# Year: 2001

### Bragg Gratings in Optical FibersOthonos A, Kalli K

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 ApplicationsOthonos A

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 hydrogenOthonos A, Kalli K, Tsai DP

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 detectionChristofides C, Kalli K, Othonos A

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 measurementsKalli K, Othonos A, Christofides C, Tardiff F

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 SensingOthonos A, Kalli K

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

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 coatingsOthonos A, Nestoros M, Palmerio D, Christofides C, Bes RS, Traverse JP

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

### 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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