Recent developments in rare-earth doped materials for optoelectronics

Femtosecond-laser micromachining (also known as inscription or writing) has been developed as one of the most efficient techniques for direct three-dimensional microfabrication of transparent optical materials. In integrated photonics, by using direct writing of femtosecond/ultrafast laser pulses, optical waveguides can be produced in a wide variety of optical materials. With diverse parameters, the formed waveguides may possess different configurations. This paper focuses on crystalline dielectric materials, and is a review of the state-of-the-art in the fabrication, characterization and applications of femtosecond-laser micromachined waveguiding structures in optical crystals and ceramics. A brief outlook is presented by focusing on a few potential spotlights.

Optical waveguides in crystalline dielectric materials produced by femtosecond-laser micromachining

Splitting image: Sustainable hydrogen production is a key target for the development of alternative, future energy systems that will provide a clean and affordable energy supply. This Minireview focuses on the development of semiconductor catalysts that enable hydrogen production via water splitting upon visible-light irradiation.

Sustainable hydrogen production is a key target for the development of alternative, future energy systems that will provide a clean and affordable energy supply. The Sun is a source of silent and precious energy that is distributed fairly all over the Earth daily. However, its tremendous potential as a clean, safe, and economical energy source cannot be exploited unless the energy is accumulated or converted into more useful forms. The conversion of solar energy into hydrogen via the water-splitting process, assisted by photo-semiconductor catalysts, is one of the most promising technologies for the future because large quantities of hydrogen can potentially be generated in a clean and sustainable manner. This Minireview provides an overview of the principles, approaches, and research progress on solar hydrogen production via the water-splitting reaction on photo-semiconductor catalysts. It presents a survey of the advances made over the last decades in the development of catalysts for photochemical water splitting under visible-light irradiation. The Minireview also analyzes the energy requirements and main factors that determine the activity of photocatalysts in the conversion of water into hydrogen and oxygen using sunlight. Remarkable progress has been made since the pioneering work by Fujishima and Honda in 1972, but he development of photocatalysts with improved efficiencies for hydrogen production from water using solar energy still faces major challenges. Research strategies and approaches adopted in the search for active and efficient photocatalysts, for example through new materials and synthesis methods, are presented and analyzed.

Water Splitting on Semiconductor Catalysts under Visible‐Light Irradiation

We report a two-step synthesis of highly luminescent CdS/ZnSe core/shell nanocrystals (emission quantum yields up to 50%) that can produce efficient spatial separation of electrons and holes between the core and the shell (type-II localization regime). Our synthesis involves fabrication of cubic-singony CdS core particles that are subsequently overcoated with a layer of ZnSe in the presence of surfactant-ligands in a noncoordinating solvent. Studies of different growth regime of the ZnSe shell indicate that one approach to obtaining high emission efficiencies is through alloying the CdS/ZnSe interface with CdSe, which leads to the formation of an intermediate ZnCdSe layer with a graded composition. We perform theoretical modeling of these core/shell nanocrystals using effective mass approximation and applying first-order perturbation theory for treating both direct electron-hole coupling and the core/shell interface-polarization effects. Using this model we determine the range of geometrical parameters of the core/shell structures that result in a type-II localization regime. We further applied this model to evaluate the degree of electron-hole spatial separation (quantified in terms of the electron-hole overlap integral) based on measured emission wavelengths. We also discuss the potential applicability of these nanocrystals in lasing technologies and specifically the possibility of single-exciton optical gain in type-II nanostructures.

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Type-II core/shell CdS/ZnSe nanocrystals: synthesis, electronic structures, and spectroscopic properties.

Periodically poled lithium niobate (PPLN) waveguides are a powerful platform for efficient wavelength conversion. Conventional PPLN converters, however, typically require long device lengths and high pump powers due to the limited nonlinear interaction strength. Here we use a nanostructured PPLN waveguide to demonstrate an ultrahigh normalized efficiency of 2600%/W−cm2 for second-harmonic generation of 1.5 μm radiation, more than 20 times higher than that in state-of-the-art diffused waveguides. This is achieved by a combination of sub-wavelength optical confinement and high-fidelity periodic poling at a first-order poling period of 4 μm. Our highly integrated PPLN waveguides are promising for future chip-scale integration of classical and quantum photonic systems.

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Ultrahigh-efficiency wavelength conversion in nanophotonic periodically poled lithium niobate waveguides

We show that proton exchanged Hx/Li1-xNbO3 single-crystalline solid solutions exhibit very complex structural chemistry, which is different from those known for powders. Seven crystallographic phases have been identified in Hx/Li1-xNbO3 layers. Correlation between the crystal structure and the ordinary and extraordinary refractive indices has been experimentally determined which allows us to explain some of the observed optical phenomena and to predict the characteristics of the great variety of proton exchanged waveguides.

Structural phase diagram of H x /Li 1-x NbO 3 waveguides: The correlation between optical and structural properties

The reverse proton-exchange (RPE) process performed in different HxLi1-xNbO3 crystalline phases of proton-exchanged and annealed proton-exchanged LiNbO3 waveguides leads to remarkable optical and compositional modifications. Analysis of ordinary and extraordinary index profiles, correlated with concentration depth profiles in RPE samples subjected to different exchange and postexchange treatments, shows how buried-index optical waveguides in LiNbO3 with symmetric mode and reduced fiber–waveguide coupling losses can be realized.

Reverse proton exchange for buried waveguides in LiNbO 3

Proton-exchanged LiNbO3 optical waveguides (HxLi1−xNbO3) exhibit very complex structural behavior which is different from that known for their powders. Seven crystallographic phases have been identified in the HxLi1−xNbO3 layers. A method to determine the proton concentration in different phases has been proposed. Correlation between the crystal structure, hydrogen concentration, and the ordinary and extraordinary refractive indices has been experimentally determined allowing one to explain some optical phenomena observed and to predict the characteristics of a great variety of proton exchanged waveguides.

Relationship between refractive indices and hydrogen concentration in proton exchanged LiNbO3 waveguides

An efficient method, the high-temperature proton exchange (HTPE), to fabricate high-quality LiNbO/sub 3/ optical waveguides is studied. The new proton exchange source, the stearic acid diluted by lithium stearate, is proposed for HTPE process. The known soft proton exchange (SPE) process can be realized by HTPE. There are no phase transitions when the /spl alpha/-phase waveguides are fabricated by SPE. This phase presents the same crystalline structure as that of LiNbO/sub 3/ and, as expected, maintains the excellent nonlinear and electrooptical properties of the bulk material. The kinetics of HTPE is studied.

LiNbO/sub 3/ optical waveguide fabrication by high-temperature proton exchange

We show how the phase diagram of the HxLi1−xTaO3 waveguiding layer on top of a crystal of a given orientation allows one to understand, and thus to control, the proton-exchange process in lithium tantalate. Different ways of producing high-quality waveguides are identified.

V. A. Fedorov

Papers

Structural phase diagram of H x /Li 1-x NbO 3 waveguides: The correlation between optical and structural properties

Reverse proton exchange for buried waveguides in LiNbO 3

Relationship between refractive indices and hydrogen concentration in proton exchanged LiNbO3 waveguides

LiNbO/sub 3/ optical waveguide fabrication by high-temperature proton exchange

Control of proton exchange for LiTaO3 waveguides and the crystal structure of HxLi1 - xTaO3.