Lasing from lanthanide-doped multiphoton upconversion nanoparticles

Abstract

Lanthanide-doped multiphoton upconversion nanoparticles, which have the capability to convert near-infrared radiation to ultra-violet luminescence, find enormous applications in photonics such as the fabrication of deep-ultra-violet upconversion lasers. However, due to the low optical gain (< 5 cm⁻¹) of lanthanide-doped upconversion materials, room-temperature multiphoton upconversion lasing was often supported in lanthanide-doped bulk fiber-assisted and/or waveguide lasers. Furthermore, it is still a challenge to realize room-temperature multiphoton upconversion lasers at deep-ultra-violet regime under near-infrared excitation due to the low multiphoton upconversion efficiency of the lanthanide-doped materials. To the best of our knowledge, there is no report on the demonstration of upconversion lasers based on upconversion nanoparticles ranging from visible to deep-ultra-violet regions. In fact, improper design of excitation scheme and laser cavities and the influence of concentration quenching are the barriers restricting the fabrication of room-temperature upconversion lasers. Therefore, the main objectives of this thesis are to investigate 1) the appropriate construction of excitation scheme, 2) the suitable design of laser cavities, and 3) the careful selection of lanthanide-doped upconversion nanoparticles for the realization of upconversion lasers. We noted that the most popular method to pump upconversion materials is the application of continue-wave diode lasers emitted at ~980 nm. By focusing the beam to a small spot, high pumping intensity can be obtained to excite upconversion. However, there are drawbacks of using continue-wave excitation sources: 1) high catastrophic optical damage and 2) thermal effects. Here we proposed and developed three-pulse train excitation scheme using 980 nm nanosecond pulses of pulse-width equal to 6 ns and repetition rate of 10 Hz to pump the upconversion nanoparticles. The advantages of high peak power and relatively short in the duration of high power irradiation can be obtained simultaneously for achieving larger population inversion. Furthermore, the use of short pulses with the pulse-width and delay time shorter than that of carrier decay lifetime of as-prepared NaYF₄:Yb/Er@NaYF₄ nanoparticles can allow releasing higher optical gain to support upconversion lasing emission. It is reported that the realization of multicolor micro-cavity upconversion lasers by using the formation of whispering gallery modes to support optical feedback inside the micro-cavity with bottle-like geometry. Lasing emission of the three color bands was demonstrated simultaneously under 3-pulse excitation. It is verified unambiguously that the NaYF₄:Yb/Er@NaYF₄ core-shell upconversion nanoparticles exhibited high UC efficiency and can be used as the gain medium to achieve lasing emission under 980 nm pulse excitation. The main difficult to achieve deep-ultra-violet upconversion emission under near-infrared excitation is the concentration quenching inside the upconversion nanoparticles. Recent investigation has shown that six-photon upconversion emission at wavelength down to 200 nm has been obtained from lanthanide-doped NaLuF4 microcrystals under near-infrared excitation. However, the corresponding emission intensity is far too low for any practical applications. Here, NaYF₄@NaGdF₄:Yb/Tm@NaYF₄ core(inert)-shell(active)-shell(inert) upconversion nanoparticles were applied to demonstrate high-intensity deep-ultra-violet emission at 311 nm followed the excitation of 976 nm lasers. The upconversion nanoparticles have been carefully design to avoid the problem of concentration quenching via the fine-tuning of exciton migration in nanostructured hosts through the control over the dimension of crystal lattice. The high-intensity emission is proved unambiguously by demonstrating lasing emission at 311 nm under 5-pulse 976 nm excitation at room temperature. A micro-cavity which supports whispering gallery modes has been fabricated by using the upconversion nanoparticles as the gain medium. To the best of our knowledge, this is the first realization of deep-ultra-violet upconversion lasers under near-infrared excitation. Hence, our study will lead to the development of near-infrared diode pumped deep-ultra-violet lasers which can 1) avoid the difficulty of shifting the operating wavelength of GaN based laser diodes below 300 nm, and 2) preserve the inherent advantages of laser diodes (i.e. compact and cost effective) to realize deep-ultra-violet lasers at room temperature for unexplored applications in the fields of information technology, biomedicine, and bio-photonics.Enhancing upconversion efficiency remains a daunting challenge in the research of upconversion nanoparticles. Efforts have been endeavored to overcome this problem including adopting core-shell nano-architectures with an inert/active host shell to minimize non-radiative decay losses on the surface, introducing metal nanostructures to enhance the output intensity due to surface plasmon resonances effects, or increasing the number of sensitizers/activators while alleviating concentration quenching in the upconversion luminescence. However, these approaches do not address the depopulation of the intermediate excited states due to the intrinsic recombination processes. We proposed the use of relatively lossy micro-cavities to enhance upconversion efficiency via lasing action. As expected, intense ultra-violet lasing emission can be obtained by lower-intensity near-infrared pumping of high-Q micro-cavity. It is observed that the required excitation power needed to support intense multiphoton upconversion emission is two orders of magnitude lower than that without the assistant of lasing effect. This is because lasing action clamps the carrier concentration of the intermediate excited states to a threshold value so that low-intensity pumping is sufficient to excite ultra-violet lasing emission from the upconversion nanoparticles. It is believed that this finding may initiate a new tactic to develop effective high-brightness upconversion emission for environmental, medical-life science and industrial applications.