-gap conductivity \ensuremath{\sigma} in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ in the 15--80 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ frequency range using coherent time-domain terahertz spectroscopy. We observe a peak in the real part ${\mathrm{\ensuremath{\sigma}}}_{1}$ similar to the coherence peak expected in an s-wave BCS superconductor. The absence of such a peak in NMR relaxation-rate experiments suggests that its origin lies in a strongly temperature-dependent inelastic-scattering rate rather in coherence factors.

Dynamic conductivity and coherence peak in YBa 2 Cu 3 O 7 superconductors

Four-pole microstrip bandpass filters have been fabricated using both postannealed and in-situ-grown thin films of Y-Ba-Cu-O. The 4-GHz, 3% bandwidth filters exhibit, at 77 K, a passband insertion loss as low as 0.3 dB, compared with the 2.8-dB loss of similar gold metallic filters at the same temperature. These results demonstrate that complex passive microwave devices can be designed and implemented using high-temperature superconducting (HTS) material. More aggressive filter designs utilizing many more poles will result in much greater performance advantages for an HTS filter technology over a normal metal technology. Ultimately, ultrasharp-skirt filters can be produced, which would allow for more closely spaced communication channels and more efficient use of microwave communication bands than is currently possible with conventional filters.

High-T/sub c/ superconductive microwave filters

We measure the complex conductivity of thin superconducting niobium films using coherent time‐domain terahertz spectroscopy. Both real and imaginary part of the conductivity are measured simultaneously without referring to the Kramers–Kronig relations. We also measure the superconducting band gap directly from the onset of absorption and compare our results with theory.

Terahertz time‐domain measurement of the conductivity and superconducting band gap in niobium

The design and fabrication of high-T/sub c/ chirp-response tapped delay line filters are discussed. The key components in this development were long delay lines with more than 10 ns of delay, impedance transformers, and backward-wave couplers. All of the typical transmission line geometries were examined, including microstrip, coplanar and stripline. Designs were developed using microwave CAD routines and superconducting niobium delay line prototypes. Tapped delay line chirp filters with up to 12 ns of total delay have been successfully fabricated in YBaCuO.

High-T/sub c/ superconductive delay line structures, and signal conditioning networks

The surface resistance and surface reactance of high-quality YBa/sub 2/Cu/sub 3/O/sub 7/ superconducting films were determined from 10 GHz to 500 GHz using time-domain propagation measurements of picosecond electrical pulses on coplanar transmission lines. For the surface resistance, the authors find a square law dependence with frequency almost up to 500 GHz and a crossover of the losses of gold and YBa/sub 2/Cu/sub 3/O/sub 7/ films at 77 K at roughly 100 GHz. The pulse propagation is successfully modeled using the Mattis-Bardeen theory for the frequency-dependent conductivity of YBa/sub 2/Cu/sub 3/O/sub 7/. Good quantitative agreement of the surface resistance and surface reactance with this theory is obtained. The potential of YBa/sub 2/Cu/sub 3/O/sub 7/ superconducting films and equivalent gold and superconducting films for high-speed applications is studied theoretically.

M. L. O’Malley

Papers

Dynamic conductivity and coherence peak in YBa 2 Cu 3 O 7 superconductors

High-T/sub c/ superconductive microwave filters

Terahertz time‐domain measurement of the conductivity and superconducting band gap in niobium

High-T/sub c/ superconductive delay line structures, and signal conditioning networks

Time-domain measurement of the surface resistance of YBa/sub 2/Cu/sub 3/O/sub 7/ superconducting films up to 500 GHz