Upconversion Laser Research Articles

Enhanced upconversion in Ho3+-doped transparent glass ceramics containing BaYbF5 nanocrystals

Novel Ho3+-doped transparent oxyfluoride SiO2–Al2O3–Na2CO3–CaO–BaF2–YbF3 glass ceramics (GC) containing BaYbF5 nanocrystals were fabricated via melt-quenching technique with subsequent heat treatment. The formation of crystalline fluoride phase was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Compared to precursor glasses, the greatly enhanced green emission (40-fold), new emission band at ultraviolet-blue region and stark splittings of emission in GC, indicate that Ho3+ enters into BaYbF5 nanocrystals with low phonon energy. Besides, the origin of the previously unconfirmed emission band at 440–460nm is clearly identified by measuring spectra from thermally coupled luminescent levels at various temperatures. The outstanding upconversion properties of Ho3+ in GC may present potential application in all-solid-state upconversion lasers operating in the visible and ultraviolet range.

Continuous modification of upconversion luminescence of fluorescent dye in the crystalline colloidal arrays

The crystalline colloidal arrays with controllable photonic bandgaps were prepared by the change of volume fraction of the polystyrene microspheres. Upconversion emission property of fluorescent dye has investigated in crystalline colloidal array, and continuous modification of the upconversion emission of fluorescent dye was observed. A significant suppression of upconversion emission of dye in the range of the photonic bandgap as well as enhancement at the bandgap edge was obtained in the crystalline colloidal arrays. In addition, upconversion emission of dye was also enhanced when the excited light overlapped with the long or short bandgap edge of the crystalline colloidal arrays, which is due to slow photons effect near the edges of a photonic bandgap. The continuous modification and enhancement of upconversion emission may be important for the development of low-threshold upconversion lasers and displays.

Physical, structural, and luminescence studies of Nd3+ doped MgO-ZnO borate glass

A series of borate glass of the system xNd2O3-5MgO-20ZnO-(75 − x)B2O3, where x = 0.5, 1.0,1.5, 2.0, and 2.5 was successfully fabricated using melt quench method. The properties of the glass were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR), absorption and luminescence spectra. The upconversion properties of Nd3+ doped borate glass were observed by using 574 nm excitation wavelength corresponding to 4I15/2 → 2H114/2 transition. The emission bands centered at 460, 500 and 620 nm which corresponding to the Nd3+ transitions, 4F7/2 → 4I15/2, 2H11/2 → 4I15/2, and 4F9/2 → 4I15/2 respectively were observed at room temperature. The presence of Nd3+ in borate based glass could intensify the upconversion luminescence spectra as it can potentially be used as host materials for upconversion lasers.

Blue upconversion laser based on thulium-doped silica m icrocavity

Whispering gallery mode optical resonant cavities fabricated from rare-earth-doped silica glasses have demonstrated lasing from the visible through the near-IR. However, achieving lasing in the blue has been elusive. In this Letter, thulium-doped silica films are synthesized and used to fabricate toroidal optical microcavities with quality factors in excess of 10 million. Despite the high phonon energy of silica, the high circulating optical intensities present in the microcavities enable upconversion of the thulium, resulting in emission in the blue and near-IR with microwatt threshold powers that scale linearly with the concentration of the thulium.

Er3+/Tm3+ codoped tellurite glass for blue upconversion—Structure, thermal stability and spectroscopic properties

The Er3+/Tm3+ codoped WO3 modified tellurite glasses with molar composition of (75−x)TeO2–20ZnO–5Na2O–xWO3–0.2wt%Er2O3–0.8wt%Tm2O3 (x=0,4,8,12mol%) were prepared to realize the blue upconversion emission. The absorption spectra, visible upconversion spectra and 1.53µm band fluorescence spectra of glass samples were measured, together with the quantitative calculations of relevant spectroscopic parameters of Tm3+ and energy transfer micro-parameters between Er3+ and Tm3+ ions, to evaluate the effects of WO3 oxide on the spectroscopic properties of Er3+–Tm3+ ions. It was found that the introduction of WO3 oxide can further improve the blue upconversion emission of Tm3+ through an enhanced phonon-assisted energy transfer process. Meanwhile, with the increase of WO3 amount, the thermal stability of glass hosts, characterized by the difference between the glass crystallization onset temperature and the transition temperature which obtained from the measured differential scanning calorimeter (DSC) curves, increased and no obvious crystallization peak was detected when the WO3 amount increased to 12mol%. Furthermore, the glass structure was briefly analyzed with the obtained Judd–Ofelt intensity parameters, the measured Raman spectra and X-ray diffraction (XRD) curves. The present results demonstrate the feasibility of realizing strong blue upconversion emission by introducing WO3 oxide with high phonon energy into the Er3+/Tm3+ codoped tellurite glass and also indicate its potential application as a glass host applied for the blue upconversion lasers.

Up-Conversion and Self-Stimulated Raman Laser in a Codoped Microsphere

In this letter, Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -Eu <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> codoped sol-gel films are applied to the surface of silica microspheres. Under the excitation with 976 nm laser, we observe the luminescence of Eu <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> with relatively low pump power. The Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> → Eu <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> energy transfer process and the pump power dependence are studied. Because of high energy density of the Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -Eu <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> codoped microsphere (YECM), the threshold power for the downconversion (Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> : <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5/2</sub> → <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7/2</sub> ) laser is as low as 8.99 μW. In addition, a self-stimulated cascaded Raman laser (SSCRL) is observed, which is stimulated by the down-conversion laser of the YECM.

Resonant Raman scattering in GaN single crystals and GaN-based heterostructures: feasibility for laser cooling

The recent progress on Raman scattering in GaN single crystals and GaN/AlN heterostructures is reviewed. Anti-Stokes Raman scattering is used to determine electron-phonon scattering time and decay time constant for longitudinal-optical phonons. In a typical high electron mobility transistor based on GaN/AlN heterostructures, strong resonances are reached for the first-order and second-order Raman scattering processes. Therefore, both Stokes and anti-Stokes Raman intensities are dramatically enhanced. The feasibility for laser cooling of a nitride structure is studied. A further optimization will enable us to reach the threshold for laser cooling. Raman scattering have potential applications in up-conversion lasers and laser cooling of nitride ultrafast electronic and optoelectronic devices.

Synthesis of hollow La2O3:Yb3+/Er3+/Tm3+ microspheres with tunable up-conversion luminescence properties

Hollow La2O3:Ln3+ (Ln3+ = Yb3+, Er3+, Tm3+) microspheres were successfully synthesized via a facile sacrificial template method by employing PS spheres as hard template followed by a subsequent heating process. The structure, morphology, and formation process are well investigated by various techniques. A systematic study on the up-conversion (UC) PL properties of La2O3:Yb3+/Er3+/Tm3+ products have been performed in detail. Under 980 nm laser diode excitation, La2O3:Yb3+/Tm3+, La2O3:Er3+ and La2O3:Yb3+/Er3+ products are mainly dominated by blue, green, and red light emissions, respectively. By increasing Er3+ concentration, the La2O3:Er3+ gives yellow emission. Furthermore, the UC white light was successfully obtained in the La2O3:Yb3+/Er3+/Tm3+ system by adjusting the relative doping concentration of Yb3+, Er3+ and Tm3+. The obtained UC white light can be tuned at a wide range, and the optimal white-emitting CIE (Commission internationale de l'eclairage [International Commission on Illumination]) chromaticity coordinate is (x = 0.3250, y = 0.3180), which is very close to the standard equal energy white light (x = 0.33, y = 0.33). The strategies for color tuning of up-conversion emission proposed in the current system may be helpful for other ones to achieve efficient multicolor (including white light) luminescence under 980 nm laser excitation. Because of abundant luminescent colors from RGB to white in La2O3:Yb3+/Er3+/Tm3+ samples under 980 nm laser diode (LD) excitation, they can potentially be used as fluorophores in the field of color displays, back light, UC lasers, and photonics.

Low threshold melt-processed two-photon organic surface emitting upconversion lasers

Herein, a low threshold, wavelength-tunable, compact, two-photon pumped upconversion laser is presented. The surface emitting lasers are composed of melt-processed 1,4-bis[2-[4-[N,N-di(p-totyl)amino]phenyl]vinyl]benzene (DADSB) as active media and two designed distributed bragg reflectors. The melting fabrication process is very simple, and the lasing threshold is as low as 150μJcm−2pulse−1, when pumped by a Ti:sapphire amplifier operating at 800nm with a 150fs pulse width. To the best of our knowledge, it is one of the lowest values for two-photon lasers. Lasing from multimode to single-mode oscillation is demonstrated. Tunable single mode oscillation was obtained at wavelength from 514nm to 523nm with a spectral width of less than 0.2nm.

A Green Upconversion Laser with Erbium-Doped LiLuF&lt;SUB&gt;4&lt;/SUB&gt; Crystal by 976 nm Fiber Laser Pump

A stable fiber laser operating at appro ximately 976 n m peak power at a 280 ns and 1.9 W pump was used as a pumping source for an upconversion laser based on an Er 3+ :doped LiLuF4 crystal. A 548 n m wavelength and 320 mW could be achieved. In this study, we used a 976 n m lo w threshold average pump power fiber laser at 8 mW, wh ich can achieve green upconversion laser output at room temperature. Th is system includes a high reflective coating at 552 n m+/-10 n m on the LiLuF4 crystal and a 96% reflective mirror forming a laser cavity fo r the 548 n m laser. Using the laser cavity setup and 976 n m fiber laser pu mp, lo w threshold pump power green erbiu m upconversion lasing was achieved. The two peak wavelengths of 548 n m and 537 n m are observed and the intensity changes by changing the input to 976 n m pu mp power.

ZnAl2O4 co-doped with Yb3+/Er3+ prepared by combustion reaction: evaluation of photophysical properties

Zinc aluminate co-doped with Yb3+/Er3+ for potential application in upconversion lasers was prepared in proportions of 2:1, 5:1, and 10:1 by combustion reaction. The samples were characterized by X-ray diffraction, transmission electron microscopy, and optical spectroscopy. The results reveal the formation of crystalline ZnAl2O4 primary phase and trace amounts of ZnO and Yb3Al5O12 secondary phases. They also indicate that increasing the Yb3+/Er3+ ratio favored the increase of secondary phases. The optical spectroscopy analysis revealed that red emission predominated over green, and that emission intensity was directly influenced by the infrared intensity of the diode laser. These results indicate that ZnAl2O4 doped with rare earth ions may also be an interesting material for luminescence obtained by energy upconversion.

Structural characteristics and optical properties of plasma assisted reactive magnetron sputtered dielectric thin films for planar waveguiding applications

Thin films of aluminum oxide (Al2O3), tantalum pentoxide (Ta2O5), titanium oxide (TiO2), yttrium oxide (Y2O3) and zirconium oxide (ZrO2) were deposited by plasma assisted reactive dual magnetron sputtering to determine their suitability as a host for a rare earth doped planar waveguide upconversion laser. The effect of deposition parameters such as cathode, plasma power and oxygen gas flows were studied and the operational working points were determined. Both power and lambda control were used to optimize the optical quality of each material. By using lambda control feedback system, the magnetron power fluctuates to sustain a fixed oxygen flow in the target area reducing the compound layer growth on the material and maintaining a healthy deposition rate. The optical properties, structure and crystalline phase of each film were found to be dependent on the process parameters. X-ray diffraction (XRD) analysis revealed that the thin films varied from amorphous to highly crystalline depending on the deposition conditions. X-ray photoelectron spectroscopy (XPS) was utilized for surface compositional analysis revealing that films had varying stoichiometric ratios which are controlled for each material by the deposition parameters chosen. The waveguide loss for the thin film layers was investigated and Ta2O5 was shown to have a slab waveguide loss of ~1dB/cm at both visible and infra-red wavelengths making it ideal for planar waveguide and laser applications. TiO2, Y2O3 and ZrO2 were found to deposit in a highly crystalline phase. Waveguiding in the TiO2 layers was not possible at 633nm or in the infrared region. The Y2O3 samples gave low loss (2–4dB/cm) at the 1.3 and 1.5μm wavelengths but no waveguiding at 633nm or 833nm was possible. Atomic force microscopy showed rough surface topography for TiO2, Y2O3 and ZrO2 akin to their crystalline growth with the SEM images confirming the regular crystalline columnar structure for the case of Y2O3 and ZrO2.

Macromol. Rapid Commun. 6‐7/2012

Front Cover: Two-photon absorption (TPA) is of interest for optical power limiting, data storage, two-photon microscopy, microfabrication, frequency up-conversion lasing, and many other applications. Highly efficient inorganic TPA molecules are now reported. Ruthenium alkynyl den-drimers up to second generation in size have been prepared by a divergent route and exploiting Sonogashira coupling. These compounds display an NLO dendrimer effect – their cubic nonlinearities and two-photon absorption cross-sections increase nonlinearly on increasing dendrimer generation. Further details can be found in the article by K. A. Green, P. V. Simpson, T. C. Corkery, M. P. Cifuentes, M. Samoc, and M. G. Humphrey* on page 573.

Frequency up-converted lasing in polymeric composites with two-photon absorbing antenna

Energy-transfer-coupled polymeric composites with donors of two-absorbing dyes and acceptors of polymer gain medium are introduced for up-converted laser applications. The two-photon pumped hybrid polymer lasers show significant performance improvement with nearly 10 times reduction of lasing threshold and over 100 times extension of lifespan.

A Model for Enhanced Up-Conversion Luminescence in Erbium-Doped Tellurite Glass Containing Silver Nanoparticles

Nanoparticles (NPs) size dependent enhancement of the infrared-to-visible frequency up-conversion (UC) and absorption coefficient in silver NPs embedded Er3+ doped tellurite glasses on pumping with the 976 nm radiation are investigated. Rate equations are derived by developing a comprehensive 4-level model in integrating the effects of quantum confinement and local field of silver NPs. Considering the spherical NPs size distribution as Gaussian, an analytical expression for the luminescence intensity and absorption coefficient are obtained for the first time. An enhancement in UC emission intensity of the green bands (2H11/2→4I15/2 and 4S3/2→4I15/2) and red band (4F9/2→4I15/2) emission of Er3+ ion at temperature 250 K and at optimized Er3+ concentration 1.0 mol% is observed up to few times in the presence of silver NPs. Furthermore, the green emission shows larger enhancement than the red emission. The observed of Er3+ luminescence is mainly attributed to the local field effects namely the surface plasmon resonance of silver NPs that causes an intensified electromagnetic field around NPs, resulting in enhanced optical transitions of Er3+ ions in the vicinity. The model is quite general and can be applied to other rare earth doped glasses containing metallic NPs. Our results on NPs size dependent emission intensity and absorption coefficient are in conformity with other findings. The present systematic study provides useful information for further development of UC lasers and sensors.

Modification of the upconversion spontaneous emission in photonic crystals

Influence of competition between excited-state absorption and spontaneous emission from intermediate excited state on upconversion emission was investigated firstly in photonic crystals. The result clearly shows that upconversion emission can be modulated by the photonic crystal. When upconversion emission band from intermediate excited state overlaps with the photonic band gap, short wavelength unconversion emission from upper state is obviously enhanced due to an inhibition of spontaneous emission from the intermediate excited state in the photonic crystal. We believe that the present work will be valuable for not only the foundational study of upconversion emission modification but also new optical devices in upconversion displays and short wavelength upconversion lasers.

From anti‐Stokes photoluminescence to resonant Raman scattering in GaN single crystals and GaN‐based heterostructures

AbstractThe progress on anti‐Stokes photoluminescence and Stokes and anti‐Stokes Raman scattering in GaN single crystals and GaN/AlN heterostructures is reviewed. Anti‐Stokes photoluminescence investigated in the past was primarily attributed to two‐photon absorption, three‐photon absorption, and phonon‐assisted absorption. On the other hand, anti‐Stokes Raman scattering was used to determine electron‐phonon scattering time and decay time constant for longitudinal‐optical phonons. In a typical high electron mobility transistor based on GaN/AlN heterostructures, strong resonances were reached for first‐order and second‐order Raman scattering processes. Therefore, both Stokes and anti‐Stokes Raman intensities were dramatically enhanced. The feasibility of laser cooling of a nitride structure has been demonstrated. Anti‐Stokes photoluminescence and Raman scattering have potential applications in upconversion lasers and laser cooling of nitride ultrafast electronic and optoelectronic devices.

Color tunability of intense upconversion emission from Er3+–Yb3+ co-doped SiO2–Ta2O5 glass ceramic planar waveguides

This work reports on the infrared-to-visible CW frequency upconversion from planar waveguides based on Er3+–Yb3+-doped 100–xSiO2–xTa2O5 obtained by a sol–gel process and deposited onto a SiO2–Si substrate by dip-coating. Surface morphology and optical parameters of the planar waveguides were analyzed by atomic force microscopy and the m-line technique. The influence of the composition on the electronic properties of the glass-ceramic films was followed by the band gap ranging from 4.35 to 4.51 eV upon modification of the Ta2O5 content. Intense green and red emissions were detected from the upconversion process for all the samples after excitation at 980 nm. The relative intensities of the emission bands around 550 nm and 665 nm, assigned to the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2 transitions, depended on the tantalum oxide content and the power of the laser source at 980 nm. The upconversion dynamics were investigated as a function of the Ta2O5 content and the number of photons involved in each emission process. Based on the upconversion emission spectra and 1931CIE chromaticity diagram, it is shown that color can be tailored by composition and pump power. The glass ceramic films are attractive materials for application in upconversion lasers and near infrared-to-visible upconverters in solar cells.

Energy transfer and photoluminescence modification in Yb–Er–Tm triply doped Y2Ti2O7 upconversion inverse opal

In this article, we fabricated Y2Ti2O7: Yb, Er, Tm upconversion inverse opal photonic crystals with energy transfer between Er3+ and Tm3+, and investigated the influence of the photonic band gap on the energy transfer between Tm3+ and Er3+. It is interesting that strong modification of the steady state upconversion emission spectra is observed, and the green or red upconversion emission from Er3+ was suppressed in the inverse opal. More significantly, the energy transfer between Tm3+ and Er3+ is enhanced by suppression of the red upconversion emission of Er3+, thus the blue upconversion emission from Tm3+ is considerably improved in the inverse opals. Additionally, the mechanisms for upconversion emission of the Y2Ti2O7: Yb, Er, Tm inverse opal are discussed. We believe that the present work will be valuable for the foundational study of upconversion emission modification and the application of upconversion displays and short wavelength upconversion lasers.

Two-Photon Upconversion Laser (Scanning and Wide-Field) Microscopy Using Ln3+-Doped NaYF4 Upconverting Nanocrystals: A Critical Evaluation of their Performance and Potential in Bioimaging

We report a simple, yet effective method to disperse NaYF4 nanocrystals (NCs) doped with luminescent Ln3+ ions in water and physiological buffers using an amphiphilic polymer poly(ethylene glycol) monooleate. These water-dispersible NCs were used for in vivo imaging by employing two-photon upconversion laser scanning microscopy (TPULSM) and two-photon upconversion wide field microscopy (TPUWFM) techniques. Using the 800 nm upconverted emission from Tm3+ ions, we show that (i) TPULSM imaging can be performed up to a depth of ∼600 μm inside an agar-milk gel tissue phantom and (ii) the edges of the object can still be identified. At depths beyond 600 μm, we observed a drastic decrease in the lateral resolution. Images of a mouse lung tissue obtained using this technique resulted in a lateral resolution with which we could observe the capillaries surrounding the alveoli air caps. The images lacked optical sectioning due to the high power density (∼2000 W/cm2) necessary to achieve an adequate signal-to-noise ratio. In addition, the time taken to obtain these images was prolonged because of the slow scanning speed necessitated by the long lifetimes and the poor quantum yield of the upconversion process. Conversely, in vivo TPUWFM imaging using the same 800 nm emission of brain blood vessels of a mouse after skull thinning gave excellent lateral resolution to differentiate blood vessels separated by a few micrometers. In addition to this, optical sectioning was observed over a depth of 100 μm, which is the first instance of optical sectioning shown in in vivo imaging employing Ln3+-doped NCs as imaging agents. Experiments with the aforementioned tissue phantom showed that imaging up to a depth of ∼400 μm could be obtained with the 800 nm emission from Tm3+/Yb3+ codoped NaYF4 NCs with a lateral resolution that allows us to distinguish micrometer-sized biological structures. In contrast, when employing the green upconverted emission from Er3+/Yb3+ codoped NaYF4 NCs, lateral resolution was completely lost at a depth of ∼300 μm.