This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. Article Cite This: ACS Nano 2019, 13, 5686−5692 www.acsnano.org Lasing in Ni Nanodisk Arrays Sara Pourjamal,† Tommi K. Hakala,*,†,‡ Marek Nečada,† Francisco Freire-Fernández,† Mikko Kataja,†,§ Heikki Rekola,⊥ Jani-Petri Martikainen,† Päivi Törmä,† and Sebastiaan van Dijken*,† † Department of Applied Physics, Aalto University School of Science, FI-00076 Aalto, Finland ‡ Institute of Photonics, University of Eastern Finland, FI-80101 Joensuu, Finland § Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus de la UAB, Bellaterra 08193, Catalonia, Spain ⊥ Smart Photonic Materials, Faculty of Engineering and Natural Sciences, Tampere University, FI-33101 Tampere, Finland * S Supporting Information ABSTRACT: We report on lasing at visible wavelengths in arrays of ferromagnetic Ni nanodisks overlaid with an organic gain medium. We demonstrate that by placing an organic gain material within the mode volume of the plasmonic nanoparticles both the radiative and, in particular, the high ohmic losses of Ni nanodisk resonances can be compensated. Under increasing pump fluence, the systems exhibit a transition from lattice-modified sponta- neous emission to lasing, the latter being characterized by highly directional and sub-nanometer line width emission. By breaking the symmetry of the array, we observe tunable multimode lasing at two wavelengths corresponding to the particle periodicity along the two principal directions of the lattice. Our results are relevant for loss-compensated magnetoplasmonic devices and topological photonics. KEYWORDS: plasmonics, nanolasing, Ni nanodisk array, surface lattice resonance, loss-compensated magnetoplasmonics at a wavelength that deviates from the DO.33−37 Despite the P lasmonic resonators and cavities provide small mode volumes and ultrafast light−matter interactions at the nanoscale. Interactions between emitters and plasmonic modes have been studied in both weak and strong coupling regimes.1,2 Theoretical and experimental investigations on plasmonic component, SLRs in arrays of noble metal nanodisks have particularly narrow line widths that can be utilized in lasing13−17,19,20,22 and Bose−Einstein condensa- tion.38 Recently, it was demonstrated that collective SLR lasing in plasmonic systems have demonstrated the feasibility modes can be excited also in arrays of higher-loss of compensating losses typical for metallic nanostructures3−23 ferromagnetic nanoparticles.39,40 and providing ultrafast operation speeds.8,24 Lasing as a loss- compensation mechanism would be particularly attractive for Here, we report on lasing in a lattice of ferromagnetic magnetoplasmonics, which enables active light manipulation at nanodisks overlaid with optically pumped organic Rhodamine the nanoscale via field-controlled breaking of time-reversal 6G (R6G) dye solution. Reduced line widths provided by the symmetry25 and versatile design of magneto-optical re- SLRs together with a carefully optimized lattice geometry and sponses.26 Potential applications of magnetoplasmonic nano- gain medium produce lasing at visible wavelengths, despite the structures include nonreciprocal optical devices,27 label-free broad plasmonic resonances of the individual nanodisks. biosensors,28,29 metrology,30 and ultrafast all-optical magnetic Lasing is characterized by a highly directional and nonlinear switching.31 However, ohmic losses in ferromagnetic metals are increase of sub-nanometer line width emission by more than 2 considerably larger than those in noble metals, and the orders of magnitude. Within the limits set by the gain profile of narrowing of intrinsically broad resonances via a transition to R6G, the lasing wavelength can be tuned by varying the lasing has not been demonstrated yet. particle periodicity. In rectangular arrays, we observe lasing at Periodic arrays of metallic nanoparticles support collective surface lattice resonances (SLRs) that originate from radiative two wavelengths corresponding to λ ≈ n × pi for different coupling of lossy single particle plasmon resonances with low- particle periodicities (px and py) along the two principle axes of loss diffracted orders (DOs) of the lattice.32−37 In optical the lattice. transmission spectra, the DOs of an array appear as maxima at λ ≈ np, where n is the refractive index of the surrounding Received: February 4, 2019 medium and p is the particle periodicity. Hybridized Fano-like Accepted: April 11, 2019 SLR modes reduce the signal, causing minimum transmission Published: April 11, 2019 © 2019 American Chemical Society 5686 DOI: 10.1021/acsnano.9b01006 ACS Nano 2019, 13, 5686−5692 ACS Nano Article Figure 1. (a) Schematic of the measurement setup. Emission spectra are measured by focusing the back focal plane of the objective lens to the entrance slit of the spectrometer. The gain medium is pumped by x-polarized 100 fs laser pulses with a wavelength of 500 nm and a 1 kHz repetition rate. The gain medium is inserted between the substrate with Ni nanodisk arrays and a cover glass. (b) Scanning electron microscopy image of the 380 nm × 380 nm array. In all experiments, the diameter and height of the Ni nanodisks are 60 nm. The scale bar corresponds to 500 nm. (c) Experimental transmission curves for the Ni nanodisk array with px = 380 nm and py = 370 nm. Data for incident polarization along the x and y directions of the array are shown. (d,e) Finite-difference time-domain simulations of near-field distributions in the same array. The simulations are performed for x- and y-polarized plane-wave excitation at the SLR wavelength. RESULTS AND DISCUSSION polarization. Coupling of a narrow DO to a broad localized In our study, we used Ni as a ferromagnetic plasmonic surface plasmon resonance (LSPR) in the Ni nanodisks material. We fabricated various arrays of Ni nanodisks on glass produces a collective SLR mode.39 The SLR wavelength substrates using electron-beam evaporation and lift-off in an (minimum transmission in Figure 1c) corresponds to the electron-beam lithography process. The nominal diameter and wavelength where 1/α − S is 0.34,35 Here, S is the so-called height of the nanodisks were 60 nm, and the size of the total array factor and α is the polarizability of a single Ni nanodisk. array was 300 μm × 300 μm. As a reference, we considered a Damping of the SLR mode depends sensitively on the square array with px = py = 380 nm. In rectangular arrays with imaginary part of 1/α. Consequently, noble metal nanodisks broken symmetry, px was kept constant and py was varied from with large polarizability produce narrow SLR modes with a line 370 to 390 nm in 5 nm steps. The gain medium consisting of width of <10 nm when ordered into periodic arrays.32−37 35 mM R6G in 1:2 dimethyl sulfoxide (DMSO)/benzyl Because of larger ohmic losses in Ni (i.e., small α), the SLRs in alcohol (BA) was inserted between the substrate with Ni our plasmonic arrays are much broader (>100 nm; see Figure nanodisk arrays and a cover glass. 1c). Finite-difference time-domain (FDTD) simulations of the A schematic of the measurement setup including excitation 380 nm × 370 nm Ni nanodisk array at the SLR wavelengths and detection lines is depicted in Figure 1a. The sample was (Figure 1d,e) show intense electric near fields in the particle excited by 100 fs laser pulses with a wavelength of 500 nm at a plane, confirming the plasmonic character of this collective 1 kHz repetition rate and from a 45° angle. The diameter of mode. Next, we demonstrate that despite the large line width the pump spot was approximately 100 μm. Emitted light from of SLR excitations, it is possible to realize lasing in Ni the sample was collected with a 10× 0.3 NA objective lens. nanostructures. The back focal plane of the objective lens was focused to the Figure 2a,b shows the ky and wavelength-resolved emission entrance slit of a spectrometer. The long axis of the slit was from the sample with a 380 nm × 380 nm Ni nanodisk array aligned along the y-axis of the sample. From 2D intensity data for a pump fluence below and above the lasing threshold (Pth ≈ collected by the CCD camera of the spectrometer, the 3.3 mJ cm−2). Below the threshold, the emission consists of wavelength and in-plane ky vector were calculated using ky = two contributions. The first contribution has no angular k0 sin (θ).19 Here k0 = 2π/λ and θ is the angle with respect to dependence and originates from the R6G molecules. Because the sample normal. We note that we did not apply a magnetic the molecules are spatially far away from the nanoparticles field during the lasing experiments. A scanning electron (and outside the SLR mode volume), they do not emit to the microscopy image of the square 380 nm × 380 nm reference SLR mode. Their emission spectrum has a line width of ∼60 array is shown in Figure 1b. nm, which is the same as that for the R6G dye solution in the Figure 1c shows experimental transmission curves of the Ni absence of the nanodisk array. The second emission nanodisk array with px = 380 nm and py = 370 nm for incident contribution follows the ⟨+1,0⟩ and ⟨−1,0⟩ DOs of the array polarization along x and y (see Supporting Information, Figure (crossed dashed lines) and, thus, depicts spontaneous emission S1 for measurements on other arrays). The intensity maxima of the molecules to the SLR modes. As expected from λ = n × are close to the DOs of the lattice. Because the DO wavelength pi and the transmission curves of Figure 1c, the DOs related to depends on the particle periodicity perpendicular to the the periodicity py cross at λ = 578 nm if py = 380 nm and n = polarization axis, the transmission curve for x-polarized light is 1.52. At a higher pump fluence of 1.3Pth, we observe an intense blue-shifted with respect to the spectrum measured with y- single emission peak at λ ≈ 580 nm with a narrow line width of 5687 DOI: 10.1021/acsnano.9b01006 ACS Nano 2019, 13, 5686−5692 ACS Nano Article Figure 2. Angle and wavelength resolved emission of a symmetric 380 nm × 380 nm Ni nanodisk array below (a) and above (b) the lasing threshold. The dashed lines indicate the DOs of the array. Figure 3. (a−e) Angle- and wavelength-resolved emission data for Because the momentum direction ky is monitored here, the ⟨+1,0⟩ samples having px = 380 nm and py ranging from 370 to 390 nm at and ⟨−1,0⟩ DOs related to the periodicity py appear as a cross a pump fluence of 4.6 mJ cm−2. The red ticks label the crossing feature, whereas the one related to px (at 578 nm) has a parabolic wavelength of the ⟨−1,0⟩ and ⟨+1,0⟩ diffracted orders related to shape. The former is sometimes called TE and the latter TM mode the periodicity py of the array. (f−j) Dispersions of respective ideal in the literature.2 (c) Emission intensity at ky = 0 as a function of infinite arrays computed using the T-matrix method for E in-plane pump fluence. (d) Line width (squares) and intensity (circles) of modes. Dispersion bands of the array are characterized by singular the emission peak showing an abrupt nonlinear change of these values of the underlying scattering problem (1) reaching near 0. parameters at a threshold pump fluence Pth ≈ 3.3 mJ cm−2. <1 nm and a small beam divergence of 5.7°. The lasing peak is emission with the reduced particle periodicity along the y slightly red-shifted from the DOs to a wavelength where the direction. The expected 15 nm blue-shift based on Δλ = n × R6G dye solution can emit to the SLR modes of the Ni Δpy supports this argument, as well as the FDTD simulations nanodisk array. The transition from spontaneous emission to of Supporting Information, Figure S3. The emission maximum lasing is manifested as an abrupt change in the emission at 580 nm is associated with the larger periodicity of the spectrum (Figure 2c). Figure 2d summarizes the variation of rectangular Ni nanodisk array along the x direction. the emission intensity and line width with increasing pump In agreement with the dependence of the two lasing peaks fluence. Most notably, we measure a strongly nonlinear on particle periodicity, the lower-wavelength emission increase of the emission intensity from ∼2 × 102 to ∼105 if maximum red shifts when py increases to 375 nm, whereas the pump fluence is enhanced from 3.2 to 3.45 mJ cm−2. the other peak remains fixed at 580 nm (Figure 3b). A further Simultaneously, the line width of the emission peak decreases increase of py to 380 nm results in a square nanodisk lattice, from ∼60 to <1 nm. We note that the lasing peak line width of and consequently, the two emissions merge into one intense our Ni system is similar to those obtained with 2D arrays of lasing peak (Figure 3c). For py > 380 nm, one would expect a Au, Ag, or Al nanoparticles (values range from 0.1 to 1.5 nm in second lasing peak to appear at λ > 580 nm. As can be seen refs 13, 18, 19, 21, and 22). Thus, despite much larger losses in from the emission data in Figure 3d,e, this is not the case. We Ni nanodisk arrays (SLR line width >100 nm compared to 4− explain the absence of the anticipated second peak by a 10 nm in noble metal lattices), intense and narrow line width reduced overlap of the SLR mode with the wavelength- lasing is still attained if enough gain is provided. This finding dependent gain profile of the R6G dye solution. At demonstrates that lasing can be used as a powerful tool to wavelengths corresponding to py > 380 nm, the R6G gain is circumvent losses in magnetoplasmonic systems. insufficient to compensate for the lossy SLR mode of the Ni Next, we break the symmetry of the Ni nanodisk array by nanodisk array, resulting in much weaker spontaneous keeping px constant and increasing py from 370 to 390 nm in 5 emission instead of lasing. For py < 380 nm, the spectrum of nm steps. Figure 3 shows emission spectra of these samples for the R6G gain medium overlaps more with the energy of the py- a pump fluence of 4.6 mJ cm−2. In (a), we observe two related SLR mode, and consequently, multimode lasing is emission maxima, one at the same wavelength, λ ≈ 580 nm, as observed. Two-mode lasing has been observed previously for that for the square array (see Figure 3c) and the other at λ ≈ noble metals.19,21 Moreover, for Al nanoparticles, it has been 565 nm. Threshold behavior and line width imply lasing action shown that highly asymmetric 1D arrays do also lase.22 for both peaks. We note that the maximum at 565 nm is 15 nm To understand the mode properties, we employ a multiple- blue-shifted from the other lasing peak. We associate this scattering T-matrix approach 5688 DOI: 10.1021/acsnano.9b01006 ACS Nano 2019, 13, 5686−5692 ACS Nano Article M (ω , k )a ν (ω , k ) = 0 (1) wavelength due to mode degeneracy. In (d), lasing takes place at the wavelength of the px-related SLR mode only. The lasing where M(ω, k) is a matrix depending on scattering and wave peak in this emission spectrum exhibits both polarizations. propagation properties of the array at a given frequency ω and Finally, in (e), single mode lasing with reduced x-polarization aν(ω, k) is a vector of coefficients describing multipole is measured. nanoparticle excitations of a given mode. The problem (1) has The polarization properties are also studied by the T-matrix a nontrivial solution; that is, a mode exists if the matrix M(ω, method, which can uncover both x- and y-polarized dipolar as k) has a 0 singular value (SV). Therefore, to find modes well as quadrupolar contributions at each wavelength. Figure supported by the array, we scan ω, k space to search for SV 4f−j summarizes the results for the different Ni nanodisk minima of M(ω, k). Due to losses, SV minima are not exactly 0 arrays. In (f), SV minima are calculated for two dipolar modes for real ω, k, but near 0 SVs nevertheless provide valuable corresponding to the experimentally observed wavelengths in information about the dispersion of the array. Figure 3f−j (a) and polarizations at high pump fluence. The same applies illustrates calculated modes with the lowest SVs of M(ω, k), for (g). In (h), the square array exhibits degeneracy of the resembling the experimental data of Figure 3a−e. The dipolar modes. This raises the question whether the Materials and Methods section provides more details on the numerical model. experimental observations in (c) are caused by lasing action Finally, we analyze the polarization properties of the lasing of two perpendicular dipolar modes or a quadrupolar mode. modes in our Ni nanodisk samples with a R6G gain medium. Finally, in (i) and (j), we note that the model predicts a large Experimentally, this was done by placing a polarizer between quadrupolar weight at the measured lasing wavelength in (d) the sample and spectrometer to determine the emission and (e). In (d), the experimental lasing peak exhibits both intensity with polarization along the x and y directions of the polarizations, and hence, the mode is indeed quadrupolar. In arrays. Figure 4a−e presents emission spectra for both contrast, lasing in (e) is almost purely y-polarized, suggesting dipolar mode lasing. To rationalize this, we point out that a priori predictions of preferred lasing modes are difficult because mode dynamics, mode competition at available gain, and mode Q-factors all play a role. Furthermore, we observe a dependence of lasing behavior on pump fluence (see Supporting Information, Figure S4 for a complete set of emission spectra at different pump fluence). A more detailed understanding of mode competition in Ni nanodisk arrays requires further studies. CONCLUSIONS In summary, we demonstrate lasing in a high-loss ferromag- netic plasmonic system. In rectangular Ni nanodisk arrays, we observe multimode lasing and the coexistence of dipolar and quadrupolar modes. The results of this paper can be used for incorporating gain into magnetoplasmonic devices and realiz- ing concepts for topological photonics. Notably, topological lasing has been demonstrated recently.42−44 In topological photonics,45−48 most lattice systems are based on nearest or next-nearest neighbor coupling via overlapping optical near- fields. Our Ni nanodisk arrays represent a radiatively coupled system where long-range couplings produce collective SLR modes. The symmetry properties of the array dictate the existence of energy degenerate modes at high-symmetry points of the Brillouin zone,41 for which the lifting of the degeneracy by a symmetry breaking mechanism can lead to topological Figure 4. (a−e) Emission spectra at ky = 0 for samples having px = features. The magnetic moment of nanodisks in a ferromag- 380 nm and py ranging from 370 to 390 nm. The pump fluence is 4.6 mJ cm−2. (f−j) Calculated singular values for arrays with the netic array can be exploited for time-reversal symmetry same particle periodicities. The blue and orange colors correspond breaking in such lattices. The Ni nanodisk arrays discussed to x- and y-polarized dipolar modes (B′1,2), respectively, and the in this paper show a magnetic circular dichroism effect of up to green color corresponds to a quadrupolar mode (A2′). 0.5% in a saturating magnetic field of 150 mT.49 MATERIALS AND METHODS polarization states at ky = 0 and a pump fluence of 4.6 mJ cm−2. In (a), the low wavelength emission related to py is x- Sample Preparation. We fabricated the samples on glass substrates using electron-beam lithography. After a poly(methyl polarized, whereas the px-related lasing peak at 580 nm exhibits methacrylate) (PMMA) layer was spin-coated and baked at 180 °C both polarizations. We note that the x-polarization of this peak for 1 min, the pattern was defined by exposing the resist layer to the disappears at higher pump fluence (see Supporting Informa- electron beam. We developed the PMMA in a 1:3 methyl isobutyl tion, Figures S4 and S5). We observe similar behavior in (b), ketone/isopropyl alcohol solution. Ni nanodisk arrays were fabricated with a red shift in the lower wavelength lasing peak because of by e-beam evaporation of a 60 nm thick film, followed by lift-off. After larger py. For the square array (c), we observe lasing at a single being patterned, the nanodisk arrays were covered by 2 nm Al2O3 5689 DOI: 10.1021/acsnano.9b01006 ACS Nano 2019, 13, 5686−5692 ACS Nano Article using atomic layer deposition to protect Ni from degrading when At the Γ point (where the band edges responsible for lasing contacted by R6G molecules. typically appear), our system exhibits D2h symmetry (or even D4h for Sample Characterization. We measured optical transmission the square array). The transformation properties of the irreducible curves using a spectrometer with a broad-band supercontinuum laser. representations under symmetry operations, encoded in the character For lasing measurements, the R6G gain medium was inserted between tables, also determine which multipole excitations are allowed in each the glass substrate with Ni nanodisk arrays and a cover glass. The gain of the irreducible representations. In this way, we can determine medium was pumped with a femtosecond laser (45° incident angle, which irreducible representations contain x and y dipolar nanoparticle 500 nm center wavelength, 100 fs pulse width, 1 kHz repetition rate, polarizations or quadrupolar polarizations. x-polarization). Emission spectra were recorded as a function of The T-matrix can be obtained using various numerical methods. wavelength and in-plane ky-vector by focusing the back focal plane of For reliability reasons and due to the fact that the aspect ratio of our a 10× 0.3 NA objective onto the slit of a spectrometer (Figure 1a). nanodisks is close to 1, we used a spherical approximation and The lasing peaks were measured with three settings of the output calculated the T-matrix analytically using the Lorenz−Mie solution, polarization filter (x filter, y filter, no filter). assuming a nanoparticle radius of 42 nm, a homogeneous background Simulations. The dispersion calculations in Figures 3f−j and 4f−j medium with refractive index n = 1.52, and nanoparticles made of Ni were performed using a multiple-scattering T-matrix approach. Here, with interpolated optical properties from Johnson and Christy.52 we briefly introduce the model. A more detailed description can be Electric field distributions in the Ni nanodisk arrays were modeled found in the Supporting Information of ref 41. using the Lumerical FDTD software. The scattering properties of a single nanoparticle are first described in terms of vector spherical wave functions (VSWFs), giving the T- ASSOCIATED CONTENT matrix of the nanoparticle at a given frequency. In the next step, electromagnetic interactions between nanoparticles are calculated as * S Supporting Information translation operators Sn,n′, which describe how VSWFs with one The Supporting Information is available free of charge on the coordinate origin (located inside the nanoparticle n′) can be ACS Publications website at DOI: 10.1021/acsnano.9b01006. decomposed in a VSWF basis at another origin (corresponding to Transmission curves for all Ni nanodisk arrays; FDTD the location of nanoparticle n). This yields a set of linear equations lasing simulations; dependence of emission spectra on describing the scattering properties of a system with many the laser pump fluence; polarization analysis of the lasing nanoparticles, which, in a per-particle matrix form, can be written as peaks (PDF) an − Tn ∑ Sn, n′an′ = Tnpext(n) n ′≠ n (2) AUTHOR INFORMATION Here, the pext(n) column vectors contain the VSWF expansion Corresponding Authors coefficients of the external electromagnetic radiation incident onto *E-mail: tommi.hakala@uef.fi. the nth nanoparticle, and an is the vector of the unknown VSWF *E-mail: sebastiaan.van.dijken@aalto.fi. expansion coefficients of the scattered field created by the nth ORCID nanoparticle. For periodic systems, the field expansion coefficients at different Päivi Törmä: 0000-0003-0979-9894 nanoparticle sites attain the Bloch waveform of ai = eik·R ia , where Ri Sebastiaan van Dijken: 0000-0001-6372-2252 are the lattice vectors and k is the Bloch wave vector. If there is only Notes one nanoparticle per unit cell and the T-matrices are the same for all The authors declare no competing financial interest. particles, eq 2 can be rewritten as ACKNOWLEDGMENTS ai − T ∑ Si , i′ei k·(R i′− R i)ai′ = Tpext(i) i ′≠ i This work was supported by the Academy of Finland under Project Nos. 303351, 307419, 318987, and 316857, by the Labeling W = ∑i′ ≠ iSi, i′eik·(Ri′ − Ri) = ∑i′ ≠ 0S0, i′eik·Ri′, this gives European Research Council (ERC-2013-AdG-340748- (I − TW )a = Tpext (3) CODE), and by the Aalto Centre for Quantum Engineering. Lithography was performed at the Micronova Nanofabrication where the only challenge remains in calculating the infinite lattice Centre, supported by Aalto University. sums contained in the operator W, which in general diverge. One can work around this using Ewald-type summation techniques. 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