# Year: 1998

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

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 oxideKalli K, Othonos A, Christofides C, Spetz AL, Lundstrom I

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 semiconductorsOthonos A

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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# Year: 1997

### Hydrogen gas detection via photothermal deflection measurementKalli K, Othonos A, Christofides C

A study of a thermal wave hydrogen sensor interrogated via transverse optical beam deflection spectroscopy is presented. The sensor is a thin film polyvinylidene fluoride film coated with a thin palladium layer. The sensitivity to hydrogen results from thermal boundary condition changes at the gas–film interface and depends upon the thermophysical properties of the gas. A simple one-dimensional model is developed to qualitatively and semi-quantitatively describe the experimental results and it shows good agreement with the experiment. Concentrations of 0.1% hydrogen in the presence of a balanced air mixture and at room temperature were measured for this sensor and they indicate possible sensitivities approaching 100 ppm.

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### Photothermal radiometric and spectroscopic measurements on silicon nitride thin filmsNestoros M, Gutierrez-Llorente A, Othonos A, Christofides C, Martinez-Duart JM

Thin films of silicon nitride of various thicknesses, deposited by radio frequency magnetron sputtering on silicon quartz substrate, have been characterized by laser-induced and frequency scanned photothermal radiometry. Fourier transform infrared spectroscopy was also used to provide a qualitative description of the behavior of the films in the infrared range which shows favourable properties of these coatings to be used in passive cooling applications.

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### Fiber Bragg gratingsOthonos A

Since the discovery of photosensitivity in optical fibers there has been great interest in the fabrication of Bragg gratings within the core of a fiber. The ability to inscribe intracore Bragg gratings in these photosensitive fibers has revolutionized the field of telecommunications and optical fiber based sensor technology. Over the last few years, the number of researchers investigating fundamental, as well as application aspects of these gratings has increased dramatically. This article reviews the technology of Bragg gratings in optical fibers. It introduces the phenomenon of photosensitivity in optical fibers, examines the properties of Bragg gratings, and presents some of the important developments in devices and applications. The most common fabrication techniques (interferometric, phase mask, and point by point) are examined in detail with reference to the advantages and the disadvantages in utilizing them for inscribing Bragg gratings. Reflectivity, bandwidth, temperature, and strain sensitivity of the Bragg reflectors are examined and novel and special Bragg grating structures such as chirped gratings, blazed gratings, phase-shifted gratings, and superimposed multiple gratings are discussed. A formalism for calculating the spectral response of Bragg grating structures is described. Finally, devices and applications for telecommunication and fiber-optic sensors are described, and the impact of this technology on the future of the above areas is discussed.

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

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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### Diagnostics of nonradiative defects in the bulk and surface of Brewster-cut Ti:sapphire laser materials using photothermal radiometry Vanniasinkam J, Munidasa M, Othonos A, Kokta M, Mandelis A

The understanding of the problem of nonradiative energy conversion in solid-state laser materials is a key factor in improving the overall efficiency of solid-state lasers. Furthermore, the reduction of the heat generated in an optically pumped laser crystal can lead to several new applications of solid-state lasers, especially in the high-power region. To improve the quality of grown crystals, laser crystal growers require accurate techniques to perform the quality control that is so vital to improving the growth process. Using a time-domain approach and a time-domain theoretical treatment of the IR radiative emission signal, it was determined that one may probe nonradiative surface and bulk processes by monitoring different time ranges. Our results show that photothermal radiometry can be used as a single-ended technique to evaluate both the bulk and surface nonradiative energy conversion rates in a solid-state laser material. This technique was compared to the standard laser cavity technique and it was concluded that photothermal radiometry can provide additional information to the standard technique by identifying the sources of heat generation as either surface- or bulk-originating

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# Year: 1996

### Ultrafast dynamics of nonlinear absorption in low-temperature-grown GaAsBenjamin SD, Loka HS, Othonos A, Smith PW

We present the results of a study of the subpicosecond dynamic behavior of optically induced absorption changes in low‐temperature‐grown GaAs. We show that the observed behavior is dominated by mid‐gap trap states, and can be accurately modeled by the rate equations previously developed to describe quasi‐cw results. Our data give the first approximate values for trap emptying times in this material

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### Distributed strain measurement based on a fiber Bragg grating and its reflection spectrum analysisLeBlanc M, Huang SY, Ohn M, Measures RM, Guemes A, Othonos A

A method of extracting the strain profile along a fiber Bragg grating from the intensity reflection spectrum is described. The procedure is based on a filter synthesis theory that relates the aperiodicity of a grating with its reflection spectrum. To illustrate the approach, we measured the strain profile near a hole in a plate and obtained a strain resolution of 80 {\textmu}$\epsilon$. The spatial resolution depends on the strain gradient; i.e., the higher the gradient, the better the resolution. A resolution of 0.8 mm was achieved for a 5-mm grating with a gradient of 250 {\textmu}$\epsilon$/mm.

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### Non-contacting measurements of photocarrier lifetimes in bulk- and polycrystalline thin-film Si photoconductive devices by photothermal radiometryMandelis A, Othonos A, Christofides C, Boussey-Said J

Laser‐induced and frequency‐scanned infrared photothermal radiometry was applied to a crystalline‐Si photoconductive device, and to polysilicon thin‐film photoconductors deposited on oxidized Si substrates by an LPCVD method. A detailed theoretical model for the radiometric signal was developed and used to measure the free photoexcited carrier plasma recombination lifetime, electronic diffusivity and surface recombination velocity of these devices, with the simultaneous measurement of the bulk thermal diffusivity. A trade‐off between detectivity/gain and frequency‐response bandwidth was found via the lifetime dependence on the wafer background temperature rise induced by Joule‐heating due to the applied bias. This effect was most serious with the bulk‐Si device, but was limited by the high resistivity of the LPCVD thin‐film devices. In the case of the bulk‐Si device, the results of photothermal radiometry were compared with, and corroborated by, frequency‐scanned photocurrent measurements. More sophisticated analysis was shown to be required for the interpretation of the polysilicon photoconductor frequency‐responses, perhaps involving the fractal nature of carrier transport in these grain‐structured devices.

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### Multi-wavelength Raman probing of phosphorus implanted silicon wafersOthonos A, Christofides C

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, DOI: 10.1016/0168-583X(96)00304-7Download
Raman spectroscopy is performed on phosphorus implanted silicon wafers with several excitation laser wavelengths ranging from 458 nm to 752.5 nm. The silicon layers were implanted with various implantation energies and doses, ranging below and over the critical dose of amorphization. A factor [kappa], relating the Raman intensity of the implanted samples with that of the pure crystalline silicon is introduced, and used to correlate the effects of ion implantation at different doses, different implantation energies and various annealing temperatures, on the silicon lattice.

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### Spectrally broadband Bragg grating mirror for an erbium-doped fiber laserOthonos A, Lee X, Tsai DP

The authors design and demonstrate a high-reflector broadband Bragg mirror for a fiber laser system. A T-matrix formalism was used to design a broadband intracore mirror consisting of a series of Bragg gratings. Ultraviolet light from a KrF excimer laser was used in an interferometric setup to inscribe these Bragg gratings in a photosensitive fiber. This resulted in a broadband Bragg mirror covering a wavelength range of approximately 3.5 nm. An all-Bragg grating, erbium-doped fiber laser cavity is demonstrated using this Bragg mirror as the high reflector and a single narrowband Bragg grating centered at 1550 nm as the output coupler. Tunability of the fiber laser was achieved over the range of the high reflector using strain-induced changes on the output coupler.

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### Photothermal radiometric investigation of implanted silicon: The influence of dose and thermal annealingOthonos A, Christofides C, Mandelis A

Photothermal radiometric measurements were performed on phosphorus implanted and annealed silicon wafers. Data were collected over modulation frequencies ranging between 0.1 and 100 kHz with the 488 nm Ar ion laser line as the excitation source. The sensitivity of this technique on implantation dose and annealing temperature is discussed. A semiquantitative analysis of the data is also carried out

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### Phase shifted Bragg gratings formed in optical fibres by post-fabrication thermal processingUttamchandani D, Othonos A

Fibre Bragg gratings have been post-fabrication processed using localised heat treatment, with significant changes to their transmission and reflection characteristics being observed. Experimental results of the spectral changes, together with an explanation of their origin, are provided in this article.

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# Year: 1995

### Photoluminescence measurements on phosphorus implanted silicon: Annealing kinetics of defectsOthonos A, Christofides C

Room‐temperature photoluminescence measurements are performed in order to study the effect of thermal annealing on phosphorus implanted silicon wafers. Measurements are carried out at near band gap excitation with a Nd:YAG laser operating at 1.06 μm. Photoluminescence measurements are also carried out with 0.488 μm laser excitation. It was found that implantation conditions (dose and energy) and annealing temperature strongly influence the intensity of the photoluminescence signal. Contribution from the bulk silicon and the effects from the ion implantation to the photoluminescence signal are discussed.

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### Novel and improved methods of writing Bragg gratings with phase masksOthonos A, Lee X

The authors demonstrate the importance of spatial coherence of the writing beam for inscribing Bragg gratings with a phase mask. Bragg gratings were written using a normal excimer laser with the fiber placed at various distances away from the phase mask. This was repeated with an excimer laser that was modified to improve the beam spatial coherence showing dramatic improvement in the ability to inscribe gratings. Tunability of the inscribed Bragg grating wavelength, utilizing a single phase mask in conjunction with the improved spatial coherence of the excitation source, is demonstrated

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### Narrow linewidth excimer laser for inscribing Bragg gratings in optical fibersOthonos A, Lee X

A spectrally narrow linewidth KrF excimer laser has been developed for the application of writing Bragg gratings in optical fibers. Two air spaced etalons 80 and 6 cm−1 have been incorporated within the laser cavity resulting in a laser line with a linewidth of approximately 4 pm. A stable wavelength setting for writing Bragg gratings has been achieved without the necessity of a feedback mechanism. A study of the formation of Bragg grating in optical fibers with this narrow linewidth KrF excimer laser has been carried out. High quality gratings have been demonstrated in hydrogen sensitized fibers with a pulse energy density as low as 20 mJ/cm2. Limitations and various problems encountered in inscribing Bragg gratings (such as two simultaneous grating lines) with the narrow linewidth excimer laser source are also discussed.

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### Determining erbium distribution in optical fibers using phase-sensitive confocal microscopyOthonos A, Wheeldon J, Hubert M

Confocal optical microscopy augmented by the phasesensitive detection technique has been used to determine the erbium ion distribution in the core of single-mode optical fibers. The fluorescence at 565 nm generated by erbium ions under 488 nm excitation has been used to map the distribution of these ions in several doped fibers and the total ion concentration was estimated. The minimum concentration of erbium ions that can be measured using this technique is estimated to be of the order of a part per million.

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### High-Resolution Quadrature Photopyroelectric Spectroscopy of a-Si:H Thin Films Deposited on Silicon WafersShen J, Mandelis A, Othonos A, Vanniasinkam J

The recently developed photothermal technique of quadrature photopyroelectric spectroscopy (Q-PPES) has been applied to measurements of amorphous Si thin films deposited on crystalline Si substrates. Direct, meaningful comparisons have been made between purely optical transmission in-phase (IP-PPES) spectra, and purely thermal-wave sub-gap spectra with the use of a novel noncontacting PPES instrument to record lock-in in-phase and quadrature spectra, respectively. FT-IR transmission spectra have also been obtained for a comparison with this IP-PPES optical method. The results of the present work showed that the FT-IR method performs the worst in terms of spectral resolution of thin films and sub-bandgap defect/impurity absorptions inherent in the Si wafer substrate. The optical IP-PPES channel, however, albeit more sensitive than the FT-IR technique, fails to resolve spectra from surface films thinner than 2100 Å, but is sensitive to sub-bandgap absorptions. The thermal-wave Q-PPES channel is capable of resolving thin-film spectra well below 500 Å thick and exhibits strong signal levels from the crystalline Si sub-bandgap absorptions. Depending on the surface thin-film orientation toward, or away from, the direction of the incident radiation, the estimated minimum mean film thickness resolvable spectroscopically by Q-PPES is either 40 Å or 100 Å, respectively.

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# Year: 1994

### Optical spectroscopy on implanted and annealed silicon wafers: Plasma resonance wavelengthChristofides C, Othonos A, Bisson M, Boussey-Said J

A study of the effects of annealing temperature on phosphorus‐implanted silicon films is carried out. Fourier transform infrared spectroscopy has been performed with two different instruments in the spectral ranges of 0.75–4 μm and 3–25 μm. In the first spectrum range special attention was given to the influence of implantation dose on reflectivity. The minimum reflectivity associated with plasma resonance has been fully employed for estimation of the electrical activation of implanted impurities. Other conclusions concerning the activation of free carriers (implanted impurities) with implantation dose and annealing temperature have been reached

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### Raman spectroscopy and spreading resistance analysis of phosphorus implanted and annealed siliconOthonos A, Christofides C, Boussey-Said J, Bisson M

Raman and electrical characterization measurements are performed in order to study the effects of thermal annealing on phosphorus implanted silicon wafers. The silicon layers were implanted for various implantation energies and doses, below, and over the critical dose of amorphization. The post‐implanted period was followed by thermal isochronal annealing at various temperatures. Special attention has been given to the amorphous/crystal transition occurring at various annealing temperatures. A bi layer model [R. Loudon, J. Phys. (Paris) 26, 677 (1965)] has been used for a quantitative determination of the annealing temperature at which a complete annihilation of implantation defects takes place. For this analysis, Raman spectra, resistivity depth profiles, as well as 1D‐SUPREM III simulation were used.

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### Determination of erbium distribution in optical fibers using confocal optical microscopyUttamchandani D, Othonos A, Alavie AT, Hubert M

Confocal fluorescence microscopy has been used to detect the 565 nm wavelength fluorescence generated from Er+3 ions present in erbium doped optical fibers, thereby enabling the concentration profile of these ions in the fiber core to be determined. The fluorescence intensity distribution also enables the total Er+3 ion concentration in the fibers to be estimated, and the estimated values are shown to be in good agreement with data supplied by the fiber manufacturers

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