Other Modulation Techniques

Other Modulation Techniques. Electric field modulated photoluminescence in a liquid junction device was reported in Ref. 75. The results agreed with the dead layer model previously discussed. Quantitative comparison of differentiation of Eq. 1 with respect to the eiectrode potentiai and impedance measurements yieided an agreement between the dead Layer and the space-charge iayer. 

Eiectroabsorption was demonstrated in Ref. 76 and the iine shape was found to fit Eq. 11. Theoreticai justification for this fit was not given. 

A summary of other modulation techniques such as waveiength moduiation, temperature moduiation and stress moduiation can befound in Ref. 77, These modulation techniques have not yet found any significant applications in the study of semiconductor-liquid junction devices. 

---

Note 2 Other modulated techniques are possible, such as modulated force TMA, modulated rate SCTA etc. 

There are several other modulation techniques available (Rashid, 1993 Sen, 1987 Mohan, Undeland, and Robins, 1995). Among them is the hysteresis control technique. 

Other modulation techniques are oscillation (tilting) of an interference filter [3] and modulation of the electron beam scan pattern in a vidicon or image-disk-sector photomultiplier spectrophotometer [34]. This was the first nonmechanical wavelength modulation. Wavelength modulation induces a synchronous modulation of the amplitude. If these intensities are expanded, for instance, in the form of a Taylor series in Aq, and the powers of the sine functions are expressed as sine and cosine functions of the corresponding multiple angles, then the derivatives can be obtained from the Fourier coefficients (see Sec. 2.1.3.3) of these series. The second derivative is obtained from the second harmonic of the induced intensity. 

As noted before, considerably more material has been published on the mathematical treatment of MDSC than on other modulated techniques, which is reflected in Fig. 2.92. 

As with other modulation techniques, the idea is to measure the reflectance while some external parameter is varied periodically. The modulation of the reflected beam can be separated from the unmodulated background, and the resulting spectrum represents some kind of derivative of the original spectrum, showing up its singularities in fine detail. The external parameter may be electric field, pressure or other stress, temperature, or even rotation of the sample. Of these, the first has proven most useful. Electroreflectance is therefore currently the best method for picking out the singularities... 

The improvement of the methodology, namely, the use of other modulation techniques such as polarization modulation and Fourier transform spectroscopy. 

The field modulation technique has been successfully applied to a broad variety of systems showing a field dependent conductance. Since a large amount of conductance data Is available for low polar solutions of tetraalkylammonium salts these systems were studied most thoroughly. In the subsequent discussion we will rely mainly on the results obtained for these systems but the general conclusions apply directly to other systems, e.g. fluorenyl-salte In ethereal solvents, metal complexes In benzene, etc. 

IRES Versus Other Reflection Vibrational Spectroscopies. In order to achieve a sensitivity sufficient to detect absorption due to molecules at submonolayer coverages, some sort of modulation technique is highly desirable. Two candidates for modulation are the wavelength and the polarization state of the incident light. The former has been successfully applied to single crystal studies by Pritchard and co-workers (5j, while the latter is the basis of the Toronto ellipsometric spectrometer and of the technique employed by Bradshaw and coworkers (6) and by Overend and co-workers (7). The two different techniques achieve comparable sensitivities, which for the C-0 stretching mode of adsorbed carbon monoxide amounts to detection of less than 0.01 monolayer. Sensitivity, of course, is very much a function of resolution, scan rate, and surface cleanliness. 

With today s continuous-wave modulation techniques in telecommunications the necessary nonlinearities are very difficult if not impossible to achieve. However, for other transmission schemes (e.g. solitons) and new potential concepts the development of organic materials fulfilling the requirements imposed by applications may is possible. 

This review has attempted to put hydrodynamic modulation methods for electroanalysis and for the study of electrochemical reactions into context with other electrochemical techniques. HM is particularly useful for the extension of detection limits in analysis and for the detection of heterogeneity on electrode surfaces. The timescale addressable using HM methodology is limited by the time taken for diffusion across the concentration boundary layer, typically >0.1 s for conventional RDE and channel electrode geometries. This has meant a restriction on the application of HM to deduce fast reaction mechanisms. New methodologies, employing smaller electrodes and thin layer geometries look to lift this restraint. 

In the recent years, sophisticated modeling tools have become available, such as the Cerius (8), where various modules aUow the analysis of crystallization, crystal growth, and material form characterization. In brief, this technique uses a simulated annealing and a rigid-body Rietveld refinement procedure, whereby the calculated and measured XRPD patterns are compared if they agree sufficiently, the structure is deemed to be solved. Other modules offered by Cerus include ... 

Spectroscopies such as UV-visible absorption and phosphorescence and fluorescence detection are routinely used to probe electronic transitions in bulk materials, but they are seldom used to look at the properties of surfaces [72]. As with other optical techniques, one of the main problems here is the lack of surface discrimination, a problem that has sometime been b q)assed by either using thin films of the materials of interest [73, 74], or by using a reflection detection scheme. Modulation of a parameter, such as electric or magnetic fields, stress, or temperature, which affects the optical properties of the sample and detection of the AC component of the signal induced by such periodic changes, can also be used to achieve good surface sensitivity [75]. This latter approach is the basis for techniques such as surface reflectance spectroscopy, reflectance difference spectroscopy/reflectance anisotropy spectroscopy, surface photoadsorption... 

The most useful direct modulation technique is the eurrent gain switching of semiconductor laser devices. This teehnique is unique to semiconduetor somees, nearly all other lasers are modulated externally. In these devices the exeitation eurrent of the laser is modulated, resulting in modulated gain and therefore modulated output power. A detailed analysis of this process is found in [27]. Simply put, an oscillating current of the form... 

Operator splitting methods require less computational resources but demand more thought. In general, stability is not guaranteed. However, operator splitting encourages modular models and allows the use of the best available numerical technique for each module. Thus advection, kinetics, cloud processes, and aerosol processes each reside in different modules. Techniques used for these subproblems will be outlined subsequently. For applications in other problems, the reader is referred to Oran and Boris (1987). 

---