Light Modulation Frequency Research Articles

First-overtone response in modulated photoluminescence emission in silicon at room temperature.

At sufficient photoexcitation, radiative relaxation of excess carriers in semiconductors becomes quadratically dependent on the excess carrier density and can lead to photoluminescence emission at the first overtone of the modulation frequency of the incident light. We have observed, for the first time, a first-overtone component in amplitude-modulated photoluminescence in silicon at room temperature and describe its origin.

Low‐density polyethylene depth profile analysis by photoacoustic spectroscopy

AbstractNear‐infrared photoacoustic spectra of polyethylene (1 mm slab) were taken in the modulation range 10–240 Hz, which corresponds to thermal diffusion layers in the 56−11 μm range. Thick‐layer spectra are very similar to polyethylene film transmission spectra, but large differences are observed between the spectra taken at various modulation frequencies. From 10 to 80 Hz, all the spectral band intensities decrease linearly with ω−a where ω is the light modulation frequency and a varies from 0.48 to 1.00 for different constituent groups. The analysis of spectral intensity as a function of modulation frequency shows that peak intensity ratios of CH3, CH2, and OH groups, relative to that of methylene groups, increase as thinner, closer‐to‐surface polymer layers are sampled. From this we conclude that near‐to‐surface layers of solid polyethylene are richer in CH3, CH2 and OH groups than the polymer bulk.

Multidimensional data formats in phase-resolved fluorescence spectroscopy

The use of phase-resolved total luminescence spectroscopy for chemical analysis is discussed. Phase-resolution allows the suppression of the scattered light signals and excitation modulation frequency is used to incorporate fluorescence lifetime selectivity into the spectral format. In the work described here, a binary system of benzo(k)fluoranthene and benzo(b)fluoranthene is used to demonstrate the selective intensity enhancement that can be accomplished through the use of multiple modulation frequencies. Phase-resolved suppression of scattered light in spectra of crude oil and human serum is also discussed.

Thermal diffusivity measurements for thin films by the photoacoustic effect

When the thermal diffusivity of materials is measured by using the photoacoustic effect method, a sound wave interference effect will inevitably occur. This interference effect causes an extra phase lag between the modulated light and the sound signal detected with a microphone. A theoretical expression has been developed to relate the phase lag caused by the sound interference effect to the modulation frequency of the incident light. Simulation results of the theoretical expression show good agreement with the experimental results. In some cases, it is convenient to use a reference film to revise the measured results. Furthermore, some theoretical relations between the phase lag and the chopping frequency as well as the thermal diffusivity of a sample have been recast according to the Rosencwaig-Gersho theory.

Induction kinetics of heat emission before and after photoinhibition in cotyledons of Raphanus sativus

Heat emitted during non-radiative de-excitation was determined in vivo by the photoacoustic method. The dependence of the photoacoustic signal on the length of the pulses (modulation frequency) of the excitation light and the effect of continuous light, which saturates photosynthesis but does not directly contribute to the signal, are described. The induction kinetic of heat emission measured with intact leaves differed only slightly from the induction kinetic of fluorescence (Kautsky effect) detected in parallel. The photoacoustic signal at high modulation frequencies (279 Hz), which represents the signal of heat emission, and the photoacoustic signal at low modulation frequencies (17 Hz), interpreted as a signal of pulsed oxygen evolution superimposed on the heat emission, were measured with leaves before and after photoinhibition. It was demonstrated that after photoinhibition the decrease in fluorescence yield and in photosynthetic activity (here detected as photoacoustic signal at 17 Hz) are paralleled by an increase in the yield of non-radiative deexcitation (photoacoustic signal at 279 Hz). The increase of heat emission, which has been hypothized for photoinhibited leaves, could now be proved by measuring the induction kinetics of the photoacoustic signal.

Photoacoustic study of Rh B dye on silica particles : Electronic spectra and determination for depth of distribution limit.

The photoacoustic spectra (PA spectra) of Rhodamine B dye adsorbed on silica particles (Rh B/ SiO2) have been measured in the wavelength region of 300-700nm. The spectral shape resembles the absorption spectrum of Rh B aqueous dilute solution although less than 9nm shifts of maxima to the high energy side are observed in the PA spectra. This similarity suggests that Rh B molecules on silica particles are as highly dispersed as those in the solution. The reasons for a slight difference in band shape are considered between the PA spectrum and the diffuse reflection spectrum of Rh B/ SiO2. When the modulation frequency of light for excitation is increased some unknown species being adsorbed very near to the surface of silica particles are detected in the PA spectra in the wavelength region of 600-700nm. The ratio of the PA signal height at the maximum of Rh B (λ=545nm) to that at the surfactant's band above (λ=700nm) is plotted vs. the thermal diffusion length. This analysis indicates that Rh B dye molecules are distributed within the depth of 64μm from the surface of silica particles.

Transport properties of doped a-Si:H films prepared by reactive evaporation

The temperature dependence of conductivity and thermopower of a series of boron and phosphorous doped hydrogenated amorphous silicon films, prepared by reactive evaporation, has been investigated. For both types of dopants we observed a shift of the Fermi level up to 0.5 eV towards the mobility edges. We have also measured the dependence of photoconductivity on the incident photons flux and on the modulation frequency of excitation light. These results show a behaviour similar to that of samples prepared by other methods, pointing out the effectiveness of the deposition technique adopted.

Recombination of carriers at n-Si/SiO2 interface via mobile centers in the oxide

A theory of the ac surface photovoltage is presented for the case of surface recombination of carriers via mobile centers located in the insulator in the region adjacent to the interface. The surface photovoltage relaxation time for this recombination mechanism is shown to be strongly dependent on the modulation frequency of the incident light, while the relaxation time is frequency independent if the recombination of carriers occurs via conventional, spatially fixed, surface states. The dependence of the ac surface photovoltage on the modulation frequency of the incident light was measured for n-Si/native oxide and n-GaAs/native oxide systems. The strong frequency dependence of the surface photovoltage relaxation time observed for an n-Si/native oxide system under depletion conditions indicated that the carrier recombination at this interface occurs mainly via mobile oxide centers. The relaxation time in n-GaAs/ native oxide system was found to be frequency independent, confirming previous conclusions that surface states in GaAs are associated with defects located on the semiconductor side of the interface.

Determination of absorption coefficients of thick semiconductor samples using photoacoustic spectroscopy

A method of measurement of the optical absorption coefficient for a semiconductor sample is given, based on the simplified pattern by Rosencwaig and Gersho in the particular case of thick samples. This method allows us to deduce directly the absorption coefficient from photoacoustic amplitude signal measurement, whatever the incident light modulation frequency. The experimental results obtained from thicker sample of GaP (0.2–1 mm) are compared with the results given by classical optical methods.

Frequency characteristics of photoacoustic signals generated in liquids

Photoacoustic signals generated in liquids are generally described with a wave equation of forced motion for which source terms are incident photoenergy. As a concrete example, the wave equation is applied to cylindrical cells, and photoacoustic signals are analyzed as a function of light modulation frequency using the assumption that the thermal diffusion length is negligibly smaller than the radius of the incident laser beam which has a Gaussian distribution. Experimental results show a monotonic decrease with increasing frequency under 10 kHz and resonance peaks beyond the 10-kHz region. These can be explained by theoretical analysis. The monotonic decrease is caused by decreased photoenergy with increasing light modulation frequency and number of the thermoelastic wave modes in the cell. The peak at 153 kHz is a resonance of the thermoelastic wave in the cell, while the other peaks correspond to structural resonance of the entire cell body and to the electrical resonance of the piezoelectric transducer.

SPECTRA OF FLUORESCENCE LIFETIME AND INTENSITY OF Rhodopseudomonas sphaeroides AT ROOM AND LOW TEMPERATURE. COMPARISON BETWEEN THE WILD TYPE, THE C 71 REACTION CENTER‐LESS MUTANT AND THE B800–850 PIGMENT‐PROTEIN COMPLEX

Abstract— Spectra of the fluorescence lifetime and intensity of chromatophores from the wild type Rhodopseudomonas sphaeroides, from the C 71 reaction center‐less mutant and of the B800–850 light harvesting pigment‐protein complex have been studied by phase fluorimetry techniques at different light modulation frequencies at room and low temperature.As already known, closed reaction centers (saturating light) are still quenchers of antenna fluorescence although with a lower efficiency than when they are opened. The fluorescence yields and lifetimes of both the C 71 mutant strain and the B800–850complex are found to increase by about 30% between room and low temperature.The fluorescence lifetimes obtained for the C 71 strain (0.65 ns at 20C; 0.85 ns at 77 K) and for the B850 complex (1 ns at 20C; 1.3 ns at 77 K) indicate that the non‐radiative deactivation pathways, in the antenna, remain important in the absence of the reaction centers even at low temperature. We suggest that these data arise from the presence of special antenna molecules which act as intrinsic quenchers of the B875 antenna fluorescence. Between room and low temperature, the fluorescence yield and lifetime of the wild type are found roughly constant. This result suggests that the energy trapping by the reaction centers is independent of the temperature. The mechanism governing the energy transfer from the antenna to the reaction centers may differ from the mechanism leading to the energy transfer within the antenna. We suggest that a partially irreversible trapping of the excitation energy, on its way to the reaction center, takes place in the vicinity of the reaction center.

Surface photovoltage measured capacitance: Application to semiconductor/electrolyte system

Surface photovoltage measured capacitance (SPMC), a novel technique for determining the properties of semiconducting materials, is introduced. SPMC permits the determination of a semiconductor depletion layer capacitance by measuring changes of the surface potential barrier induced by low intensity chopped light whose photon energy exceeds the band-gap energy. The theory of the SPMC technique is derived, and an equivalent circuit for analyzing the measurements is described. The present technique is compared with the conventional current measured capacitance method, and it is shown that using SPMC the separation of the depletion layer capacitance from the influence of the surface states can be performed by measurements at only one frequency of light modulation. This is a simpler procedure than the frequency dispersion measurement required of conventional capacitance techniques. Measurements of the capacitance-voltage characteristics of a semiconductor/electrolyte system, in particular n-type WS2 exposed to an inert electrolyte, are used as examples to illustrate the capabilities of SPMC.

High resolution photoacoustic microscopy on a surface acoustic wave device

Photoacoustic imaging of microscopic metal film test structures on a surface acoustic wave device at 76-MHz light modulation frequency is reported. The samples are evaporated on a YZ LiNbO3 crystal surface and are scanned through the focus spot of a mode-locked dye laser beam. The optically generated surface acoustic waves are detected by means of a resonant interdigital transducer. Synchronization of the focus spot position to the corresponding piezoelectric signal yields a high resolution photoacoustic image of the samples (lateral resolution 5 μm).

A reflectance spectrophotometer-surface fluorometer suitable for monitoring changes in hemoprotein spectra and fluorescence of flavins and nicotinamide nucleotides in intact tissues

A spectrophotometer-fluorometer was constructed for simultaneous measurement of reflectance spectrum changes and surface fluorescence of intact tissues and organs. The apparatus employs monochromators for dual-wavelength spectrophotometry and optical filters for fluorometry. Three light beams are time-shared by a novel type of modulator to hit the same area of the tissue to be measured. The analog circuitry was controlled by a digital logic circuitry clocked by an optically derived frequency based on the angular velocity of the rotating light modulator and produced a maximum of three sets of differences in the parameters measured. The optical design has the advantage of allowing the use of monochromators rather than filters in a multichannel apparatus. The electronics accept a wide range of light modulation frequencies without difficulties in the phasing of the detector gates, so that signal sampling can be optimized for the recording of fast and slow phenomena. The apparatus was used successfully for the simultaneous monitoring of the mitochondrial redox state and tissue oxygenation in isolated perfused rat hearts.

Picosecond-resolution fluorescence lifetime measuring system with a cw laser and a radio

A procedure for measurement of fluorescence lifetimes with picosecond time resolution is described. A cw laser beam is modulated with a standing-wave acousto-optic modulator. The modulated beam is split; one part serves as a reference beam, the other part excites the fluorescent sample. The sample flourescence and the reference beam, attenuated and delayed optically to be equal in amplitude and opposite in phase to the fluorescence, are incident onto a single photomultiplier tube. The thus achieved photodetector ac null is monitored either by an AM radio, whose intermediate-frequency signal is displayed on an oscilloscope, or by a spectrum analyzer. With 30-MHz light modulation and the radio, lifetimes could be determined with resolution better than 15 ps. With the spectrum anlyzer and 170-MHz light modulation frequency we have achieved 4-ps lifetime resolution. Correction for photomultiplier transit time versus incident wavelength is made.

Frequency dependence of the photo-EMF of strongly inverted Ge and Si MIS structures—I. Theory

The dependence of the real and imaginary photo-EMF components on frequency of the incident light modulation has been calculated for Ge and Si MIS structure in the inversion regime at the interface. It is shown that physical parameters of a semiconductor, such as doping levels in the bulk and near the interface, the rate of surface recombination proceeding at the backside of the sample, diffusion coefficient of minority carriers and light absorption coefficient can be derived from the experimental results.

Solvation Shell Relaxation and Ion-Electron Recombination in Strongly Associated Solvents

The relaxation properties of the region in a solution in which there is strong interaction between the solute and solvent molecules (the cybotactic region) were studied with electron paramagnetic resonance spectroscopy used as the detection system. In the system under investigation, perylene radical cation was used as the solute, and either sulfuric acid or fluorosulfonic acid was used as the solvent. The properties studied were the solvent cage reorientation time and the ion-electron recombination time. Modulated light of a known wavelength distribution was used to irradiate the sample and synchronize a lock-in detector, which was used to detect the light-induced effects, to the light modulation frequency. Signal averaging techniques were used so that accurate values of the output as functions of the sample temperature, irradiation light wavelength, and irradiation light modulation frequency were obtained. Two extreme model systems were used to interpret the light-induced signal data, a reversible and an impulse model. A range of times was specified for the solvent cage reorientation and ion-electron recombination as a function of the sample temperature. In sulfuric acid, the cage reorientation time varies from 1176 μsec at 6°C to 99 μsec at 77°C, while the ion-electron recombination time varies from 649 to 42 μsec. The temperature range studied for fluorosulfonic acid was −36–95°C. In this range, the cage reorientation time varied from 857 to 81 μsec and the ion-electron recombination from 815 to 35 μsec. Over the temperature ranges studied, the intermediate coupling constant for the perylene radical cation increased from 13.7 to 224 mG in sulfuric acid and 19.7 to 345 mG in fluorosulfonic acid.

Proposed method for measuring picosecond pulsewidths and pulse shapes in CW mode-locked lasers

A new method is proposed for measuring the widths, and possibly even the detailed shapes, of picosecond pulses in CW mode-locked lasers. The pulses are passed through an electro-optic modulator, which is biased for zero transmission with zero applied voltage, and which is driven at a harmonic of the pulse-repetition frequency. The variation in average light transmission through the modulator is monitored while the relative phase between the light pulses and the modulator drive is varied. The pulsewidth may be deduced from one such measurement made at a microwave light-modulation frequency sufficiently high that the pulsewidth is a finite fraction of a period at the modulation frequency. By making such measurements at a number of harmonics, the complete Fourier expansion (including both amplitudes and phases) of the picosecond pulse envelope can, at least in principle, be determined without ambiguity.

Behaviour of photomultipliers against high-frequency modulated light

The frequency-response of several phototubes to light modulated in intensity by an ultrasonic device has been examined. Frequency of light-modulation up to 48 Mc/sec was used. The method can be applied in order to select photomultipliers for fast-counting rates.