Esmaeil
Heydari
ab,
Isabel
Pastoriza-Santos
c,
Luis M.
Liz-Marzán
def and
Joachim
Stumpe
*bg
aInstitut für Physik und Astronomie, University of Potsdam, 14476 Potsdam, Germany. E-mail: stumpe@uni-potsdam.de
bDepartment of Polymers and Optics, Fraunhofer Institute for Applied Polymer Research (IAP), 14476 Potsdam, Germany
cDepartamento de Química Física and CINBIO, Universidade de Vigo, 36310, Vigo, Spain
dDepartamento de Química Física, Universidade de Vigo, 36310 Vigo, Spain
eBioNanoPlasmonics Laboratory, CIC biomaGUNE, 20014 San Sebastian, Spain
fIkerbasque, Basque Foundation for Science, 48013 Bilbao, Spain
gFaculty of Science, University of Potsdam, 14476 Potsdam, Germany
First published on 9th June 2017
We demonstrate plasmonically nano-engineered coherent random lasing and stimulated emission enhancement in a hybrid gain medium of organic semiconductors doped with core–shell plasmonic nanoparticles. The gain medium is composed of a 300 ± 2 nm thin waveguide of an organic semiconductor, doped with 53 nm gold nanoparticle cores, isolated within silica shells. Upon loading the nanoparticles, the threshold of amplified spontaneous emission is reduced from 1.75 μJ cm−2 × 102 for an undoped gain medium, to 0.35 μJ cm−2 × 102 for a highly concentrated gain medium, and lasing spikes narrower than 0.1 nm are obtained. Most importantly, selection of silica shells with thicknesses of 10, 17 and 21 nm enables engineering of the plasmon–exciton energy coupling and consequently tuning of the laser slope efficiency. With this approach, the slope efficiency is increased by two times by decreasing the silica shell from 21 nm down to 10 nm, due to the enhancement of the localized electric field.
Conceptual insightsA simple approach for engineering laser performance in the nanoscale is essential for further development of efficient nanoplasmonic lasers. Core–shell plasmonic-dielectric nanoparticles with their unique optical characteristics hold a huge potential for fabrication of such lasers. Plasmonic nanoparticles focus electromagnetic fields in a nanoscale volume, by coupling excitation light to the vibration of free electrons in the nanoparticles, which can be used for efficient pumping of the gain medium. Silica shells enable the distribution of nanoparticles in the gain medium and control the energy transfer between the gain medium and plasmonic sites, by changing the shell thickness. Here we report on nano-engineering the performance of random lasers formed in organic semiconductor thin films doped with Au@SiO2 core–shell nanoparticles, by modifying their slope efficiency and threshold. We demonstrate that, by applying different thicknesses of silica shell, it is possible to tune the excitation rate of the gain medium. This is important for further development of nanoplasmonic random lasers based on organic semiconductors. |
Fig. 1a schematically shows a plasmonically nano-engineered random laser formed in a thin film waveguide of the F8BT/MEH-PPV polymer. Au@SiO2 NPs with an average Au core size of 53 nm and different thicknesses of silica shell are embedded in a 300 ± 2 nm thin film of F8BT/MEH-PPV. The selection of particle size was based on the degree of overlap between the LSPR of Au NPs and both the excitation and emission spectra of F8BT/MEH-PPV, as well as a trade-off between having a large scattering cross-section and preserving the film forming properties. A 532 nm laser beam with a pulse length of 300 ps and repetition frequency of 10 Hz was employed as the pump source. The pump beam impinged onto the random laser with an incident angle (∼30 degrees) to facilitate the actual test. The intensity of the laser beam could be continuously tuned using a variable optical attenuator. The emission was collected perpendicular to the random laser surface by a lens and focused onto an optical fiber that was coupled to the spectrometer. Fig. 1b presents the excitation, emission and ASE spectra of the F8BT/MEH-PPV semiconductor polymer, and the LSPR spectrum of Au@SiO2 NPs. In this hybrid plasmon–exciton system, in order to satisfy the condition of plasmonic emission enhancement, the NPs are designed in such a way that their LSPR spectrum (purple line) overlaps with both the excitation (blue line) and emission (orange line) spectra of the gain medium. ASE (red line) was captured when the nanoplasmonic random laser was excited resonantly (green line) with 0.75 μJ cm−2 × 102 laser pulses. In this case, the pump wavelength lies within the excitation spectrum of the F8BT/MEH-PPV layer, and the LSPR band of the NPs. The full width at half maximum (FWHM) of the ASE is 2.5 nm for this excitation energy, and it appeared close to the shoulder of the F8BT/MEH-PPV emission, far from the excitation spectrum.
Fig. 2a–c show TEM images of the Au@SiO2 core–shell NPs with 53 nm Au cores and SiO2 shells with thicknesses of 10, 17, and 21 nm, which were used in our experiments. Hereafter they will be referred to as Au@SiO2 (53:
10), Au@SiO2 (53
:
17), and Au@SiO2 (53
:
21).
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Fig. 2 TEM images of Au@SiO2 core–shell NPs with 53 nm Au cores coated with 10 nm (a), 17 nm (b) and 21 nm (c) average silica shells. |
Fig. 3a presents the emission of the F8BT/MEH-PPV thin films for different concentrations of Au@SiO2 (53:
10) core–shell NPs, based on excitation pump pulse energies. For a thin waveguide without NPs (blue color), the emission increased at a low rate by increasing the pump pulse energy, until it reached the threshold value of 1.75 μJ cm−2 × 102, and from there the ASE increased with a higher rate. The appearance of ASE is due to the waveguide confinement in a high refractive index polymer thin film. As shown in Fig. 3a, addition of Au@SiO2 NPs decreases the threshold while increasing the slope efficiency of ASE. For instance, by adding 10 μL of 8 mM Au@SiO2 (53
:
10) NPs (black dots), the threshold reduced down to 0.65 μJ cm−2 × 102, and the slope efficiency increased. However, for intense pumping above 2 μJ cm−2 × 102, the slope efficiency was changed due to the reduction of the refractive index and formation of triplet energy transfer.21 This was accompanied by an increase in the FWHM and a blue shift of the emission. The threshold and slope efficiency are a function of the number of particles, thus the addition of NPs up to 80 μL reduced the threshold down to 0.35 μJ cm−2 × 102 and increased the slope efficiency, but no sharp spikes were observed. Quantum yield (QY) measurements revealed that it increased with the number of particles. For 80 μL, the QY reached the value of 0.128, from 0.106 for the undoped case. Fig. 3b shows the ratio of emission intensity for Au@SiO2 NP-doped thin films as compared to the undoped thin film, based on pump pulse energies, regardless of the nature of emission. For 10 μL of Au@SiO2 NPs, this ratio was initially increased by increasing the pump pulse energy, and reached a maximum of 50 times at 1.25 μJ cm−2 × 102. This enhancement was expected because, according to Fig. 3a, for the undoped thin film the threshold is around 1.75 μJ cm−2 × 102 and the emission intensity increases at a low rate, while for the thin film doped with 10 μL of NPs the threshold is around 0.65 μJ cm−2 × 102 and the intensity grows at a higher rate. By passing 1.25 μJ cm−2 × 102 the ratio starts to decrease at a low rate until reaching 1.75 μJ cm−2 × 102, the threshold for the undoped thin film. For pump pulse energies beyond 1.75 μJ cm−2 × 102 the enhancement ratio decreases at a higher rate, revealing different dynamics induced by triplet energy transfer. Higher ratios were achieved for higher concentrations of the doping NPs. For ratios under 1.25 μJ cm−2 × 102, the enhancement rate was increased by increasing the concentration, while for ratios beyond 1.25 μJ cm−2 × 102 the reduction rate was increased. The maximum enhancement, as large as 200 times, was observed for the thin film doped with 80 μL of NPs. In this experiment, Au@SiO2 NPs, as nanoscale antennas, strongly scatter the emission of the gain medium and consequently enhance the ASE because of their role in increasing the optical path and confinement of emitted light by multiple scattering and reflection in the thin film waveguide.
By increasing the concentration of Au@SiO2 NPs, they eventually shaped a feedback mechanism due to plasmonic scattering and confinement, which is further supported by total internal reflection.
Fig. 4a demonstrates typical coherent nanoplasmonic random lasing (red line) when 200 μL of Au@SiO2 (53:
10) core–shell NPs was loaded in the semiconductor polymer matrix, as compared with the output signal for excitation below the lasing threshold (black line). Multiple lasing spikes with FWHM less than 0.1 nm, associated with different modes of the random laser, were shaped as a result of the interference of waves in the organic semiconductor thin film. Fig. 4b illustrates the surface profile of the plasmonic random laser, each large peak representing a single NP. Based on these peaks, the average distance was calculated to be shorter than 280 nm. The corresponding characteristics diagram (red line) is shown in Fig. 4c. In this figure, the laser slope efficiency of the plasmonically nano-engineered random laser, using 53 nm Au NP cores coated with 10 nm silica shell (red), is compared with slope efficiencies of Au NPs coated with 17 nm (orange) and 21 nm (black) silica shells, in order to demonstrate that it is possible to tune the energy coupling between the plasmonic nano-antennas and excitons in the organic semiconductor thin film. The slope efficiency was increased by two times, by reducing the silica shell thickness from 21 nm to 10 nm, while there is a minor change in the threshold. Therefore, despite the earlier investigations in a bulk of dye solution,22 it was observed that in a thin film of organic semiconductor the silica shell dominantly influences the stimulated emission by increasing the slope efficiency, rather than reducing the lasing threshold. Here, the enhanced electric field in the vicinity of NPs can increase the excitation and/or radiative decay rate of the gain medium, however, the QY values show a minor contribution of the radiative decay rate in the enhanced emission. FDTD simulations were carried out for Au@SiO2 (53
:
10) and Au@SiO2 (53
:
21), to obtain a better insight into the distribution of the electric field in these resonantly coupled nano-engineered systems.
Nanoplasmonic random lasers can be engineered to work efficiently if the core–shell system is designed to exploit the enhanced and nanoscale electric field. Comparison between Fig. 5a and b shows that for NPs with 10 nm silica shell thickness, a stronger electric field is available outside the silica shell. The excitation rate of the gain medium comprising Au@SiO2 NPs doped in an organic semiconductor polymer matrix is a function of the distance between the NPs and semiconductor polymer, and it decreases by increasing this distance. In fact, the maximum value of the electric field is located on the surface of the NPs, however, because of the high absorption loss due to direct electron transfer, in this case, it quenches the emission of the semiconductor polymer molecules.23,24 Since the change in the QY is not significant, the enhancement of the excitation rate25 in the close vicinity of the NPs is dominantly responsible for the observed behavior in Fig. 4c through providing the highest available localized electric field for the organic semiconductor molecules. This experiment clearly demonstrates that it is possible to plasmonically nano-engineer the function of random lasers in organic semiconductor thin films with a random distribution of plasmonic NPs and a sufficient overlap of plasmonic and organic semiconductor energy levels to form low threshold and efficient random lasers.
This journal is © The Royal Society of Chemistry 2017 |