Phase-locking

Figure Al.6,8 shows the experimental results of Scherer et al of excitation of I2 using pairs of phase locked pulses. By the use of heterodyne detection, those authors were able to measure just the mterference contribution to the total excited-state fluorescence (i.e. the difference in excited-state population from the two units of population which would be prepared if there were no interference). The basic qualitative dependence on time delay and phase is the same as that predicted by the hannonic model significant interference is observed only at multiples of the excited-state vibrational frequency, and the relative phase of the two pulses detennines whether that interference is constructive or destructive.

Scherer N F, Carlson R J, Matro A, Du M, Ruggiero A J, Romero-Rochin V, Cina J A, Fleming G R and Rice S A 1991 Fluorescence-detected wave packet interferometry time resolved molecular spectroscopy with sequences of femtosecond phase-locked pulses J. Chem. Rhys. 95 1487... 

Historically, the first and most important capacitance method is the vibrating capacitor approach implemented by Lord Kelvin in 1897. In this technique (now called the Kelvin probe), the reference plate moves relative to the sample surface at some constant frequency and tlie capacitance changes as tlie interelectrode separation changes. An AC current thus flows in the external circuit. Upon reduction of the electric field to zero, the AC current is also reduced to zero. Originally, Kelvin detected the zero point manually using his quadrant electrometer. Nowadays, there are many elegant and sensitive versions of this technique. A piezoceramic foil can be used to vibrate the reference plate. To minimize noise and maximize sensitivity, a phase-locked... 

The ability to create and observe coherent dynamics in heterostructures offers the intriguing possibility to control the dynamics of the charge carriers. Recent experiments have shown that control in such systems is indeed possible. For example, phase-locked laser pulses can be used to coherently amplify or suppress THz radiation in a coupled quantum well [5]. The direction of a photocurrent can be controlled by exciting a structure with a laser field and its second harmonic, and then varying the phase difference between the two fields [8,9]. Phase-locked pulses tuned to excitonic resonances allow population control and coherent destruction of heavy hole wave packets [10]. Complex filters can be designed to enhance specific characteristics of the THz emission [11,12]. These experiments are impressive demonstrations of the ability to control the microscopic and macroscopic dynamics of solid-state systems. 

For the case of a flowing fluid in the a phase, Locke [215] showed that the effective transport equation is given by... 

Before concluding the discussion of the time-domain two-pathway approach, it is appropriate to expand on the relation of this experiment to the two-photon interferometry of Refs. 78-80, where excitation with two phase-locked,... 

Weber FIP (1967) Method for pulsewidth measurement of ultrashort light pulses generated by phase-locked lasers using nonlinear optics. J Appl Phys 38 2231-2234... 

Automatic adjustment of mirrors and monochromators by intensity monitors and phase-locked loops is called stabilization. If the feedback breaks down, the beam will slowly move as the temperature of the optical part is changing. 

C, HPLC-relative retention correlation, different stationary and mobile phases, Locke 1974)... 

C, HPLC-relative retention correlation, different stationary or mobile phases, Locke 1974) 0.075 (extrapolated value, shake flask-UV, measured range 25-74.7°C, Wauchope Getzen 1972)... 

OFRR lasers and phase-locked OFRR lasers will be investigated. Finally, applications of the OFRR in highly sensitive intracavity bio/chemical sensors will also be explored. 

Photomultipliers are generally used to convert the spectral radiation to an electrical current and often phase-sensitive lock-in amplifiers are used to amplify the resulting current. AES and AFS require similar read-out systems because both methods are measuring small signals. The difficulty associated with both these methods is the separation of the signal for the atomic transition of interest from the background radiation emitted by excited molecular species produced in the atom reservoir. AFS phase locks the amplifier detection circuit to the modulation frequency of the spectral source. Modulation of the source is also used in AAS. 

Fig. 15.2a Two tunable vacuum tight cylindrical resonant cavities that were phase-locked and their impedance were matched using the impedance tuning coil. The plungers seen on the top were to aid us in tuning these cylinders to resonate in TE011 mode (See Color Plates)...

Similarly, when arrays of these test resonant cavities loaded with functionalized wicks for various toxins are phase-locked to the reference cavity as shown in Fig. 15.5 they can act as detectors for targeting various toxins as well as their precursors through an array of suitably tuned cavities with specifically functionalized nanotubes. By using the approach as demonstrated in this document, it has been shown that the apparatus can be used to successfully detect low levels of toxin vapors associated with the drag Methamphetamine, in a laboratory-controlled environment. Some of the results of this study are highly sensitive in nature and are not reported in this document. These results can be obtained by other avenues. 

Besides the two main characteristics of sensitivity as well as specificity of a sensor, the industrial, military, and other standards demand the device to be portable, economical, autonomous, and power efficient. In order to address some of these characteristics, the authors in their respective laboratories have been working on improving the design of the prototype, as shown in Figs. 15.6 and 15.7, respectively. The necessaiy electronics consisting of local oscillators, beat oscillators, smaller cavities, mixers, and phase-locking loops have been assembled in prototypes. As of this date the device needs further evaluation in an operational environment to establish a set of encyclopedic data and for comparison with unknown toxins. 

Fig. 15.5 Arrays of cavities phase-locked to the reference cavity and loaded with functionalized wicks to specifically determine the toxin and its precursors...

Figure 11.10. Phase-locked detection of fluorescence lifetime using single reference signal. FID = fluorescence inducing and detecting devices LPF = low-pass electronic filter VCO = voltage-controlled oscillator v, = signal to modulate the output intensity of the excitation light source v/= the fluorescence signal.

Phase-Locked Detection Using a Single Reference Signal... 

Phase-Locked Detection Using Two Reference Signals... 

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