High frequency modulation

The light produced by a laser has a much more narrow wavelength than the light of an LED or other light sources (Fig. 15.3). In addition, laser light is coherent, i.e., the photons travel in parallel paths from the source. Lasers made of thin-films are similar to bulk lasers (He-Ne, ruby) except that they are more compact and efficient. Due to the short lifetime of the photons, high-frequency modulations are possible. 

Schreiber, U. (1986). Detection of rapid induction kinetics with a new type of high frequency modulated chlorophyll fluorometer. Photosynthesis Research, 9, 261-72. 

Skogerboe, K. J. and Yeung, E. S., Single laser thermal lens detector for microbore liquid chromatography based on high-frequency modulation, Anal. Chem., 58, 1014, 1986. 

The reduction of zinc ions at d.m.e. has widely been studied and the reaction has been reported to be quasi-reversible.94 Van Der Pol and co-workers54 studied this reaction by the faradaic rectification polarographic technique using high-frequency modulated signals. The kinetic parameters have been evaluated by the... 

A more complex but faster and more sensitive approach is polarization modulation (PM) IRLD. For such experiments, a photoelastic modulator is used to modulate the polarization state of the incident radiation at about 100 kHz. The detected signal is the sum of the low-frequency intensity modulation with a high-frequency modulation that depends on the orientation of the sample. After appropriate signal filtering, demodulation, and calibration [41], a dichroic difference spectrum can be directly obtained in a single scan. This improves the time resolution to 400 ms, prevents artifacts due to relaxation between measurements, and improves sensitivity for weakly oriented samples. However, structural information can be lost since individual polarized spectra are not recorded. Pezolet and coworkers have used this approach to study the deformation and relaxation in various homopolymers, copolymers, and polymer blends [15,42,43]. For instance, Figure 7 shows the relaxation curves determined in situ for miscible blends of PS and PVME [42]. The (P2) values were determined... 

Since TIRF produces an evanescent wave of typically 80 nm depth and several tens of microns width, detection of TIRF-induced fluorescence requires a camera-based (imaging) detector. Hence, implementing TIRF on scanning FLIM systems or multiphoton FLIM systems is generally not possible. To combine it with FLIM, a nanosecond-gated or high-frequency-modulated imaging detector is required in addition to a pulsed or modulated laser source. In this chapter, the implementation with of TIRF into a frequency-domain wide-field FLIM system is described. 

At first sight, it may seem difficult to get high-frequency modulated fundamental light, especially when still thinking about 10 Hz nanosecond lasers, as used in nanosecond HRS experiments. However, the repetitive pulse laser, as used in femtosecond HRS, naturally provides high frequencies The harmonic content in the frequency domain of the very short pulse in the time domain extends to well in the GHz range. 

For the measurement of differential absorption spectra in the visible and ultraviolet spectral region, a light modulation technique was introduced over two decades ago [7]. The principle of this measurement also governs the measurement of infrared VCD. This technique involves the high frequency modulation of the exciting beam between left and right circular polarization states via a photoelastic modulator, and measuring the differential absorption with a lock-in amplifier tuned to the modulation frequency. 

Very fast fluorescence signals can be examined by a phase-resolved frequency domain technique. A high-frequency modulated light source is used to excite the sample according to... 

SCHREIBER U., SCHLIWA U. BILGER W. (1986) Continuous recording of photochemical and non-photochemical chlorophyll fluorescence quenching with a new type of modulation fluorometer. - Photosvn. Res. 10 51-62. SCHREIBER U. (1986) Detection of rapide induction kinetics with a new type of high-frequency modulated chlorophyll fluorometer. - Photosvn. Res. 9 261-272. 

High-frequency modulators use traveling wave electrodes, for reasons described in the Frequency Response section. In this configuration the modulator impedance is ideally determined by the termination resistance at the end of the traveling wave electrodes. In practice the electrodes have loss that affects the impedance. 

Y (668), external cavity diode laser (ECDL). high frequency modulation spectroscopy A1 (394), second harmonic generation (SHG) of ECDL... 

Microelectronics [large-scale integrated (LSI) circuits, sensor carriers, high-frequency modules]... 

Fig. 10.22. Sideband generation in high-frequency modulation spectroscopy and resulting imbalance when one of the sidebands is absorbed by a gas [10,84]...

Very recently. Block et al. [133] reported on optical modulators with ring diameters smaller than 50 pm in a silicon nitride based waveguide system on silicon oxide with a top cladding of an electro-optic polymer, namely AJTB141 chromophore 28% wt into an amorphous polycarbonate polymer. Using Cu electrodes they attained a high frequency modulator with modulation up to 10 GHz with low drive voltage (2.7 Vpp). 

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