Frequency modulated detection

T. R. Albrecht, P. Griitter, D. Horne and D. Rugar, Frequency modulation detection using high-Q cantilevers for enhanced force microscope sensitivity, J. Appl. Phys. 69(2), 668 (1991). 

The pyrolysis of CR NH (<1 mbar) was perfomied at 1.3 atm in Ar, spectroscopically monitoring the concentration of NH2 radicals behind the reflected shock wave as a fiinction of time. The interesting aspect of this experiment was the combination of a shock-tube experiment with the particularly sensitive detection of the NH2 radicals by frequency-modulated, laser-absorption spectroscopy [ ]. Compared with conventional narrow-bandwidth laser-absorption detection the signal-to-noise ratio could be increased by a factor of 20, with correspondingly more accurate values for the rate constant k T). 

Votsmeier M, Song S, Davidson D F and Hanson R K 1999 Sensitive detection of NH2 in shock tube experiments using frequency modulation spectroscopy int. J. Chem. Kinetics 31 445-53... 

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

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. 

Micro-Doppler analyses can also be used for people identification. In that case, it is possible to detect Doppler frequency modulation caused by heartbeats, breathing and body motion (walk, run, head turns etc.). 

When a component of the modulated absorption wave which synchronizes with the fcth harmonic of the modulation frequency is detected by a phase-sensitive detector, the signal obtained is proportional to the coefficient of the kih term ak (// ). 

The graphical representation of this protocol is shown schematically in Fig. 10.15. Signals from two amperometric electrodes, representing channel 1 (blue) and channel 2 (red) detect to electroactive species, which is delivered to them with frequency modulation of, for example, 1 Hz. The experiment is performed in the benchtop fluid setup shown in Fig. 10.16. The first interesting observation is the presence of higher harmonics in the coherence spectrum. They arise as the effect of nonsinusoidal modulation. A pure sine wave would transform to the frequency domain as a single line. Any other waveform of the same frequency will contain higher harmonics in the spectrum. 

Fig. 6.10 The principle of the quartz crystal microbalance gold surface modification (left) and analyte detection by frequency modulation (right)... 

We detected the saturated fluorescence emitted by a beam of 23S metastable atoms as they cross at right angle the slave laser light. A 1015 atoms/s.sterad flux of metastable helium atoms was produced by electronic collisions in a DC discharge of a helium atomic beam, similar to that described in [15]. To improve the precision of the linecenter determination, we increased the signal-to-noise ratio S/N by means of standard frequency modulation the third harmonic demodulated lineshape is shown in Fig. 4. The function expected for a Lorentzian spectrum was fit and linecenters were calculated with an uncertainty ranging between 10 kHz and 20 kHz, that is consistent with the observed S/N, mainly limited by the stability of the reference frequency and of the metastable helium beam. The reproducibility was two or three times worse than the uncertainty,... 

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