We demonstrate controlled transitions between competing radiative and nonradiative decay channels in the up-conversion luminescence of NaYF4:Er3+/Yb3+ nanocrystals placed in proximity to metallic nanowires. The nanocrystal-nanowire separation is used as a key control parameter governing the optical response. An essential aspect of our approach is the reduction of inhomogeneous residual polymer layers from the nanowires, thereby reducing uncertainties associated with distance control and improving reproducibility in emitter-metal hybrid nanostructures. Replacing them with a well-defined polymer spacer yields controlled access to three qualitatively distinct interaction regimes: luminescence quenching, plasmonic enhancement, and effective decoupling. Transitions between these regimes are shown to reflect changes in the dominant energy relaxation pathways: from nonradiative losses in direct contact with the metal, through modification of the local photonic density of states and coupling to plasmon-mediated modes, to behavior characteristic of quasi-isolated emitters. The plasmonic origin of the emission enhancement in the intermediate regime is evidenced by an increase in luminescence intensity accompanied by shortened decay times, as revealed by fluorescence lifetime imaging microscopy of up-conversion nanocrystals. All experiments were performed on single nanostructures, thereby excluding artefacts arising from aggregation effects and strengthening the interpretation of the observed phenomena. The presented approach provides controlled access to plasmon-modified decay channels and offers a basis for the rational design of functional nanophotonic and sensing structures with tailored optical properties.

In the quest to construct photonic circuits, the key issue is the coupling of light into subwavelength devices. While plasmonic waveguides can be used as building blocks in novel nanodevices, for efficient energy transmission, the distance between the plasmonic nanostructures must not exceed a few nanometers due to the rapidly decreasing coupling. Here, we study a system of two silver nanowires positioned precisely in order to control the separation between their ends. In such a structure, any measurable energy propagation between the nanowires is possible upon direct contact, as when the ends of the nanowires are away, they are also isolated optically from each other. We show that by combining precise positioning of the nanowires with the deposition of aqueous microdroplets, the coupling between two silver nanowires can be restored using colloidal quantum dots (QDs) as an interfacing medium. This effect is observed even if the QD-interfaced nanowires are separated by over a micrometer. The energy transfer between QDs in the microdroplet, as evidenced by shortening of the decay time of the QD fluorescence, is efficient enough to consider extending this method to facilitate and control the coupling between any type of nanostructure in nanophotonic devices.

Blue laser diodes (LDs) have gained significant attention as cost-effective pump sources for efficient visible rareearth (RE) doped fiber lasers, owing to their high power and brightness. In this study, we investigate the potential of blue LDs as pump sources for deep-red holmium-doped fluorozirconate glass (Ho:ZrF4) fiber lasers. We report, for the first time, watt-level, high-efficiency operation of a 75X nm Ho:ZrF4 fiber laser directly pumped by 442 nm LDs. This diode-pumped laser achieved a record output power of 1.04 W, with a maximum slope efficiency of about 53 % for a 25-cm long fiber-corresponding to approximately 90 % of the Stokes efficiency limit. Additionally, we demonstrate the shortest cavity length ever used for a deep-red fiber laser: several centimeters level 75X nm cavity fiber laser pumped by blue LDs. Operating under dual-wavelength pumping at 442 and 449 nm, this laser achieved an output power of 0.15 Wand a slope efficiency of 15 % with respect to the 442 nm pump power. The experimental results are in close agreement with numerical model used to optimize all cavity configurations.

In this work, we present an in-depth analysis of a Ho3+-doped ZrF4 fiber laser operating at 1.2 mu m, pumped using a 640 nm laser diode. First, we present spectroscopic studies on fiber emission under red excitation, discussing and interpreting a series of emission lines attributed to Stokes and anti-Stokes transitions, with the latter involving up-conversion processes. We analyze their impact on the laser's operating characteristics. Next, we report on the laser built from the same fiber, operating at 1.2 mu m with an output power of approximately 60 mW and a slope efficiency slightly above 25%. Further, using experimental data and spectroscopic parameters, we develop a mathematical model that incorporates holmium ion population rate equations alongside pump and signal power propagation equations. The agreement between theoretical predictions and experimental results confirms the model's validity. In the final part of this study, we implement a genetic algorithm to optimize the laser's operating parameters, including pumping efficiency, reabsorption, fiber length, and mirror parameters. In particular, we propose a double-clad fiber structure that significantly reduces excited-state absorption of the pump. Finally, by globally optimizing the laser's operating conditions, we numerically demonstrate that an output power of up to 18.7 W and a slope efficiency exceeding 31% are achievable. The presented results demonstrate a promising semi-empirical approach for optimizing laser performance parameters.

In this work, we present an experimental approach for monitoring the temperature of submicrometric, real-time operating electrical circuits using luminescence thermometry. For this purpose, we utilized lanthanide-doped up-converting nanocrystals as nanoscale temperature probes, which, combined with a highly sensitive confocal photoluminescence microscope, enabled temperature monitoring with spatial resolution limited only by the diffraction of light. To validate our concept, we constructed a simple model of an electrical microcircuit based on a single silver nanowire with a diameter of approximately 100 nm and a length of about 50 µm, whose temperature increase was induced by electric current flow. By driving electric current only along one half of the nanowire, we created a dual-function microstructure, where one section is a resistive heater, while the other operates as a radiator. Such a combination realistically reflects the electronic circuit and its thermal behavior. We demonstrated that nanocrystals distributed around this circuit allow for remote temperature readout and enable precise monitoring of the thermal energy propagation and heat dissipation processes, which are crucial for designing and developing highly integrated electronic on-chip devices.

Photon avalanche (PA) is one of energy up-conversion processes, which is characterized by highly nonlinear relation between the luminescence intensity and the excitation power. Demonstrating PA in nanocrystals paved the way to new potential applications in optoelectronics, sensorics, and imaging. We present the essential aspects of photon avalanching in the context of super-resolution imaging, including the mechanism of improved lateral resolution in raster scanning with a single Gaussian excitation beam. Moreover, we discuss the impact of the luminescence rise dynamics on the imaging based on the photon avalanche effect.

Lanthanide-doped nanoparticles (NPs) exhibit temperature-dependent luminescence, enabling the design of luminescent nanothermometers for industrial and medical applications. This research demonstrates the temperature-sensing properties of NaYF4:7.5%Er3+@NaYF4 and NaErF4@NaYF4 NPs, which have a hexagonal shape and average size of 17 nm. Their core@shell structure is confirmed using high-resolution transmission electron microscopy, and they exhibit intense upconversion (UC) emission under 1532 nm excitation in H2O and D2O colloids. The recorded spectra show Er3+ emission bands with varying intensity ratios depending on the Er3+ concentration, chosen solvent, and temperature. The spectroscopic properties of the studied NPs allow for their excitation and observation of emission within biological windows, which makes them useful for bio-related applications. The emission of prepared NPs is analyzed as a function of temperature from 298 up to 358/363 K in H2O and D2O. The ratios for thermally-coupled levels and non-TCLs and their relative sensitivities are studied. For the high dopant concentration sample in water, the O & horbar;H vibrations and blue shift in the absorption spectrum lead to a record relative sensitivity of 2.50% K-1 (at 363 K) for the 2H11/2/4I11/2 ratio. The use of synthesized NPs for bioimaging under 1550 nm excitation is also demonstrated to observe their accumulation in the guts of Daphnia magna.

Fluorescent photochromic molecules have two photocyclization states and the change between the fluorescent form and non-fluorescent form is reversible and depends on the wavelength of light. By placing these molecules in the vicinity of a silver nanowire, featuring the plasmon resonance, we can control the photoswitching properties of these emitters. In this work, we deposited a submicron droplet of molecules on one end of a nanowire and used focused laser to remotely change the photocyclization state of the molecule via surface plasmon polariton propagation. The effects observed for such a unique system can be applied for quantum optics and sensing.

Photon avalanche is one of the anti-Stokes upconversion processes characterized by highly nonlinear response of the emission intensity to excitation power density changes. By exceeding the critical pumping power threshold, even minute increase of this power results in a steep increase (by 2-3 orders of magnitude) of the emission intensity. While photon avalanche has been observed in bulk materials since 80-ties, it was reported in thulium-doped nanocrystals only recently, enabling to use them for single excitation beam super-resolution imaging. In current work we explore new perspectives for nanoscale avalanche phenomenon by combining the avalanching materials with plasmonically active metallic nanostructures.

For decades, researchers have been interested in developing efficient rare-earth-doped laser sources using dual-wavelength pumping. However, dual-wavelength pumping of rare-earth-doped lasers and amplifiers are currently restricted to pump wavelengths longer than the visible. Extending the use of dual-wavelength pumping for directly diode-pumped rare-earth-doped lasers is of great importance for both academic and practical purposes. We have developed the first blue dual-wavelength pumping of Holmium-doped ZrF 4 -BaF 2 -LaF 3 -AlF 3 -NaF (Ho:ZBLAN) fiber lasers operating at 75X nm. A dual-wavelength pumping method was proposed by pumping two separate wavelengths of 449 and 442 nm into a Ho:ZBLAN fiber to populate the upper laser state $^{5}{\mathrm{F}}_{4} + ^{5}{\mathrm{S}}_{2}$ and depopulate the lower laser state 5 I 7 , respectively. The influence of the 449 and/or 442 nm pumping schemes on the efficiency and threshold of the 75X nm laser was studied by varying the active fiber length. For dual-wavelength pumping with blue laser diodes, a shorter fiber and a lower 449 nm pump power are favoured. Our approach expands the applicability of dual-wavelength pumping to visible laser systems with energy-level structures consisting of long-lived lower-level bottlenecks.

Luminescence thermometry is a powerful technique for monitoring temperature in a sensitive, remote (through light), and minimally invasive manner. Up to now, many macroscopic and microscopic luminescence temperature probes exploiting different temperature sensing schemes have been investigated, with the majority of the studies using aggregates of nanothermometers. This work presents isolated single up-converting NaYF4:Er3+/Yb3+ nanocrystals as functional temperature indicators operating in a standard confocal microscopy configuration. More specifically, the nanocrystals were used to monitor the temperature of a single silver nanowire, whose temperature was controlled electrically via the Joule process. We demonstrate that individual nanocrystals placed near the nanowire can precisely determine the temperature distribution in its surroundings. These results, which combine nanoscopic heat generation with temperature readout using isolated nanocrystals, represent an essential step for the application of isolated single nanoprobes for luminescence thermometry at the nanoscale.

Photon avalanche (PA) is a highly nonlinear mode of upconversion that is characterized by 100–1000‐fold increase in luminescence intensity upon minute increments of pumping power. The practical realization of numerous possible nano‐bio‐technology applications utilizing the PA phenomenon will require information on its susceptibility to the material volume and surface. Here, these parameters are investigated via experimental and theoretical PA. The two‐color, highly nonlinear PA emission at 475 and 800 nm is clearly observed in bulk single crystal, individual microcrystals, and ensembles of colloidal core and core–shell nanoparticles of LiYF 4 host doped with either 3 or 8% of thulium ions. The properties of PA emission, such as PA nonlinearity, PA gain, PA intensity, and luminescence kinetics in these materials show dependence on crystal volume and surface quenching. Theoretical simulations provide understanding of key physical processes that influence PA performance. Moreover, photon avalanche single beam super‐resolution imaging is realized for the first time in 3% Tm 3+ doped LiYF 4 core–shell nanoparticles. The obtained insights and predictions form a solid background for further development and applications of new optimized PA materials.

This work presents an alternative experimental approach to directly measure propagation losses of surface plasmon polaritons (SPPs) in a single silver nanowire (AgNW). The methodology synergistically combines several techniques, including single nanowire optical shaping, accurate triggering of SPPs by upconversion nanocrystals (NCs), and precise deposition of sub-micrometer-size droplets with emitters. Indeed, a femtoliter volume of colloidal NCs deposited at one end of the nanowire and excited with a laser through an objective provides a stable source of polaritons, which propagate in the nanowire. The intensity of radiation released by the SPPs scattered out at the opposite AgNW end is a direct measure of propagation losses. This is enabled through a method of precise laser cutting of the nanowire, where the length of the nanowire can be gradually reduced by an optically controlled melting process. At the same time, both the length and diameter of the nanowire are constantly monitored using optical imaging of the complex interaction between the nanowire and polarized Gaussian beam. The optical cutting technique, implemented in this experiment, shows great potential for advanced and inexpensive shaping of the metallic nanostructures for nanophotonic applications.

Single nanocrystal spectroscopy is employed to demonstrate metal-enhanced optical response of Er 3+ /Yb 3+ doped up-conversion nanocrystals deposited on graphene upon coupling with silver nanowires. Direct interaction between nanocrystals and graphene results in quenching of up-conversion emission and shortening of luminescence decay times, due to the energy transfer to graphene. The amount of the energy absorbed by graphene can be enhanced by coupling Er 3+ /Yb 3+ doped up-conversion nanocrystals with silver nanowires. Microscopy studies with high spatial resolution together with time-resolved analysis of nanocrystal luminescence show increase of the emission rates with fourfold enhancement of the intensity for nanocrystals placed in the vicinity of silver nanowires. This strong enhancement emerges despite simultaneous interaction with graphene. The hybrid nanostructure provides thus a way to combine optical activity of up-conversion nanocrystals and enhancement provided by metallic nanowires with excellent electrical and mechanical properties of graphene.

We observe correlation between the length of surface plasmon polariton propagation in silver nanowires and the enhancement of fluorescence intensity due to coupling with localized plasmon resonances. The results of excitation wavelength-dependent fluorescence imaging of CdTe QDs uniformly deposited over silver nanowires indicate the strongest enhancement of fluorescence intensity for wavelengths close to the maximum of localized plasmon resonance, with modest reduction of this effect when the excitation shifts toward longer wavelengths. In contrast, when one of the ends of a silver nanowire is excited with a focused laser, we find a reverse relation for surface plasmon polariton propagation. In fact, the energy propagates most efficiently for the longest excitation wavelength of 635 nm. This inverse correlation points toward an apparent trade-off between electric field confinement and plasmon propagation distance in silver nanowires, introducing thus a limit for efficient remote fluorescence detection when fluorescence enhancement is also desired.

In this paper, we demonstrate plasmonic substrates prepared on demand, using a straightforward technique, based on laser-induced photochemical reduction of silver compounds on a glass substrate. Importantly, the presented technique does not impose any restrictions regarding the shape and length of the metallic pattern. Plasmonic interactions have been probed using both Stokes and anti-Stokes types of emitters that served as photoluminescence probes. For both cases, we observed a pronounced increase of the photoluminescence intensity for emitters deposited on silver patterns. By studying the absorption and emission dynamics, we identified the mechanisms responsible for emission enhancement and the position of the plasmonic resonance.

Nonlinear optical properties of nanocapsules co-loaded with up-converting NaYF4 NPs and CdSe QD are investigated. Large two-photon absorption and low cytotoxicity determine studied nanocapsules for effective luminescence visualisation applications.

In this paper, we demonstrate remote activation and detection of the 2-photon up-conversion luminescence via surface plasmon polaritons propagating in a long silver nanowire. The hybrid nanostructure was assembled by locally depositing a submicron droplet of nanocrystal-containing colloidal solution on one of the ends of the metallic nanowire. When - using a classic confocal microscope - the second end of the nanowire, without the nanocrystals, is illuminated with infrared laser light, we observe strong emission from the same end. Therefore, it indicates that surface plasmon polaritons activated with infrared light at the second end of the nanowire propagate along it and can excite nanocrystals in the droplet at the opposite end. Subsequently, the excited nanocrystals up-convert the energy and by launching surface plasmon polaritons can guide the up-converted luminescence back to the starting point. The emergence of this effect is much more pronounced for a laser polarized along the nanowire. The spectral and temporal character of this emission reveals strong interactions between surface plasmon polaritons and electronic states of the nanocrystals. The details of local and non-local aspects of the effects of remote excitation and guiding of energy in a silver nanowire are elucidated using a unique experimental setup, based on two microscope objectives for spatial separation and control of both excitation and emission beams.