Light Modulation Frequency Research Articles

Pyroelectric Response of PZT-PVDF Nanocomposites of (0-3) Connectivity

Pyroelectric response of PVDF and P(VDF 0.5 /TrFE 0.5 )loaded with Pb(Zr 0.5 Ti 0.5 )O 3 ceramic powder of crystallite size ∼30 nm and aggregate size ∼100 nm was studied in light modulation frequency range 1 Hz - 1 kHz. As the ceramic crystallite size is comparable with intrinsic heterogeneity of the polymers we studied also dielectric dispersion and absorption of polymer and composite samples processed the same way. The effect of polymer heterogeneity is discussed and figures of merit of PZT-PVDF and PZT-P(VDF/TrFE) composites are given.

Morphological Dependence of Radiative and Nonradiative Relaxation Energy Balance in Photoexcited Aryl Ether Dendrimers as Observed by Fluorescent and Thermal Lens Spectroscopies

Radiative and nonradiative relaxation processes after excitation by ultraviolet light were measured for o-, m-, and p-aryl ether dendrimers of fourth generation (o-, m-, and p-Ar(L4)2) by using fluorescent and thermal lens spectroscopies. Samples were dissolved in CH2Cl2 to provide concentrations of a constant absorbance at excitation light wavelength (244 nm). Regarding the nonradiative process, we investigated thermal lens signal dependence on the light modulation frequency to determine the nonradiative relaxation rate. When frequency ranged from 4 to 200 Hz, the thermal lens signal became larger in the order p-, o-, m-Ar(L4)2, while the fluorescent intensity became larger in the order p-, m-, o-Ar(L4)2. We transformed these results into an energy balance of the radiative and nonradiative relaxation processes. Our analysis showed that 50% of the excitation energy was not released from p-Ar(L4)2 for a 100-ms order. Next, the measured fluorescence decay times of the three Ar(L4)2 were obtained as 1.7 ns which revealed that the anomalous properties of dendrimers did not originate in long-lived electronic excitation states, but in long-term storage of internal energy. To explain this phenomenon, a novel mechanism for intramolecular energy storage with nonergodic energy transfer should be considered. Last, we proposed that the nonlinear conjugated oscillator model of Fermi−Pasta−Ulam theory would be suitable for the intramolecular energy storage.

Photoacoustic Characterization of Thermal and Electronic Transport Properties of CdInGaS4 in a Transmission Detection Configuration

Photoacoustic (PA) measurement under a transmission detection configuration (TDC) was applied to study the thermal and electronic transport properties of CdInGaS4. It was shown that the PA signal in the TDC originated from both electronic transport process and heat transport process in the semiconductor sample, and these could be separated by changing the modulation frequency of the excited light. By fitting the theoretically calculated PA signal amplitudes as a function of the modulation frequence to the experimental ones for a variety of thicknesses, the values of the excess-carrier lifetime, carrier diffusivity, surface recombination velocity, and thermal diffusivity of the CdInGaS4 were determined. The contributions of three heat sources to the PA signals were investigated. We found that the PA signals of CdInGaS4 mainly resulted from nonradiative surface recombination under our experimental conditions.

Graphite Photoelectrochemistry: 3. Photoelectrochemical Oxidation of Surface-Confined Hydroquinones at Highly Oriented Pyrolytic Graphite Basal Plane Electrodes

Photocurrent measurements at irradiated highly oriented pyrolytic graphite basal plane electrodes with adsorbed quinone/hydroquinone (benzoquinone, 1,4-naphthoquinone, and 9,10-phenantherenequinone) couples are reported. The photocurrent was generated by modulated UV-vis light. The potential dependence of the photocurrent, the photocurrent light action spectra, and photocurrent dependence on light modulation frequency are reported. The peak of the anodic photocurrent curve was found close to the formal potential of the adsorbed redox couple. The peak is ascribed to the action of two opposite processes: (1) oxidation of adsorbed hydroquinones by hot holes that are photogenerated in the graphite; (2) relaxation of the adsorbed redox couple to equilibrium by a dark reduction process. A model describing the dependence of the photocurrent magnitude and phase angle on the light modulation frequency as a function of parameters of the dark reduction process is presented. Rate constants calculated by the proposed model for the dark process are consistent with reported values.

Dependence of the Photoacoustic Signal Intensity on Modulation Frequency for CdInGaS4 Crystals under a Transmission Detection Configuration

Dependence of the photoacoustic (PA) signals on the modulation frequency was measured for CdInGaS4 crystals in the reflection and transmission detection configurations (RDC and TDC). The PA signals originate from both the thermal diffusion and photo-excited carrier diffusion processes under TDC. The two processes can be separated by changing the modulation frequency of the excited light. The experimental results show the possibility of characterizing the thermal and electronic properties of CdInGaS4 crystals by the PA technique in TDC.

Voltage responsivity of pyroelectric sensor

Composites polymer–ceramic using castor oil-based polyurethane (PU) as non-ferroelectric matrix and Lead Zirconate Titanate (PZT) as ceramic powder have been prepared at thin films form by spin coating. The samples are poled by appropriated electric field to show piezo and pyroelectric activity. The pyroelectric coefficient p( T) at 343 K is obtained to be equal 5.8×10 −5 C m −2 K −1 for a composite with 32 vol.% of ceramic. The figure of merit of this composite is six times higher than of PZT ceramic. The voltage responsivity of the pyroelectric is reduced when the thickness of the sample increases. It was used modulated white light as radiation source to excite the sensor film. The electric signal of the sensor decreases with the light modulation frequency by 1/ f.

Optoelectronic standardization of laser Doppler perfusion monitors

An optoelectronic method for standardization of laser Doppler (LD) instruments is proposed and tested. The optoelectronic standardization is based on detection of modulated near-infrared light from a light emitting diode (LED) emitted into the optic probe (detecting fiber) of the LD instrument. Changes of the frequency and amplitude of the light enables evaluation of the sensitivity and linearity of the instrument response. Measurements of the LD instrument response show linear increase of perfusion signal with the modulation frequency of light. Analysis of the LD signal components (flux, speed, concentration) show that for fixed amplitude and varying frequency of the modulation, the flux and speed are linearly related to the frequency and the concentration is constant. The linear increase of flux and concentration signals was observed for increasing amplitude of modulation with fixed frequency. The proposed technique allows for automatic evaluation of the instrument response.

Reflectance measurements of layered media with diffuse photon-density waves: a potential tool for evaluating deep burns and subcutaneous lesions

The basic principles of a non-contact, near-infrared technique for the mapping of layered tissues are discussed theoretically and verified experimentally. The propagation properties of diffuse photon-density waves in tissues depend on the optical properties of the tissue. When a layered medium is irradiated by amplitude modulated light, the difference in optical properties between the layers is evident in the phase and amplitude of the diffuse reflection coefficient, which is a result of the interference of the partial waves propagating in the different layers. Thus, diffuse photon-density waves are applicable to the analysis of the structure of layered tissue. The probing depth is determined by the modulation frequency of the incident light. For modulation frequencies between several hundred megahertz and a few gigahertz, this allows us to analyse the properties of muscle tissue of up to 4-8 mm below the surface. Experimental results based on chicken breast muscle are given. As an example, the technique might be of use for evaluating the depth of necrosis and the blood volume fraction in deep burns.

The scanning microscope for semiconductor characterization (SMSC): electrolyte electroreflectance and photovoltage imaging study of the electrochemical activation of RuS 2 photoelectrodes for oxygen evolution

Abstract The photoelectrochemical behaviour of the oxygen evolving n-RuS 2 ∣ H 2 SO 4 interface was investigated with the help of the scanning microscope for semiconductor characterization. Electrolyte electroreflectance and photovoltage digital images were obtained as a function of the pretreatment of the RuS 2 surface (polishing and electrochemical etching). Polishing gives rise to an inhomogeneous, scarcely photoactive surface, while electrochemical etching destroys the damaged overlayer generated by polishing, leaving an inhomogeneous, highly photoactive surface in contact with the electrolyte. In general, more photoactive zones are characterized by a higher electrolyte electroreflectance signal (EER-s) generated at the interfacial hydroxide layer, and a smaller signal coming from the space charge layer. According to the observed influence of the donor concentration on the EER-s signal amplitude, the contrast observed in the EER-s image was attributed to lateral heterogeneities in the donor concentration. The difficulty of modulating the electric field in the hydroxide layer at relatively high frequencies was attributed to the low relaxation time constant of surface states involved in the EER-s signal. The fact that at low light modulation frequencies those zones with the highest EER-s amplitude also show the highest photovoltage indicates that the photovoltage intensity depends on the time constant.

Influence of structural characteristics on the thermal conductivity of polycrystalline diamond films

By analyzing the signal formed by the photoacoustic effect as a function of the light modulation frequency, it is shown that this effect may be used to determine the thermal conductivity of diamond materials. The method is checked experimentally for two types of polycrystalline diamond films grown by chemical vapor deposition with the gaseous medium activated by a dc discharge and a microwave discharge. The data obtained on the thermal conductivity of the films are discussed with reference to the results of an investigation of the optical absorption, Raman light scattering, and cathodoluminescence of similar films. It is shown that the thermal conductivity of polycrystalline diamond films depends on the structural characteristics, which are determined by the deposition conditions.

Action spectra of the photopotential generation for pigment and dye solutions in nematic liquid crystals located in the electrochemical cell

The method of measurements of the action spectra of photopotential generation using a modulated acting light beam was elaborated and applied to two systems: the solutions of stilbazolium merocyanines and to the mixture of chlorophyll a and luteine. Both types of samples were dissolved in nematic liquid crystal and located either between two semiconducting electrodes, or between a transparent gold electrode and a semiconducting electrode. By changing the frequency of light modulation or the phase shift between a modulated light beam and a measured photopotential signal, information about the kinetics of photopotential generation is obtained. The free base and protonated merocyanine forms exhibit different kinetics of potential generation. The amplitude of the photopotential of the chlorophyll and luteine mixture is higher than the sum of the amplitudes of the photopotential of these pigments located in separated cells. In a cell with two semiconducting electrodes, the light gradient (e.g., different illumination of front and back electrodes due to light absorption) plays an important role in photopotential generation.

Potential distribution in a bounded piezoelectric in the presence of the photothermoacoustic effect

The photothermoacoustic effect in a bounded piezoelectric is studied theoretically and experimentally. Calculations are made of the potential distribution over the thickness and the potential difference in a thin piezoelectric layer as a function of the coordinate of this layer. The amplitude distribution of the potential oscillations over the thickness of the piezoelectric reveals two maxima. The phase of the oscillations at these maxima is shifted by approximately 180°. It is shown that the potential difference on the opposite surfaces of the plate is zero regardless of the propagation constant of the thermal wave. An experiment was carried out using a layered disk formed by seven identical piezoelements. The amplitude and phase shift of the potential difference were investigated for each element at different light modulation frequencies. The experimental results show good agreement with the theory.

Pyroelectric homogeneity of corona-charged P(VDF/TrFE) films

Temperature variation of pyroelectric response of P(VDF/TrFE) (75/25)-14 μm was studied in light modulation frequency range 1 Hz to 50 kHz at various excitation-response configurations. Since the dielectric behaviour of the film is involved in the dynamic pyroelectric characteristics important in the applications we measured also dielectric dispersion and thermally stimulated current. Pyroelectric coefficient p of the copolymer samples in form of diaphragm fixed on its perimeter does not depend on whether the positively, or negatively charged surface of the film is illuminated. Moreover, the p value is linearly dependent on the light modulation frequency f in the range 1-50 Hz and is frequency independent in the range 50 Hz – 1 kHz. The pyroelectric coefficient is constant in the temperature range 10°C – 100°C and thermal annealing causes a small decrease in the p-value.

Photoacoustic spectroscopy of diamond powders and polycrystalline films

Photoacoustic spectroscopy is used to study optical absorption in diamond powders and polycrystalline films. The photoacoustic spectra of diamond powders with crystallite sizes in the range from ∼100 µm to 4 nm and diamond films grown by chemical vapor deposition (CVD) had a number of general characteristic features corresponding to the fundamental absorption edge for light with photon energies exceeding the width of the diamond band gap (∼5.4 eV) and to absorption in the visible and infrared by crystal-structure defects and the presence of non-diamond carbon. For samples of thin (∼10 µm) diamond films on silicon, the photoacoustic spectra revealed peculiarities associated with absorption in the silicon substrate of light transmitted by the diamond film. The shape of the spectral dependence of the amplitude of the photoacoustic signal in the ultraviolet indicates considerable scattering of light specularly reflected from the randomly distributed faces of the diamond crystallites both in the polycrystalline films and in the powders. The dependence of the shape of the photoacoustic spectra on the light modulation frequency allows one to estimate the thermal conductivity of the diamond films, which turns out to be significantly lower than the thermal conductivity of single-crystal diamond.

Optical Absorption in Porous Silicon Studied by Photoacoustic Spectroscopy

Nanoporous Si layers have been investigated by photoacoustic (PA) spectroscopy. The optical absorption spectra for porous Si on p-type Si substrates were obtained in the region of 300 to 1500 nm. The absorptivity became remarkable at 600 nm and increased drastically up to the saturation value while the wavelength decreased from 500 to 300 nm. On the basis of the comparison of photoacoustic spectra registered for different light modulation frequencies the value of thermal conductivity of porous Si was estimated to be 1.55 x 10 -3 W/(cm K).

Pyroelectric response of TGS-PEO composites

0-3 type composites of triglycine sulphate, with high molecular weight polyethylene oxide were prepared by hot-pressing. FT Raman spectroscopy was used to control the interaction of glycine ions with the polymer. Pyroelectric and electric response was studied for composite samples in the form of diaphragm fixed on its perimeter and poled in low dc field (E p = 6MV/m). Pyroelectric response in the current mode of TGS/PEO composites decreases on increasing the light modulation frequency. Pyroelectric coefficient evaluated for TGS/PEO composites at 10 Hz amounts to ∼ 7.10−5 C/m2K.

Comparative study of optical absorption in diamond powders and polycrystalline CVD films

Abstract Optical absorptivity of powders and polycrystalline films is difficult to measure by common methods because of the strong diffuse light scattering. In this report we present the results of studies of diamond powders and polycrystalline CVD films by photoacoustic (PA) spectroscopy which allows us to measure the light absorption in the light scattering substances due to the photothermal effect. Diamond powders and polycrystalline CVD films with a crystallite size ranging from hundreds of microns to a few nanometers were investigated. The optical absorption in the spectral range 200 to 1500 nm was estimated from PA spectra registered at the various light modulation frequencies (of 20 to 1500 Hz). The efficiency of PA spectroscopy for investigating diamond-related materials was shown. The spectral features corresponding to the different kinds of defects in the diamond crystal lattice, and to non-diamond carbon inclusions, were detected. The dependence of the PA spectrum shape on the modulation frequency exhibited a correlation of the light scattering and the crystallite size in samples studied.

Frequency Domain Analysis of Photon Migration Based on the Microscopic Beer-Lambert Law

Exact analytic solutions for frequency domain responses of photon migration in variously shaped turbid media are derived based on a model in which the microscopic Beer-Lambert law holds. We show that the system function specified by the Fourier transform of the impulse response is a regular function, and that the temporal or spatial changes in the macroscopic absorption coefficient and the concentration of the absorber can therefore be determined from macroscopic observables such as amplitude, phase and modulation frequency of the probe light. The feasibility of using this technique in spectroscopy to determine the absolute concentration of an absorber in turbid media is also discussed. The advantage of these methods is that we do not need to take the boundary conditions into account. Similar approaches may also yield simple techniques to determine physical properties in various scientific fields.

Optical response of single-crystal (La,Ca)MnOδ thin films

(La,Ca)MnOδ thin film can be used as a bolometer operated over a wide temperature range around its metal–semiconductor transition where its resistance (R) changes rapidly. This material has a high optical absorption coefficient over a broad wavelength range and is chemically stable in air. We measured the optical response of single-crystal (La,Ca)MnOδ thin films to a He–Ne laser (632.8 nm) as a function of temperature (T), light modulation frequency (f), bias current (I), and laser power (P). The temperature dependence of the measured signal follows the dR/dT curve and the signal depends linearly on I and P. These results indicate that the response is mainly bolometric. Room temperature operation is possible if the sample composition and oxygen stoichiometry are adjusted appropriately.

Photoelectrochemical reduction of carbon dioxide in aqueous solutions on p-GaP electrodes: an a.c. impedance study with phase-sensitive detection

The mechanism of the photoelectrochemical reduction of water and CO 2 on p-GaP electrodes in aqueous solutions was studied using photocurrent spectroscopy with chopped light excitation, revealing both a photo-anodic sub-bandgap response and a photo-cathodic current due to the bandgap response. A.c. power curves were recorded for p-GaP in pH 7 phosphate buffer electrolyte using modulated irradiation and a lock-in amplifier. At a light modulation frequency of 150 Hz, CO 2 bubbling effected a 15% increase in the photo-cathodic current, relative to that in an Ar flushed solution. This effect disappeared at a light modulation frequency of 1560 Hz. Ruthenium pretreatment of p-GaP resulted in a doubling of both the cathodic photocurrent and the photovoltage. Pretreatment of p-GaP by dipping in a solution of CuSO 4 caused a marked enhancement in both the dark current and photocurrent at reverse bias. In preparative reduction of aqueous bicarbonate, using light chopped at 22 Hz and −1.08 V between the p-GaP and a stainless steel counter electrode separated by a cation selective membrane, the production of formic acid was determined. The efficiency of conversion of the incident light energy to the Gibbs energy of formation of the formic acid produced amounted to 0.3%, both with bare and with Cu treated p-GaP.