Study of the pump dependence of the emission properties of a plasmonic array nanolaser

. Lattice plasmon lasers demonstrated to have many degrees of freedom useful to tailor the lasing properties, as the tuning of the morphological properties of the array or the coupling of proper emitters to band-edge states of the plasmonic crystal. Here, we present the results of the study of the lasing emission properties of a hexagonal array of aluminum nanoparticles as a function of the pumping conditions. We demonstrate how the geometrical and dynamic features of the pumping system have a significant impact on the lasing properties without a ff ecting the temporal coherence of the emission. Moreover, by combining di ff erent pumping systems and studies at di ff erent incidence angles, the relationship between nanoarray, dye and pump has been clarified.


Introduction
Metallic nanoparticles have been the object of intense research in the last two decades thanks to their properties to confine light below the diffraction limit, with strong near-field enhancement effects, which make these systems good candidates for a wide range of applications.However, the high losses induced by the excitation of localized surface plasmons (LSPs) in the nanoparticles can significantly limit their applicability.On the other hand, when the nanoparticles are arranged in ordered arrays, a strong reduction of the losses is obtained via the hybridization of LSP to diffractive modes of the periodic array, generating extended resonant modes, called Surface Lattice Resonances (SLRs).The lasing properties of lattice plasmon lasers are strongly dependent on the SLR structure and can be engineered by tuning the nanoparticle properties such as shape, size, and material, as well as, the lattice parameters (symmetry and periodicity) [1,2].In this work, we studied the effect of the pumping conditions on the lasing properties at Γ point of a plasmonic nanolaser.We demonstrate that by linking the momentum of the incident photon to the structure, we can increase the lasing intensity, while by selectively changing the pump pulse duration the lasing threshold can be decreased.Moreover, we evidenced that although some lasing features are reliant on the pumping mechanism, others, such as coherence, are not.

Experimental
The study has been made on a 5x5 mm 2 hexagonal array of Al tapered nanocones (lattice parameter a 0 = 475nm) coupled with an organic dye (Lumogen Red 305) in a polymeric matrix (PMMA, 3%w/w).Figure 1a displays * e-mail: mirko.trevisani@phd.unipd.it the SEM image in cross-section of the nanoarray.The nanoparticles have a bottom radius of 75 nm, a top radius of 35 nm and height of 150 nm.The dye was chosen due to its high quantum yield in the visible range and its stability in a polymeric film.The band structure has been obtained by angle-resolved spectroscopy, using white light as a source, and collecting the transmitted beam through an optical fiber on a spectrometer (Ocean Optics HR4000).The incident angle was swept in the range (−40 • , 40 • ) to scan the first Brillouin zone, whereas the collection angle was fixed at the position of the lasing emission (0 • , Γ point).The lasing emission properties have been studied by exciting with three different lasers: a 18 ps-pulsed laser, a 10 ns-pulsed laser, both with 10 Hz of repetition rate, and a CW diode laser.The excitation wavelength was set at 532 nm, close to the dye maximum absorption.The emission spectrum was collected at different angles θ respect to the normal to the sample.

Results and discussion
In figure 1b we report the experimental extinction map of the hexagonal array coupled to the emitter, taken in TE polarization and along ΓM direction.Three different situations can be highlighted at the excitation wavelength (532 nm), namely (A) photons coupled to the (±1,±1) mode, (B) photons coupled to the (0,±1) mode, and (C) photons not coupled to the structure.Figure 1c shows the lasing emission spectra, taken with ps excitation above threshold (at a fluence F=1.4mJ/cm 2 ) at the three incidence angles A, B, C. It is evident from the collected spectra that the lasing intensity is strongly dependent on the incident angle.The highest enhancement takes place when the photons are coupled to the structure, in particular when the inplane wave vector of the incident beam perfectly matches the diffractive mode [3].In addition to the conservation of in-plane momentum, other factors should be considered in the coupling mode, such as the polarization state of the optical modes.This explains the differences between A and B positions.The pulse duration effect has been investigated over a broad temporal range by using the three lasers: ps-pulsed, ns-pulsed and continuous wave.Comparing the dye lifetime, which is of the order of few ns, the pumping system can be classified as: faster, comparable and slow system.The response of the sample is strongly affected by the excitation system: as figure 2 shows, the collected spectrum at Γ point drastically changes according to the pulse duration.At the picosecond and nanosecond timescale, the spectrum evolution as a function of the photon fluence shows the emergence of a narrow peak above a critical threshold, combined with the line width reduction, confirming the lasing action.Whereas, for longer pulse duration, i.e.CW laser, the stimulated emission is completely suppressed and only directional fluorescence occurs.These different behaviors are mainly due to the competitive process of non-radiative intersystem crossing among the dye molecules [4] that can avoid lasing emission in the case of slow system and can increase the lasing threshold in the case of ns excitation.The temporal coherence properties of the lasing emission have been studied with both ns and ps excitation, by using a modified Michelson interferometer.Above the threshold, interference images were taken at different positions of one of the two mirrors in the interferometer.The visibility was calculated for each image and the temporal coherence was extracted.In both the excitation (ps ad ns pulsed laser) we obtained a coherence length of the orders of mm.This result proves that temporal coherence does not depend on the pumping system and, above threshold the system response is identical.

Conclusion
We have investigated the effect of the pumping conditions on the lasing properties of a plasmonic nanolaser.Depending on the incident angle and the pulse duration, it is possible to couple the excitation beam to the array in a resonant way.This can modify the lasing emission, reducing the threshold and increasing the emitted intensity.The results demonstrate how, in addition to modeling lattice characteristics and selecting an appropriate gain medium, pump parameters are also an important aspect of the design of efficient nanolasers.

Figure 1 .
Figure 1.(a) SEM image in cross-section of the hexagonal array (a 0 = 475 nm).(b) Experimental extinction map of the hexagonal array coupled to the emitter, taken in TE polarization and along ΓM direction.The white line indicates the excitation wavelength (532 nm) and the gray dots are the coordinates of the incident angles investigated.(c) Lasing spectra collected at the Γ point (0 • ) at fluence F = 1.4mJ/cm 2 , TE polarization and three different incident angles (A,B,C coordinates in panel b).

Figure 2 .
Figure 2. Emission spectra collected at the Γ point for excitation with different pump pulse duration.