Monochromatic Illumination Research Articles

Application of an optical interferometer for measuring the surface contour of micro-components

The application of an optical interferometric system using a Mireau objective to measure the surface profile of micro-components is described. The proposed system produces a uniform monochromatic illumination over the test area and introduces an interference fringe pattern localized near the test surface. Both the interference fringes and the 2D image of the test surface can be focused by an infinity microscope system consisting of a Mireau objective and a tube lens. A piezoelectric transducer (PZT) attached to the Mireau objective can move precisely along the optical axis of the objective. This enables the implementation of phase-shifting interferometry without changing the focus of a CCD sensor as the combination of the Mireau objective and the tube lens provides a depth of focus which is deep in comparison to the phase-shifting step. Experimental results from surface profilometry of the protrusion/undercut of a polished fibre within an optical connector and of the curved surface of a micromirror demonstrate that features in the order of nanometres are measurable. Measurements on standard blocks also show that the accuracy of the proposed system is comparable to an existing commercial white-light interferometer and a stylus profilometer.

Uphill energy transfer from long-wavelength absorbing chlorophylls to PS II in Ostreobium sp. is functional in carbon assimilation

From the algal genus Ostrobium two species are known which express a chlorophyll antenna absorbing between 710 and 725 nm to a different extent. In a comparative study with these two species it is shown that quanta absorbed by this long wavelength antenna can be transferred to PS II leading to significant PS II-related electron transfer. It is documented that under monochromatic far red light illumination growth continues with rather high efficiency. The data show that the uphill-energy transfer to PS II reduces the quantum yield under white light significantly. It is discussed that this strategy of energy conversion might play a role in special environments where far red light is the predominant energy source.

Achromatic spatial frequency multiplication: A method for production of nanometer-scale periodic structures

Fabrication of periodic structures with resolution below 100nm is required in many emerging applications such as subwavelength optical components, self-assembly templates, or patterned magnetic media. The achromatic spatial frequency multiplication technique that we have developed uses wideband illumination and self-imaging property (Talbot effect) of gratings to create periodic structures. The depth of field is practically unlimited, unlike the Talbot images obtained with monochromatic illumination. The technique requires limited spatial coherence making it suitable for use with sources other than lasers or synchrotrons which are normally used in interference lithography. We demonstrated the feasibility of the method using simulations and exposures in the extreme ultraviolet (EUV) region. One- and two-dimensional patterns with 50nm feature sizes were obtained. The technique is especially suitable for use with EUV light where the newly developed sources (laser or discharge produced plasma) have a rather wide bandwidth and limited spatial coherence. The technique is extendible to the production of much higher-resolution structures with the fundamental limit for the pattern period being equal to one-half of the wavelength.

A method to study uniformity of electrophysical properties of high-resistance CdZnTe crystals

Uniformity of properties of a semiconductor material is one of the important conditions to achieve high spectrometric performance of ionizing radiation detectors. To study uniformity of electrophysical properties of crystals, an original method for acquisition of distributions of local values of a dielectric permittivity /spl epsiv/' and tangent of a dielectric loss angle tan/spl delta//sub M/ in high-resistance semiconductors is offered. The method was approved on Cd/sub 1-x/Zn/sub x/Te (x=0.1 to 0.16) crystals grown by Bridgman method under high pressure of inert gas. Locality and reproducibility of measurements are provided by a special design of a movable electrode, with both the area and the hold-down pressure of the electric contact being unvaried. Differences in /spl epsiv/' and tan/spl delta//sub M/ distributions over the sample's area and an effect of uniform monochromatic illumination at a varied wavelength on these distributions are revealed. The physical nature of revealed irregularities is analyzed with involving results of researching CdZnTe crystals by diverse methods.

Introduction and recovery of point defects in electron-irradiated ZnO

We have used positron annihilation spectroscopy to study the introduction and recovery of point defects in electron-irradiated $n$-type ZnO. The irradiation (${E}_{\mathrm{el}}=2\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$, fluence $6\ifmmode\times\else\texttimes\fi{}{10}^{17}{\mathrm{cm}}^{\ensuremath{-}2}$) was performed at room temperature, and isochronal annealings were performed from 300 to 600 K. In addition, monochromatic illumination of the samples during low-temperature positron measurements was used in identification of the defects. We distinguish two kinds of vacancy defects: the Zn and O vacancies, which are either isolated or belong to defect complexes. In addition, we observe negative-ion-type defects, which are attributed to O interstitials or O antisites. The Zn vacancies and negative ions act as compensating centers and are introduced at a concentration $[{V}_{\mathrm{Zn}}]\ensuremath{\simeq}{c}_{\mathrm{ion}}\ensuremath{\simeq}2\ifmmode\times\else\texttimes\fi{}{10}^{16}{\mathrm{cm}}^{\ensuremath{-}3}$. The O vacancies are introduced at a 10-times-larger concentration $[{V}_{\mathrm{O}}]\ensuremath{\simeq}3\ifmmode\times\else\texttimes\fi{}{10}^{17}{\mathrm{cm}}^{\ensuremath{-}3}$ and are suggested to be isolated. The O vacancies are observed as neutral at low temperatures, and an ionization energy of 100 meV could be fitted with the help of temperature-dependent Hall data, thus indicating their deep donor character. The irradiation-induced defects fully recover after the annealing at 600 K, in good agreement with electrical measurements. The Zn vacancies recover in two separate stages, indicating that the Zn vacancies are parts of two different defect complexes. The O vacancies anneal simultaneously with the Zn vacancies at the later stage, with an activation energy of ${E}_{V,\mathrm{O}}^{m}=1.8\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The negative ions anneal out between the two annealing stages of the vacancies.

Near-field scanning photocurrent microscopy of a nanowire photodetector

A near-field scanning optical microscope was used to image the photocurrent induced by local illumination along the length of a metal-semiconductor-metal (MSM) photodetector made from an individual CdS nanowire. Nanowire MSM photodetectors exhibited photocurrents ∼105 larger than the dark current (<2pA) under uniform monochromatic illumination; under local illumination, the photoresponse was localized to the near-contact regions. Analysis of the spatial variation and bias dependence of the local photocurrent allowed the mechanisms of photocarrier transport and collection to be identified, highlighting the importance of near-field scanning photocurrent microscopy to elucidating the operating principles of nanowire devices.

Response of c-Si PV arrays under monochromatic light for MEMS power supply

There is an increasing number of portable or ubiquitous applications where compact and reliable energy supply systems are required in the low end of the power range, but where the output voltage has to be compatible with conventional electronic equipment. In this work, the option of small-sized photovoltaic arrays designed to convert monochromatic light is explored along with improvements in the fabrication process of c-Si ‘flip-chip’ assembly and c-Si monolithic arrays. Arrays having nine cells connected in series (0.8–1.4 cm2 each) have been fabricated. Photovoltaic response under several monochromatic light wavelengths has been measured and compared to reference solar AM1.5 light. Electrical results under standard solar spectrum AM1.5 and 1 SUN irradiance show open circuit voltages, Voc, short circuit current densities, Jsc, and maximum delivered powers, Pm, of: 5 V and 4.1 V, 21.7 mA cm−2 and 24.5 mA cm−2, and 4.5 mW and 3.8 mW for the ‘flip-chip’ and monolithic technologies respectively. Typically, open circuit values in the range of 4.5 V, short circuit current densities in the vicinity of 45 mA cm−2 and maximum power in the range of 2 mW can be obtained for monochromatic illumination of 830 nm wavelength with 20 mW of incident power.

Activation Energy of Electron Transport in Dye-Sensitized TiO2 Solar Cells

Various characteristics of dye-sensitized nanostructured TiO2 solar cells, such as electron transport and electron lifetime, were studied in detail using monochromatic illumination conditions. The electron transport was found to be a thermally activated process with activation energies in the range of 0.10-0.15 eV for light intensities that varied 2 orders of magnitude. Electron lifetimes were determined using a new method and found to be significantly larger (>1 s) than previously determined. An average potential was determined for electrons in the nanostructured TiO2 under illumination in short-circuit conditions. This potential is about 0.2 V lower than the open-circuit potential at the same light intensity. The electron transport time varies exponentially with the internal potential at short-circuit conditions, indicating that the gradient in the electrochemical potential is the driving force for electron transport in the nanostructured TiO2 film. The applicability of the conventionally used trapping/detrapping model is critically analyzed. Although experimental results can be fitted using a trapping/detrapping model with an exponential distribution of traps, the distribution parameters differ significantly between different types of experiment. Furthermore, the experimental activation energies for electron transport are smaller than those expected in a trapping/detrapping model.

Dedicated mammography: Imaging with monochromatic X-rays and a clinical mammography unit

The aim of the current studies is to implement monochromatic X-rays on a conventional diagnostic mammography unit for the improvement of image quality and/or the reduction of radiation dose. The experimental setup consists of a conventional mammography unit with monochromator module mounted at the exit of the X-ray tube and a digital linear array detector instead of the screen–film system for image acquisition. The heart of the monochromator module is a curved HOPG (highly oriented pyrolytic graphite) crystal and a slit collimator. The monochromator is adjusted to 17.5 keV (K α-emission line of the Mo-anode). For image acquisition the object moves through the X-ray beam, which is fixed in space and directed on the detector (12 bit sensitivity, operated at 54 μm resolution). The final images were reconstructed from the measured images, a reference image and the detector background. Lead calibration grids, a contrast detail phantom and gadolinium (Gd) or iodine (I) containing contrast media were investigated. The exposure dose was measured. The results reveal that only the 17.5 keV K α-emission line of the Mo-anode passes the monochromator while the K β-emission line and the bremsstrahlung spectrum are rejected. The photon flux is considerably lower for monochromatic than for polychromatic X-rays under identical geometric conditions but sufficient to investigate 5 cm thick objects. The contrast response functions (CRF) for polychromatic and monochromatic X-rays are very similar while contrast values are in favor of the monochromatic illumination. In comparison to polychromatic X-rays, the 17.5 monochromatic X-rays lead to higher contrast for Gd or I containing contrast media. Monochromatic imaging has a high potential in diagnostic mammography: image contrast is improved and radiation dose can be reduced in comparison to polychromatic imaging. For clinical installation, high-power X-ray tubes are mandatory and the imaging procedure should be adapted to the slot-scan technique

Electromagnetic modeling of near-field phase-shifting contact lithography with broadband ultraviolet illumination

Near-field phase-shifting contact lithography is modeled to characterize electromagnetic absorption in a photoresist layer with one face in contact with a quartz binary phase-shift mask. The broadband ultraviolet illumination is represented as a frequency-spectrum of normally incident plane waves. A rigorous coupled-wave analysis is carried out to determine the absorption spectrum of the photoresist layer. The specific absorption rate in the photoresist layer is calculated and examined in relation to the geometric parameters. Columnar features in the photoresist layer are of higher quality on broadband illumination in contrast to monochromatic illumination, in conformity with some recent experimental results. Feature resolution and profile are noticeably affected by the depth of the grooves in the phase-shift mask. Ideally, the feature linewidth can be less than about 100 nm for broadband illumination in the transverse-magnetic mode. These conclusions are subject to modification by the photochemistry-wavelength characteristics of the photoresist.

Organic Polymer Thin-Film Transistor Photosensors

We present our study of the effects of monochromatic illumination on the electrical performance of organic polymer thin-film transistors (OP-TFTs) and the use of these devices as photosensors. In the case of monochromatic illumination that is strongly absorbed by the polymer, the drain current of a device biased in the OFF-state is significantly increased and the threshold voltage is reduced. Light that is not strongly absorbed by the polymer has little effect on the electrical performance of the OP-TFTs. We explain these effects in terms of the photogeneration of excitons in the polymer channel region of the device. The density of excitons generated in the polymer depends on the energy of the incident photons, as well as on the irradiance level of the incident illumination. The photogenerated excitons subsequently dissociate into electrons and holes. The electrons can be trapped by positively charged states, thereby reducing the threshold voltage, while the photogenerated holes contribute to the excess photocurrent measured at the drain. To demonstrate the possible use of OP-TFTs as photosensors, we also present the responsivity, photosensitivity (signal-to-noise ratio), external quantum efficiency, noise-equivalent power, and specific detectivity of these devices. The dependence of these parameters on the incident photon energy and irradiance level is described.

Measurement of built-in potential in polymorphous Si:H p–i–n solar cells

The performance of a solar cell, especially open-circuit voltage ( V oc), is critically related to the built-in potential ( V b). In this work, the open-circuit voltage V oc and the short-circuit current density J sc of hydrogenated polymorphous silicon (pm-Si:H) p–i–n solar cell, deposited by radio-frequency powered plasma-enhanced chemical vapour deposition (RF-PECVD), have been measured. From the measured data at different monochromatic illumination intensities and temperatures, the value of V b of cell was determined by using two different methods: activation energy and differential temperature. The results from both methods were analyzed and compared as a function of excitation wavelength. It was observed that there is a good agreement between the two methods. It was found that V b increases until about 650 nm, and then decreases with increasing wavelength. This behaviour is explained and interpreted in different ways.

Poly(3-methylthiophene)-based photovoltaic devices prepared onto tin-oxide/sulfonated-polyaniline electrodes

We characterize poly(3-methylthiophene), PMT, photovoltaic devices prepared onto tin oxide substrates covered with sulfonated polyaniline, SPAN. Both, PMT and SPAN are prepared by electrochemical methods. Under monochromatic illumination ( λ=580 nm, I λ =0.8 W m −2) TO/SPAN/PMT/Al devices present power conversion efficiency up to 0.8%. When submitted to white light irradiation (75 W m −2, tungsten halogen lamp) these devices present power conversion efficiency up to 0.1%.

Multiple-beam interference with nearly grazing waves in dielectric wedges: some polarization properties

By nearly grazing waves we understand travelling waves having wavevectors forming an angle of nearly π/2 with respect to the normal of a planar surface. Multiple-beam superposition of nearly grazing waves has been recognized as giving rise to stepwise constant phase behaviour of the total field after reflections in dielectric gaps. This behaviour of the phase is polarization dependent. When two dielectric interfaces form a wedge with nearly grazing waves within, the formation of bright interference patterns can be easily achieved. One of the reasons for this is the above mentioned phase behaviour. An additional effect favouring pattern formation comes from the values of the Fresnel coefficients of reflection, which are close to unity for nearly grazing waves. Because both features have polarization dependence, a corresponding polarization dependence of the resulting interference patterns is also expected. Numerical results employing basic considerations of multiple-beam superposition of nearly grazing waves in dielectric wedges for s and p polarizations are presented under monochromatic and polychromatic illuminations. Experimental observations are also shown.

Spectrophotometric error in colour coordinates introduced by fluorescence of white calibration tile

AbstractFluorescence has been detected in some white standard tiles intended for the calibration of spectrophotometers for colour measurements. The problem is that usually no warning is given to the user about it, giving raise to errors if they are calibrated using conventional spectrophotometric techniques. This work analyzes the differences on the colour coordinates of one of these standards measured with a polychromatic illumination spectrophotometer and with a monochromatic illumination one. This work also analyzes the colour coordinates error obtained when other coloured standards are calibrated against one of these white fluorescent standards, when reflectance measurement is done with a polychromatic illumination spectrophotometer as well as with a monochromatic illumination instrument. These are the two most typical situations found in practice where the use of fluorescent standards may produce errors. © 2004 Wiley Periodicals, Inc. Col Res Appl, 29, 111–114, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.10233

Grating pseudo-imaging with polychromatic and finite extension sources

The pseudo-imaging process in a system consisting of two periodic gratings and illuminated by an incoherent polychromatic and finite extension source placed at a finite distance from the gratings is studied. An analytical expression of the irradiance distribution on a plane, also located at a finite distance from the gratings, has been obtained from previous results on monochromatic illumination. In the analysis presented, different imaging regimes are found and related to the parameters which characterize the double grating system. The pseudo-imaging phenomenon strongly depends on both the spatial and temporal coherence of the incident illuminating field. Certain pseudo-images are observed with polychromatic and incoherent incident light under some restrictions. On the other hand, pseudo-image process analogous to Talbot effect appears only by monochromatic and plane illuminating wavefront.

A genetic optical interferometric inspection on micro-deformation

Summary This paper describes a feasibility study of an optical method for measuring nanoscale deformation of micro-components that are commonly employed in the field of microelectromechanical systems (MEMS). Both theoretical and experimental results demonstrate that an optical interference fringe pattern resulted from an air gap consisting of two surfaces (object and reference surfaces) is a simple function of the deformation of the micro-component. A microscopic system incorporating a coaxial monochromatic light illumination and a high resolution CCD sensor is utilized to record the interference fringe pattern. The experimental results on different micro-components show that the proposed technique is applicable to the deformation measurement on micro-components of MEMS devices.

Pentacene-based photodiode with Schottky junction

We have fabricated a metal/organic semiconductor Schottky photodiode based on Al/pentacene junction. Since the energy band gap of thin solid pentacene was determined to be 1.82 eV, as characterized by direct absorption spectroscopy, we measured spectral photoresponses on our Schottky photodiode in the monochromatic light illumination range of 325–650 nm applying a reverse bias of −2 V. The main features of photo-response spectra were found to shift from those of direct absorption spectra toward higher photon energies. It is because the direct absorption spectra mainly show exciton level peaks rather than the true highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gaps while the photo-response spectra clearly represents the true HOMO–LUMO gap. Our photo-response spectra reveal 1.97 eV as the HOMO–LUMO gap.

Polyfluorene copolymer based bulk heterojunction solar cells

Bulk heterojunction solar cells based on blends of photoactive layers of polyfluorene copolymer Poly((2,7-(9-(2′-ethylhexyl)-9-hexyl-fluorene)- alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole))- co-(2,7-(9-(2′-ethylhexyl)-9-hexyl-fluorene)- alt-2,5- thiophene)) (LBPF3) acting as electron donor, and [6,6]-phenyl-C 61-butyric acid methylester (PCBM), acting as electron acceptor, were constructed and studied. The power conversion efficiency for a 1:4 (by weight) blend of LBPF3:PCBM under simulated solar light illumination having light intensity of 100 mW/cm 2 was 1.7%, and 9.2% under monochromatic (565 nm) light illumination with light intensity of 0.145 mW/cm 2. The maximum external quantum efficiency (incident photons to converted electrons) for this device was found to be above 40% from 400 to 560 nm. The effects of blend composition and film thickness on the photovoltaic parameters were also studied. The incident light intensity dependence of the short circuit current showed a linear relationship.

Rigorous electromagnetic modeling of near-field phase-shifting contact lithography

Electromagnetic absorption in a photoresist layer sandwiched between a silicon substrate and a quartz binary phase-shifting mask (employed in near-field phase shift contact lithography) is theoretically analyzed. A rigorous coupled-wave analysis is used to compute the distribution of specific absorption rate (SAR) in the photoresist layer on either monochromatic and polychromatic illumination. The nonuniform distribution of the SAR in the photoresist layer for the printing of high-aspect-ratio features is systematically examined in relation to the geometric dimensions of the binary phase mask, the photoresist layer thickness, and the polarization state of the incident plane wave. Two methods for the improvement of feature resolution and profile are also discussed.