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Tb 3Sc 2Al 3O 12 (TSAG) is a paramagnetic insulator which, in spite of its congruent nature and excellent magneto-optical properties as Faraday rotators for optical isolators (OIs), cannot be industrially produced. The reason is that its lattice structure is so stressed that single crystals easily crack upon cooling after growth or during the cutting process. By the incorporation of a small amount of Lu, it has been demonstrated that the stress in the structure can be effectively released, and, therefore, the crystal cracking can be avoided. This has enabled the substitution of standard Tb 3Ga 5O 12 (TGG), achieving better high-power performance and compacter OIs. In this study, the charge state and the lattice site location of Lu as well as Y dopants in TSAG are investigated by X-ray Absorption Near Edge Structure and Extended X-ray Absorption Fine Structure Analysis. Measurements on single crystal and powdered samples indicate that both dopants are incorporated in the 3+ charge state and substitute predominantly in the dodecahedral Tb-site ( >87.5%), while a minor fraction is found in the octahedral Sc-site.

Substitutional disordered garnet crystal hosts such as (Y1-xGdx)3(Al1-yScy)5O12 (YGSAG) produce inhomogeneous broadening of the optical spectra of the dopant, making them good candidates for solar-pumped laser materials. In the case of codoping with Nd and Cr ions, the spectral overlapping between the Cr absorption bands and the solar spectrum and between the Cr emission lines/bands and the Nd absorption lines leads to the increase of the pumping efficiency for the Nd emission. As a consequence of energy transfer from Cr to Nd, the threshold of the Nd laser oscillation is expected to be reduced when compared with that of the single doped Nd crystal. The laser oscillation of the Nd stimulated emission for Nd:Cr:YGSAG has been evaluated using an Ar ion laser, giving evidence of the energy transfer excitation processes involved in the laser output.

Microvoids are well-known crystal defects, usually exhibiting a rounded and/or irregular shape, which act as undesired scattering centers in optical applications. Here, for the first time, residual microvoids in the shape of perfect regular hexagonal-prisms are observed in apparently scattering-free CeF3 bulk single crystals. These microvoids reflect exactly the natural crystal habit in “negative” form: the prisms stand along the c-axis and the hexagonal sides are parallel to the six equivalent planes perpendicular to the a-axis. They distribute randomly with sizes up to 13 µm and heights in the submicron order with a maximum around 500 nm. The microvoids are formed by the embedding of bubbles during the crystallization process. The bubbles’ gas is possibly caused by the high vapor pressure of CeF3 itself at the high melting point.

Garnet crystals codoped with Nd3+ and Cr3+ ions are a candidate for solar-pumped laser materials. Substitutional disordered crystals Y3GaxAl5-xO12 were prepared to improve the pumping efficiency of Nd3+ luminescence and energy transfer from Cr3+ to Nd3+ ions. The substitutional disordered crystal host produced inhomogeneous broadening of the Nd3+ and Cr3+ optical spectra. Enhancement of overlapping between the Cr3+ absorption bands and the solar spectrum, and between the Cr3+ luminescence bands and the Nd3+ absorption lines led to the increases of the pumping efficiency and the energy transfer rates, respectively. The excitation spectrum of the Nd3+ luminescence, the nonexponential decay curves of the Cr3+ luminescence, and the quantum yields of the Cr3+ and Nd3+ luminescence have given evidence on the energy transfer from Cr3+ to Nd3+ ions.

High optical quality LiREF4(RE = Tb3+, Dy3+, Ho3+, Er3+and Yb3+), PrF3and CeF3single crystals have been grown by the Czochralski technique. Their magneto-optical properties have been measured and analyzed in detail in the ultraviolet-visible wavelength region, and their figures of merit as Faraday rotators have been determined. CeF3presents superior properties above 300 nm, showing a figure of merit higher than that of the reference material, terbium-gallium-garnet, which is nowadays used in the visible-near infrared. PrF3is the best rotator for the 220-300 nm range. Towards shorter wavelength and in the vacuum ultraviolet, it is shown that the LiREF4crystals are unique rotators.

Czochralski grown Ce and Gd-doped Y 3 Al 5 O 12 (Ce,Gd:YAG) single crystal demonstrated superior performance features in a new concept of white LED where it is used as thin single crystal phosphor plates (SCPPs). New SCPP-based white LED realizes epoxy resin free package, which allows to eliminate the photodegradation issue of the organic material. Optical, thermal and electrical properties of SCPPs and SCPP-based white LEDs are discussed with the powder phosphor and commercial LED reference. SCPPs showed significantly higher values and better stability under the high temperatures, irradiation and current. Quantum efficiency was found to be as high as 93%.

White light-emitting diodes (wLEDs) are the new environmental friendly sources for general lighting purposes. For applications requiring a high-brightness, current wLEDs present overheating problems, which drastically decrease their emission efficiency, color quality and lifetime. This work gives an overview of the recent investigations on single-crystal phosphors (SCPs), which are proposed as novel alternative to conventional ceramic powder phosphors (CPPs). This totally new approach takes advantage of the superior properties of single-crystals in comparison with ceramic materials. SCPs exhibit an outstanding conversion efficiency and thermal stability up to 300°C. Furthermore, compared with encapsulated CPPs, SCPs possess a superior thermal conductivity, so that generated heat can be released efficiently. The conjunction of all these characteristics results in a low temperature rise of SCPs even under high blue irradiances, where conventional CPPs are overheated or even burned. Therefore, SCPs represent the ideal, long-demanded all-inorganic phosphors for high-brightness white light sources, especially those involving the use of high-density laser-diode beams.

β-Ga 2 O 3 crystals are one of the transparent conductive oxides (TCOs). The n-type conductivity of β-Ga 2 O 3 is attributed to an oxygen deficit. In order to examine a mechanism of the conductivity, electron-spin resonance (ESR) measurements were carried out for β-Ga 2 O 3 doped with various impurity ions (Mn, Cr, Si, Zr, Hf). A single narrow ESR line with an anisotropic g value (~ 1.96) was observed for Si, Zr, or Hf doped β-Ga 2 O 3 and ascribed to “ motional narrowing ” of electrons trapped around oxygen vacancies, namely, conduction electrons. Extinction of the narrow ESR line for Mn or Cr doped β-Ga 2 O 3 may be due to a decrease of the conduction electron density and/or a strong spin–spin interaction between the conduction electron and the paramagnetic ions. The fine structure of the ESR spectrum for Mn or Cr doped β-Ga 2 O 3 gives evidence that Mn 2 + and Cr 3 + ions substitute for octahedral Ga 3 + ions in the lattice without charge compensators.

Ferroelectric fluoride BaMgF 4 , β -Ga 2 O 3 and F-doped core-free Y 3 Al 5 O 12 single crystals for ultraviolet and vacuum ultraviolet (UV/VUV) applications, are introduced. 2-in. size BaMgF 4 single crystals have been grown. Subsequently, second harmonic generation (SHG) in different wavelengths was demonstrated by the quasi-phase matching (QPM) devices. 2-in. size β -Ga 2 O 3 single crystals, which have large bandgap, 4.8 eV, have been grown. Blue light emitting diodes (LEDs) on β -Ga 2 O 3 were demonstrated. F-doped core-free Y 3 Al 5 O 12 single crystals have been grown as a potential new lens material for UV/VUV.

High optical quality terbium calcium fluoride (Tb 0.81 Ca 0.19 F 2.81 ) and terbium strontium fluoride (Tb 0.76 Sr 0.24 F 2.76 ) single crystals have been grown by the Czochralski method under controlled atmosphere. While both compounds present similar optical and magneto-optical properties (determined by the high concentration of Tb 3+ ions), the lower absorption losses and high Faraday rotation exhibited by terbium calcium fluoride, compared to those of terbium strontium fluoride, lead to a notable figure of merit for Faraday isolators operating in the visible wavelength region. In particular at short wavelengths, below 500 nm, the figure of merit of terbium calcium fluoride is remarkably higher than that of the industrial standard terbium gallium garnet.

The oxide garnet {Tb3}[Sc2-xLux](Al3)O12 (TSLAG) is developed as alternative to Tb3Ga5O12 (TGG). TSLAG exhibits better transmittance, Verdet constant, and radiation resistance. Further, PrF3 and CeF3 fluoride crystals are investigated as unique Faraday rotators for the UV-visible region.

The growth characteristics of large size SrAlF 5 and BaMgF 4 single crystals by the Czochralski technique are described and compared. Transmission spectra in the vacuum ultraviolet wavelength region indicate a high transparency of SrAlF 5 (about 90% without considering surface reflection loses) down to 150 nm, in contrast to the optical loses observed for BaMgF 4 . The ferroelectric character of SrAlF 5 is evidenced by the reversal of the spontaneous polarization in a hysteresis loop. The higher potential of SrAlF 5 in comparison with BaMgF 4 for the realization of all-solid-state lasers in the ultraviolet wavelength region by the quasi-phase matching technique is pointed out. SrAlF 5 , besides a higher grade of transparency, shows a nonlinear effective coefficient similar to that of quartz and uniaxial nature, in contrast to the one-order smaller nonlinear coefficient and biaxial character of BaMgF 4 .

The growth characteristics and properties of large size SrAlF5 single crystals are described and compared with those of BaMgF4. SrAlF5 shows a higher grade of transparency and a larger nonlinear effective coefficient as well as uniaxial and ferroelectric nature, on contrast to the one order smaller nonlinear coefficient and biaxial character of BaMgF4. The Sellmeier and Cauchy coefficients are obtained fitting the measured ordinary and extraordinary refractive indices, and the grating period for the first order quasi-phase matching (QPM) is estimated as a function of the wavelength. The higher potential of SrAlF5 in comparison with BaMgF4 for the realization of all-solid-state lasers in the ultraviolet wavelength region by the QPM technique is pointed out.

β-Ga2O3 is the most transparent conductive oxide, well known since several decades for its large bandgap of 4.8 eV. Its potential as semiconductor material, however, is just emerging in recent years. Present work shows the development of βGa2O3 for semiconductor applications and its current state-of-the-art. The discussion is focused on three different aspects: (1) Advantageous growth from melt of large-size β-Ga2O3single-crystals. High-crystalline quality and carrier control make possible the production of conductive and semi-insulating wafers. (2) β-Ga2O3as substrate for homoepitaxy as well as for heteroepitaxial deposition of GaN-based devices. High-brightness blue-LEDs with vertical current injection are demonstrated. (3) Potential of β-Ga2O3for high-power devices with higher breakdown voltage than GaN and SiC counterparts. The first Schottky barrier diode is shown, as well as first transistors (MESFET and MOSFET) are indicated. Single-crystal phosphors are introduced as novel alternative to currently used powder phosphors. In connection with high-brightness white light-sources, based on LEDs or LDs plus phosphor converters, single-crystal phosphors possess advantageous features. These avoid the use of resins and exhibit a very high internal quantum efficiency, which remains stable with the temperature increase.

BaMgF4 single crystals doped with Nd3+ are of great interest as possible active media for VUV–UV and mid-IR all solid-state lasers. The luminescence spectroscopy and the excited state dynamics of these crystals are investigated for the first time upon VUV excitation using synchrotron radiation. The material shows fast Nd3+ 5d–4f emission upon direct VUV excitation into Nd3+ 5d levels. The decay kinetics of the Nd3+ 5d–4f emission in this host lattice upon direct VUV excitation of 5d levels is characterized by a lifetime of about 14 ns with no significant rise after the excitation pulse. No significant temperature dependence of the lifetime is observed within the range 8–300 K.

LuLiF4 single crystals codoped with thulium (Tm) and holmium (Ho) were successfully grown by the Czochralski method. The excellent laser performance, in terms of highest slope efficiency, and highest optical-to-optical efficiency and lower laser threshold, was studied in comparison with 5% Tm, 0.5% Ho-codoped YLiF4, and Lu3Al5O12 under the same conditions.

CeF3 and PrF3 single crystals are investigated as Faraday rotators for the UV-visible region. Their properties are compared with those of the industrial standard reference terbium-gallium-garnet (TGG) single crystal. In contrast to TGG, CeF3 exhibits a higher transparency in the whole near UV-visible-IR, and a remarkable higher figure of merit which rapidly increases towards the cutoff. In the case of PrF3, the transparency extends to even shorter wavelengths, and analogously to CeF3 its figure of merit increases notably in the UV. These results indicate the potential of CeF3 and PrF3 as optical isolators in the UV-visible, where at present there are no alternative candidates.

International audience; Cd1−xMnxTe (x=0.1, 0.3, 0.5, 0.7 and 0.9) (CMT) single crystals were grown by the vertical Bridgman method. The optical studies reveal that with the increase in Mn concentration, the band gap values increase, which is attributed to s, p-d exchange interaction between the band carriers and Mn ions. Faraday rotation angle of the grown CMT (x=0.5) crystals were measured at the following wavelengths: 825, 1060 and 1575 nm. It was inferred that CMT exhibit larger Faraday effect (3-6 times larger than terbium-gallium garnet (TGG) currently used for optical isolators) making it as an efficient material for optical isolator at longer wavelengths. Field-cooled and zero field-cooled magnetizations of CMT were measured as a function of temperature and magnetic field. The spin-glass like behavior of CMT and their tendency to decrease in magnitude with increasing Mn concentration have been analyzed. The metal contacts on the Cd1−xMnxTe (x=0.1, 0.5, 0.7 and 0.9) crystals have been made with various metals and metal alloys to establish the ohmic contact. The detector characteristics of CMT have been tested using γ-rays with 511 keV (22 Na) and 59.5 keV (241 Am).

International audience; {Tb3}[Sc2−xLux](Al3)O12 crystals have been grown and investigated for the first time for magneto-optical applications. Tb3Al5O12 exhibits the best magneto-optical features; however, its incongruent melting nature has led to the industrial use of Tb3Ga5O12. Tb3Sc2Al3O12 had been proposed as an alternative to Tb3Al5O12, but unfortunately its growth also presents several drawbacks. The present investigation shows that by the isovalent substitution of Sc3+ by Lu3+ in the octahedral site of the garnet structure, it is possible to improve the growth characteristics while preserving the superior magneto-optical properties. We demonstrate that {Tb3}[Sc2−xLux](Al3)O12 crystals show a higher visible transparency and a larger Faraday rotation than Tb3Ga5O12 crystals. {Tb3}[Sc2−xLux](Al3)O12 is therefore a very promising material in particular for new magneto-optical applications in the visible-near IR wavelength region.

The first ultraviolet (UV) and visible optical isolators based on CeF3 are demonstrated. CeF3 possesses unique properties as Faraday rotator for the UV-visible wavelength region: a wide transparency range (wavelength of >300 nm) and an outstanding Verdet constant. In contrast, currently used terbium-gallium garnets and magneto-optical glasses possess a low transparency in the visible and a small Verdet constant in the UV, respectively. The optical isolator prototypes consist of a CeF3 rod, a single ring magnet, and a couple of beam splitters. The ring magnets have been designed to guarantee a homogeneous magnetic field; for it, numerical simulations have been carried out. The two prototypes are very compact and operate in the UV at 355 and in the visible at 405 nm, respectively. The performance of these devices indicates the high potential of CeF3 as a new UV-visible Faraday rotator, specially for shorter wavelengths where at present there are no optical isolators available.

The growth of Ce:Y3Al5O12(Ce:YAG) single-crystal phosphors (SCPs) by the Czochralski technique is analyzed in terms of segregation coefficient, solubility and absorption cross-section. The emission characteristics of these SCPs are investigated in a wide temperature range, from liquid He temperature up to 500 °C. The internal quantum efficiency of SCPs achieves its maximum at about 250 °C. Thermal quenching of SCPs at high temperature is attributed to the Mott–Seitz mechanism. In the case of ceramic powder phosphors, a continuous droop is observed with the temperature due to defect-related non-radiative recombination paths. It is shown that (Ce:YAG SCPs + blue LEDs/LDs) can cover a wide range of color temperatures 5500–7000 K, with color rendering indices around 70. In conclusion, it is shown that Ce:YAG SCPs are the most efficient and temperature stable converters to fabricate high-brightness white light sources with high-power blue LEDs and LDs.

The electron paramagnetic resonance (EPR) study of $\ensuremath{\beta}\ensuremath{-}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ crystals gives evidence that donors can be regarded as ${\mathrm{O}}^{2\ensuremath{-}}$ vacancies trapping single electrons. The Lorentzian line shape of the EPR spectra observed in the range of 5--300 K, which exhibit anisotropic g values, suggests that motional narrowing occurs in this temperature range. For any magnetic-field orientation a single EPR line is observed, indicating that donor electrons are predominantly created in one of the three different oxygen sites in the $\ensuremath{\beta}\ensuremath{-}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ crystal. A previous transmission electron microscopy study suggested that a break of symmetry in domains of 2--3 nm correlates with a preceding cluster model of oxygen vacancies. From the temperature dependence of the EPR linewidth and the electrical conductivity it is found that the electron conduction in the clusters and/or between them is governed by a tunneling process at low temperatures, whereas at temperatures above 50 K, the transport of electrons through hopping between the clusters is thermally activated.

Ce:(YLu)AlO single-crystals are investigated in detail as phosphors for high-brightness LEDs and LDs. It is demonstrated that single-crystal phosphor plates (SCPPs) present very advantageous characteristics in comparison with standard powder phosphors. On the one side, the very high quantum efficiency of the SCPPs does not decrease with the temperature increase, and on the other side, the SCPPs possess a high thermal conductivity, so that these can be efficiently cooled to avoid device overheating. The optical properties of these SCPPs are investigated from room temperature to . Ce:YAlO SCPPs are adequate for high-brightness cool white light sources. Instead, Ce:(YLu)AlO SCPPs, whose emission shifts to the green with the Lu concentration, are very promising for neutral and warm white light sources in conjunction with a red-phosphor. It is shown that these white sources exhibit a color rendering over 90 for a wide range of excitation and SCPP emission wavelengths.

High optical quality LiREF(4) (RE = Tb(3+), Dy(3+), Ho(3+), Er(3+) and Yb(3+)), PrF(3) and CeF(3) single crystals have been grown by the Czochralski technique. Their magneto-optical properties have been measured and analyzed in detail in the ultraviolet-visible wavelength region, and their figures of merit as Faraday rotators have been determined. CeF(3) presents superior properties above 300 nm, showing a figure of merit higher than that of the reference material, terbium-gallium-garnet, which is nowadays used in the visible-near infrared. PrF(3) is the best rotator for the 220-300 nm range. Towards shorter wavelength and in the vacuum ultraviolet, it is shown that the LiREF(4) crystals are unique rotators. Overall, the rare-earth fluoride single crystals studied here exhibit better properties than other materials considered so far, and therefore they have potential to cover the increasing demand for new and improved Faraday rotators in the ultraviolet-visible wavelength region.

The optical and magneto-optical properties of the {Tb3}[Sc1.95Lu0.05](Al3)O12 garnet (TSLAG) crystal are analyzed in detailed and compared with those of the reference compound Tb3Ga5O12 (TGG). TSLAG presents a very high transmittance, particularly higher than that of TGG in the visible region, and its Verdet constant is 20% larger at any wavelength. The figure of merit and extinction features point out the superior characteristics of TSLAG respect to TGG. A prototype, polarization independent optical isolator based on a TSLAG rotator is demonstrated at 1080 nm. Its performance confirms the high optical quality of this crystal, indicating its mature development for Faraday rotator applications.

The incorporation of Nd3+ in the ferroelectric fluoride BaMgF4 crystal is investigated for its potential application as self-frequency-converter solid state laser. Low temperature high-resolution optical spectroscopy shows that Nd3+ is located at two well distinguished sites referred as A and B. The whole set of energy levels in the excited F43/2, I411/2, and ground I49/2 states are identified for both Nd3+ sites and their crystallographic location in BaMgF4 lattice is discussed. The radiative lifetime of the F43/2 metastable laser level is obtained to be 1430 and 800 μs for site A and B, respectively, and the luminescence quantum efficiency of the F43/2 metastable state for the dominant A site is 87%. The possibility of annihilating the contribution of the B site in the optical spectrum is demonstrated by means of the incorporation Na+ into the crystal.

BaMgF(4) is a ferroelectric fluoride which shows a very wide transparency range extending from 125 nm to 13 microm. The conjunction of these properties confers to BaMgF(4) a unique chance for optical applications in the UV and mid-IR wavelength regions, where other nonlinear materials cannot be used. In particular its application as frequency converter in all solid-state lasers is considered. The wavelength dispersion of the refractive indices along the three optical principal axes is measured in the transparent region, and the Sellmeier coefficients for the three refractive indices are determined. The conditions for nonlinear optical processes are calculated for birefringent-matching and quasi phase-matching, with special emphasis in the UV and IR wavelength regions. Quasi phase-matching can be achieved in the whole transparent wavelength region, in contrast to birefringent-matching, which can be obtained in a limited range 573-5634 nm. First demonstration of second harmonic generation by quasi phase-matching with a ferroelectric fluoride is shown by frequency-doubling the emissions of a 1064 nm Nd:YAG laser and a tunable Ti:sapphire laser. The shortest emission is obtained in the UV at 368 nm, indicating the potential of BaMgF(4) as nonlinear medium for the fabrication of all solid-state lasers in the vacuum-UV/UV and mid-IR wavelength regions.

The temperature dependence of the electrical properties of β-Ga2O3 is analyzed from liquid helium to room temperature by Hall measurements. Below 100K, the carrier motion takes place within the impurity conduction band and it shows a power-law dependence. An electrostructural coupling is ascertained. Thermal activation in the conduction band is observed above 100K, with an activation energy Ea=7meV. Thermal conductivity is measured to 13Wm−1K−1. Further, it is shown that Ti is a good Ohmic electrode for n-type β-Ga2O3 substrates.

The ferroelectric properties of bulk BaMgF4 are determined by polarization hysteresis measurements. The spontaneous polarization Ps is found to be 6.6μC∕cm2, while the coercive field Ec varies between 4 and 24kV∕cm, depending exponentially on the frequency. Periodical poling of domains, with a 24μm period, is demonstrated and evidenced by a selective domain etching.