1. Introduction

Recently, remarkable advances have been made in the exploiting and demonstrating plasmonic nanodevices such as single-photon emitter based on the conversion of single-plasmon [1], single-photon transistor [2], all-optical modulator and surface plasmon (SP) amplification by stimulated emission of radiation (SPASER) [3, 4], which are all based on the interaction of nanoscale optical emitter and SP of metallic nanostructure and have promising applications in quantum information processing and communication [5–7]. A single plasmon is generated in the exciton-plasmon-photon conversion system by the efficiently coupling of emission from a single quantum dot, it propagates along Ag NW and emits out at the end of NW. Single-photon transistor is also designed based on the strong coupling of quantum emitter and propagating SPs confined in a metallic NW.

Au and Ag nanostructures have large local field enhancement and optical nonlinearities due to the SP resonant absorption [8–14], which could be used to modulate emission properties of the optical emitters [15–18]. Comparing to a single Ag NW, the arrayed Ag NWs with appropriate separations more significantly strengthen the coupling with the optical emitters [19]. The resonant excitation wavelength of Nd³⁺ (⁴F_5/2 states) is around 805 nm, it is far off the resonance of transverse SP (TSP) of Ag NWs, which is helpful to effectively depress the direct excitation of SPs by the excitation laser with a polarization vertical to the long axis of the NWs. The Ag NW arrays act as very efficient nanolens arrays to direct the majorities of the radiative emissions of proximal optical emitters into the SP modes of NWs [20–24]. The SPs are waveguided in subwavelength dimension and propagate along the arrayed NWs, the propagating SPs could be stimulated amplified by the population-inversed optical emitters around the NWs.

2. Exprimental

Ag NWs array in Nd³⁺ doped AAO template (Ag/Nd:AAO) were grown by using electrochemistry deposition. The templates were prepared by using two-step anodization processes. The Silver was deposited in the pores of membranes by dc electrolysis in an electrolyte containing AgNO₃ (0.45 g/10 mL) and H₃BO₃ (0.45 g/10 mL) with Pt counter electrodes.

The scanning electron microscopy (SEM) was performed using a FEG SEM Sirion 200 operated at an accelerating voltage of 10.0 kV. The transmission electron microscopy (TEM) was performed using a JEOL 2010HT operated at 100 kV. The absorption spectra were recorded by UV-VIS-NIR spectrophotometer (Varian Cary 5000). The excitation source for PL was a mode lock Ti:Sapphire picosecond pulsed laser (Mira 900, Coherent) with pulse width ~3 ps and repetition rate 76 MHz. The PL from samples were recorded by spectrometer (SpectraPro 2500i, Acton) with liquid nitrogen cooled CCD (SPEC-10, Princeton).

3. Results and discussion

The SEM image of Nd³⁺ doped AAO template is shown in Fig. 1(a). All the samples used in this study were annealed 4 hrs at 600°C in N₂ atmosphere to improve the photoactivity of Nd³⁺ in AAO. The SEM image of arrayed Ag NWs and TEM image of a single Ag NW are shown in Fig. 1(b) and 1(c), respectively. The Ag NWs have average length ~3µm and diameter ~80 nm with separation ~30 nm. The electron diffraction pattern of a single NW shown in Fig. 1(d) and XRD pattern of the whole sample are attributed to metallic Ag NWs of the annealed sample. Fig. 1(e) shows the absorption spectra of and Ag/Nd:AAO sample and Nd:AAO template, which were recorded by using unpolarized source with normal incident. The absorption bands of Ag/Nd:AAO around 460 nm are caused by the TSP of Ag NW arrays.

 

Fig. 1. Nanostructures and absorption spectra of Ag/Nd:AAO (all the samples in this study were annealed 4 hours in N₂ atmosphere at 600°C). (a) SEM of Nd:AAO template. (b) SEM image of Ag NWs embedded in Nd:AAO. (c) TEM image of a single Ag NWs. The Ag NWs have an average diameter of ~80 nm and a separation of ~30 nm. (d) Electron diffraction pattern of a single Ag NW. (e) The absorption spectra of Ag/Nd:AAO sample and Nd:AAO template. The absorption peak around 460 nm is attributed to TSP of Ag NW arrays.

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The input laser (cw or ps) is normally incident and the PL is recorded by using transmission model as shown in Fig. 2(a). The normal incident laser depressed the direct excitation of LSP of Ag NWs and effectively populated Nd³⁺ in different depth of the sample.

The similar PL spectra of Ag/Nd:AAO sample and Nd:AAO template are shown in Fig. 2(b). In which, Nd³⁺ were resonantly excited to ⁴F_5/2 levels by using Ti:Sapphire laser at wavelength λ _exc=805 nm. The PL peak around 870~880 nm is attributed to the radiative transitions ⁴F_3/2→⁴I_9/2 of Nd³⁺. The excitation wavelength dependence of PL intensity (usually called PLE) of Ag/Nd:AAO is shown in Fig. 2(c). In which, a narrow band pass filter (BPF) is used to record the peak PL intensity at 900 nm. Two peaks around 750 and 805 nm in PLE are corresponding to the metastates ⁴F_7/2 and ⁴F_5/2 of Nd³⁺, respectively.

 

Fig. 2. Excitation and recoding of PL from Ag/Nd:AAO sample and Nd:AAO template. (a) Illustration of excitation and recording of PL in transmittance mode. (b) PL spectra of Nd:AAO template and Ag/Nd:AAO sample with excitation wavelength λ _exc=805 nm (P _exc=5 mW). The PL peak around 870~880 nm is attributed to the radiative transitions ⁴F_5/2→⁴I_3/2 of Nd³⁺. (c) PL excitation (PLE) spectrum of Ag/Nd:AAO recorded at the emission wavelength λ _emi=900 nm (P _exc=60 mW). Two peaks around 750 and 805 nm in PLE are corresponding to the metastates ⁴F_7/2 and ⁴F_5/2 of Nd³⁺.

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The PL spectra of Ag/Nd:AAO recorded with excitation power P _exc=60, 80 and 100 mW are shown in Fig. 3(a). The peak PL intensity around 880 nm super-linearly increased and the spectral width slightly decreased as P _exc increased to 100 mW. The detailed excitation power dependence of PL intensity is surprisingly interesting (see Fig. 3(b)). Linear, sub-linear and super-linear dependences are all exhibited. One can clearly see different power dependences in the four power regions. Region I: ν ₁=∂logI _PL/∂P _exc=0.92, P _exc<P _C2=33 mW. Region II: ν ₂=0.52, P _C2≤P _exc<P _C3=84 mW. Region III: ν ₃=3.4, P _C3≤P _exc<P _C4=115 mW. And region IV: ν ₄=8.1, P _exc≥P _C4=115 mW. We will discuss the originations of four different power dependences later.

 

Fig. 3. Excitation power dependence of PL. (a) PL spectra from Ag/Nd:AAO with P _exc=60, 80 and 100 mW. As P _exc increases from 80 to 100 mW, the peak intensity increases ~50% while the spectral width decreases ~12%. (b) Peak PL intensity (leakage SPs) at λ _emi=872 nm as a function of excitation power. The SPs in Ag nanocavity arrays are linearly (ν ₁=0.92), sublinearly (ν ₂=0.52), stimulated (ν ₃=3.4) and avalanched (ν ₄=8.1) amplified in the region I, II, III and IV. The threshold power for the region II, III and IV are P _C2=33 mW, P _C3=84 mW and P _C4=115 mW, respectively.

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The polarization distribution of PL from Ag/Nd:AAO recorded with P _exc=20 and 90 mW are shown in Fig. 4(a). The polarization ratio is defined as r _p=(I _V–I _H)/(I _V+I _H), where I _V and I _H represent vertical and horizontal component of PL intensity, respectively. The value of r _p is around 0 in region I, it approximately linearly increases with the power P _exc in region II, reaches the maximum 0.082 and then decreases with P _exc in region III, and finally dramatically decreases to the negative value in region IV(see Fig. 4(b)).

 

Fig. 4. Polarization behaviors of output PL. (a) Polarization distribution of output PL with excitation power P _exc=20 and 90 mW. (b) Polarization factor r _p as a function of P _exc. The value of r _p is around 0 in region I, approximately linearly increases in region II, reaches the maximum 0.082 in region III and dramatically decreases to the negative in region IV.

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In region I (P _exc<P _C2=33 mW), the output PL intensity approximately linearly increased with the input power and the slope ν ₁=0.92. Ag/Nd:AAO and Nd:AAO have the same power dependences. Considering that the thickness of Nd:AAO is about 20 µm, and the average length of Ag NWs in Nd:AAO is only about 3 µm, so we believe that the PL is mainly from the Nd³⁺ in the layer without Ag NWs (or we say that only the Nd³⁺ in the layer without Ag NWs is populated by the input laser) in the weak excitation region, which means that the interaction between populated Nd³⁺ and Ag NWs is very weak.

In region II (P _C2≤P _exc<P _C3=84 mW), the Nd³⁺ in the layer with Ag NWs was also partially populated. This portion of populated Nd³⁺ strongly interacted with Ag NWs due to the small distance (the average gap between the NWs is only about 30 nm). On the one hand, the dipole emissions from this portion of Nd³⁺ with revised populations generated the SPs. On the other hand, the propagating SPs were absorbed by the other portion of Nd³⁺ unpopulated. This interconversion of Nd³⁺ dipoles and Ag SPs leads to a sublinear power dependences (ν ₂=0.52). The partial polarization of output PL is additional evidence that the dipole transitions of Nd³⁺ coupled to the propagating SPs along the NWs and the SPs emit out from the end of NWs.

As the input power increasing to region III (P _C3≤P _exc<P _C4=115 mW), almost all the populations of Nd³⁺ in AAO were reversed. The output PL intensity superlinearly increased with the input power, the slope ν ₃ increased from 0.52 to about 3.4. The polarization factor r _p increased to the maximum 0.082 at P _exc≈(P _C3+P _C4)/2. The value of r _p will be equal to (1-cos²45°)/(1+cos²45°)=0.333 if all the radiative transitions of Nd³⁺ coupled to the NWs and all far field emissions are from the ends of NWs when the relatively larger reflection of I _H is neglected. So the observed polarization factor 0.082 implied an efficient conversion from the dipole emission of Nd³⁺ to propagating SPs. In the PL spectra of Ag/Nd:AAO (see Fig. 2(a)), the half spectral width of right side at half maximum (HWHM) is about 34.1 and 34.2 nm at P _exc=60 and 80mW, it decreased to 30.2 nm at P _exc=100 mW. The decreasing of spectral width is about 12%. The increasing of slope (power index) ν ₃ and decreasing of spectral width of PL implied stimulated processes were partially involved [25, 26] in region III.

When the input power further increased to region IV (P _exc≥P _C4=115 mW), an additional amplification processes caused by strong coupling between the Ag NWs were involved. These amplification processes lead to a broadened PL spectrum with a higher slope ν ₄=8.1 and a larger negative polarization ratio r _p=-0.15. The broadened PL spectrum in this strong excitation region may be partially caused by the direct plasmon emission and intraband transitions from Ag NWs [27–30].

Comparing to a single Ag NW, the Ag NWs array system has larger plasmonic coupling efficiency and smaller SP propagation loss [19]. By improving the design of plasmonic nanocavity system (such as optimizing the diameters of Ag NWs and the gap distances between the Ag NWs array, increasing the concentration of Nd³⁺ ions, increasing the coupling between the radiative emission of Nd³⁺ ions and the SPs of Ag NWs array, and decreasing the propagating loss of SPs along the Ag NWs), it is possible to realize stimulated amplifications of SPs in this nanosystem [31–33].

4. Conclusions

In summary, the plasmonic nanocavity system consisting of Ag NWs arrays embedded in Nd³⁺ doped AAO exhibited linear, sublinear and superlinear PL. The input laser with λ _exc=805 nm resonantly pump the population to ⁴F_5/2 states of Nd³⁺, the radiative transitions ⁴F_3/2→⁴I_9/2 of Nd³⁺ centered at 880 nm were strongly coupled to the propagating SPs. The slope ν=∂logI _PL/∂P _exc increased from 0.52 to 3.4, the polarization ratio of output PL reached the maximum 0.082 and the corresponding spectral width decreased about 12%. The observed nonlinear PL should find the applications in plasmonic nanodevices.