Near-Field Tunability

Brighter, tunable ultrafast light sources would benefit many of the areas discussed in the report, particularly infrared-terahertz (between visible light and radio waves) vibrational and dynamical imaging, near-field scanning optical microscopy (NSOM), and X-ray imaging. 

IR measurements on doped Si and shown that the subsurface mobile carriers can be probed by their response to an IR near-field with a spatial resolution of 30nm [48]. The group of Havenith presented a scanning near-field infrared microscopy (SNIM) system this is an IR s-SNOM set-up based on a continuous-wave optical parametric oscillator (OPO) as an excitation source with a much wider tunability compared to the usually applied CO2 lasers [49]. With this set-up, a subsurface pattern of implanted gallium ions in a topographically fiat silicon wafer was imaged with a lateral resolution of <30 nm. 

An approach to full spectroscopic near-field imaging in the IR range was presented by Michaels et al. [62], whose illumination-mode a-SNOM set-up was based on a Ti sapphire-pumped optical parametric amplifier (OPA) coupled into a tapered fluoride glass optical fiber. The OPA system provides tunable broadband IR radiation with output powers in the miUi Watt range. After passing the thin film sample, the transmitted fight is collected by a Cap2 lens and coupled into a monochromator with a detector array. Broadband illumination with a bandwidth... 

TERS and theory TERS also provides excellent stimulation and a playground for theoretical studies on enhanced optical processes. The tip-substrate configuration can be considered as a unique photonic system with tunable optical resonances that provides huge near-field enhancements for species located in the tip-substrate cavity. A suitable theoretical description of such a photonic system is, even today, a great challenge, although the general features of such a system are well understood. 

With the development of more versatile tunable semiconductor lasers in the near infrared, high-resolution absorption spectroscopy in this spectral range has been greatly improved. Magnetic field tuned semiconductor lasers as well as current tunable devices have been used. 

Alexandrite, the common name for Cr-doped chrysoberyl, is a laser material capable of continuously tunable laser output in the 700-800 nm region. It was established that alexandrite is an intermediate crystal field matrix, thus the non-phonon emitting state is coupled to the 72 relaxed state and behaves as a storage level for the latter. The laser-emitted light is strongly polarized due to its biaxial structure and is characterized by a decay time of 260 ps (Fabeni et al. 1991 Schepler 1984 Suchoki et al. 2002). Two pairs of sharp i -lines are detected connected with Cr " in two different structural positions the first near 680 nm with a decay time of approximately 330 ps is connected with mirror site fluorescence and the second at 690 nm with a much longer decay of approximately 44 ms is connected with inversion symmetry sites (Powell et al. 1985). The group of narrow lines between 640 and 660 nm was connected with an anti-Stokes vibronic sideband of the mirror site fluorescence. 

A resonance ionization mass spectrometer (RIMS) uses a tunable, narrow bandwidth laser to excite an atom or molecule to a selected energy level that is then analyzed by MS. The selective ionization often is accomplished by absorption of more photons from the exciting laser, but can also be effected by a second laser or a broadband photon source. Multiple photon absorption can result in direct ionization or in production of excited species that can then be ionized with a low-energy photon source (IR laser) or by a strong electric field. Resonance ionization methods have been applied to nearly all elements in the periodic table and to many radionuclides, including Cs (Pibida et al., 2001), Th (Fearey et al., 1992), U (Herrmann et al., 1991), Np (Riegel et al., 1993), Pu (Smith, 2000 Trautmann et al., 2004 Wendt et al., 2000), radioxenon and radiokrypton (Watanabe et al., 2001 Wendt et al., 2000), and 41Ca (Wendt et al., 1999). 

Electric field spectroscopy with a tunable laser uses the Stark effect to provide selective modulation of specific branches, but does not permit the direct measurement of dipole moments. There are a number of molecules in which a tunable laser has been used to scan Stark-split line profiles of lines of known assignment. One such example is illustrated in figure 7 this shows the five M components of the P3 (3) line in the electronic 0-0 band of the HCF radical near 17238.4 cm , recorded using a single-mode dye laser and an electric field of 19.25 kV cm in parallel polarisation. The analysis of many Stark-split spectra using eqns 2-4 leads to the following values for the a-axis component of the dipole moment in the two states ... 

---