Incident Light Source Research Articles

Highly sensitive thin-film organic phototransistors: Effect of wavelength of light source on device performance

Organic phototransistors (OPTs) were fabricated from pentacene and copper phthalocyanine (CuPC) based on the geometry of organic field-effect transistors (OFETs); and the effect of the wavelength of the incident light source on their performance was examined. High performance OFETs with pentacene and CuPC were fabricated and the characteristics of the OPTs were examined under UV and visible-light irradiations with top illumination. The CuPC and pentacene OPTs show a high responsivities of 0.5–2 and 10–50A∕W and maximum IPh∕IDark of 3000 and 1.3×105, respectively, under 365nm UV light. However, under visible light, at a wavelength of 650nm, the pentacene OPTs had 100 times less responsivity, 0.15–0.45A∕W, and a IPh∕IDark of 1000, even though an absorption coefficient three times larger was observed at this wavelength than at 365nm. A strong correlation was found between the performance of the OPTs and the incident photon to current conversion efficiency spectra of an organic semiconductor. The strong dependence on the wavelength of incident light of the performance of the prepared OPTs can be explained by an internal filter effect in which light with a large absorption coefficient is filtered at the top surface and through the bulk of the film when light is directed onto the opposite side of the OFET gate electrode. Thus, light cannot efficiently contribute to the generation of charge carriers in the channel regions that were formed in the first two molecular layers adjacent to the dielectric interface. Consequently, the most efficient OPTs were produced when the following conditions of incident light were satisfied: The photon energies (or frequencies) should be (i) larger than the band gap and (ii) have a relatively small absorption coefficient, since the light can penetrate down to the channel layer more efficiently when it is near the dielectric interface without any loss in absorption through the film.

Optical properties of hair: detailed examination of specular reflection patterns in various hair types

Details of the specular reflection of curved hair tresses, resulting from illumination with a collimated incident light source, were examined both qualitatively and quantitatively using high-resolution photography and image analysis. The reflections were found to consist of a multitude of light dots aligned with the fibers and typically separated by a distance of 81-145 microm. The contrast between the dots (specular reflection) and the darker regions (diffuse reflection) of the entire reflection band was found to increase with increasing pigmentation of hair. Highly pigmented Oriental hair provided more contrast within the specular reflection band than unpigmented natural white hair. A quantitative description of the light reflection patterns within the specular reflection band included two-dimensional distribution of luminosity, histograms of the frequency of appearance for peak maxima and minima in luminosity distribution plots, and histograms of absolute maxima and minima of luminosity along the length of the fibers. Specular reflection from African hair, which consists of many curls that provide multiple and randomly distributed reflection centers, have also been investigated. Using microscopy software, Image Tool 2.0, and a method termed image threshold, the number of reflection sites and their shapes could be quantified. For example, treatment of African hair with synthetic sebum was shown to significantly affect the reflection patterns, resulting in a decrease in the overall hair luster. Comparison of reflection patterns from Caucasian frizzy, very curly, and curly hair is also discussed.

Estimation of Chromatic Characteristics of Scene Illumination in an Image by Surface Recovery from the Highlight Region

This article proposes an illuminant estimation algorithm that estimates the spectral power distribution of an incident light source using its chromaticity determined based on the perceived illumination and highlight region. The proposed algorithm is composed of three steps. First, the illuminant chromaticity of the global incident light is estimated using a hybrid method that combines the perceived illumination and highlight region. Second, the surface spectral reflectance is then recovered from the image after decoupling the global incident illuminant for each channel. The surface spectral reflectance calculation is limited to the MAR (maximum achromatic region), which is the most achromatic and brightest region in the image, and estimated using the PCA (principal component analysis) method along with a set of given Munsell samples. Third, the closest colors are selected from a spectral database composed of reflected-lights generated by the given Munsell samples and a set of illuminants. Finally, the illuminant of the image is calculated using the average spectral distributions of the reflected-lights selected for the MAR region and its average surface reflectance. Experimental results confirmed the accuracy of the estimates produced by the proposed method for various illuminants.

The response of squid and fish to changes in the angular distribution of light

This paper describes the responses of a squid (Alloteuthis subulata) and a fish (Trachurus trachurus) to changes in the angular distribution of light. An apparatus was made that simulated the angular distribution of daylight in the sea. The apparatus enabled the direction of the brightest light to be changed and the positions of the animals in response to these changes were observed. Squid viewed head-on were observed to roll by a maximum of 20° when the incident light source was at angles between 20° and 90° (where 0° is vertically downwards). When viewed laterally, i.e. in the pitch plane, the squid were observed to position themselves more closely with respect to the angle of the light source, they swam in a near vertical plane when the incident light source was at an angle of 90°. Swimming movements in the roll and pitch plane became more horizontal with positions of the light source between 90 and 180°. Horse mackerel, in contrast, inclined their dorsal surfaces to almost perfectly match the angle of the incident light source, even swimming upside-down when light came from below. These experiments also revealed that squid display a counter-shading chromatophore pattern (‘Flexible Countershading’) in response to light coming from the sides, which involves darkening the side of the body facing the brightest light. The use of chromatophores in this way may explain why the dorsal light reflex in squid is so weak compared to that of fish.

Fabrication of microbowtie structures for optical probing of nanoscale objects

A microbowtie array critical to a newly proposed optical probe is studied. The optical probe is used for high-speed detection of nanoscale objects. Based on the concept of Wave Interrogated Near Field Array (WINFA), an array of microbowtie structures acts as a detector in amplifying the incident light source to overcome the difficulties of the conventional optical probe that are limited by the diffraction limits of the optical light. In the present study, the micro bowties are fabricated by an electron beam lithography technique. The paper begins with an introduction of the probe concept and then the design considerations are given followed by the fabrication procedure and results. Two different substrates are considered to study the charge accumulation problem caused by an insulating substrate, required by the probe. Results indicate that the bowtie structure with an indium-tin-oxide coated glass substrate can minimize the charge accumulation problem, making it a suitable substrate for the bowtie structures considered. The paper concludes with recommendations for making even smaller nano-sized bowtie structures.

An Arctangent Unwrapping Technique of Photoelasticity Using Linearly Polarized Light at Three Wavelengths

Abstract: In the automatic measurements of the principal stress directions ψ and the total relative phase retardation ρtol, ψ and ρtol can be easily obtained by the phase unwrapping method using the arctangent function. However, only some published works exist on the measurements of ψ and ρtol by photoelastic techniques using a linearly polarised incident light source. This paper presents the technique of applying an unwrapping by means of the arctangent function to photoelasticity. The technique overcomes the error associated with the quarter‐wave plate by using an incident light of different wavelength and does not need an interpolation for the determination of ψ . The technique is validated by determining ψ and ρtol for the well‐known circular disk subjected to a diametral compressive load model.

Local variation of the work function of Cu(1 1 1) surface deduced from the low energy photoemission spectra

We have investigated the photoemission spectra very close to the vacuum level of the Cu(1 1 1) surface with high resolution, using a tunable laser as the incident ultraviolet light source. The incident photon energy was tuned very close to the work function of the sample surface. A small emission peak was found ∼100 meV below the vacuum level cutoff that corresponds to the work function. The origin of the small peak is attributed to the emission from the areas whose local work function is depressed below the work function of the majority area. Variation of the spectrum caused by surface roughening and oxygen adsorption suggests that the areas with lower work function are the step edges.

レーザ計測法による燃焼器出口の微粒子（ＰＭ）濃度測定法の開発

A research in which laser light scattering and attenuation techniques are applied simultaneously in a complimentary fashion is fully investigated to allow the determination of size and number density of particulate matter concentration in exhaust gas from steady flame at real time. The particulate matters, however, seem to be dispersed or cluster-like, spherical or maybe of any sizes, including Rayleigh-size, for which it is particularly difficult to infer simultaneously particle size and concentration. This study is based on the theories of Rayleigh-Debye about light scattering and the form of the particulate matter in exhaust gas is cluster-cluster-aggregation, called Cluster-Rayleigh-Debye model. The main point of this technique is combined of using laser light scattering for the case of scattering angles of 40deg. and 140deg. and attenuation technique with two laser light sources individually. 50 (mW) and 15 (mW) Helium-Neon laser light sources emitting monochromatic radiation at a wave-length of 632.8 (nm) are used as incident light sources. In the present study, the detailed profiles of size and number density of particulate matter concentrations in exhaust gas from steady flame are obtained. The dependent on intensity level and refractive index variations can be neglected. The application and results of this method are fully investigated, compared to those of system using filter and discussed.From results of the experiments, it can be concluded that the implementation of the Cluster-Rayleigh-Debye model system has been realized successfully to provide particulate matter concentration in exhaust gas from any steady flame at real time and those are fundamental for next stage experiments on unsteady flame and diesel engines. There is no significant difference between the result of this optical method and filter system.

Estimation of Spectral Distribution of Scene Illumination from a Single Image

This article proposes an illuminant estimation algorithm that estimates the spectral power distribution of an incident light source from a single image. The proposed illumination recovery procedure has two phases. First, the surface spectral reflectances are recovered. In this case, the surface spectral reflectances recovered are limited to the maximum achromatic region (MAR) which is the most achromatic and highly bright region of an image, after applying intermediate color constancy process using a modified gray-world algorithm. Next, the surface reflectances of the maximum achromatic region are estimated using the principal component analysis method along with a set of given Munsell samples. Second, the spectral distribution of reflected lights of MAR is selected from the spectral database. That is, a color difference is compared between the reflected lights of the MAR and the spectral database, which is the set of reflected lights built by the given Munsell samples and a set of illuminants. Then the closest colors from the spectral database are selected. Finally, the illuminant of an image can be calculated dividing the average spectral distributions of reflected lights of MAR by the average surface reflectances of the MAR. In order to evaluate the proposed algorithm, experiments with artificial and real captured color-biased scenes were performed and numerical comparison examined. The proposed method was effective in estimating the spectral distribution of the given illuminants under various illuminants and scenes without white points.

Dynamic Near-Infrared Light Scattering from Blue-Colored Polystyrene Latex

A new type instrument for dynamic light scattering (DLS) measurements was designed and set up to avoid the influences of absorption effect and emission from scatterers. The instrument was equipped with two kinds of incident laser light sources: He-Ne laser (λ = 632.8 nm) and Nd:YAG laser (λ = 1064 nm). The reproducible autocorrelation function (ACF) of blue-colored polystyrene latex (BPSL), as well as an ordinary colorless polystyrene latex (PSL), was measured successfully in the near-infrared region, and the diameters of BPSL could be evaluated accurately. On the other hand, the relationship between absorption coefficients and diffusive coefficients of BPSL measured in the visible region could be presented empirically.

Multiple-route and multiple-state current-voltage characteristics of an InP/AlInGaAs switch for multiple-valued logic applications

A novel multiple-state switching device based on an InP/AlInGaAs heterojunction bipolar transistor (HBT) structure has been successfully fabricated and demonstrated. The common-emitter current gain up to 25 is obtained under the forward operation mode. However, the anomalous multiple-negative-differential-resistance (MNDR) phenomena controlled either by electrical or optical input signals are observed under the inverted operation mode. The studied device exhibits a single-route S-shaped NDR behavior in the dark and a distinct significant S-shaped MNDR phenomena by introducing an incident light source at room temperature. Moreover, the anomalous multiple-route and multiple-step current-voltage (I-V) characteristics are also observed at 77 K. The switching behaviors are attributed to the avalanche multiplication, barrier lowering effect and potential redistribution process. Experimental results show that the studied device provides a good potentiality for multiple-valued logic and optoelectronic switching system applications.

Raman Spectroscopy with a Single-Frequency, High-Power, Broad-Area Laser Diode

A high-power, broad-area laser diode was used as an incident light source in a Raman spectrometer. The laser was forced to lase in a very narrow frequency range (⩽ 400 MHz) by the use of a Metcalf–Littman arrangement. All side bands and light-emitting diode (LED) modes were suppressed; their intensities are lower than the noise of recorded rotational spectra. The incident wavelength was tuned to coincide with the D1 transition of 85Rb. Due to this choice, the elastically scattered light (Rayleigh component) can be eliminated by a Rb cell in front of the spectrometer. The usefulness of this arrangement is demonstrated for CO2, N2, and air. Because of the suppression of the dominating Rayleigh intensity, this setup is ideally suited for a Fourier transform Raman spectrometer.

Continuous and non-invasive study of brain oxygenation in the calf by near infrared spectroscopy

Near infrared spectroscopy (NIBS) was used to detect changes in brain oxygenation in five tranquilised calves that were placed on a hypoxic gas mixture (10 per cent O 2/90 per cent N 2) and hyperoxic gas mixture (30 per cent O 2/70 per cent N 2) for five minutes at each concentration. A NIRO 500 (Hamamatsu, Japan) was used for the NIBS, with the incident light source and separate detector (optodes) placed on shaved skin on the most dorsal surface of the frontal bone. Sequential arterial blood gas sample analyses provided confirmation of the appropriate change in systemic oxygenation status. By the end of the five-minute-period of breathing 10 per cent oxygen, NIBS of the calf head detected highly significant changes in haemoglobin oxygenation reflective of hypoxaemia, with oxyhaemoglobin decreasing by 23·5 units (P<0·01) and deoxyhaemoglobin increasing by 45·6 units, (P<0·01) from the baseline of breathing room air. Total haemoglobin (oxyhaemoglobin + deoxyhaemoglobin) showed a significant increase of 22·1 units (P<0·05) but there was no significant change in LAIRS determined cytochrome aa 3 oxygenation. Concomitant blood gas alterations included significant decreases in Pa) 2 (−27·8 mmHg, P<0·01), haemoglobin saturation (−29·0 per cent, P<0·05), and PaCO 2 (−7·8 mmHg, P<0·05) and significantly increased blood pH (0·059, P<0·05). At the end of the five minutes of breathing 30 per cent oxygen NIBS of the calf head detected significantly increased oxyhaemoglobin (13·1 units, P<0·01) and decreased deoxyhaemoglobin (−13·7 units, P<0·05) when compared with baseline breathing of room aim. Total haemoglobin and cytochrome aa 3 were unchanged from baseline. The accompanying arterial blood gas changes included significant increases in PaO 2 (30·9 mmHg, P<0·05), arterial O 2 saturation (11·7 per cent, P<0·05), and significantly decreased pH (−0·026, P<0·05), This study showed that NIRS can be used to continuously and non-invasively detect cerebral oxygenation changes in the live calf in response to both increased and decreased systemic arterial oxygen. Additionally, despite induction of profound hypoxaemia, cytochrome aa 3 in the brain did not appear to become reduced.

Optical tomographic reconstruction from diffuse remittance in scattering media using partial least squares estimation

The long-term goal of this research is to develop a method for quantitative, three-dimensional optical imaging in scattering media. Towards this end, a reconstruction technique for depth-resolved absorption determinations based on diffuse reflectance measurements was developed. The approach employs an array of radial measurements of intensity to obtain multi-perspective information through a specimen. Both Monte Carlo simulations and experimentally obtained reflectance measurements of known phantoms were used to evaluate the sensitivity of the method to changing sample absorption. Diffuse reflectance experiments were made using a collimated incident light source and layered absorbing/scattering samples. A fiber optic detection system measured radial responses for a series of different phantom compositions. Results demonstrate that a well defined ray path is obtained for light measured from each source and detector pair. For image reconstruction, a comparison to volume element estimation was made for classical back projection, inverse least squares and partial least squares methodologies. Overall, PLS gave significantly better results with a 50% reduction in the coefficient of variation compared to the back projection method. This depth resolved tomographic approach is a new tool to study spatial relationships of chromogenic constituents in scattering media.

Optical-Model-Based Analysis of Consecutive Images

Most existing algorithms for analyzing a moving scene use a 3-dimensional geometricmodel consisting of an object and a viewer and disregard optical processes. So, they cannot be applied to realistic scenes, such as those with light sources. This paper therefore presents a comprehensive image analysis based on a 3-dimensional opticalmodel that includes light sources as well as the object and the viewer. The optical model provides a photometric constraint describing the geometric relationship between light sources and surface shape, and the constraint serves as the basis for a new algorithm that estimates optical flow, object shape, surface reflection coefficients, and illuminant direction from consecutive images. It is shown that the parameter is not just a correction term in estimating optical flow but also a photometric term having physical meaning. This is proved by showing that incident light source and surface reflection coefficients can be estimated using the parameter. The analysis can provide enough parameters for regeneration of a scene using computer-graphics techniques.

Amplitude Sensitive Optical Heterodyne and Phase Lock-in Technique on Small Optical Rotation Angle Detection of Chiral Liquid

A novel method in which an optical polarized heterodyne polarimeter and lock-in technique are used to measure low concentration glucose solution based on the detection of an amplitude sensitive heterodyning signal is proposed. The linearity and the sensitivity of the measurements are enhanced in comparison with those of conventional methods in which a Pockels cell, Faraday rotator, and rotating linear polarizer are used to modulate the incident light source. A pseudo heterodyning carrier and delayed optical heterodyning technique is also discussed in comparison with the proposed amplitude sensitive optical heterodyne method. During the detection of glucose solution, the measurable rotation angle reaches 10-4 deg and a sensitivity of a concentration to 10 mg/d l in 1 cm optical path length is obtained. Further improvements such as using a better intensity stabilized laser and a high extinction ratio Glan-Thompson polarizer will allow the detection sensitivity of the measuring optical rotation angle to reach 10-5 deg.

Nonphotochemical, Polarization-Dependent, Laser-Induced Nucleation in Supersaturated Aqueous Urea Solutions.

We report a new photophysical phenomenon in which 1.06 mm pulses from a Q-switched Nd:YAG laser induce crystallization in supersaturated solutions of urea in water. Because the solutions are transparent at the incident wavelength, a photochemical mechanism is unlikely. The needle-shaped crystals that initially form tend to be aligned parallel to the electric field vector of the light, suggesting a Kerr-like field-induced alignment of urea molecules that aids in organizing prenucleating clusters. The effect has application to pump-probe nucleation studies and to clean nucleation in sealed systems. [S0031-9007(96)01456-1] PACS numbers: 81.10.Dn, 42.50.Vk The study of the light-induced condensation of supersaturated vapors dates back to the work of Tyndall in 1869 [1]. More recent interest has centered on the formation of atmospheric aerosols [2], laser-induced chemical vapor deposition [3], and laser-induced clustering in atomic and molecular systems [4]. The mechanism typically involves the photochemical generation of a nonvolatile product that acts as a nucleus for the growth of the condensed phase [5]. Nucleation in liquid solutions is a more complex problem involving two components, and, to our knowledge, there have been no reports of light-induced nucleation from supersaturated solutions. Nevertheless, this problem is of great theoretical and commercial importance, such as in industrial crystallization processes [6]. Recently, while attempting to observe second harmonic generation in supersaturated solutions of urea in water, we have noticed that pulses from a Q-switched Nd:YAG laser can induce nucleation in such solutions. Because the incident light source is near infrared, a photochemical mechanism is unlikely. The orientation of the crystallites that are formed depends on the plane of polarization of the incident radiation, suggesting an electric-field-induced effect. Aqueous urea solutions, with concentrations, c, in the range of 11.5 -13.5M, were prepared by combining solid urea and water in a 1.3-cm diameter pyrex test tube, which was then heat sealed with a torch. Great care was taken to exclude dust from samples. Supersaturated solutions were generated and regenerated by heating the tubes to 45 ‐ C and holding them at that temperature for

Optical scattering of diamond films

One of the important applications for diamond films is the protective coating for optical windows, which require good transparency. Light scattering causes transmittance loss and cannot be ignored if the surface roughness or grain sizes are comparable with the wavelength. The microwave plasma technique was used to grow diamond films of well-defined {111} and {100} facets on silicon substrates by controlling the CH 4 H 2 ratio at 0.7% and 2.0%, respectively. The flow rate was 100 sccm and substrate temperatures were at 600 °C. An etched, free-standing window was used to study optical scattering by FTIR spectra and an integrating sphere. The FTIR transmittance spectra show that the scattering effect starts at a wavelength of around 5 μm, where well-grown crystallites appear. Such effect is even more serious for rougher {111} facets. This scattering can be eased by longer deposition and the so-called two-step process to obtain higher nucleation density. Optical properties of the diamond windows studied by integrating sphere show that only about 5% of incident visible laser beam is kept on its direction after passing the films, this number being somewhat higher for 1300 nm diode laser beam. Most of the loss can be attributed to body scattering inside the films. These results are in consistence with that of FTIR studies. It is safe to say that more directional trasmittance can be anticipated if a longer wavelength infrared beam is used as incident light source.

Hot spot locations and temperature distributions in a forced convection photochemical reactor

We have computed steady axisymmetric temperature distributions in a tubular photoreactor in which fluid in laminar flow absorbs light from an azimuthally uniform, radially incident light source. In addition to the primary on-axis temperature maximum, a second hot spot forms off the centerline as the optical density γ increases. This results from a shift of light absorption toward the wall, which is held at a fixed temperature (e.g. by forced convection cooling on the outside). At still larger γ the centerline hot spot disappears, leading to a decrease in the maximum temperature and accompanied by an increase in the temperature near the wall. There is a critical value of γ, depending on the Peclet number Pe, below which the hot spot is located on the centerline and above which a second hot spot forms and moves off the centerline. The reduction in maximum centerline temperature may be advantageous for product selectivity and yield in the central core of the reactor, but may be disadvantageous with respect to the formation of light-absorbing deposits on the interior of the tube wall. Axial heat conduction has significant effects for Pe < 50, including displacement of the hot spot upstream from the exit plane of the illuminated section, as well as an increase in the maximum temperature.

Coal examination on a mesoscopic scale

The use of a binocular microscope and incident light source to examine whole, oriented coal samples at a mesoscopic (× 10) scale allows the rapid appraisal of coal type. Microscopic studies may subsequently be used to support and amplify the mesoscopic facies. This method has the following advantages over other techniques of coal facies analysis: 1) It is rapid and not hampered by subjective macroscopic brightness criteria that may bear little relationship to the maceral content. 2) The orientation, sedimentary structure and texture of the coal is preserved, and the relationships of coal facies with associated mineral matter are retained. 3) Stratigraphic information such as facies association, facies sequences, and the nature of facies boundaries can be used to interpret the origin of coal types. 4) Microscopic studies can be carried out on a few examples of each facies identified at a mesoscopic scale, thus decreasing the amount of time necessary for analysis. -Author