Excited State Absorption Process Research Articles

Linear photophysics, two-photon absorption and femtosecond transient absorption spectroscopy of styryl dye bases

The steady-state and time-resolved linear spectral properties, two-photon absorption spectra and fast relaxation processes in the excited states of styryl base-type derivatives were investigated. The nature of linear absorption, fluorescence and excitation anisotropy spectra were analyzed in solvents of different polarity at room temperature and specific dependence of the solvatochromic behavior on the donor-acceptor strength of the terminal substituents was shown. Two-photon absorption (2PA) efficiency of styryl dye bases was determined in a broad spectral range using two-photon induced fluorescence technique, and cross-sections maxima of ~ 100 GM were found. The excited state absorption (ESA) and fast relaxation processes in the molecular structures were investigated by transient absorption femtosecond pump-probe methodology. The role of twisted intramolecular charge transfer (TICT) effect in the excited state of styryl dye base with dimethylamino substituent was shown. The experimental spectroscopic data were also verified by quantum chemical calculations at the Time Dependent Density Functional Theory level, combined with a polarizable continuum model.

The modulation of upconversion in BaTiO3:Yb3+, Er3+ inverse opal structures

Under 980nm excitation, it is observed that green upconversion emission is suppressed whereas red emission is enhanced when the photonic band gap (PBG) covers the spectral region of green emission in the BaTiO3: Yb3+, Er3+ inverse opal structure. The investigation of the PBG effect on upconversion dynamics reveals that the population of the low level 4I13/2 is increased by both cross relaxation and energy back transfer processes when spontaneous emission of higher levels is suppressed. Then, following energy transfer or excited state absorption processes starting from 4I13/2, the intensity of red emission is enhanced. The ratio of 525nm versus 550nm emission presents the temperature of samples when the PBG complete or absent covers the green emission band. However, the branching ratio deviates the temperature index in condition of a part of overlap between PBG and green emission band.

(Invited) Investigation of Blue Excitation Flux Dependent Conversion Efficiency in Phosphors for High Luminance Applications

Solid State Lighting (SSL) devices based on phosphor conversion have significantly evolved on a scale of output flux and white light luminance and extend now to the range where competition with the brightest arc discharge-based sources is ongoing. Such sources are unique in the SSL world in that they provide both an expanded emission spectrum and high luminance. The efficiency of high luminance semiconductor pumped phosphor sources depends not only on efficient generation of primary radiation from the semiconductor (LEDs or laser diode), but also strongly on the conversion material which is being pushed to optical and thermal limits. Such conversion materials are typically operating under heavy irradiance and thermal load and nearly always exhibit sublinear behavior in output at high input fluxes, due to a multitude of reasons. In the present study, we have separated contributions from thermal and non-thermal quenching mechanisms, in particular investigated phosphor saturation, or excitation quenching, in the converter light output as a function of blue pump intensity. It is important here to distinguish between the true loss mechanisms (for example via Auger-like or excited state absorption processes [1-6]) and the reduction in converted output due to increased transparency for the pump light (e.g. activator ground state depletion). We discuss the behavior of measured conversion efficiency as a function of excitation intensity (and parameters like temperature, activator type and concentration or host material). A model is proposed to explain the observed dependencies in different host-activator systems. [1] W.J. Miniscalco et al, J.Appl.Phys., 49, 6109 (1978)[2] D.S. Hamilton et al, Phys.Rev.B 39(13), 8807 (1989) [3] K. Wisniewski et al, Acta Phys. Polonica A, 95(3), 403 (1999)[4] K.C. Mishra et al, J.Electrochem.Soc. 151(5), H105 (2004)[5] A. Lenef et al, SPIE 884107 (2013) [6] O.B. Shchekin et al, Phys.Status Solidi RPL 1-5 (2016)

Sub-10 ns single-shot dynamic recording in holographic polymeric medium by nonlinear absorption using excited state absorption process

Optical recording by a single shot of an 8 ns laser pulse in a dye-doped holographic polymeric medium was demonstrated with a peak power as low as 71 W at a wavelength of 402 nm. Nonlinear absorption triggered by two-photon absorption and enhanced by the succeeding process allows high-speed recording corresponding to 125 Mbps (3.4 times the recording speed of a Blu-ray Disc). The preformed holographic grating in the medium enhanced the readout signal through diffraction, resulting in a signal-to-noise ratio of about 15 dB. The recording capacity was estimated at ca. 80 GB/side for 100 µm/side by recording 20 layers, but that of over TB/side class would be possible for optical optimization. Theoretically, it can be improved to 10 TB/side for 800 µm/side by recording 400 layers.

Understanding the features in the ultrafast transient absorption spectra of CdSe quantum dots

We describe a model to explain the features of the ultrafast transient absorption (TA) spectra of CdSe core type quantum dots (QDs). The measured TA spectrum consists of contributions by the ground state bleach (GSB), stimulated emission (SE) and excited state absorption (ESA) processes associated with the three lowest energy transition of the QDs. We model the shapes of the GSB, SE and ESA spectral components after fits to the linear absorption. The spectral positions of the ESA components take into account the biexcitonic binding energy. In order to obtain the correct weightage of the GSB, SE and ESA components to the TA spectrum, we enumerate the set of coherence transfer pathways associated with these processes. From our fits of the experimental TA spectra of 65Å diameter QDs, biexcitonic binding energies for the three lowest energy transitions are obtained.

Metal-Organic Frameworks Modulated by Doping Er(3+) for Up-Conversion Luminescence.

Here we present metal-organic frameworks prepared by a one-step synthesis method, possessing both architectural properties of MOF building and up-conversion luminescence of rare earth Er(3+) (hereafter denoted as Up-MOFs). Up-MOFs have characteristic up-conversion emissions at 520, 540, and 651 nm under the excitation of 980 nm owing to the multiple photon absorption. The up-conversion mechanism of these Up-MOFs has been discussed, and it can be attributed to the excited state absorption process. The design and synthesis of Up-MOF materials possessing near-infrared region excitation and up-conversion luminescence are fully expected to be candidates for the advancement of applications in bioimaging, sensors, optoelectronics, and energy conversion/storage devices.

Visible up-conversion and near-infrared luminescence of Er3+/Yb3+ co-doped SbPO4-GeO2 glasses

Recent advances in glass chemistry have led to new multifunctional optical glasses of great technological importance. Glasses containing high amounts of antimony have been studied for use in nonlinear optics, near-infrared transmission, and as hosts for rare-earth ions in photonic devices. This work describes a luminescence study of Er3+ and Er3+/Yb3+ co-doping in a new SbPO4-GeO2 binary glass system. Near-infrared and visible up-conversion emissions were observed in the green and red regions, which are enhanced when the samples are co-doped with Yb3+. Near-infrared emissions have good quantum efficiency and full width half maximum of 61 nm. Visible up-conversion emissions are governed by two photons and described by excited state absorption, energy transfer and cross-relaxation processes.

Two-wavelength two-photon process for optical selection of rare-earth ions

The past decades has witnessed the blossom of upconversion, which is based on the ground state absorption (GSA) and excited state absorption (ESA) processes. Herein, we propose a brief strategy based on two-photon process with two simultaneous excitation wavelength for optical selection of rare earth (RE) ions. RE ion can be transferred to its intermediate state with resonant frequency from the first laser, and then furthered to the target state with the second laser of appropriate frequency, and every ion is separately excited by simultaneous two-wavelength independently. Herein, the resulting 2H9/2 → 4I15/2 transition of Er3+ in NaYF4 microprisms is employed as a monitoring probe, and this two-photon process is demonstrated by the excitation and emission spectra, and further verified by the dependences on excitation power and the luminescence decay curves. This conception will also provide a new strategy on precise optical selection of RE ions, especially for quantum information technology.

Upconversion emissions from high energy levels of Tb^3+ under near-infrared laser excitation at 976 nm

Ultraviolet upconversion emissions from high energy levels (5D1, 5L8, 5L9,5G5, and 5L10) of Tb3+ were observed in the NaYF4:Tb3+, Yb3+ microcrystals under excitation of a 976 nm laser diode at room temperature. These energy levels were populated by energy transfer and excited state absorption processes. Power dependence measurements confirmed it was caused by a three-photon upconversion process. Concentration quenching did not strongly affect the population of these levels.

Luminescence studies and infrared emission of erbium-doped calcium zirconate phosphor.

The near-infrared-to-visible upconversion luminescence behaviour of Er(3+)-doped CaZrO3 phosphor is discussed in this manuscript. The phosphor was prepared by a combustion synthesis technique that is suitable for less-time-taking techniques for nanophosphors. The starting materials used for sample preparation were Ca(NO3)2.4H2O, Zr(NO3)4 and Er(NO3)2, and urea was used as a fuel. The prepared sample was characterized by X-ray diffraction (XRD). The surface morphology of prepared phosphor was determined by field emission gun scanning electron microscopy (FEGSEM). The functional group analysis was determined by Fourier transform infrared (FTIR) spectroscopy. All prepared phosphors with variable Er(3+) concentrations (0.5-2.5 mol%) were studied by photoluminescence analysis. It was found that the excitation spectra of the prepared phosphor showed a sharp excitation peak centred at 980 nm. The emission spectra with variable Er(3+) concentrations showed strong peaks in the 555 nm and 567 nm range, with a dominant peak at 555 nm due to the ((2)H(11/2),(4)S(3/2)) transition and a weaker transition at 567 nm associated with 527 nm. Spectrophotometric determination of the peak was evaluated by the Commission Internationale de I'Eclairage (CIE) method These upconverted emissions were attributed to a two-photon process. The excitation wavelength dependence of the upconverted luminescence, together with its time evolution after infrared pulsed excitation, suggested that energy transfer upconversion processes were responsible for the upconversion luminescence. The upconversion mechanisms were studied in detail through laser power dependence. Excited state absorption and energy transfer processes were discussed as possible upconversion mechanisms. The cross-relaxation process in Er(3+) was also investigated.

Synthesis and up-conversion luminescence properties of Er3+/Yb3+ co-doped Sr2CeO4 phosphors

Er3+/Yb3+ co-doped Sr2CeO4 phosphors were synthesized using a high-energy ball milling method at 1100°C. The X-ray diffraction patterns confirmed their orthorhombic structure. The up-conversion emissions corresponding to 2H11/2→4I15/2, S3/2→4I15/2 and 4F9/2→4I15/2 transitions peaking at 526nm, 552nm and 669nm have been observed under 975nm excitation. With the help of a schematic energy level diagram the up-conversion mechanisms were discussed in detail. The excited state absorption and energy transfer process were discussed as possible up-conversion mechanisms. The energy back transfer (4S3/2+2F7/2→4I13/2+2F5/2) contribute the increased intensity ratio of red to green emission in Sr2CeO4:Er3+/Yb3+ samples. A decay curve analysis for the UC emission corresponding to the 4S3/2→4I15/2 and 4F9/2→4I15/2 transitions in the Er3+/Yb3+ co-doped Sr2CeO4 phosphors was performed. The CIE chromaticity coordinates of the Er3+/Yb3+ co-doped Sr2CeO4 phosphors were located in the green region. The results indicate that the material may be suitable for applications in the fields of color displays and light emitting devices.

Blue and green upconversion emissions of Zr:Nd:LiNbO3 single crystals

Zr:Nd:LiNbO3 crystals codoped with 0.1 mol% of Nd2 O3 and three concentrations of ZrO2 (0, 2 and 4 mol%) were grown by the Czochralski method from the congruent melt. The X-ray diffraction (XRD) patterns, UV–visible absorption and infrared (IR) spectra were measured to analyze the crystal composition and defect structure. The blue and green upconversion emissions of Nd3+ ions under 598 nm excitation were observed. The intensity of upconversion emissions was increased by the introduction of 2 mol% zirconium ion (Zr4+) and decreased by the introduction of 4 mol% Zr4+ ions. The luminescence decay measurement indicated that the 4G7/2 state of Nd3+ ion was mainly populated by excited state absorption process. It was proposed that Nd:LiNbO3 crystals doped with approximately 2 mol% Zr4+ ions could be applied as laser materials at 522/535 nm.

Largely enhanced saturable absorption of a complex of plasmonic and molecular-like au nanocrystals.

A saturable absorber is a nonlinear functional material widely used in laser and photonic nanodevices. Metallic nanostructures have prominent saturable absorption (SA) at the plasmon resonance frequency owing to largely enhanced ground state absorption. However, the SA of plasmonic metal nanostructures is hampered by excited-state absorption processes at very high excitation power, which usually leads to a changeover from SA to reversed SA (SA→RSA). Here, we demonstrate tunable nonlinear absorption behaviours of a nanocomplex of plasmonic and molecular-like Au nanocrystals. The SA→RSA process is efficiently suppressed, and the stepwise SA→SA process is fulfilled owing to energy transfer in the nanocomplex. Our observations offer a strategy for preparation of the saturable absorber complex and have prospective applications in liquid lasers as well as one-photon nonlinear nanodevices.

Spectroscopic properties of tellurite glasses co-doped with Er3+ and Yb3+

Spectroscopic characterization of Er3+/Yb3+ co-doped tellurite glasses 70.8TeO2–5Al2O3–13K2O–(11−x)–BaO–0.2Er2O3–xYb2O3, where x=0, 0.4, 0.8, 1.2 and 2mol% has been carried out through X-ray diffraction, Raman, absorption and luminescence spectra. The Judd–Ofelt intensity parameters were calculated for 0.2mol% Er3+-doped glass and are used to evaluate radiative properties such as transition probabilities, branching ratios and radiative lifetime. The emission cross-section of the 4I13/2→4I15/2 transition has been calculated from the absorption data using McCumber׳s theory. The emission intensity of both, visible and infrared signals as a function of Yb2O3, have been studied under 980nm and 375nm laser excitation. The physical mechanisms responsible for both, visible and infrared signals in the tellurite samples have been explained in terms of the energy transfer and excited state absorption process. The FWHM of the 4I13/2→4I15/2 transition as a function of Yb2O3 mol% and distance (δ) between the laser focusing point and the end-face of the glass has been reported. It was observed both, experimentally and numerically, a change in the FWHM with variations of δ less than 8mm. The latter was attributed to the radiation trapping effect.

Infrared spectroscopy and upconversion luminescence behaviour of erbium doped yttrium (III) oxide phosphor

The paper reports upconversion luminescence behaviour and infra-red spectroscopic pattern of erbium doped yttrium (III) oxide phosphor. Sample was synthesized by solid state reaction method with variable concentration or erbium (0.5–2.5mol%). The conventional solid state method is suitable for large scale production and eco-friendly method. The prepared sample was characterized by X-ray diffraction (XRD) technique. From structural analysis by XRD technique shows cubic structure of prepared sample with variable concentration of erbium and no impurity phase were found when increase the concentration of Er3+. Particle size was calculated by Scherer’s formula and it varies from 67nm to 120nm. The surface morphology of prepared phosphor was determined by field emission gun scanning electron microscopy (FEGSEM) technique. The surface morphology of the sample shows good connectivity with grains as well as some agglomerates formation occurs in sample. The functional group analysis was done by Fourier transform infra-red technique (FTIR) analysis which confirm the formation of Y2O3:Er3+ phosphor was prepared. The results indicated that the Y2O3:Er3+ phosphors might have high upconversion efficiency because of their low vibrational energy. Under 980nm laser excitation sample shows intense green emission at 555nm and orange emission at 590nm wavelength. For green emission transition occurs 2H11/2→4I15/2, 4S3/2→4I15/2 for upconversion emissions. Excited state absorption and energy transfer process were discussed as possible upconversion mechanisms. The near infrared luminescence spectra was recorded. The upconversion luminescence intensity increase with increasing the concentration or erbium up to 2mol% after that luminescence intensity decreases due to concentration quenching occurs. Spectrophotometric determinations of peaks are evaluated by Commission Internationale de I’Eclairage (CIE) technique. From CIE technique the dominant peak of from PL spectra shows intense green emission so the prepared phosphor is may be useful for green light emitting diode (GLED) application.

Spectroscopic properties and energy transfer parameters of Er3+-doped fluorozirconate and oxyfluoroaluminate glasses.

Er3+- doped fluorozirconate (ZrF4-BaF2-YF3-AlF3) and oxyfluoroaluminate glasses are successfully prepared here. These glasses exhibit significant superiority compared with traditional fluorozirconate glass (ZrF4-BaF2-LaF3-AlF3-NaF) because of their higher temperature of glass transition and better resistance to water corrosion. Judd-Ofelt (J-O) intensity parameters are evaluated and used to compute the radiative properties based on the VIS-NIR absorption spectra. Broad emission bands located at 1535 and 2708 nm are observed, and large calculated emission sections are obtained. The intensity of 2708 nm emission closely relates to the phonon energy of host glass. A lower phonon energy leads to a more intensive 2708 nm emission. The energy transfer processes of Er3+ ions are discussed and lifetime of Er3+: 4I13/2 is measured. It is the first time to observe that a longer lifetime of the 4I13/2 level leads to a less intensive 1535 nm emission, because the lifetime is long enough to generate excited state absorption (ESA) and energy transfer (ET) processes. These results indicate that the novel glasses possess better chemical and thermal properties as well as excellent optical properties compared with ZBLAN glass. These Er3+- doped ZBYA and oxyfluoroaluminate glasses have potential applications as laser materials.

Anomalous NIR Luminescence in Mn2+‐Doped Fluoride Perovskite Nanocrystals

An anomalous near‐infrared (NIR) upconversion (UC) emission band at approximately 770 nm is demonstrated in KZnF3:Yb3+,Mn2+ nanocrystals with heavy Mn2+ doping. This band would enable advanced biological imaging with improved resolution and enhanced penetration depth. Careful studies based on structure analysis, excitation and emission spectra, and luminescence decay curves indicate that this unusual NIR emission (770 nm) originates from the 6A1g(S)4T1g(G)→6A1g(S)6A1g(S) transitions of the Mn2+–Mn2+ dimers. The influence of Mn2+ concentration and temperature on the Stokes and UC luminescence properties are also investigated. The proposed mechanism for the observed NIR UC emission involves ground state absorption and excited state absorption processes. The present results not only provide a useful and effective approach to achieving pure NIR UC emission, and also new insights into the development of advanced photonic devices and technologies.

Using pulse transit delay in Z-scan to discriminate between excited-state absorption and other nonlinear processes in ZnO nanocones

We report a new approach that uses the single beam Z-scan technique, to discriminate between excited state absorption (ESA) and two and three photon nonlinear absorption. By measuring the apparent delay or advance of the pulse in reaching the detector, the nonlinear absorption can be unambiguously identified as either instantaneous or transient. The simple method does not require a large range of input fluences or sophisticated pulse-probe experimental apparatus. The technique is easily extended to any absorption process dependent on pulse width and to nonlinear refraction measurements. We demonstrate in particular, that the large nonlinear absorption in ZnO nanocones when exposed to nanosecond 532 nm pulses, is due mostly to ESA, not pure two-photon absorption.

Synthesis and Characterization of Sm$^{3+}$–Yb$^{3+}$ Codoped Y$_{2}$O$_{3}$ Phosphor

The Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> phosphors doped/codoped with Sm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> /Sm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> ions at different concentrations have been prepared using combustion technique. The prepared phosphors are characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. The average crystallite size of the phosphors calculated based on the XRD is observed to increase with annealing. The FTIR analysis suggested that the impurity is reduced on annealing the sample. NIR to visible frequency upconversion emission has been observed from synthesized phosphors upon a 980 nm diode laser excitation. The concentration dependence study shows a tuning in light color emitted from samples. The upconversion mechanism involved in the Sm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> /Sm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> doped/codoped Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> phosphors has been explained on the basis of excited state absorption (ESA) and energy transfer (ET) process.

Resonant excited state absorption and relaxation mechanisms in Tb^3+-doped calcium aluminosilicate glasses: an investigation by thermal mirror spectroscopy

Resonant excited state absorption (ESA) and relaxation processes in Tb(3+)-doped aluminosilicate glasses are quantitatively evaluated. A model describing the excitation steps and upconversion emission is developed and applied to interpret the results from laser-induced surface deformation using thermal mirror spectroscopy. The fluorescence quantum efficiency of level (5)D(4) was found to be close to unity and concentration independent while, for the level (5)D(3), it decreases with Tb(3+) concentration. Emission spectroscopy measurements supported these results. ESA cross sections are found to be more than three orders of magnitude higher than the ground state absorption cross section.