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Rare-earth ion-doped upconversion (UC) micro/nanoparticles (UCM/NPs) could enable the conversion of near-infrared photons to high-energy emissions in the visible and ultraviolet regions. This unique property makes UCM/NPs highly attractive for applications in diverse fields, including materials science, photophysics, and biomedicine. However, ensemble spectroscopy ignores the heterogeneity in terms of size, shape, defects, surface groups, and charges of single UCM/NPs. Recently, the rapid development of single-particle spectroscopic techniques results in a gradual shift in the investigation of UC materials to heterogeneous nanoscale structures, unique photophysical phenomena, and advanced applications of single UCM/NPs, and the optical performance of a single UCM/NP has become a central topic in the community of UC materials. In this Perspective paper, we outline the characterization methods of a single UCM/NP and provide an overview of recent and on-going progress in investigations on single UCM/NPs, with a focus on their spectroscopic properties and applications in the polarization, waveguide, micro/nano-laser, super-resolution nanoscopy, and nanobarcode. Finally, current challenges and perspectives in this field are highlighted for future research. [ABSTRACT FROM AUTHOR]

Enhanced broadband near-infrared luminescence has been observed in Bi-doped oxyfluoride glasses excited from UV to near-infrared regions with the addition of AgCl. Enhancement factors depend greatly on excitation wavelength and maximal enhancement factor over three times occurs at the excitation wavelength around 320, 640, and 800 nm. Ag species play dual functions. The mechanism of the enhancement is discussed in depth combing the energy transfer from Ag+, molecular-like, nonplasmonic Ag species, Bi3+ and Bi2+ to near-infrared bismuth active centers, and the redox reaction of Bi species with Ag species. These results offer a valuable way to enhance the near-infrared luminescence efficiency of Bi-doped glasses, and the dual functions of Ag species may also be employed to enhance luminescence of rare-earth and transition metal ions doped materials. [ABSTRACT FROM AUTHOR]

Here, we present three facile approaches to achieve wavelength tunable luminescence in the same host material with single dopant, i.e., by modulating doping level, preparation temperature, and atmosphere. Based on these methods, ultra-broadband tunable near-infrared luminescence with the largest full width at half maximum of about 500 nm covering the whole windows of optical communication has been obtained in bismuth-doped germanate glasses. Wavelength tunable luminescence is also observed with the change of excitation wavelength. Systematical strategy was followed to approach the physical origin of the near-infrared luminescence and we proposed that three different bismuth active centers contribute to the near-infrared luminescence in the germanate glasses. A comprehensive explanation for the tunable luminescence is given, combining the concentration, energy transfer, and chemical equilibrium of these active centers in the glasses. With the increase of melting temperatures and the increase of reducing extent of the preparation atmosphere, bismuth species transform from Bi3+ to Bi2+, Bi+, Bi0 and bismuth clusters, and then to bismuth colloid. Of particular interest is that red tunable luminescence was also observed by modulating doping level, preparation atmosphere, and excitation wavelength. Besides, the trapped-electron centers in germanate glasses can interact with bismuth species of high valence states leading to the formation of bismuth active centers of low valence states and the decrease of trapped-electron centers. This tunable ultra-broadband luminescence is helpful for a better understanding of the origin of the near-infrared luminescence in Bi-doped glasses and may have potential applications in varieties of optical devices. [ABSTRACT FROM AUTHOR]

In this paper, we theoretically show that the field-free molecular alignment can be controlled by shaping the femtosecond laser pulse with a periodic phase step modulation, involving the maximum degree and temporal structure of the molecular alignment. We show that the molecular alignment can be completely suppressed or reconstructed as that by the transform-limited laser pulse, the temporal structure of the alignment transient can be controlled with a desired shape, and the molecular alignment and antialignment for any temporal structure can be switched. Furthermore, we also show that both the degree and direction of the molecular alignment at a fix time delay can be continuously modulated. [ABSTRACT FROM AUTHOR]

This work discusses the response of Na to both high-energy electrons and femtosecond-laser (fs-laser) pulses in the soda-lime glass. The evidence for different responses of Na to high-energy electron irradiation between glasses with and without fs-laser irradiation suggests that the chemical and/or physical states of Na in the fs-laser irradiated glass are different from those in the original glass. Fs-laser pulses in the glass may be able to neutralize Na, which may form clusters. These results suggest that close attention should be paid to the defects associated with Na when optical or physical data are interpreted in fs-laser irradiated Na glasses. [ABSTRACT FROM AUTHOR]

Tm3+ doped and Tm3+/Yb3+ codoped lanthanum aluminum germanate (LAG) glasses are prepared by melt-quenching method and characterized optically. Based on the measurement of absorption spectrum, Judd–Ofelt intensity parameters (Ω2,Ω4,Ω6) are calculated. The radiation emission rates, branching ratios, and lifetimes of Tm3+ are calculated to evaluate the spectroscopic properties of Tm3+ in LAG glass. The infrared emission properties of the samples are investigated and the results show that the 1.8 μm emission can be greatly enhanced by adding proper amount of Yb3+ under the excitation of 980 nm. The energy transfer processes of Yb3+–Yb3+ and Yb3+–Tm3+ are analyzed, and the results show that Yb3+ ions can transfer their energy to Tm3+ ions with high efficiency and large energy transfer coefficient. [ABSTRACT FROM AUTHOR]

Ho3+/Yb3+ codoped lanthanum aluminum germanate glasses are prepared simply by melt-quenching method. The 2 μm emission characteristic and energy transfer from Yb3+ to Ho3+ upon excitation of a conventional 980 nm laser diode is investigated and the energy transfer efficiency is as high as 83%. The spectroscopic parameters are calculated based on Judd–Ofelt (J-O) theory, the intensity parameters Ω2, Ω4, and Ω6 are 4.44, 1.92, and 1.11×10-20 cm2, respectively. The transition probabilities and branching ratios are also estimated by using the J-O parameters. Beer–Lambert and Fuchtbauer–Ladenburg theories are applied to calculate the absorption, emission, and gain cross sections of 5I7→5I8 transition. [ABSTRACT FROM AUTHOR]

In this paper, we report on spectroscopic studies of Cr3+/Ni2+ codoped transparent glass ceramics containing β-Ga2O3 nanocrystals. Energy transfer between Cr3+ and Ni2+ in the Cr3+/Ni2+ codoped glass ceramics has been investigated by steady state emission spectra, time-resolved emission spectra, and excitation spectra. The nonradiative energy transfer has been confirmed from the decrease in the Cr3+ fluorescence lifetime after Ni2+ codoping. The energy transfer efficiency is estimated and the energy transfer mechanism is discussed. [ABSTRACT FROM AUTHOR]

We demonstrated the conversion of near-ultraviolet radiation of 250–350 nm into visible emission of 450–600 nm and near-infrared emission of 970–1100 nm in the Yb2O3 doped transparent 40SrO-20TiO2-40SiO2 glasses. The observed broad visible emission band centered at 510 nm is associated with silicon-oxygen-related defects in the glassy matrix, and the near-infrared emission originated from the Yb3+ 2F5/2→2F7/2 transition is due to the energy transfer from silicon-oxygen-related defects to Yb3+. The energy transfer process was studied by both the steady state spectra and the time-resolved spectra of Yb3+ at 15 K. The temperature dependent energy transfer rate was calculated. The Yb2O3 concentration dependent energy transfer efficiency has also been evaluated, and the maximum value is 56% for 12 mol % Yb2O3 doped glass. [ABSTRACT FROM AUTHOR]

Bi, Ga, and Al codoped germanium glass was prepared and its optical properties were investigated by absorption, photoluminescence excitation (PLE), and photoluminescence spectra. Two active centers which occupy strong and weak crystal field environment are identified by using the PLE spectrum. The tunable and ultrabroadband luminescence properties are originated from electron transitions of these two active centers. Internal optical gain around 1300 and 1560 nm has been detected. The wavelength-dependent internal gains excited with 808 and 980 nm laser diodes show different characteristics, and the relative flat optical amplification can be realized by choosing 980 nm pumping. [ABSTRACT FROM AUTHOR]

Transparent Ni2+-doped MgO–Al2O3–TiO2–SiO2 glass ceramics were prepared, and the optical properties of Ni2+-doped glass ceramics were investigated. Broadband emission centered at 1320 nm was observed by 980 nm excitation. The longer wavelength luminescence compared with Ni2+-doped Li2O–Ga2O3–SiO2 glass ceramics is ascribed to the low crystal field hold by Ni2+ in MgO–Al2O3–TiO2–SiO2 glass ceramics. The change in optical signals at the telecommunication bands with or without 980 nm excitation was also measured when the seed beam passes through the bulk gain host. [ABSTRACT FROM AUTHOR]

All-optical control of the noncentrosymmetry of azodye-doped polymer films was demonstrated by nonresonant dual-frequency coherent excitation. Three kinds of noncentrosymmetries of the polymer films were optically tailored using appropriate combinations of the writing beam polarizations, linearly polarized, and circularly polarized light. The tensor analysis of the photoinduced noncentrosymmetry was performed by monitoring second harmonic generation (SHG) of the samples. The quadratic dependence of SHG on film thickness was observed, indicating that a χ[sup (2)] grating satisfying the phase-matching condition for SHG was optically induced in the polymer films and that this nonresonant all-optical poling can be used to tailor the symmetry of bulk samples. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Phase-matched second-harmonic generation (SHG) in azodye-doped polymethylmethacrylate films was demonstrated using the nonresonant optical poling technique. During the seeding process, samples were irradiated simultaneously by coherent superposition of the fundamental and the second-harmonic light of a femtosecond laser. Optical poling experiments were performed at three wavelengths of 1500, 1320, and 1300 nm, respectively. The quadratic dependence of SHG on the film thickness showed that a χ[sup (2)] grating that satisfied the phase-matching condition for SHG was optically induced in the polymer films. The SHG conversion efficiency of a 105 μm thick film was estimated to be about 2%. A relaxation retardation effect of the photoinduced χ[sup (2)] was also observed in the thick films. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Deals with a study which examined wavelength dispersions in the Faraday effect of GaS[sub3/2]-GeS[sub2]-LaS[sub3/2]-based glasses containing various rare-earth ions. Experimental details; Results and discussion; Conclusion.

Eu2+-doped high silica glass (HSG) is fabricated by sintering porous glass which is impregnated with europium ions. Eu2+-doped HSG is revealed to yield intense blue emission excited by ultraviolet (UV) light and near-infrared femtosecond laser. The emission profile obtained by UV excitation can be well traced by near-infrared femtosecond laser. The upconversion emission excited by 800 nm femtosecond laser is considered to be related to a two-photon absorption process from the relationship between the integrated intensity and the pump power. A tentative scheme of upconverted blue emission from Eu2+-doped HSG was also proposed. The HSG materials presented herein are expected to find applications in high density optical storage and three-dimensional color displays. [ABSTRACT FROM AUTHOR]

We report the self-formation of quasiperiodic void structure with the length of several hundred micrometers inside the CaF2 crystal. The quasiperiodical voids along the propagation direction of the laser beam were formed spontaneously after the irradiation of a single femtosecond laser beam which was focused at a fixed point inside the crystal sample. The length of the void array varied with the focal depth beneath the sample surface. The possible mechanism of the self-formed void structure was discussed. [ABSTRACT FROM AUTHOR]

We theoretically and experimentally demonstrate the quantum coherent control of the resonance-mediated two-photon absorption in rare-earth ions by the phase-shaped femtosecond laser pulse. Our theoretical results show that the resonance-mediated two-photon absorption can be effectively controlled, but the control efficiency depends on the laser repetition rate in real experiment due to the long lifetime and the short decoherence time of the excited state, and the larger laser repetition rate yields the lower control efficiency. These theoretical results are experimentally confirmed in glass sample doped with Er3+ by utilizing the femtosecond lasers with low repetition rate of 1 kHz and high repetition rate of 80 MHz. [ABSTRACT FROM AUTHOR]

Nonlinear optical responses of bismuth (Bi) doped silicon-rich silicon dioxide (Si-rich SiO2) films were studied by a z-scan and an optical Kerr gate method under femtosecond excitation around 800 nm. It was found that the Bi-doping enhances the nonlinear optical response of Si-rich SiO2 films by several orders of magnitudes. The nonlinear refractive index was of the order of 10-11 cm2/W and the response time was shorter than our time resolution of 100 fs. The nonlinear refractive index was independent of the wavelength in the range from 750 to 835 nm, suggesting that virtual transitions are involved in the nonlinear optical processes. [ABSTRACT FROM AUTHOR]

Quantum cutting (QC) is a promising approach for enhancing the energy conversion efficiency of solar cells since it allows for conversion of one ultraviolet photon from the sun into two near-infrared photons with energy comparable to the band gap of silicon solar cells. We find that QC can occur by cooperative energy transfer from Eu2+ to Yb3+ in soda-lime-silicate glasses subjected to thermal reduction around the glass transition temperature. Besides the QC effect, the thermal reduction results in improvement of surface performances, e.g., hardness. This synergy effect potentially makes the thermally reduced glass an ideal solar cell substrate material. [ABSTRACT FROM AUTHOR]

In this paper, we theoretically demonstrate that the (2+1+1) resonance enhanced multi-photon ionization photoelectron spectroscopy in sodium atom can be effectively controlled by shaping femtosecond laser pulse with a π phase step modulation in weak laser field, involving its total photoelectron energy, maximal photoelectron intensity, and spectroscopic bandwidth. Our results show that the total photoelectron energy can be suppressed but not enhanced, the maximal photoelectron intensity can be enhanced and also suppressed, and the photoelectron spectroscopy can be tremendously narrowed. These theoretical results can provide a feasible scheme to achieve the high-resolution photoelectron spectroscopy and study the excited state structure in atomic and molecular systems. [ABSTRACT FROM AUTHOR]

Multilayer structures consisting of bismuth (Bi)-doped silica thin films and different kinds of spacer, i.e., Si, silica, Si-rich silica, layers are grown and the luminescence properties are studied. When samples were annealed at a low temperature, Bi-related near infrared active centers (BRACs) were formed at interfaces between Bi-doped silica and Si-rich silica (or silicon) due to the reduction of Bi3+ to BRACs by silicon. On the other hand, films annealed at a high temperature showed similar emission behaviors to bulk glasses. The results demonstrated here establish a new strategy for the control of BRACs and building peculiar Bi activated film structures. [ABSTRACT FROM AUTHOR]

We report on the conversion of near-ultraviolet radiation of 250–350 nm into near-infrared emission of 970–1100 nm in Yb3+-doped transparent glass ceramics containing Ba2TiSi2O8 nanocrystals due to the energy transfer from the silicon–oxygen-related defects to Yb3+ ions. Efficient Yb3+ emission (2F5/2→2F7/2) was detected under the excitation of defects absorption at 314 nm. The occurrence of energy transfer is proven by both steady state and time-resolved emission spectra, respectively, at 15 K. The Yb2O3 concentration dependent energy transfer efficiency has also been evaluated, and the maximum value is 65% for 8 mol % Yb2O3 doped glass ceramic. These materials are promising for the enhancement of photovoltaic conversion efficiency of silicon solar cells via spectra modification. [ABSTRACT FROM AUTHOR]

Spectral properties of Yb3+/Ni2+ codoped transparent silicate glass ceramics containing LiGa5O8 nanocrystals were investigated. The near-infrared emission intensity of Ni2+ was largely increased with Yb3+ codoping due to Yb3+→Ni2+ energy transfer. The qualitative calculation of the energy transfer constant Cs-a and rate Ps-a showed that the Yb3+→Ni2+ energy transfer was much greater than in the opposite direction. Yb3+/Ni2+ codoped glass ceramics with 0.75 mol % Yb2O3 exhibited a near-infrared emission with full width at half maximum of 290 nm and fluorescent lifetime of 920 μs. The glass ceramics are promising for broadband optical amplification. [ABSTRACT FROM AUTHOR]

Bi-doped germanium silicate glass is prepared and their optical properties are investigated. The glass sample shows broadband and flat emission characteristics compared with germanate glass. The single-pass optical amplification was measured on a traditional two-wave mixing configuration. Ultrabroadband optical amplification at 1272 and 1560 nm is observed simultaneously. The highest gain at 1272 nm of germanium silicate glass reaches to 6.73 dB excited with single commercially available 980 nm laser diode. The glass is promising for optical amplification covering almost all the O, E, S, C, and L bands. [ABSTRACT FROM AUTHOR]

The nonlinear optical crystal Ba2TiSi2O8 is induced three-dimensionally inside a glass by a femtosecond pulse laser with 800 nm, 250 kHz, and 150 fs. Micro-Raman and photoluminescence spectra confirm that crystals can grow in the focal point of the laser beam. Continuous crystalline lines were written through moving the focal point of the laser beam inside the glass. The results demonstrate a convenient method to engrave three-dimensional patterns of crystals with nonlinear optical properties for integrated optical devices. [ABSTRACT FROM AUTHOR]

We report on the precipitation control of Au nanoparticles in periodic arrays in silicate glass. Au[sub 2]O-doped glass samples were first irradiated by two 800 nm interfered femtosecond laser pulses at room temperature and then heat treated at 550 °C for the Au nanoparticle precipitation in the laser irradiation areas. One-dimensional periodic arrays of the Au nanoparticles were controlled by changing the pulse energy and the incident angle between the interfered laser pulses. The smallest dimension in the obtained arrays was a width of 300 nm. The mechanism of the metal nanoparticle precipitation by this technique was discussed. Only two pulses are required to encode these periodic microstructures, which are applicable to emerging nanostructure devices such as optical memory with ultrahigh storage density, micrograting with high diffractive efficiency and integrative micro-optical switches. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the precipitation and control of metal nanoparticles inside transparent glasses. An Ag[sup +]-doped silicate glass sample was first irradiated by using an 800 nm femtosecond laser at room temperature and then annealed at 550 °C. The area near the focal point of the laser beam became gray after laser irradiation and yellow after further annealing at 550 °C for 10 min. Absorption and electron spin resonance spectra of the glass sample showed that a portion of silver ions near the focused part of the laser beam inside the glass were reduced to silver atoms after the laser irradiation. These silver atoms aggregated to form nanoparticles after further annealing at temperatures above 500 °C. A mechanism is suggested that consists of multiphoton reduction, which is induced by the fundamental light of the laser beam and supercontinuum white light, and diffusion of silver atoms driven by heat energy to form nanoparticles. The observed phenomenon may have promising applications for the fabrication of three-dimensional multicolored images inside a transparent material and for integrative micro-optical switches. [ABSTRACT FROM AUTHOR]

Long-lasting phosphorescence was observed at 510 nm in a Sn[sup 2+] -Cu[sup 2+] codoped Na[sub 2]O-CaO-SiO[sub 2] glass at room temperature under UV illumination of 254 nm. When the glass was compressed under 3, 6, and 9 GPa, the phosphorescence shifted to 465 nm and its decay rate became shorter. The optical absorption spectra of the samples changed after compression, showing that the cupric ions were reduced to the cuprous ions. The high-pressure treatment also resulted in a lower-energy shift in the absorption edge. It was suggested that Sn[sup 2+] ions act as hole trapping centers, while oxygen vacancies surrounding by Ca[sup 2+] ions as well as active sites in the glass matrix, i.e., as electron trapping centers. [ABSTRACT FROM AUTHOR]

We report the recording, readout, and erasure of a three-dimensional optical memory using the valence-state change of samarium ions to represent a bit. A photoreduction bit of 200 nm diam can be recorded with a femtosecond laser and readout clearly by detecting the fluorescence as a signal (excitation at 488 nm, 0.5 mW Ar[sup +] laser). A photoreduction bit that is stable at room temperature can be erased by photo-oxidation with a cw laser (514.5 nm, 10 mW Ar[sup +] laser). Since photoreduction bits can be spaced 150 nm apart in a layer within glass, a multilayer structure with several hundred layers could be used to record data. A memory capacity of as high as 1 Tbit could thus be achieved in a glass piece with dimensions of 10 mm x 10 mm x 1 mm. [ABSTRACT FROM AUTHOR]

In this letter, we demonstrate rapid decomposition in Ge-doped SiO[sub 2] glass under high-energy electron irradiation, and comment on possible mechanisms for the interaction of the electron-beam with the glass. Nanometer-scale modifications in Ge-doped SiO[sub 2] glasses can be obtained by the redistribution of Ge in the glasses as a result of patterned electron-beam writing. [ABSTRACT FROM AUTHOR]

Permanent holographic gratings were fabricated in bulk azodye-doped polymethylmethacrylate by the coherent field of a femtosecond laser. Optical microscopy and atomic force microscopy observations revealed that the gratings consist of two parts, surface relief gratings on both surfaces and refractive index modulated volume gratings in the interior of the polymers. The diffraction efficiency of the first-order Bragg for the gratings was estimated to be 90%. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the observation of space-selective oxidation of Mn[sup 2+] to Mn[sup 3+] in a transparent and colorless Mn and Fe ions codoped silicate glass at room temperature by using an 800-nm-femtosecond laser. Difference absorption spectrum of the glass after and before the irradiation of the focused infrared femtosecond pulsed laser showed that a portion of Mn[sup 2+] ions near the focused part of the laser beam inside the glass were oxidized to Mn[sup 3+] ions after the laser irradiation. Mn[sup 3+] ions were stable at the temperatures below 300 °C. It is suggested that Mn[sup 2+] ions act as hole trapping centers while Fe[sup 3+] ions as well as active sites in the glass matrix act as electron trapping centers. A promising application was demonstrated for the fabrication of three-dimensional colored image inside a transparent material. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

All-optical control of the noncentrosymmetry of azodye-doped polymer films was demonstrated by nonresonant dual-frequency coherent excitation. The tensor analysis of the photoinduced noncentrosymmetry was performed by monitoring second harmonic generation (SHG) of the samples. The quadratic dependence of SHG on the film thickness showed that a χ[sup (2)] grating satisfying the phase-matching condition for SHG was optically induced in the polymer films, indicating this nonresonant all-optical poling can be used to tailor the symmetry of bulk samples. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this letter, we report precipitation of nanometer scale Zn crystalline particles in Zn borosilicate glass induced by 100 keV electron irradiation. Nanometer Zn crystalline particles precipitate rapidly in the glass under high intensity electron irradiation. A double ionization mechanism is proposed in interpretation of the formation of Zn particles. Thermal effects are less important than ionization interactions. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the observation of memorized polarization-dependent light scattering in a Eu[sup 2+]-doped fluoroaluminate glass sample. Anisotropic light scattering along the plane of the light polarization was observed in the glass sample after the excitation of a focused 800 nm, 150 fs laser beam at a repetition rate of 200 kHz. When we changed the direction of the light polarization and irradiated the same location, we observed an anisotropic light-scattering pattern identical to the original one at the beginning, but then observed a new pattern along the new direction of the light polarization while the original light-scattering pattern disappeared gradually with the passage of time. This observed phenomenon was considered to be due to the light scattering of the polarization-dependent permanent microstructure induced by the polarized ultrashort pulsed laser itself. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on a novel phenomenon in calcium aluminosilicate glasses doped with Ce[sup 3+], Tb[sup 3+], and Pr[sup 3+]. After irradiation by an 800 nm femtosecond pulsed laser, the focused part of the laser in the glasses emits bright and long-lasting phosphorescence able to be clearly seen with the naked eye in the dark even one hour after the removal of the activating laser. Moreover, by selecting appropriate glass compositions and species of rare earth ions, optional three-dimensional image patterns emitting long-lasting phosphorescence in various colors, including blue, green, and red, can be formed within glass samples by moving the focal point of the laser. Based on absorption spectra, the long-lasting phosphorescence is considered to be due to the thermostimulated recombination of holes and electrons at traps induced by the laser irradiation, which leave holes or electrons in a metastable excited state at room temperature. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]