Publications

Year: 2006

Study of the annealing kinetic effect and implantation energy on phosphorus-implanted silicon wafers using spectroscopic ellipsometry
Lioudakis E, Christofides C, Othonos A

Journal of Applied Physics, DOI: 10.1063/1.2207688Download
In this work, we have studied the changes in the optical properties on crystalline silicon implanted wafers (1×1013–1×1016 P+/cm2) using an extensive ellipsometric analysis. The effects of implantation energy (20–180 KeV) and subsequent isochronical annealing temperature (300–1100 °C) on the electronic band structure of material are investigated. The evolution of pseudodielectric functions is studied using a temperature dependent multilayer model for each implantation dose and energy. The temperature evolution of integrated damage depth profile for each wafer is presented depicting the amorphous/crystalline transition temperatures. Finally, the critical implantation dose and energy of crystalline to amorphous silicon phase are given.

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Year: 2005

Effects of Ge concentration, boron co-doping, and hydrogenation on fiber Bragg grating characteristics
Konstantaki M, Tamiolakis G, Argyris A, Othonos A, Ikiades A

, DOI: 10.1002/mop.20572Download
In this paper, we compare photosensitivity, refractive-index modulation depth, spectral bandwidth, and excess loss through the inscription of fiber Bragg gratings in unloaded and hydrogen-loaded fibers with low- or high-germanium concentration and boron co-doping. Gratings in hydrogenated boron co-doped fibers possess the highest reflectivity, but with wider bandwidth and strong excess loss

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Ellipsometry on optically thin palladium films on silicon-based substrate: effects of low concentration of hydrogen
Lioudakis EE, Othonos A

Optical Engineering, DOI: 10.1117/1.1840955Download
Optically thin palladium films evaporated on silicon substrates are investigated following exposure to low concentrations of hydrogen gas in nitrogen using spectroscopic ellipsometry. Changes in the parameters tan Psi and cos Delta are observed for concentrations as low as 0.01% hydrogen in nitrogen. A nonlinear behavior of the change in the ellipsometry parameters as a function of hydrogen concentration is demonstrated, with saturation occurring at a flow of 0.05% hydrogen in nitrogen.

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Prism-based ultrafast pulse-shaping apparatus

Optical Engineering, DOI: 10.1117/1.1873432Download
We report on a computer-controlled pulse-shaper setup with high-efficiency throughput, based on dispersive prisms, as an alternative to the conventional grating ultrafast pulse-shaping apparatus. A detailed description of the experimental apparatus and operation of this system is given. The advantages and disadvantages of this configuration are discussed. Experimental implementation of the prism pulse shaper in recovering nearly bandwidth-limited laser pulses from chirped ultrafast laser pulses is demonstrated using both closed-loop and open-loop operation

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Ultrafast carrier dynamics in highly implanted and annealed polycrystalline silicon films
Lioudakis E, Nassiopoulou AG, Othonos A

Journal of Physics: Conference Series, DOI: Download
We have studied the ultrafast optical response of highly implanted and annealed polycrystalline silicon films. One-micron thin polycrystalline silicon samples-on-quartz implanted with As ions at a high dose of 2 × 10 16 cm −2 at 100 keV and annealed at various temperatures have been investigated using ultrafast pulses. Frequency doubled amplified femtosecond pulses at 400 nm have been used in a pump-probe configuration to measure the temporal reflective response from the polysilicon samples. A super-continuum of ultrafast laser pulses were generated and used in probing the samples at various wavelengths in the visible part of the spectrum. Transient reflection measurements reveal negative and positive contributions to the change in the index of refraction of the samples attributed to an increase in the carrier density and lattice temperature. Ion induced defects and subsequent annealing are key contributing factors in the dynamic behavior of these samples.

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Year: 2004

Fine art painting characterization by spectroscopic ellipsometry: preliminary measurements on varnish layers
Christofides C, Castellon B, Othonos A, Polikreti K, De Deyne C

Thin Solid Films, DOI: DOI: 10.1016/j.tsf.2004.02.033Download
The long-term conservation of the original state of paintings requires high quality multi-spectral digital image technologies. CRISATEL Program addresses fine art professionals as primary end-users base. The analysis by spectroscopic ellipsometry of the varnish on fine art paintings will allow the curator to correct the effect of aged or glossy varnish on the digital images and to simulate the original state of the masterpiece through this superficial layer. Sets of varnish samples have been measured first at the University of Cyprus using a SOPRA GES5 Spectroscopic Ellipsometer and then at SOPRA facilities with a Fourier transform infrared spectroscopic ellipsometer. The capability to measure a varnish layer (thickness and optical index) on glass and to distinguish synthetic from natural varnishes has been proved. The goal of this paper is to review the first results obtained in the characterization of varnishes for fine art paintings, and to present some problems that are still open and can represent a challenging research field for the future.

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Year: 2003

Photomodulated thermoreflectance investigation at elevated temperatures: plasma versus thermal effect
Christofides C, Othonos A, Loizidou E

Applied Physics Letters, DOI: 10.1063/1.1541935Download
Photomodulated thermoreflectance measurements were performed at elevated temperatures (294 to 623 K), on crystalline silicon lightly doped with boron. The temperature dependence is qualitatively and quantitatively discussed. The “competition” between thermal and plasma contribution, as a function of temperature, is one of the main subjects of this letter.

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Photomodulated thermoreflectance detection of hydrogen at elevated temperatures: a detection limit
Othonos A, Christofides C

Applied Physics Letters, DOI: 10.1063/1.1543254Download
Laser photomodulated thermoreflectance has been used as a means of detecting, at elevated temperatures, low concentration of hydrogen using an optically thin film of palladium. Data indicate that concentrations as low as a few parts per billion can easily be detected at 100 °C. A semi-quantitative interpretation of the photothermal signal has been achieved using a Langmurian isothermic model.

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High-temperature photomodulated thermoreflectance measurements on phosphorus implanted and annealed silicon wafers
Othonos A, Christofides C, Loizidou E

Journal of Applied Physics, DOI: 10.1063/1.1621723Download
Photomodulated thermoreflectance measurements between 300 and 650 K on phosphorus implanted and annealed silicon wafers are reported. The change of the photothermal amplitude and phase as a function of temperature is discussed. Several measurements have been performed on silicon wafers annealed at various temperatures in the range of 300 to 1100 °C. The activation energy of the local annealing process was also estimated to be 0.17 eV

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Year: 2002

Influence of temperature and modulation frequency on the thermal activation coupling term in laser photothermal theory
Christofides C, Othonos A, Loizidou E

Journal of Applied Physics, DOI: 10.1063/1.1484232Download
A quantitative analysis of the influence of temperature and modulation frequency on the thermal activation coupling term in laser photothermal theory is performed. Until now it was taken for granted that the coupling term is negligible only in the case of “relatively low” temperatures and generally when the equilibrium free-carrier density n0 satisfies the Sablicov's, Vasil'ev, and Sandomirskii inequality. In this work an extensive computational study of this inequality in the temperature range of 300–1000 K was performed and a precise “map” is given concerning the violation of the inequality under various conditions including modulation frequency (0.1–106 Hz) and doping concentration (intrinsic to 1020 cm−3). Some experimental photomodulated measurements have been performed in order to test the validity of the “map

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Characterization of reflectivity inversion, alpha- and beta-phase transitions and nanostructure formation in hydrogen activated thin Pd films on silicon based substrates
Kalli K, Othonos A, Christofides C

Journal of Applied Physics, DOI: 10.1063/1.1417992Download
Optically thin palladium metal films evaporated on different silicon based substrates are investigated following exposure to different concentrations of hydrogen gas in air. Laser modulated reflectance off the palladium surface of silicon oxide and silicon nitride substrates is used to recover information regarding the reflectivity inversion and α/β-phases of the palladium complex after both first and multiple gas cycling. Atomic force microscopy confirms the formation of metal nanostructures following exposure to hydrogen of the optically thin palladium films.

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Ultrafast dynamics in phosphorus-implanted silicon wafers: The effects of annealing
Othonos A, Christofides C

Phys. Rev. B, DOI: 10.1103/PhysRevB.66.085206Download
Carrier relaxation in phosphorus-implanted silicon wafers (1016P+/cm2) annealed at different temperatures ranging from 350 to 1100 °C is investigated by near-infrared ultrafast time-resolved reflectivity measurements. A kinetic model based on four couple differential equations describing the carrier density, electron, hole, and lattice temperatures is used to evaluate the expected changes in the time-resolved reflectivity. This model was used to fit the experimental data having the trap recombination time constant and the optical absorption coefficient as fitting parameters. Our measurements reveal a complicated recovery of the ion-implanted silicon samples to crystallinity following annealing with carrier lifetimes ranging from

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Spatial dependence of ultrafast carrier recombination centers of phosphorus-implanted and annealed silicon wafers
Othonos A, Christofides C

Applied Physics Letters, DOI: 10.1063/1.1497723Download
In this letter, the spatial dependence of the carrier recombination centers induced in phosphorus-implanted and annealed silicon wafers have been examined. Ultrafast time-resolved reflectivity measurements of a set of phosphorus-implanted annealed silicon wafers (1016 P+/cm2) as a function of position on the wafer have been carried out, and an x–y map of the carrier lifetime for each of the samples has been obtained. Measurements reveal distinct features of the distribution of carrier recombination centers for the nonannealed and annealed samples between 350 °C and 1100 °C in an area of 36×36 μm2 with resolution better than 3 μm. The presence of islands of clusters in ion-implanted and annealed samples is also discussed in this letter

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Year: 2001

Bragg Gratings in Optical Fibers
Othonos A, Kalli K

, DOI: Download
Optical fiber networks have been developed to the point where they are now synonymous with modern telecommunications and optical sensing. A major drawback to their evolution has been the reliance on bulk optics for conditioning and controlling the guided light beam, requiring the use of high quality, bulk-optic components, and placing stringent tolerance on optical alignment–thus making conceptually simple systems complicated and expensive in practice. Replacing a bulk optic mirror or beam splitter with a fiber equivalent can dramatically increase system stability and portability, whilst reducing overall size. The most successful fiberized technology to date is the optical fiber laser and amplifier and fused tapered coupler. With the significant discovery of photosensitivity in optical fibers, a new class of in-fiber component has been developed, called the fiber Bragg grating. Photosensitivity refers to a permanent change in the index of refraction of the fiber core when exposed to light with characteristic wavelength and intensity that depend on the core material. The fiber Bragg grating can perform many primary functions, such as reflection and filtering, in a highly efficient, low loss manner. This versatility has stimulated a number of significant innovations

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Year: 2000

Bragg gratings in optical fibers: Fundamentals and Applications

, DOI: Download
The discovery of fiber optics has revolutionized the field of telecommunications making possible high-quality, high-capacity, long distance telephone links. Over the past three decades, the advancements in optical fiber has undoubtedly improved and reshaped fiber optic technology. Today optical fibers are synonymous with the word “telecommunication”. In addition to applications in telecommunications, optical fibers are also utilized in the rapidly growing field of fiber sensors. Despite the improvements in optical fiber manufacturing and advancements in the field in general, basic optical components such as mirrors, wavelength filters, and partial reflectors have been a challenge to integrate with fiber optics. Recently, however, all these have changed with the ability to alter the core index of refraction in a single mode optical fiber by optical absorption of UV light.

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Optically thin palladium films on silicon-based substrates and nanostructure formation: effects of hydrogen
Othonos A, Kalli K, Tsai DP

Applied Surface Science, DOI: DOI: 10.1016/S0169-4332(00)00140-9Download
Optically thin palladium films evaporated on different silicon-based substrates are investigated following exposure to different concentrations of hydrogen gas in air. Laser modulated reflectance off the palladium surface of silicon oxide, silicon nitrite and polycrystalline silicon substrates is used to recover information regarding changes in optical properties of the samples due to the absorption of hydrogen. Simple index of refraction arguments are sufficient to explain the results. Structural changes of the palladium films have been investigated using atomic force microscopy before and after hydrogen exposure. An interesting nanostructure formation is evident in some of the samples, leading to a possible means of fabricating nanodevices.

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Year: 1999

Optical response of thin supported palladium films to hydrogen: Non-destructive testing for hydrogen detection
Christofides C, Kalli K, Othonos A

Platinum Metals Review, DOI: Download
The properties of thin palladium films on silicon dioxide/silicon substrates are investigated. As the electrical properties of very thin palladium films cannot be measured, due to highly discontinuous nature of the films resulting from the low level of surface coverage, simple laser diagnostic techniques are used. Results showed the changes in the absolute reflectivity of Pd-SiO2 films upon exposure to various concentrations of hydrogen gas as a function of temperature

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Nondestructive evaluation of metal contaminated silicon wafers using radiometric measurements
Kalli K, Othonos A, Christofides C, Tardiff F

Journal of Applied Physics, DOI: 10.1063/1.371168Download
We have performed nondestructive measurements on metal contaminated silicon wafers using photothermal radiometric measurements. Data were collected as a function of modulation frequency and time, showing clear distinctions between the different samples examined. The sensitivity of this technique to different forms of metallic contamination is examined. A qualitative and semiquantitative comparison is made between theory and experiment.

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Fiber Bragg Gratings: Fundamentals and Applications in Telecommunications and Sensing
Othonos A, Kalli K

, DOI: Download
Following the realization of low loss optical waveguides in the 1960s, optical fibers have been developed to the point where they are now synonymous with modern telecommunication and optical sensor networks. A major drawback to the evolution of optical fiber-based networks has been the reliance on bulk optics for conditioning and controlling the guided light beam. The necessity of coupling light out of the waveguides to perform, for example, reflection, diffraction, and filtering (spatial, polarization, etc.) is an inherently lossy process. Moreover, coupling light in and out of fiber significantly increases the number of high-quality, bulk optic components, often requiring stringent tolerance on optical alignment, thus making conceptually simple systems complicated and expensive in practice. Replacing a bulk optic mirror or beam splitter with a fiber equivalent can dramatically increase system stability and portability, while reducing overall size, thus pushing laboratory-based experiments into real world environments. The most successful fiberized technology to date is the optical fiber laser and amplifier and fused tapered coupler. The intrinsic low loss nature of these components and their compatibility with integrated-optic waveguide structures have made them indispensable to the continued development of optical systems as a whole.

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Year: 1998

Photomodulated thermoreflectance detection of hydrogen gas using optically thin palladium film on silicon oxide
Kalli K, Othonos A, Christofides C, Spetz A, Lundstrom I

Review of Scientific Instruments, DOI: 10.1063/1.1148787Download
The sensitivity of various thicknesses of optically thin film palladium layers evaporated onto silicon oxide substrate is investigated in the presence of a hydrogen/air atmosphere at room temperature. The magnitude of the resulting reflectivity change is measured using an excite-probe technique, through laser excited photothermal modulation of a probe beam. This allows for the recovery of information from both the amplitude and phase channels of the hydrogen sensor output. Data indicates that concentrations of 0.1% hydrogen in the presence of a balanced air mixture and at room temperature may be measured with an 8 nm palladium film. The presence of inhomogeneities in the palladium layers leads to anomalous behavior

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Temperature-induced reflectivity changes and activation of hydrogen sensitive optically thin palladium films on silicon oxide
Kalli K, Othonos A, Christofides C, Spetz AL, Lundstrom I

Review of Scientific Instruments, DOI: 10.1063/1.1149116Download
The optical properties of several thin metal film palladium-silicon oxide structures are examined at room temperature before and after annealing to 200 °C and also at 90 °C—in all cases in the presence of hydrogen gas. Multicycling sample activation is shown to occur in the presence of hydrogen at room temperature with an increase in reflectivity on exposure to hydrogen, in contrast to thicker 80 Å films. The reflectivity change increases with increasing film thickness (1–10 Å). The surface activation at room temperature, before and after annealing to 200 °C, is compared with the performance at 90 °C, where it is shown that heat treatment strongly influences the behavior of the metal film

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Probing ultrafast carrier and phonon dynamics in semiconductors

Journal of Applied Physics, DOI: 10.1063/1.367411Download
Over the past 2 decades there has been tremendous advancements in the field of ultrafast carrier dynamics in semiconductors. The driving force behind this movement other than the basic fundamental interest is the direct application of semiconductor devices and the endless need for faster response and faster processing of information. To improve and develop microelectronics devices and address these needs, there must be a basic understanding of the various dynamical processes in the semiconductors which have to be studied in detail. Therefore, the excitation of semiconductors out of their equilibrium and the subsequent relaxation processes with various rates has become a key area of semiconductor research. With the development of lasers that can generate pulses as short as a few femtoseconds the excitation and subsequent probing of semiconductors on an ultrashort timescale have become routine. Processes such as carrier momentum randomization, carrier thermalization, and energy relaxation have been studied in detail using excite-and-probe novel techniques. This article reviews the status of ultrafast carrier and phonon dynamics in semiconductors. Experimental techniques such as excite-and-probe transmission, time-resolved up-conversion luminescence, and pump-probe Raman scattering along with some of the significant experimental findings from probing semiconductors are discussed. Finally, a selfconsistent theoretical model, which correlates the carrier and phonon dynamics in germanium on an ultrashort time scale, is described in detail.

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Superimposed grating wavelength division multiplexing in Ge-doped SiO[sub 2]/Si planar waveguides
Othonos A, Bismuth J, Sweeny M, Kevorkian A, Xu JM

Optical Engineering, DOI: 10.1117/1.601873Download
An improved model of wavelength division multiplexing (WDM) by superimposed gratings in planar waveguides has been developed. Based on this theory, principal design rules of N-channel WDM are established and a fanout capacity of 100 is estimated. Finally, four-channel WDM at 840 nm has been demonstrated on a Ge:SiO2/SiO2/Si planar waveguide, which was hydrogenated to enhance its photosensitivity. The superimposed gratings were written using a narrow linewidth KrF excimer laser in an interferometric setup used in inscribing fiber Bragg gratings

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Photothermal radiometry on nickel (pigmented aluminium oxide) selective solar absorbing surface coatings
Othonos A, Nestoros M, Palmerio D, Christofides C, Bes RS, Traverse JP

Solar Energy Materials and Solar Cells, DOI: DOI: 10.1016/S0927-0248(97)00232-8Download
Photothermal radiometry (PTR) is applied to characterize nickel-pigmented aluminium oxide solar absorbing coatings. A modulated laser beam is used to heat the solar samples. The subsequent emission of thermal radiation is measured as a function of modulated frequency in the range of 10 Hz to 10 kHz. A simple one-dimensional model is used to fit the experimental PTR results, allowing for the extraction of some thermal parameters for the solar absorbing coatings. Finally, comparison of the emissivity measured by traditional technique and the photothermal radiometry is made.

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Year: 1997

Laser photothermal diagnostics of genuine and counterfeit British and United States banknotes
Othonos A, Mandelis A, Nestoros M, Christofides C

Optical Engineering, DOI: 10.1117/1.601212Download
Laser-induced, frequency-scanned IR photothermal radiometry was used to investigate the thermophysical properties of the paper on which several genuine and counterfeit British (£10) and U.S. ($50, $100) currency bills were printed. The radiometric photothermal amplitudes and phases were further compared with a theoretical model, which yielded simultaneous quantitative measurements of the thermal diffusivities and conductivities of the bills. Both statistical and single-specimen results demonstrated the excellent thermophysical resolution of the technique with prospects for its use in the nonintrusive, on-line identification of counterfeit banknotes.

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