Publications

Year: 1997

Hydrogen gas detection via photothermal deflection measurement
Kalli K, Othonos A, Christofides C

Review of Scientific Instruments, DOI: 10.1063/1.1148320Download
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 films
Nestoros M, Gutierrez-Llorente A, Othonos A, Christofides C, Martinez-Duart JM

Journal of Applied Physics, DOI: 10.1063/1.366506Download
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 gratings

Review of Scientific Instruments, DOI: 10.1063/1.1148392Download
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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Chapter 3 Photoluminescence and Raman Scattering of Ion Implanted Semiconductors. Influence of Annealing
Othonos A, Christofides C

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Noncontact Carrier Lifetime Depth Profiling of Ion-Implanted Si Using Photothermal Radiometry
Othonos A, Salnick A, Mandelis A, Christofides C

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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

Quantum Electronics, IEEE Journal of, DOI: 10.1109/3.644114Download
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 GaAs
Benjamin SD, Loka HS, Othonos A, Smith PW

Applied Physics Letters, DOI: 10.1063/1.116178Download
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 analysis
LeBlanc M, Huang SY, Ohn M, Measures RM, Guemes A, Othonos A

Opt. Lett., DOI: 10.1364/OL.21.001405Download
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 radiometry
Mandelis A, Othonos A, Christofides C, Boussey-Said J

Journal of Applied Physics, DOI: 10.1063/1.363472Download
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 wafers
Othonos 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 laser
Othonos A, Lee X, Tsai DP

Optical Engineering, DOI: 10.1117/1.600725Download
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 annealing
Othonos A, Christofides C, Mandelis A

Applied Physics Letters, DOI: 10.1063/1.117903Download
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 processing
Uttamchandani D, Othonos A

Optics Communications, DOI: 10.1016/0030-4018(96)00059-4Download
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 defects
Othonos A, Christofides C

Journal of Applied Physics, DOI: 10.1063/1.360337Download
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 masks
Othonos A, Lee X

Photonics Technology Letters, IEEE, DOI: 10.1109/68.466584Download
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 fibers
Othonos A, Lee X

Review of Scientific Instruments, DOI: 10.1063/1.1145538Download
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 microscopy
Othonos A, Wheeldon J, Hubert M

Optical Engineering, DOI: 10.1117/12.215512Download
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 Wafers
Shen J, Mandelis A, Othonos A, Vanniasinkam J

Appl. Spectrosc., DOI: Download
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

Large ultrafast optical nonlinearities in As-rich GaAs
Benjamin SD, Othonos A, Smith PW

Electronics Letters, DOI: 10.1049/el:19941154Download
The measurement of large ultrafast bandgap-resonant optical nonlinearities in As-rich samples of GaAs that have been grown at low temperatures is reported. Light-induced refractive index changes of magnitude greater than 0.1 and with picosecond response times have been observed. These materials appear to be promising candidates for the fabrication of compact, ultrafast all- optical devices

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

Journal of Applied Physics, DOI: 10.1063/1.356097Download
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 silicon
Othonos A, Christofides C, Boussey-Said J, Bisson M

Journal of Applied Physics, DOI: 10.1063/1.356543Download
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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Superimposed multiple Bragg gratings
Othonos A, Lee X, Measures RM

Electronics Letters, DOI: 10.1049/el:19941359Download
The authors demonstrate the inscription of seven Bragg gratings at the same location on a photosensitive fibre. These superimposed Bragg gratings were written over a 60 nm span in the region of 1530 nm with reflectivities better than 45%. Effects due to the inscription of multiple gratings within the same location on the fibre such as wavelength shifting and linewidth narrowing are studied.

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Raman spectroscopy using a fiber optic probe with subwavelength aperture
Tsai DP, Othonos A, Moskovits M, Uttamchandani D

Applied Physics Letters, DOI: 10.1063/1.111802Download
Raman spectroscopy with subwavelength spatial resolution of a diamond sample was recorded using a tapered fiber optical probe in conjunction with a conventional Raman spectrometer. The experiment demonstrates the potential of suboptical wavelength resolution analytical spectroscopy. The tapered fiber optical probe with an aperture of around 100 nm, served as the means for delivering pump radiation while simultaneously collecting the Stokes radiation from the diamond specimen. Comparing the magnitude of the Raman scattering measured with the submicron single mode fiber probe to similar signals obtained with a nontapered probe made of the same type of fiber, illustrates the potential increase in effective optical aperture resulting from the close approach of the fiber to the surface.

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

Photonics Technology Letters, IEEE, DOI: 10.1109/68.275511Download
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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Year: 1993

A Multiplexed Bragg Grating Fiber Laser Sensor System
Alavie AT, Karr SE, Othonos A, Measures RM

Photonics Technology Letters, DOI: 10.1109/68.257209Download
A technique for multiplexing Bragg gratings in a fiber laser arrangement is described. This technique has successfully been used to multiplex two and three Bragg gratings with very little crosstalk. The Bragg grating laser sensors were used to measure both strain and temperature. Independent strain and temperature tuning of the gratings shows no crosstalk

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