Band tuning

Walling JC, Jenssen HP, Morris RC, Odell EW, Peterson OG (1979) Broad-band tuning of solid-state Alexandrite laser. J Opt Soc Am 69 373... 

Laser Light Modulation and Side Band Tuning... 

For example, noise figure values on the order of 2-2.5 dB are readily achievable at most microwave frequencies from L-band through X-band. By tighter selection process, and narrow-band tuning, noise... 

In the case of a radio operating in the FM wavelength band, or indeed any wavelength band, the aerial receives a signal which contains all the transmitted frequencies. What the radio does is, effectively, to Fourier transform the signal so that we can tune in to any of the frequencies without interference from any others. 

Dye lasers, frequency doubled if necessary, provide ideal sources for such experiments. The radiation is very intense, the line width is small ( 1 cm ) and the wavenumber may be tuned to match any absorption band in the visible or near-ultraviolet region. 

Figure 9.33 shows examples of SVLF spectra obtained by tuning a frequency-doubled dye laser to the Og absorption band of the system of pyrazine (1,4-diazabenzene)... 

Electronic transitions in molecules in supersonic jets may be investigated by intersecting the jet with a tunable dye laser in the region of molecular flow and observing the total fluorescence intensity. As the laser is tuned across the absorption band system a fluorescence excitation spectrum results which strongly resembles the absorption spectrum. The spectrum... 

More commonly, the resonant two-photon process in Figure 9.50(c) is employed. This necessitates the use of two lasers, one at a fixed wavenumber Vj and the other at a wavenumber V2 which is tunable. The first photon takes the molecule, which, again, is usually in a supersonic jet, to the zero-point vibrational level of an excited electronic state M. The wavenumber of the second photon is tuned across the M to band system while, in principle, the photoelectrons with zero kinetic energy are detected. In practice, however, this technique cannot easily distinguish between electrons which have zero kinetic energy (zero velocity) and those having almost zero kinetic energy, say about 0.1 meV... 

The Ziegler and Nichols closed-loop method requires forcing the loop to cycle uniformly under proportional control. The natural period of the cycle—the proportional controller contributes no phase shift to alter it—is used to set the optimum integral and derivative time constants. The optimum proportional band is set relative to the undamped proportional band P , which produced the uniform oscillation. Table 8-4 lists the tuning rules for a lag-dominant process. A uniform cycle can also be forced using on/off control to cycle the manipulated variable between two limits. The period of the cycle will be close to if the cycle is symmetrical the peak-to-peak amphtude of the controlled variable divided by the difference between the output limits A, is a measure of process gain at that period and is therefore related to for the proportional cycle ... 

FIGURE 13.55 (sensor based on an elecmcalty tunable micromachined Eabry-Perot interferomeced and (b) the siitfcing of the pass band controlled by the tuning voltage. ... 

Tuni 7g The interferometer is tuned by electrode voltage control. The band-pass center wavelength is displaced accordingly. The FWHM of the transmission pass band is approximately 70 nm at the 4.2 pm wavelength. The stable tuning range of L is... 

Figure 13.55 shows the principle of a gas sensor based on an electrically tunable micromachined Fabry-Perot interferometer, and the shifting ol the pass band controlled by rhe tuning voltage. 

Another possible modification of poly(sulfur nitride) that is expected to produce conducting polymers is the replacement of alternating sulfur in the thiazyl chain by an RC unit, i.e., [(R)CNSN]x. This type of polymer would have five r-electrons per four atoms in the repeating unit and, consequently, would have a partially occupied conducting band. The prospect of tuning the electronic properties of this polymer by... 

The careful control of electronic properties is, of course, a key motivation of such structural changes the so-called band-gap tuning being a particularly important concern. Efficiency of synthesis and structural homogeneity of the products are essential ingredients of such an approach since failure to achieve e.g. quantitative transformation of precursor polymers or to couple benzene units exclusively in a para-fashion interrupts the extensive -conjugation and hampers a reliable structure-propcrty-relalion. 

A major and growing use of the minor metalloids is in semiconductor fabrication. Germanium, like silicon, exhibits semiconductor properties. Binary compounds between elements of Groups 13 and 15 also act as semiconductors. These 13-15 compounds, such as GaAs and InSb, have the same number of valence electrons as Si or Ge. The energy gap between the valence band and the conduction band of a 13-15 semiconductor can be varied by changing the relative amounts of the two components. This allows the properties of 13-15 semiconductors to be fine-tuned. 

Semiconductors have a considerably smaller band gap (e.g. silicon 1.17 eV). Their conductivity, which is zero at low temperatures but increases to appreciable values at higher temperatures, depends greatly on the presence of impurities or, if added advertently, dopants. This makes it possible to manipulate the band gap and tune the properties of semiconductors for applications in electronic devices [C. Kit-tel. Introduction to Solid State Physics (1976), Wiley Sons, New York N. Ashcroft and N.D Mermin, Solid State Physics (1976), Saunder College]. 

---