Frequency Conversion Techniques

Jianwen, F. Shufen, and L. Miaohang, Appl. Phys. Lett., 1980, 37, 883. 

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In order to learn about the true quantum efficiency of photogeneration one therefore has to study the photoinduced charge generation mechanism at faster time scales. Pump probe spectroscopy utilising a few optical-cycle laser pulses (5-6 fs) in the visible spectral range with broadband frequency conversion techniques [89] now makes it possible to study extremely fast optically-initiated events with unprecedented time resolution. Such a setup was used to time-resolve the kinetics of the charge transfer process from a polymer chain to a fullerene moiety in thin films of poly[2-methoxy, 5-(3, 7 -dimethyl-octyloxy)]-p-phenylene vinylene (MDMO-PPV) and [6,6]-phenyl C6i butyric acid methyl ester (PCBM). Solutions prepared from 1 wt% solutions of toluene on thin quartz substrates were studied. 

The laser-based methods make use of different types of laser sources. Many of them, such as the solid-state laser or the semiconductor laser, are under continuous development because of the discovery of new lasing materials or the optimization of frequency conversion techniques. The type of the analytical application and the nature of the required information determine the choice of a special laser source, particularly pulsed or continuous-wave (CW) operating mode. Pulsed laser sources with fixed wavelength are usually used for LA, whereas both tunable pulsed and CW laser sources are preferred for element-specific detection or plasma diagnostics. 

The progress of both technology and applications in the field of ultrafast processes within the last 20 years has been of remarkable dimensions. Not least because of the advent of all-solid-state femtosecond laser sources and because the extension of laser wavelengths by frequency conversion techniques has provided a variety of high-performance sources for extremely short light pulses. These excellent sources have enabled researchers all over the world extensively and quite successfully to investigate ultrafast phenomena in physical, chemical, and biological systems. 

Much effort is presently concentrated to extend the nonlinear frequency conversion techniques to cover the far-IR or terahertz region, roughly the spectral region 0.1-10 THz, where along with the XUV and... 

As another example, conversion to furnish either DC or AC, or combinations of either, at whatever frequencies are required, is easily accomplished by standard conversion techniques and add-on systems. 

As already indicated Eq. (39) (Eq. (40)) gives the rate of ideal time and frequency resolved emission. If compared with experimental data gained by single photon counting, F(iv t) has to undergo a time averaging with the respective apparatus function which determines the possible time resolution of the measurement (for up conversion techniques see [44]). 

Frequency up-converted lasing is an important research area and has become more promising in recent years because this technique provides several main advantages when compared to other coherent frequency up-conversion techniques ... 

The sequence of the processing steps between the beam former and the scan conversion is different among the various commercial systems but the goals of the steps remain the same. The beam former output will be a wideband RF, an IF, or a complex baseband signal, which will usually be bandpass filtered to reduce out-of-band noise contributions. In systems with very wideband processing, frequency diversity techniques (e.g., split spectrum processing) can be brought into play to try to reduce the impact of coherent interference or speckle. 

Various frequency-conversion interactions can also be used for spectroscopy, image conversion, control of various properties of laser beams, and certain optical information-processing techniques. 

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