Confining energy migration in upconversion nanoparticles towards deep ultraviolet lasing

Manipulating particle size is a powerful means of creating unprecedented optical properties in metals and semiconductors. Here we report an insulator system composed of NaYbF4:Tm in which size effect can be harnessed to enhance multiphoton upconversion. Our mechanistic investigations suggest that the phenomenon stems from spatial confinement of energy migration in nanosized structures. We show that confining energy migration constitutes a general and versatile strategy to manipulating multiphoton upconversion, demonstrating an efficient five-photon upconversion emission of Tm3+ in a stoichiometric Yb lattice without suffering from concentration quenching. The high emission intensity is unambiguously substantiated by realizing room-temperature lasing emission at around 311 nm after 980-nm pumping, recording an optical gain two orders of magnitude larger than that of a conventional Yb/Tm-based system operating at 650 nm. Our findings thus highlight the viability of realizing diode-pumped lasing in deep ultraviolet regime for various practical applications.

: A comparison of luminescence lifetimes of (a) 7 F 5/2 state of Yb 3+ and (b) 1 G 4 state of Tm 3+ in the NaYF 4 @NaYbF 4 :Tm(1%)@NaYF 4 core-shell-shell nanoparticles as a function of the inner shell thickness. The decay curves of localized Tm 3+ transition ( 1 G 4 → 3 H 6 ) show marginally dependence on the shell thickness, implying a similar defect density in these nanoparticles. The marked increase in Yb 3+ lifetime with decreasing inner shell thickness is thus attributed to the spatial confinement of energy migration that suppresses energy loss to the host lattice.    Tm/Gd(1/y%)@NaYF 4 core-shell-shell nanoparticles as a function of Gd 3+ content in the inner shell layer. Emission intensity at 311 nm increased with increasing dopant concentration of Gd 3+ from 0 to 30 mol%. When dopant concentration of Gd 3+ was further increased, the emission intensity at 311 nm declined due to the drop in overall emission intensity ascribed to low Yb 3+ concentrations. The spectra were recorded on water dispersions of corresponding nanoparticles (0.03 M) by excitation with a 980-nm CW laser diode at a power density of 20 W cm -2 .
Supplementary Figure 13: Upconversion emission intensity at 311 nm of the NaYF 4 @ NaYbF 4 :Tm/Gd (1/30 mol%)NaYF 4 core-shell-shell nanoparticles as a function of excitation power for different excitation schemes (i.e.; 1-to 5-pulse trains). We generate a train of pulses to excite the upconversion nanoparticles because we want to 1) obtain high peak excitation power, 2) avoid thermal and catastrophic optical damage, and 3) maximize excitation efficiency especially for the 5-photon upconversion process. The results show that 5-pulse excitation can significantly improve the upconversion emission intensity of the nanoparticles. We therefore used the 5-pulse train excitation scheme throughout the laser experiment. General procedure for the synthesis of the core-shell-shell nanoparticles: The procedure is identical to the synthesis of core-shell nanoparticles, except that the as-synthesized core-shell nanoparticles were used as seeds to mediate the shell growth.
General procedure for the synthesis of cubic phase NaYbF 4 :Tm@NaYF 4 nanoparticles: The procedure is similar to the synthesis of the hexagonal phase counterparts, except that metal trifluoroacetates were used as precursors to thermally decompose in a ternary solvent mixture of where I 0 and I(t) represents the maximum luminescence intensity and luminescence Intensity at time t after cut off the excitation light, respectively. Photographs of luminescent samples were taken with Nikon D90 camera. Unless otherwise stated, all measurements were carried out at room temperature.

Supplementary Note 2: Monte-Carlo modeling of energy migration
The energy migration process is described using the random-walk model. The Monte-Carlo calculation of the random walk was executed using C++ and performed in a simulated 3D coordinate matrix.
In crystalline materials, each atom can be simplified as a  The Monte-Carlo simulation can then be generated using the following criteria, (1) Every excitation energy initiates at lattice point L(t=0)=(0, 0, m), where m is nearest number to (r 1 + r 2 ) / 2 with m = kr oa (k is an positive integer), and r 1 and r 2 are the inner and outer radius of the Yb shell.
(2) The migration proceeds for 5000 steps. For each step of migration, we generate a random number R between 0 and 1 and let 2P o→a +6P 0→b =1, If 0 < R < P o→a , the migration goes to case 1 If P 0→a < R < 2P 0→a , the migration goes to case 2 If 2P 0→a +(i-3)P 0→b < R < 2P 0→a +iP 0→b , the migration goes to case i, where i=3, 4,5,6,7,8. In case of the boundary situation, some direction(s) may not be available, then probabilities of all remaining directions will be normalized accordingly.
(4) To generate sufficient samples, in total 10,000,000 independent energy migration undergo random walk.
In Figure 2c, we plot the probability of finding the excitation energy with |L y |<6 to the y=0 plan for three cases r 1 =13 nm, r 2 =16 nm, 19 nm, and 25 nm. With r 2 increases , the energy migrates to a larger area and the probability of finding the excitation energy in the vicinity of the starting point drops significantly.