Time-domain multiplexing

The concept of using decoupled coils has been developed further by Li et al. [19]. In this case, an RF switch was used for time-domain multiplexing of the signals into a single receiver channel. A four-coil system was constructed for operation at 6 T in a wide-bore (89 mm) magnet. Each coil was fabricated by using 17 turns of 50 pm diameter copper wire wrapped around a 180 pm i.d., 355 pm o.d., polyimide-coated fused silica capillary, giving an observe volume... 

Beyond imaging, the combination of CRS microscopy with spectroscopic techniques has been used to obtain the full wealth of the chemical and the physical structure information of submicron-sized samples. In the frequency domain, multiplex CRS microspectroscopy allows the chemical identification of molecules on the basis of their characteristic Raman spectra and the extraction of their physical properties, e.g., their thermodynamic state. In the time domain, time-resolved CRS microscopy allows the recording of the localized Raman free induction decay occurring on the femtosecond and picosecond time scales. CRS correlation spectroscopy can probe three-dimensional diffusion dynamics with chemical selectivity. 

While in the frequency domain all the spectroscopic information regarding vibrational frequencies and relaxation processes is obtained from the positions and widths of the Raman resonances, in the time domain this information is obtained from coherent oscillations and the decay of the time-dependent CARS signal, respectively. In principle, time- and frequency-domain experiments are related to each other by Fourier transform and carry the same information. However, in contrast to the driven motion of molecular vibrations in frequency-multiplexed CARS detection, time-resolved CARS allows recording the Raman free induction decay (RFID) with the decay time T2, i.e., the free evolution of the molecular system is observed. While the non-resonant contribution dephases instantaneously, the resonant contribution of RFID decays within hundreds of femtoseconds in the condensed phase. Time-resolved CARS with femtosecond excitation, therefore, allows the separation of nonresonant and vibrationally resonant signals [151]. 

The main problem with periodic TDFRS is, that the different frequencies are measured at different times. This requires a long-time stability, especially of the heterodyne reference, lasting about as long as the entire experiment. Time domain experiments, on the other hand, are frequency multiplexed, and stability of the heterodyne background is only required for one homodyne/heterodyne separation cycle as described in the experimental section, which is only of the order of seconds, not hours. No stability of the signal amplitude is required for the averaging of C lel(t) over arbitrary times. 

We shall conclude this chapter with a few speculative remarks on possible future developments of nonlinear IR spectroscopy on peptides and proteins. Up to now, we have demonstrated a detailed relationship between the known structure of a few model peptides and the excitonic system of coupled amide I vibrations and have proven the correctness of the excitonic coupling model (at least in principle). We have demonstrated two realizations of 2D-IR spectroscopy a frequency domain (incoherent) technique (Section IV.C) and a form of semi-impulsive method (Section IV.E), which from the experimental viewpoint is extremely simple. Other 2D methods, proposed recently by Mukamel and coworkers (47), would not pose any additional experimental difficulty. In the case of NMR, time domain Fourier transform (FT) methods have proven to be more sensitive by far as a result of the multiplex advantage, which compensates for the small population differences of spin transitions at room temperature. It was recently demonstrated that FT methods are just as advantageous in the infrared regime, although one has to measure electric fields rather than intensities, which cannot be done directly by an electric field detector but requires heterodyned echoes or spectral interferometry (146). Future work will have to explore which experimental technique is most powerful and reliable. 

For NMR measurements, pulsed excitation is routinely used. It exploits the multiplex advantage by simultaneous acquisition of the response at all frequencies, and it enables convenient manipulation of the excitation by conducting the experiment in the time domain. The time axis can, for instance, be separated into different intervals, during which the nuclear spins of the sample can be manipulated by rf irradiation and mechanical... 

Figure 1. Current Nanoscale Optofluidic Sensor Arrays, (a) 3D rendering of the NOSA device, (b) 3D rendering after association of the corresponding antibody to the antigen immobilized resonator, (c) Experimental data illustrating the successful detection of 45 pg/ml of anti-streptavidin antibody. The blue trace shows the initial baseline spectrum corresponding to Fig. la where the first resonator is immobilized with streptavidin. The red trace shows the test spectra after the association of anti-streptavidin. (d) Finite difference time domain (FDTD) simulation of the steady state electric field distribution within the 1-D photonic crystal resonator at the resonant wavelength, (e) SEM image demonstrating the two-dimensional multiplexing capability of the NOSA architecture.

Successful concepts for "reaction-modulated" IR difference spectroscopy use the multiplex advantage of FTIR spectroscopy or the availability of high-intensity laser IR sources. A kinetic photometer using tunable IR diode lasers as sources for the mid-infrared has been developed in our laboratory and will be described elsewhere [6]. It covers the time-domain from approx. 500 nsec to some seconds. A second approach is time-resolved FTIR spectroscopy using a rapid-scanning interferometer, several scans can be recorded per second and the time-domain of slow reactions thus be covered [7]. The following schemes illustrate both concepts ... 

Because there is no motion of the mirror at the retardation points, Fourier frequencies are eliminated and do not contribute to the measurement. In this manner, spectral multiplexing is decoupled from the time domain. Thus the entire spectral range is modulated at a single frequency. 

Recently Liebert et al. have demonstrated that advanced TCSPC is able to record effects of brain activity with 50 ms time resolution, clear separation of scattering and absorption, and probably better depth resolution than CW or frequency-domain techniques [324, 327, 328]. A system of four parallel TCSPC modules with four individual detectors and several multiplexed laser diode lasers is used. A fast sequence of time-of-flight distributions is recorded in consecutive time intervals of 50 to 100 ms. Variations of the optical properties in the brain are derived from the intensity and the first and second moments of the time-of-flight distributions [325]. 

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