Publications

Year: 2008

Time-resolved ultrafast carrier dynamics in as-grown nanocrystalline silicon films: the effect of film thickness and grain boundaries

, DOI: 10.1002/pssr.200701219Download
In this letter, we have studied transient photoinduced absorption in as-grown nanocrystalline silicon films with thickness varied from 5 to 30 nm. Effects of quantum confinement (QC) in z -direction and grain boundary distortions alter the carrier dynamics of these films considerably. Based on the determination of critical points in the first Brillouin zone of the band structure of materials, we have time-resolved the relaxation times of surface-related states and indirect valleys. When decreasing the film thickness down to the QC limit (∼10 nm) new ultrafast relaxation mechanisms start to play a dominant role in carrier dynamics due to the topological disordering of these ultrathin films. These relaxation mechanisms seem to be related with the traping/de-traping of the excited carriers prior to recombination

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Direct observation of excitons in polymer/carbon nanotube composites at room temperature: The influence of nanotube concentration
Lioudakis E, Kanari C, Othonos A, Alexandrou I

Diamond and Related Materials, DOI: DOI: 10.1016/j.diamond.2008.01.028Download
In this work, we have employed spectroscopic ellipsometry technique to study the optical properties of polymer/carbon nanotube (CN) composites as a function of nanotube concentration. Using a two-layer structural model based on Airy rigorous equations, the optical constants in various CN concentrations have been extracted. In the optical absorption spectra, we have observed a tuning of the excitonic transitions with the addition of single wall CN concentration. Based on theoretical calculations for the interaction of CNs with the surrounding effective media, the spectral evolution of the binding energy of optically active excitonic transitions with the addition of CNs suggests that the effective dielectric function of surrounding media decreases. Furthermore, using an oscillator Lorentz model, the absorption peaks, oscillator strengths and energy broadenings of the excitonic transitions have been extracted

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Optical properties of polyelectrolyte quantum dot multilayer films prepared using the layer by layer self-assembly method
Lioudakis E, Koupanou E, Kanari C, Leontidis E, Othonos A

Journal of Applied Physics, DOI: 10.1063/1.2906121Download
In this work, we have used spectroscopic ellipsometry to study the optical properties of polyelectrolyte-PbS quantum dot (QD) multilayer films prepared using the layer by layer self-assembly method. The optical results provide information about the absorption coefficients of the materials as a function of the number of layers deposited on a quartz substrate. We have found that the fundamental energy gap of the films decreases linearly upon addition of each layer due to the formation of nanoclusters at the surface. Furthermore, the influence of PbS QD concentration in colloidal dispersion on the energy gap of the materials is examined in detail, and it is found that the optical band gap in the films is in agreement with the linear absorption measurements in the PbS colloidal dispersion from which the film deposition takes place. Finally, the observed electronic transitions of the films corroborate that nanoparticles in the regime of strong quantum confinement are present in the films. This comprehensive fundamental study provides important information, necessary for photovoltaic applications, about the absorption tunability of these nanofilms.

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Determination of critical points on silicon nanofilms: surface and quantum confinement effects
Lioudakis E, Othonos A, Nassiopoulou AG

, DOI: 10.1002/pssc.200780109Download
In this work, we present a comprehensive study of the optical properties of nanocrystalline silicon films with thickness varied from 5 to 30 nm. Spectroscopic ellipsometry is employed to determine the dielectric functions of these films using a structural two-layer model based on the rigorous Airy formula. Our investigation gives an important insight of the origin of critical points for direct and indirect gaps of nanocrystalline silicon films as well as the evolution of them with decreasing the film thickness. The influence of the quantum confinement effect due to the nanoscale grain size and the surface vibrations at the interface on the optical properties are examined in detail

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Femtosecond Dynamics in Single Wall Carbon Nanotube/Poly(3-Hexylthiophene) Composites
Lioudakis E, Othonos A, Alexandrou I

Nanoscale Research Letters, DOI: 10.1007/s11671-008-9149-xDownload
Femtosecond transient absorption measurements on single wall carbon nanotube/poly(3-hexylthiophene) composites are used to investigate the relaxation dynamics of this blended material. The influence of the addition of nanotubes in polymer matrix on the ultrashort relaxation dynamics is examined in detail. The introduction of nanotube/polymer heterojunctions enhances the exciton dissociation and quenches the radiative recombination of composites. The relaxation dynamics of these composites are compared with the fullerene derivative-polymer composites with the same matrix. These results provide explanation to the observed photovoltaic performance of two types of composites.

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Transient Photoinduced Absorption in Ultrathin As-grown Nanocrystalline Silicon Films
Lioudakis E, Othonos A, Lioutas C, Vouroutzis N

Nanoscale Research Letters, DOI: 10.1007/s11671-007-9105-1Download
We have studied ultrafast carrier dynamics in nanocrystalline silicon films with thickness of a few nanometers where boundary-related states and quantum confinement play an important role. Transient non-degenerated photoinduced absorption measurements have been employed to investigate the effects of grain boundaries and quantum confinement on the relaxation dynamics of photogenerated carriers. An observed long initial rise of the photoinduced absorption for the thicker films agrees well with the existence of boundary-related states acting as fast traps. With decreasing the thickness of material, the relaxation dynamics become faster since the density of boundary-related states increases. Furthermore, probing with longer wavelengths we are able to time-resolve optical paths with faster relaxations. This fact is strongly correlated with probing in different points of the first Brillouin zone of the band structure of these materials.

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Surface-Related States in Oxidized Silicon Nanocrystals Enhance Carrier Relaxation and Inhibit Auger Recombination
Othonos A, Lioudakis E, Nassiopoulou A

Nanoscale Research Letters, DOI: 10.1007/s11671-008-9159-8Download
We have studied ultrafast carrier dynamics in oxidized silicon nanocrystals (NCs) and the role that surface-related states play in the various relaxation mechanisms over a broad range of photon excitation energy corresponding to energy levels below and above the direct bandgap of the formed NCs. Transient photoinduced absorption techniques have been employed to investigate the effects of surface-related states on the relaxation dynamics of photogenerated carriers in 2.8 nm oxidized silicon NCs. Independent of the excitation photon energy, non-degenerate measurements reveal several distinct relaxation regions corresponding to relaxation of photoexcited carriers from the initial excited states, the lowest indirect states and the surface-related states. Furthermore, degenerate and non-degenerate measurements at difference excitation fluences reveal a linear dependence of the maximum of the photoinduced absorption (PA) signal and an identical decay, suggesting that Auger recombination does not play a significant role in these nanostructures even for fluence generating up to 20 carriers/NC.

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Year: 2007

Instrumentation for the monitoring of toxic pollutants in water resources by means of neural network analysis of absorption and fluorescence spectra
Kuzniz T, Halot D, Mignani AG, Ciaccheri L, Kalli K, Tur M, Othonos A, Christofides C, Jackson DA

Sensors and Actuators B: Chemical, DOI: DOI: 10.1016/j.snb.2006.09.012Download
The concentration of several pollutants, usually present in industrial waste waters, is predicted by the neural network data processing of absorption and fluorescence measurements in the visible spectral range. Proper network training provides quantitative analysis of many pollutants with sub-ppm resolution. Compact optical fibre instrumentation for absorption spectroscopy and an innovative flowcell for fluorescence measurements enable cost-effective, in situ, nonstop monitoring of waste waters.

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Time-resolved ultrafast carrier dynamics in as-grown nanocrystalline silicon films: the effect of film thickness and grain boundaries
Lioudakis E, Nassiopoulou AG, Othonos A

physica status solidi (RRL) – Rapid Research Letters, DOI: Download
We have studied the ultrafast optical response of highly implanted and highly annealed polycrystalline silicon films using 400 nm ultrashort amplified pulses with fluence ranging between 8 mJ cm−2 to 56 mJ cm−2. Transient reflection measurements reveal differences both in the short and long temporal behaviour between the implanted non-annealed and annealed samples. Important contributing factors to the dynamics of the non-annealed sample are the carrier recombination centres and traps induced by ion implantation. In contrast to the non-annealed sample, the Auger recombination process is a key factor in the dynamics in the first few picoseconds for the sample annealed at 1100 °C. A model based on two coupled differential equations has been employed to investigate in detail the carrier dynamics in these systems. Parameters including carrier trapping times, diffusion coefficients and the Auger coefficient have been extracted

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Femtosecond carrier dynamics of InxGa1- xN thin films grown on GaN (0001): Effect of carrier-defect scattering
Lioudakis E, Iliopoulos E, Georgakilas A, Othonos A

Journal of Applied Physics, DOI: Download
Ultrafast carrier dynamics in the In0.33Ga0.67N epilayer were investigated in detail, using femtosecond transient differential non-degenerate optical absorption measurements. Following an excitation at 400 nm with fluence ranging from 25 µJ cm−2 to 3000 µJ cm−2, probing was carried out above and near the bandgap using different wavelengths generated from a super continuum source. We have found that bandgap renormalization plays a key role when probing at photon energies well above the bandgap and it is clearly distinct from other effects at the lowest fluence. The critical carrier density for the onset of noticeable bandgap renormalization effects in this material when probing well above the bandgap is approximately 5 × 1018 carriers cm−3. We have observed a decrease in the energy loss rate of this material as a function of photogenerated carrier density which is attributed to phonon bottleneck effect. For the lowest carrier density, we have extracted an optical phonon lifetime to be approximately 45 ± 9 fs.

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Quantum confinement and interface structure of Si nanocrystals of sizes 3-5 nm embedded in a-SiO2
Lioudakis E, Othonos A, Hadjisavvas GC, Kelires PC, Nassiopoulou AG

Physica E: Low-dimensional Systems and Nanostructures, DOI: DOI: 10.1016/j.physe.2006.12.020Download
Spectroscopic ellipsometry and Monte Carlo simulations are employed to answer the fundamental question whether the energy gaps of Si nanocrystals with sizes in the range of 3–5 nm, which are embedded in amorphous silica, follow or deviate from the quantum confinement model, and to examine their interfacial structure. It is shown that the optical properties of these nanocrystals are well described by the Forouhi–Bloomer interband model. Analysis of the optical measurements over a photon-energy range of 1.5–5 eV shows that the gap of embedded nanocrystals with a mean size of not, vert, similar3.9 nm follows closely quantum confinement theory. A large band gap expansion (not, vert, similar0.65 eV) compared to bulk Si is observed. The Monte Carlo simulations reveal a non-abrupt interface and a large fraction of interface oxygen bonds. This, in conjunction with the experimental observations, indicates that oxygen states and the chemical disorder at the interface have a negligible influence on the optical properties of the material in this size regime.

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Optical properties of conjugated poly(3-hexylthiophene)/[6,6]-phenylC[sub 61]-butyric acid methyl ester composites
Lioudakis E, Othonos A, Alexandrou I, Hayashi Y

Journal of Applied Physics, DOI: 10.1063/1.2799049Download
In this work, we present the evolution of optical constants as a function of [6,6]-phenyl C61-butyric acid methyl ester (PCBM) concentration for conjugated poly(3-hexylthiophene)/[6,6]-phenylC61-butyric acid methyl ester composites. The PCBM concentration of the utilized samples varies from 1 to 50 wt %. The dielectric functions for all these composites reveal electronic structural changes as a result of the addition of PCBM. We have deconvoluted the contribution of the substrate using a two-layer Fabry-Pérot structural model. The extracted optical properties contain crucial absorption peaks of singlet exciton states and vibronic sidebands for poly(3-hexylthiophene) (P3HT) conjugated polymer as well as two PCBM-related states at higher energies. With the addition of PCBM, we have observed a limit of 20 wt % PCBM beyond which two discrete energy levels (3.64 and 4.67 eV) appear in the spectrum. For the highest concentration composite, the results suggest that the interchain interactions provide a small excitonic contribution in the absorption spectrum at energies where the conjugated polymer absorbs (1.85–2.7 eV) and a strong rise of PCBM states (3.64 and 4.67 eV) which are responsible for the subsequent exciton dissociation. In addition, the energy gap between the higher occupied molecular orbitals and the lower unoccupied molecular orbitals of the highest concentration composite (50 wt %) is 1.85 eV. The tuning of the optical properties of P3HT with the addition of PCBM shows that ellipsometry can be used to monitor layer concentration toward optimization of plastic solar cells.

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The role of surface vibrations and quantum confinement effect to the optical properties of very thin nanocrystalline silicon films
Lioudakis E, Antoniou A, Othonos A, Christofides C, Nassiopoulou AG, Lioutas CB, Frangis N

Journal of Applied Physics, DOI: 10.1063/1.2800269Download
We report on a spectroscopic study of very thin nanocrystalline silicon films varying between 5 and 30 nm. The role of quantum confinement effect and surface passivation of nanograins in optical properties are examined in detail. The coupling between surface vibrations and fundamental gap Eg as well as the increase of interaction between them at the strong confinement regime ( ⩽ 2 nm) are proposed for the observable pinning of Eg in luminescence measurements.

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Femtosecond carrier dynamics of In[sub x]Ga[sub 1 - x]N thin films grown on GaN (0001): Effect of carrier-defect scattering
Lioudakis E, Othonos A, Iliopoulos E, Tsagaraki K, Georgakilas A

Journal of Applied Physics, DOI: 10.1063/1.2786610Download
Ultrafast carrier dynamics in ultrathin InxGa1−xN ternary alloys were investigated in detail, using femtosecond transient differential optical absorption measurements. Measurements were performed with probing wavelengths above and below the band edge of the materials. Furthermore, we performed a structural characterization by x-ray diffraction reciprocal space maps and we found that the alloys with the higher InN mole fraction (x = 0.89 and x = 0.43) present large lattice mismatch-strain relaxation whereas the lower InN content alloy was fully coherent with the underlying GaN (0001) layer. Our analysis showed that the observed nonradiative carrier dynamics was strongly related with the carrier-defect scattering of the materials. Our measurements in conjunction with the numerical analysis showed that when we excite these alloys with ultrashort laser pulses the background carrier concentration for In-rich InGaN samples participates to the photoexcited carrier relaxation process via carrier momentum scattering. For the higher InN content alloys (x = 0.89 and 0.43) the relaxation times were short (0.4–1.4 ps) whereas for the full strained alloy (x = 0.07) a slower nonradiative relaxation time ( ∼ 25 ps) was observed. Finally, the energy loss rate of this material (2.05±0.10 meV/fs) as well as the optical phonon lifetime (44±2 fs) were extracted.

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Ultrafast carrier dynamics on conjugated poly(3-hexylthiophene)/[6,6]-phenylC[sub 61]-butyric acid methyl ester composites
Lioudakis E, Othonos A, Alexandrou I, Hayashi Y

Applied Physics Letters, DOI: 10.1063/1.2785120Download
The authors have studied ultrafast carrier dynamics on poly(3-hexylthiophene)/[6,6]-phenylC61-butyric acid methyl ester (PCBM) composites up to 50% PCBM concentration. They have resolved the exciton intraband relaxation of composites as well as the subsequent electron/polaron relaxation of dissociated excitons. The observed fast time constant of intraband relaxation is found to be 0.5–1.5 ps, whereas the second long-live relaxation (0.5–1 ns) is strongly fullerene related. A wavelength dependent ultrafast study is performed giving fundamental information on the nonradiative exciton relaxation, exciton dissociation, and electron relaxation of PCBM-related states.

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Ultrafast transient photoinduced absorption in silicon nanocrystals: Coupling of oxygen-related states to quantized sublevels
Lioudakis E, Othonos A, Nassiopoulou AG

Applied Physics Letters, DOI: 10.1063/1.2728756Download
The authors have studied transient photoinduced absorption in single monolayers of oxidized silicon nanocrystals. Transient photoinduced absorption measurements along with optical absorption and photoluminescence (PL) emission reveal that the light-absorption process takes place in defects related to strong PL emission, suggesting that the photoexcited carriers are in oxygen-related interface states. They have time-resolved ultrafast relaxation paths in oxygen-related states and quantized sublevels, which have important implications in the understanding of fundamental optical properties for this system.

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Influence of grain size on ultrafast carrier dynamics in thin nanocrystalline silicon films
Lioudakis E, Othonos A, Nassiopoulou AG, Lioutas CB, Frangis N

Applied Physics Letters, DOI: 10.1063/1.2738383Download
The ultrafast carrier dynamics in thin nanocrystalline silicon films (10 and 20 nm thick) on quartz over a broad spectral range using optical pumping at a moderate fluence of 2.5 mJ/cm2 were studied. The films were composed of randomly oriented silicon nanocrystals of various sizes and shapes. The authors probed a fast and a slow relaxation mechanism. The slow decay ( ∼ 300 ps) was attributed to bulk nanocrystal states similar to those of bulk silicon, while the faster one ( ∼ 3 ps) was attributed to surface states at grain boundaries, more dominant in the smaller nanocrystals due to their larger surface/volume ratio.

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Ultrafast carrier dynamics in band edge and broad deep defect emission ZnSe nanowires
Othonos A, Lioudakis E, Philipose U, Ruda HE

Applied Physics Letters, DOI: 10.1063/1.2825290Download
Ultrafast carrier dynamics of ZnSe nanowires grown under different growth conditions have been studied. Transient absorption measurements reveal the dependence of the competing effects of state filling and photoinduced absorption on the probed energy states. The relaxation of the photogenerated carriers occupying defect states in the stoichiometric and Se-rich samples are single exponentials with time constants of 3–4 ps. State filling is the main contribution for probe energies below 1.85 eV in the Zn-rich grown sample. This ultrafast carrier dynamics study provides an important insight into the role that intrinsic point defects play in the observed photoluminescence from ZnSe nanowires.

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Year: 2006

Ellipsometric analysis of ion-implanted polycrystalline silicon films before and after annealing
Lioudakis E, Nassiopoulou A, Othonos A

Thin Solid Films, DOI: DOI: 10.1016/j.tsf.2005.08.339Download
A study of arsenic ion-implanted polycrystalline silicon films before and after annealing at various temperatures has been performed using spectroscopic ellipsometry in the ultraviolet to the visible spectral region. Using the Bruggeman effective medium approximation, an optical/structural model is presented for all the annealed samples explaining the measurements. Ellipsometric measurements reveal important structural changes as a function of annealing temperature which provide an interesting inside into the annealing kinetics of ion-implanted polycrystalline silicon films. This work also demonstrates the importance of spectroscopic ellipsometry in determining non-destructively the dielectric functions in materials that have undergone complex processing.

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

Semiconductor Science and Technology, DOI: Download
We have studied the ultrafast optical response of highly implanted and highly annealed polycrystalline silicon films using 400 nm ultrashort amplified pulses with fluence ranging between 8 mJ cm −2 to 56 mJ cm −2 . Transient reflection measurements reveal differences both in the short and long temporal behaviour between the implanted non-annealed and annealed samples. Important contributing factors to the dynamics of the non-annealed sample are the carrier recombination centres and traps induced by ion implantation. In contrast to the non-annealed sample, the Auger recombination process is a key factor in the dynamics in the first few picoseconds for the sample annealed at 1100 °C. A model based on two coupled differential equations has been employed to investigate in detail the carrier dynamics in these systems. Parameters including carrier trapping times, diffusion coefficients and the Auger coefficient have been extracted.

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Optical and structural properties of implanted Si wafers: the effects of implantation energy and subsequent isochronal annealing temperature
Lioudakis E, Christofides C, Othonos A

Semiconductor Science and Technology, DOI: 10.1088/0268-1242/21/8/013Download
We have studied the influence of implantation energy and subsequent isochronal annealing temperature on the optical and structural properties of implanted Si wafers employing a multiwavelength spectroscopic ellipsometer. A temperature-dependent multilayer optical model is used to explain the ellipsometric data for all implantation energies (20 to 180 keV) and annealing temperatures (300 to 1100 °C) of this work. This work completely characterizes the structural and optical properties of these implanted samples via the pseudodielectric functions and the integrated damage depth profile. For the highest implantation sample self-annealing phenomena have appeared, reducing the integrated damage depth profile. Finally, the dynamics of isochronal annealing temperature on the integrated damage depth profile of these wafers exhibit an abrupt drop in the transition temperature where a long-range ordering is obtained and pseudodielectric functions approach the crystallinity shapes.

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Femtosecond time-resolved study in In[sub x]Ga[sub 1 - x]N (0001) ultrathin epilayers: Effects of high indium mole fraction and thickness of the films
Lioudakis E, Othonos A, Dimakis E, Georgakilas A

Applied Physics Letters, DOI: 10.1063/1.2405413Download
In view of promising full-solar-spectrum photovoltaic systems based on InxGa1−xN ternary alloys, femtosecond time-resolved study in ultrathin epilayers was employed in order to extract the fundamental properties of material. Two different thicknesses of epilayers were employed with relative high indium mole fractions. State filling effect at various probing energy states has been observed for both epilayers. Saturation of state filling as well as enhanced photoinduced absorption occurred at higher probing wavelengths. Furthermore, coherent acoustic phonon oscillations were also observed for both ultrathin epilayers with a thickness dependent oscillation frequency. Finally, absorption band edge of these alloys has been determined.

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Probing carrier dynamics in implanted and annealed polycrystalline silicon thin films using white light
Lioudakis E, Othonos A, Nassiopoulou AG

Applied Physics Letters, DOI: 10.1063/1.2200745Download
Polycrystalline silicon thin film samples implanted and annealed at various temperatures have been studied using ultrafast laser pulse excitation. Nondegenerate pump-probe technique has been utilized to investigate carrier dynamics in the highly implanted samples at a relatively small fluence. A model based on two coupled differential equations has been used to fit the experimental data, giving a simple but adequate picture of the dynamics of this system. Basic sample parameters such as carrier trapping times, diffusion coefficient, and penetration depths have been extracted, providing a dependence on the annealing temperature for the samples under investigation

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Ultrafast carrier dynamics in InxGa1 - xN (0001) epilayers: Effects of high fluence excitation
Lioudakis E, Othonos A, Dimakis E, Iliopoulos E, Georgakilas A

Applied Physics Letters, DOI: 10.1063/1.2190456Download
Ultrafast carrier dynamics in InxGa1−xN (0001) epilayers were investigated, using femtosecond transient differential transmission and reflection measurements for x = 0.07, 0.15, and 0.33, over a fluence range of 1–12 mJ/cm2. Stimulated emission as well as band gap renormalization play a crucial role in the dynamics of the photogenerated carriers. Threshold fluence leading to saturation of the differential reflectivity and transmission signals related to the In mole fraction has been observed, which is attributed to band gap renormalization, Auger process, and carrier recombination through In-rich nanoclusters. Furthermore, coherent acoustic phonon oscillations have also been observed in the In0.15Ga0.85N at high fluence excitation

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Fibre Bragg Gratings
Othonos A, Kalli K, Pureur D, Mugnier A

, DOI: Download
Today optical fibres plus related components are synonymous with the word “telecommunication”. In addition to applications in telecommunications, optical fibres are also utilized in the rapidly growing field of fibre sensors. Despite the improvements in optical fibre 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 fibre optics. Recently, however, all this has changed with the ability to alter the core index of refraction in a single mode optical fibre by optical absorption of UV light

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