Modulation light

Another group of conjugated thiophene molecules for future appHcations are those being developed as nonlinear optical (NLO) devices (75). Replacement of benzene rings with thiophene has an enormous effect on the molecular nonlinearity of such molecules. These NLO molecules are able to switch, route, and modulate light. Technology using such materials should become available by the turn of the twenty-first century. 

With a modulated light source, the phase difference between the perpendicular and parallel components of the emission is measured. The time delay of the perpendicular component of emission is longer than that of the parallel component because the molecule requires a certain period of time to rotate into the perpendicular orientation where it can be detected through the perpendicular polarizer. 

Luminescence lifetimes are measured by analyzing the rate of emission decay after pulsed excitation or by analyzing the phase shift and demodulation of emission from chromophores excited by an amplitude-modulated light source. Improvements in this type of instrumentation now allow luminescence lifetimes to be routinely measured accurately to nanosecond resolution, and there are increasing reports of picosecond resolution. In addition, several individual lifetimes can be resolved from a mixture of chromophores, allowing identification of different components that might have almost identical absorption and emission features. 

Halavaty, A. S. and K. Moffat (2007). N- and C-terminal flanking regions modulate light-induced signal transduction in the LOV2 domain of the blue-light sensor photo tropin 1 from Avena sativa. Biochemistry 46 14001-14009. 

In phase-fluorimetric oxygen sensors, active elements are excited with periodically modulated light, and changes in fluorescence phase characteristics are measured. The delay or emission (phase shift, ( ), measured in degrees angle) relates to the lifetime of the dye (x) and oxygen concentration as follows ... 

A constant bias potential is applied across the sensor in order to form a depletion layer at the insulator-semiconductor interface. The depth and capacitance of the depletion layer changes with the surface potential, which is a function of the ion concentration in the electrolytic solution. The variation of the capacitance is read out when the semiconductor substrate is illuminated with a modulated light and the generated photocurrent is measured by means of an external circuit. 

The PAS phenomenon involves the selective absorption of modulated IR radiation by the sample. The selectively absorbed frequencies of IR radiation correspond to the fundamental vibrational frequencies of the sample of interest. Once absorbed, the IR radiation is converted to heat and subsequently escapes from the solid sample and heats a boundary layer of gas. Typically, this conversion from modulated IR radiation to heat involves a small temperature increase at the sample surface ( 10 6oC). Since the sample is placed into a closed cavity cell that is filled with a coupling gas (usually helium), the increase in temperature produces pressure changes in the surrounding gas (sound waves). Since the IR radiation is modulated, the pressure changes in the coupling gas occur at the frequency of the modulated light, and so does the acoustic wave. This acoustical wave is detected by a very sensitive microphone, and the subsequent electrical signal is Fourier processed and a spectrum produced. 

In situations where absorption of the incident radiation by the transducing gas is troublesome a piezoelectric transducer (made from barium titanate, for example) can be attached to the sample (or sample cuvette in the case of liquids) to detect the thermal wave generated in the sample by the modulated light (8,9). The low frequency, critically damped thermal wave bends the sample and transducer thus producing the piezoelectric response. The piezoelectric transducer will also respond to a sound wave in the solid or liquid but only efficiently at a resonant frequency of the transducer typically of the order of 10 to 100 KHz (see Figure 4). Thus neither in the case of microphonic nor piezoelectric detection is the PA effect strictly an acoustic phenomenon but rather a thermal diffusion phenomenon, and the term "photoacoustic" is a now well established misnomer. 

FIGURE 9.10 An illustration of the single-beam flame atomic absorption optical path. The modulated light from the source is created either by a chopper or through electronic pulsing. 

Pulse fluorometry uses a short exciting pulse of light and gives the d-pulse response of the sample, convoluted by the instrument response. Phase-modulation fluorometry uses modulated light at variable frequency and gives the harmonic response of the sample, which is the Fourier transform of the d-pulse response. The first technique works in the time domain, and the second in the frequency domain. Pulse fluorometry and phase-modulation fluorometry are theoretically equivalent, but the principles of the instruments are different. Each technique will now be presented and then compared. 

Phase-modulation fluorometry The sample is excited by a sinusoidally modulated light at high frequency. The fluorescence response, which is the convolution product (Eq. 6.9) of the pulse response by the sinusoidal excitation function, is sinusoidally... 

Prior to describing the possible applications of laser-diode fluorometry, it is important to understand the two methods now used to measure fluorescence lifetimes these being the time-domain (Tl)/4 5 24 and frequency-domain (FD) or phase-modulation methods.(25) In TD fluorometry, the sample is excited by a pulse of light followed by measurement of the time-dependent intensity. In FD fluorometry, the sample is excited with amplitude-modulated light. The lifetime can be found from the phase angle delay and demodulation of the emission relative to the modulated incident light. We do not wish to fuel the debate of TD versus FD methods, but it is clear that phase and modulation measurements can be performed with simple and low cost instrumentation, and can provide excellent accuracy with short data acquisition times. 

In the frequency domain, any periodic excitation, r.(t), can be described by a sum of sinusoidally modulated light waveforms at harmonics of the fundamental frequency of the excitation... 

Each sinusoidally modulated light intensity of the response is phase delayed and demodulated with respect to the excitation such that... 

Phase-modulation immunoassay measurements are made with sinusoidally modulated light. Since the emission is a forced response to the excitation, the emitted light has the same periodicity as the excitation. Due to the time lag between absorption and emission, the emission is delayed in comparison with the excitation. The time delay between the zero crossing of one period of the excitation and of the emission is measured as the phase angle (Figure 14.11). The emission is also demodulated, due to a decrease in the alternating current (AC) component of the AC to direct current (DC) ratio. 

It is possible to resolve the emission from one species in the presence of another by phase-suppression techniques. The emission from a single fluorophore excited by sinusoidally modulated light is described by... 

Berry BC, Stafford CM, Pandya M, Lucas LA, Karim A, Fasolka MJ (2007) Versatile platform for creating gradient combinatorial libraries via modulated light exposure. Rev Sci Instrum 78 072202... 

An applied electric field can also change a material s linear susceptibility, and thus its refractive index. This effect is known as the linear electro-optic (LEO) or Pocket s effect, and it can be used to modulate light by changing the voltage applied to a second-order NLO material. The applied voltage anisotropically distorts the electron... 

In a laser or LED printer, modulated light is projected onto the drum surface to create a latent image. The modulated light is used only to create the positive image, hence the term black writing. ... 

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