Terahertz (THz) radiation, which lies in the far-infrared region, is at the interface of electronics and photonics. Narrow-band THz radiation can be produced by free-electron lasers and fast diodes. Broadband THz radiation can be produced by thermal sources and, more recently, by table-top laser-driven sources and by short electron bunches in accelerators, but so far only with low power. Here we report calculations and measurements that confirm the production of high-power broadband THz radiation from subpicosecond electron bunches in an accelerator. The average power is nearly 20 watts, several orders of magnitude higher than any existing source, which could enable various new applications. In particular, many materials have distinct absorptive and dispersive properties in this spectral range, so that THz imaging could reveal interesting features. For example, it would be possible to image the distribution of specific proteins or water in tissue, or buried metal layers in semiconductors; the present source would allow full-field, real-time capture of such images. High peak and average power THz sources are also critical in driving new nonlinear phenomena and for pump-probe studies of dynamical properties of materials.

Keyword: boron nitride nanotubesBoron nitride nanotubes (BNNTs) are desired for their exceptional mechanical, electronic, thermal, structural, textural, optical, and quantum properties. Golberg [1] gives an excellent review of possible applications. To date, BNNTs have been grown by a number of techniques which can be divided roughly into two categories based on the class of material produced.One is the high temperature category in which energy is concentrated into a B or BN target at a level which can vaporize elemental boron. BNNTs form in the deposits of the liberated vapors. The energy is input by laser [2][3][4][5][6] or by arc discharge [7][8][9] . Only small quantities (mg's) of material have been produced by this method, but the tubes are high quality. They have one or just a few walls, and most importantly, the tube walls are low in defects and parallel to the axis of the nanotube. The second category is low temperature synthesis, between about 600 C and 1700 C, well below the vaporization temperature of pure boron (~4000 C). These low temperature synthesis methods can be further divided into two catagories. In the first category, ball-milled precursor powders of boron and catalyst are annealed in a nitrogen or ammonia gas atmosphere, sprouting nanostructures on their

Selective photothermal targeting of fatty tissues is feasible using infrared lipid absorption bands. Potential clinical applications are suggested by this FEL study.

Jefferson Laboratory's kW-level infrared free-electron laser utilizes a superconducting accelerator that recovers about 75% of the electron-beam power. In achieving first lasing, the accelerator operated "straight ahead" to deliver 38-MeV, 1.1-mA cw current for lasing near 5 &mgr;m. The waste beam was sent directly to a dump while producing stable operation at up to 311 W. Utilizing the recirculation loop to send the electron beam back to the linac for energy recovery, the machine has now recovered cw average currents up to 5 mA, and has lased cw with up to 1720 W output at 3.1 &mgr;m.

No abstract