Increasing Modulation Frequency Research Articles

Three Dimensional Photothermal Deflection and Thermal Lensing in Solids: The Effect of Modulation Frequency

In this paper, a detailed theoretical treatment of the three dimensional photothermal deflection, under modulated cw excitation, is presented for a three layer system (backing-solid sample-fluid). By using a technique based on Green's function and integral transformations we find the explicit expressions for laser induced temperature distribution function and the photothermal deflection of the probe beam. Numerical analysis of those expressions for certain solid samples shows that the laser induced temperature of the sample surface, the effective thermal length and the deflection signal and its width decrease with increasing modulation frequency. Furthermore, increasing the diffusivity/conductivity of solid sample results in a decrease in the deflection signal intensity and slight increase in the signal width. Finally, we find that the focal length of the photothermal lens in normal direction is much greater than that of in tangential direction.

All-optical switching effect in PVK-based optoelectronic composites

We report the all-optical switching effect of photoconductive polymer composites based on poly(vinyl carbazole) (PVK) and azobenzene dyes. The optical switching effects were measured by using 514 nm pump beam and 632.8 nm probe beam. The reversible and repeatable change in the transmittance of the probe beam is attributed to the photoinduced anisotropy due to the photoisomerization of azobenzene dyes. The influencing factors of the optical switching effect, such as the modulation frequency and pump beam intensity, were studied experimentally. The experimental results indicate that, as the modulation frequency increases, the switching response becomes quicker, while as the pump beam power increases, the modulation depth becomes deeper.

Experimental observation of chirp reduction in bandwidth-enhanced semiconductor lasers subject to strong optical injection

An optically injection-locked semiconductor laser under direct current modulation is experimentally studied. Significant increase in the relaxation resonance frequency is first demonstrated, implying the same degree of enhancement in modulation bandwidth. Under bandwidth-enhanced operating conditions, substantial reduction in frequency chirping is observed, which demonstrates that significant bandwidth enhancement and chirp reduction are simultaneously achieved in experiment. Chirp reduction as a function of modulation frequency is also investigated. Substantial chirp reduction happens at low modulation frequency. However, the frequency chirping reduces less with increasing modulation frequency. The frequency chirping of the injection-locked laser approaches to that of the free-running laser at high modulation frequency. Effective chirping parameter is introduced and analyzed for the injection-locked laser to study the physical mechanism behind the chirp reduction.

Subsurface mapping of thermal properties with optical heterodyne force microscopy (abstract)

Optical heterodyne force microscopy (OHFM) is a novel local probe microscopy that combines an atomic force microscope (AFM) with a periodically modulated laser source. The amplitude and phase of the vibrations of the sample surface induced by the periodic heating can be imaged with OHFM. The thermal diffusivity, rather that just the thermal conductivity, has a significant effect on the image contrast. Surface and subsurface variations in thermal properties can be mapped to depths of the order of the thermal diffusion length, which decreases as the modulation frequency increases. The use of an incident laser beam chopped at megahertz frequencies typically allows ∼1 μm lateral or depth resolution in thermal properties. However, the thermal near field can be imaged to a lateral resolution of that of the AFM, namely a few nanometers. In this study we have detected a buried aluminum film with OHFM and done quantitative comparisons with a one-dimensional theoretical model based on the thermal wave propagation in the sample. We will show the topography and OHFM images for a 2.5×2.5 μm region obtained when scanning across a boundary between the Cr/SiO2 structure and the Cr/Al/SiO2 structure. The top layer consists of a 90 nm Cr film. For the Cr/Al/SiO2 structure only there is a buried intermediate layer, a 180 nm Al film. The films are deposited on a silica substrate. An infrared laser beam of wavelength 830 nm is chopped at ∼4 MHz to give an average incident power of 3 mW focused to a ∼3 μm diameter spot on the tip. A commercial transparent Si3N4 cantilever of spring constant ∼0.1 N m−1 and a tip radius ∼20 nm is used. Compared with the one-dimensional model of the thermoelastic response of the structure, the experimental contrast between the two regions is somewhat smaller than predicted for the amplitude images and larger than predicted for the phase images. The discrepancy may be due to a thermal boundary resistance or to three-dimensional effects not accounted for in the model.

Pyroelectric Properties of the B-Polyvilnylidene Fluoride (PVDF) Thin Film Prepared by Vacuum Deposition with Electric Field Application

Polyvinylidene fluoride (PVDF) thin film having β phase is prepared by vacuum deposition with electric field application and its pyroelectric properties are studied using a dynamic method to examine the possibility of its application to a pyroelectric IR sensor. The pyroelectric responses of the PVDF thin film are characterized as the frequency dispersion in both low and high modulation frequency regions, and their frequency dependences are observed. In the low frequency region (2–100 Hz), the polarization can easily rotate with the increase of modulation frequency and show the maximum since the reorientation rate of domains is higher than the modulation frequency. On the other hand, in the high frequency region (100–1000 Hz), the pyroelectric response decreases as the frequency increases, because the reorientation rate of domains is suppressed and thus, the change of polarization decreases. Pyroelectric coefficient, figure of merits for noise equivalent power and detectivity of the PVDF thin film are measured as 4.39×10-9 C/cm2·K, 7.72×10-10 C·cm/J, and 2.29×10-9 C·cm/J, respectively. Also, the noise equivalent and the detectivity are 1.66×10-7 W/Hz1/2, and 6.03×105 cm·Hz1/2/W, respectively.

Dispersion of photon density waves in a strongly scattering medium

In the approximation of the radiation transfer equation, the damping decrement and the velocity of photon density waves propagating in a strongly scattering medium are studied as functions of the increasing modulation frequency of the source of radiation up to the microwave frequency range. The results obtained for microwave frequencies are found to be in qualitative agreement with those derived from the Bethe-Salpeter equation.

Nonlinear dynamics of a CO2laser with current modulation and cavity detuning

A detailed numerical study of the dynamic behaviour of a CO2 laser under current modulation and cavity detuning has been performed on the basis of the complete laser model which takes into account vibrational–vibrational and vibrational–rotational energy exchanges in the active medium. We have shown that due to a relatively fast decrease in the laser response with increasing modulation frequency, nonlinear regimes (period doubling and chaos) can be obtained with cavity detuning either from the line centre or near the laser threshold. Comparison is made with a two-level equation model for the active medium.

Temporal modulation transfer functions obtained using sinusoidal carriers with normally hearing and hearing-impaired listeners.

Temporal modulation transfer functions were obtained using sinusoidal carriers for four normally hearing subjects and three subjects with mild to moderate cochlear hearing loss. Carrier frequencies were 1000, 2000 and 5000 Hz, and modulation frequencies ranged from 10 to 640 Hz in one-octave steps. The normally hearing subjects were tested using levels of 30 and 80 dB SPL. For the higher level, modulation detection thresholds varied only slightly with modulation frequency for frequencies up to 80 Hz, but decreased for high modulation frequencies. The decrease can be attributed to the detection of spectral sidebands. For the lower level, thresholds varied little with modulation frequency for all three carrier frequencies. The absence of a decrease in the threshold for large modulation frequencies can be explained by the low sensation level of the spectral sidebands. The hearing-impaired subjects were tested at 80 dB SPL, except for two cases where the absolute threshold at the carrier frequency was greater than 70 dB SPL; in these cases a level of 90 dB was used. The results were consistent with the idea that spectral sidebands were less detectable for the hearing-impaired than for the normally hearing subjects. For the two lower carrier frequencies, there were no large decreases in threshold with increasing modulation frequency, and where decreases did occur, this happened only between 320 and 640 Hz. For the 5000-Hz carrier, thresholds were roughly constant for modulation frequencies from 10 to 80 or 160 Hz, and then increased monotonically, becoming unmeasurable at 640 Hz. The results for this carrier may reflect "pure" effects of temporal resolution, without any influence from the detection of spectral sidebands. The results suggest that temporal resolution for deterministic stimuli is similar for normally hearing and hearing-impaired listeners.

Measurement of auditory temporal processing using modified masking period patterns.

A common metric of auditory temporal processing is the difference in the threshold for a pure-tone signal masked by either unmodulated or amplitude-modulated noise. This technique may be viewed as a modification of the masking period pattern technique. Such measurements have been proposed as an efficient means of estimating auditory temporal resolution in a clinical setting, although in many cases threshold differences may reflect additional spectro-temporal processes. The primary purpose of the present experiment was to examine interactions among signal frequency and masker bandwidth and the effects of modulation frequency on modified masking period patterns. The results revealed unmodulated-modulated threshold differences that increased with increasing masker bandwidth and decreased with increasing modulation frequency. There was little effect of signal frequency for narrow-band noise maskers that were equal in absolute bandwidth across frequency. However, unmodulated-modulated threshold differences increased substantially with increasing signal frequency for bandwidths proportional to the signal frequency and for wideband maskers. Although the results are interpreted in terms of a combination of both within-channel and across-channel cues, the specific contributions of these cues in particular conditions are difficult to ascertain. Because modified masking period patterns depend strongly upon a number of specific stimulus parameters, and because it is difficult to determine with any precision the underlying perceptual processes, this technique is not recommended for use as a clinical measure of auditory temporal processing.

A high-precision magnetoencephalographic study of human auditory steady-state responses to amplitude-modulated tones

The cerebral magnetic field of the auditory steady-state response (SSR) to sinusoidal amplitude-modulated (SAM) tones was recorded in healthy humans. The waveforms of underlying cortical source activity were calculated at multiples of the modulation frequency using the method of source space projection, which improved the signal-to-noise ratio (SNR) by a factor of 2 to 4. Since the complex amplitudes of the cortical source activity were independent of the sensor position in relation to the subject's head, a comparison of the results across experimental sessions was possible. The effect of modulation frequency on the amplitude and phase of the SSR was investigated at 30 different values between 10 and 98 Hz. At modulation frequencies between 10 and 20 Hz the SNR of harmonics near 40 Hz were predominant over the fundamental SSR. Above 30 Hz the SSR showed an almost sinusoidal waveform with an amplitude maximum at 40 Hz. The amplitude decreased with increasing modulation frequency but was significantly different from the magnetoencephalographic (MEG) background activity up to 98 Hz. Phase response at the fundamental and first harmonic decreased monotonically with increasing modulation frequency. The group delay (apparent latency) showed peaks of 72 ms at 20 Hz, 48 ms at 40 Hz, and 26 ms at 80 Hz. The effects of stimulus intensity, modulation depth, and carrier frequency on amplitude and phase of the SSR were also investigated. The SSR amplitude decreased linearly when stimulus intensity or the modulation depth were decreased in logarithmic steps. SSR amplitude decreased by a factor of 3 when carrier frequency increased from 250 to 4000 Hz. From the phase characteristics, time delays were found in the range of 0 to 6 ms for stimulus intensity, modulation depth, and carrier frequency, which were maximal at low frequencies, low intensities, or maximal modulation depth.

COLOSSAL MAGNETORESISTANCE BOLOMETER

Based on the sharp resistance change near the metal-insulator transition (M-I) of the colossal magnetoresistance (CMR) materials, we design and test a new type CMR bolometer. La0.67Ca0.33MnO3 epitaxial thin films were used as the photosensitive element of the bolometer, and the optical response of this device to the He-Ne laser and the blackbody were measured. The measured signal and noise decrease with increasing modulation frequency; however, the signal-to-noise ratio shows no obvious change for the He-Ne laser. The temperature dependence of signal was measured in both ferromagnetic metallic and paramagnetic insulating states. It shows that the strongest signal appears in the ferromagnetic state very close to the M-I point. The film's Curie temperature can span from 70 to 350K, depending on the manufacture conditions. So, working near or at room temperature characterizes this CMR thin film bolometer. In this sense, it compensates the weakness of the high-temperature superconductor bolometer that can be used only around the liquid Nitrogen (LN) temperature.The measured detectivity is about 107 cmHz1/2/W and the thermal response constant is 20ms for this device. Compared with the high-temperature superconductor bolometer, the time constant of CMR bolometer is longer and the detectivity is lower. However, there is much room to improve yet.

Improved Method for Three-Dimensional Imaging in Photoacoustic Microscope

The proposed new method partially solves the problem of low signal-to-noise-ratio (SNR) when modulation frequency increases. In this paper, we suggest to keep constant the amplitude of the detected photoacoustic (PA) signal by linearly increasing the modulation voltage versus frequency. The PA signal of the sample can be easily estimated by fast Fourier transform. It is also demonstrated that the presented method is able to detect a subsurface flaw in a graphite in the 10–100 Hz frequency range.

Upper threshold of burst duration for continuous apparent motion. Effects of amplitude modulation and frequency band.

The upper thresholds of burst duration that produced continuous apparent motion were measured when stimuli were successively presented using three loudspeakers arranged in the horizontal plane. Stimuli tested were broadband noises with amplitude modulation, tones and bandpass noises. The upper threshold depended on stimulus and tended to be longer than that of 72 ms for an unmodulated broadband noise. For amplitude-modulated broadband noises, there was an increase in the upper threshold from 72 to 290 ms as the modulation index increased from 0.0 to 1.0. However, the upper threshold decreased from 360 to 135 ms with an increase in modulation frequency from 4 to 64 Hz. For tones, the upper threshold displayed a concave function of frequency with a minimum of 71 ms at a frequency of 3 kHz. For bandpass noises, the upper threshold fell from 347 to 101 ms as the bandwidth broadened from 1/12 to 3.0 octaves. These results are discussed in connection with perception for amplitude modulation and mechanisms to detect motion.

Temporal versus spectral cues in AM detection

Several recent studies have shown that modulation thresholds for detecting sinusoidal AM are approximately constant out to much higher modulation frequencies for tonal carriers than for broadband noise carriers. It has been suggested that with tonal carriers there is a range of modulation frequencies for which both temporal (envelope) cues and spectral cues are used for detection. The present study attempts to assess the relative importance of these cues. Modulation thresholds for AM, quasifrequency modulation (QFM), and beats were measured for modulation frequencies of 8–512 Hz and carrier frequencies of 2 and 5 kHz. The results indicate that at intermediate modulation frequencies, AM thresholds are approximately constant while those for QFM and for beats decrease with increasing modulation frequency. This indicates that the detection of AM is not based solely on temporal cues. The implications of these findings for envelope detection and modulation filter-bank models will be discussed. [Work supported by NIDCD Grant No. DC00683.]

A photopyroelectric method with air as the thermal coupling fluid

The photopyroelectric (PPE) technique is usually used as a contact photothermal method to characterize thin solid materials. We performed theoretical and experimental investigations of a non-contact PPE configuration to analyse the effects of an air gap between the thermal source and a PVDF sensor using a modulated laser beam as optical excitation. The detection of the thermal wave developed within the sample was possible across an air gap of up to 15 mm. The PPE signal was described well thanks to a two-dimensional (2D) model with axial symmetry, which could be reduced in some particular cases to a simpler 1D propagation model. Border effects can be artificially taken into account in the calculations by increasing the adiabatic lateral limits in the model, keeping the active sensor area constant. Despite there being strong attenuation with increasing modulation frequency, the PPE method can be used as well as can other non-contact photothermal techniques. New perspectives for the application of the PPE technique have been opened.

Amplitude modulation detection thresholds and associated psychometric functions.

Amplitude modulation detection thresholds for broadband stimuli were measured as a function of modulation frequency in seven listeners with normal hearing. Two adaptive procedures (1-down, 1-up and 4-down, 1-up) were used to track the 50% and the 84% correct levels in a three-interval, three-alternative, forced-choice task. Preliminary data show that the temporal modulation transfer functions for the two criteria are parallel for lower modulation frequencies but tend to converge with increasing modulation frequency. The resultant two-point psychometric functions associated with the modulation detection data demonstrate an increase in slope with increasing modulation frequency. Time constants were calculated from a least-squares fit of the equation y=k/(f+a) and further illustrate the effects of the changes in psychometric function slope. These data have implications for comparisons of temporal acuity estimates across data sets.

High-Frequency Response of Intersubband Infrared Photodetectors with a Multiple Quantum Well Structure

We have theoretically studied the response of quantum well intersubband photodetectors to high-frequency modulated infrared radiation. The small-signal responsivity dependent on the modulation frequency of infrared radiation and device parameters has been derived using the proposed analytical model. It has been shown that with increasing modulation frequency the roll-off of the small-signal responsivity is associated with the electron transit and capture effects which limit the device bandwidth.

Detection of frequency modulation (FM) and amplitude modulation (AM) by budgerigars (Melopsittacus undulatus)

Many bird vocalizations contain elements consisting of both frequency-modulated (FM) and/or amplitude-modulated (AM) compo- nents. In humans, differences in perception of FM versus AM sounds are related to other psychoacoustic phenomena and have provided an interesting test of the excitation pattern model of hearing. Here, budgerigars were trained by operant conditioning and tested with the method of constant stimuli to detect FM and AM. The birds were tested at three carrier frequencies (1.0, 2.86, and 4.0 kHz) and the threshold for detecting modulation was measured at eight different modulation frequencies from 5 to 640 Hz. The threshold for detection of frequency modulation decreased with increasing modulation frequency, while the threshold for detecting amplitude modulation was fairly constant over the entire range of modulation frequencies. This pattern of results is similar to that found for humans. Critical modulation frequencies calculated from these functions will be compared to other psychoacoustic phenomena in budgerigars and the relevance of these findings for mechanisms of hearing in birds and the characteristics of vocal signals will be discussed. [Work supported by NIH DC-00198 and MH-00982 to RJD.]

Photothermal measurement of thermal anisotropy in pyrolytic graphite

We investigated the anisotropic thermal conductivity in pyrolytic graphite by thermoreflectance. A laser-heated circular spot on a surface perpendicular to the planes developed into an elliptical temperature distribution which was recorded by a raster scanning technique at modulation frequencies ranging from 600 Hz to 100 kHz. The ratio of in-plane and perpendicular thermal conductivity was determined by fitting the phase of the temperature data with an analytical model, and was found to decrease with increasing modulation frequency. Highest conductivity values were considerably smaller than previously published data based on steady-state measurements. The frequency dependence and additional features in the phase profiles at high frequencies are discussed in view of sample surface preparation and the local nature of the thermoreflectance measurement.

Modulated potential electrogenerated chemiluminescence of luminol and Ru(bpy) 3 2+

Chemiluminescence emission intensity is modulated by modulating the potential of a working electrode which is used to generate a key species in the electrogenerated Chemiluminescence (ECL) reaction. The emission is monitored synchronously using a lock-in amplifier. The reactions used in the characterization are luminol with hydrogen peroxide and tris(2,2′-bipyridyl)ruthenium (II) (or Ru(bpy)32+) with oxalate. Modulation widths of ± 50 mV yield maximum signals for luminol when centered at 0.45 V (vs Ag/AgCl) and for Ru(bpy)32+ when centered at 1.05 V. The resulting signal decreases with increasing modulation frequency and shows that luminol/H2O2 is a faster ECL system than Ru(bpy)32+/oxalate. Working curves for luminol and for oxalate have essentially the same linear range and slope with the modulated potential approach as with a DC electrode potential. This approach provides capability for differentiating the analytical signal from constant background emission or stray light.