Todd I. Smith

Reducing the sensitivity of a free‐electron laser to electron energy

A technique is described which allows a free‐electron laser to utilize an electron beam with a wide energy spread. A laser using this technique can be powered by a greater variety of electron sources than would otherwise be possible. In addition, the techniques ease some of the problems encountered when designing a laser to operate in conjunction with a storage ring.

IR microscopy with a transient photo-induced near-field probe (tipless near-field microscopy)

Photo-induced reflectivity generated by picosecond pulses of light incident on the surface of semiconductors has been used to create transient mirrors with dimensions determined by the spot size of the visible light. These mirrors were used as near-field probes for scattering of the infrared (IR) laser beam. It has been verified that the IR light reflected from this transient mirror has a spatial resolution determined by the spot size of the visible light. This methodology enables IR microscopy of thin samples with the resolution of a visible microscope. Advantages of this approach are: (i) no need for near-field distance control, (ii) possibility of fast sample scanning, (iii) no attenuation of the IR beam in a tapered fiber probe, and (iv) the sample to be imaged can be covered by or encased in a transparent liquid or solid. Preliminary results, prospects and limitations are discussed.

Realizable free-electron lasers

We analyze the general class of constant period free-electron lasers (FELs) based on single-pass linear accelerator technology. The emittance and energy spread of the electron beam used to drive an FEL must be chosen to match the acceptance of the FEL wiggler. This acceptance determines the attainable current, and the current determines the gain and power output. For an optimized system in which the optical mode size in the interaction region is minimized, the gain is found to be independent of the laser length, while the efficiency and power output scale as the inverse and inverse cube of the length. Very high power output and good efficiencies are predicted.

Low Temperature Aspects of a Cryogenic Accelerator

The object of this paper is to identify the many low temperature aspects of a cryogenic accelerator and to indicate how these are related to the operating characteristics which might ultimately be achieved.

Wide-field coherent anti-Stokes Raman scattering microscopy with non-phase-matching illumination

We have developed and tested a wide-field coherent anti-Stokes Raman scattering (CARS) microscopy technique, which provides the simultaneous imaging of an extended illuminated area without scanning. This method is based on the non-phase-matching illumination of a sample and imaging of a CARS signal with a CCD camera using conventional microscope optics. We have identified a set of conditions on the illumination and imaging optics, as well as on sample preparation. Imaging of test objects proved high spatial resolution and chemical selectivity of this technique.

Laser‐driven electron acceleration by means of two‐wave interaction

Two‐wave electron acceleration is analyzed where one field is a slow microwave signal and the other is a laser light beam. An acceleration gradient of 1.0 GeV/m is achievable with a laser field strength of approximately 1010 V/cm.

On contrast parameters and topographic artifacts in near-field infrared microscopy

Near-field microscopy overcomes the diffraction limit through the partial conversion of the evanescent fields, formed around the subwavelength sources of light, into propagating waves by interactions between the probe and the sample. Contrast parameters in this imaging technique are quite different from those in conventional (far-field) optics. We study the mechanisms of image formation in the transmission mode of a near-field microscope in the mid-infrared part of the spectrum (6–10 μm). The amount of light propagating from a subwavelength aperture through a flat substrate (“allowed” light) is found to strongly increase as the tip approaches the sample, generating topographic artifacts in near-field images. Such artifacts can be eliminated by flat sample preparation techniques. The transmitted power is strongly influenced by the refraction index of the sample resulting in a substantial difference of the near-field spectrum from the far-field one. A model describing tunneling of light through a subwavelen...

The Stanford Superconducting Recyclotron

With four orbits of recirculation, the HEPL superconducting recyclotron is expected to provide a high duty factor (> 70%) electron beam at 300 MeV by 1982. At present the beam has been recirculated twice and extracted. To date, maximum energy has been 156 MeV with best energy resolution (FWHM) 0.018%. The dependence of beam breakup starting currents on orbit optics has been investigated. Installation of the components for the third and fourth orbits will be completed this year.

The Free Electron Laser

We review the status of our research on the Free Electron Laser

The source issue in infrared microspectroscopy

Infrared spectroscopy using a Fourier Transform Infrared (FTIR) spectrometer is routine. These instruments are sophisticated and mature, and generally use a blackbody radiator as their infrared source. However, because the brightness of a thermal source is limited, the signal-to-noise ratio of these instruments begins to degrade at spatial resolutions not much better than 1 mm and they are rarely useful at resolutions smaller than 20 μm. Synchrotrons provide much brighter infrared beams than thermal sources, and Free-Electron Lasers (FELs) provide even brighter beams than synchrotrons. We will discuss the limitations of thermal sources, and show that a synchrotron is an excellent source for infrared spectroscopy at spatial resolutions on the order of the wavelength (λ). Even better spatial resolution, about λ/10, can be expected if an FEL is used as a source.