1. Introduction

The main properties of light interacting with nanostructured materials can be modified substantially due to the appearance of various types of light-matter interaction mechanisms. A special place in this field belongs to spatially periodic metal-dielectric nanostructures known as plasmonic and photonic crystals [1–3], exhibiting spectral selectivity of their properties due to the existence of plasmonic and photonic band gaps, plasmon-driven waveguide modes and field localization effects. Moreover, introduction of a magnetic medium in such structures forms the so-called magnetoplasmonic crystals (MPC) and results in a drastic enhancement of the magneto-optical (MO) effects in specific spectral ranges [4–6]. These properties are intensively studied during the last years and are extremely attractive for possible applications in magneto-plasmonics [7–13].

The main mechanisms underlying high MO activity of magnetoplasmonic crystals consist in the modification of the dispersion relations for the SPP excitation in a magnetic medium [6]. The investigated structure consists of a gold grating on top of a bismuth iron garnet (BIG) film. It was shown [12] that such a structure supports excitation of various resonant modes through the diffraction on the grating. Namely, two types of surface plasmon-polaritons (SPPs) can be excited on different sides of the gold grating and waveguide (WG) modes can propagate in the dielectric slab. The spectral position and the efficiency of the SPPs and WG modes can be controlled by varying the structural parameters of the MPC, so the possibility of the development of a versatile method for the MPC fabrication seems to be very attractive. To the best of our knowledge, the comparative studies of MPCs with a continuous variation of the MPC period and slits width has not been performed yet.

In this paper we discuss in detail the combined ion-beam etching technique used for the fabrication of a series of 1D Au/garnet MPCs and perform a study on optical and magneto-optical properties of the obtained samples. High quality of the fabricated MPCs results in the excitation of high-order SPPs (up to m = +3), anomalous transmission effect and resonant enhancement of the magneto-optical response due to excitation of SPPs.

2. Experimental results

In the experiment, we studied optical properties of a series of planar 1D MPC fabricated using an advanced combined ion-beam low energy sputter-deposition and focused ion-beam etching technique, first suggested in [14]. The fabricated Au/BIG MPCs had the periods from 660 nm up to 820 nm, the slit width of 90 ÷ 160 nm and a 60 nm thick Au layer. Previous studies have shown that such MPC parameters allow to observe the excitation of SPP and WG modes in the visible - near IR spectral ranges.

Schematic of the etching method is shown in Fig. 1a. We used the substrates consisting of 2.2 μm thick garnet films of the composition (BiTm)₃Fe₅O₁₂ (BIG) grown by liquid-phase epitaxy on a (110) gallium-gadolinium garnet (Gd₃Ga₅O₁₂) substrate. In order to achieve flat defectless surface of BIG layer it was planarized by dual ion-beam sputter technique, described in detail elsewhere [11]. A 120 nm thick gold film was deposited as 100×100 μm pixels by sputtering of an Au target with a 1.5 kV Ar⁺ ion beam at a current density of 0.25 mA/cm² and introducing the double ion-beam sputter-deposition technique. Preliminary low-energy ion-beam treatment of BIG surface and repeating of the Au deposition resulted in high adhesion of the gold layer to the garnet. Atomic force microscopy (AFM) image of the Au surface is shown in Fig. 1d along with the cross-section, the mean-square surface roughness is around 0.3 nm. It is worth noting that after the single deposition of gold, this value was at least 3 times larger. Note that in order to obtain an MPC with ≈60 nm thick Au layer, we apply a novel two-stage technique.

 

Fig. 1 (a) Three stages of the combined ion-beam etching method employed for the MPC fabrication. SEM images of the BIG layer (b) before and (c) after planarization. (d) AFM image of the surface of the deposited Au film after introducing the double sputter-deposition technique.

Download Full Size | PDF

Initially, thicker Au layer (120 nm) was deposited. The 60 nm difference in Au thickness was made equal to the depth of the Au subsurface damage layer, formed by a 30 kV Ga⁺ focused ion beam (FIB) used for the initial formation of the gold grating (stages I–II in Fig. 1a). The thickness of ion-beam irradiated damage layer was estimated using the SRIM 2012 program package.

After the formation of the metal grating by focused high-energy ion beam, the upper Au layer was removed using a broad ion beam of low energy 0.3 kV oxygen ions. The scanning electron microscopy (SEM) images of the MPC at these two stages are shown in Fig. 1b,c. As a result, the stripes were formed in Au film, with the bottom of the stripes corresponding to the BIG/air interface as is shown in Fig. 1a, stage III. This delicate process can be employed, because the etching rate of BIG is several times lower as compared to that of gold, and the irradiation of low-energy O⁺ ions does not lead to the formation of a secondary surface relief. In other words, the surface of the BIG layer remained flat and the thickness of the final ion-beam damage surface layer did not exceed a few nanometers. We would like to underline that this stage is extremely important for procuring defectless surface of the magnetic garnet layer, which is responsible for the magnetic field control over the SPP propagation on Au/BIG interface.

Excellent periodicity of the composed MPCs was proven by the optical diffraction measurements, as well as directly by scanning electron microscopy. High quality of the samples should also result in rich optical properties, caused by the excitation of various resonant modes that can propagate in this type of structures.

Optical spectra were measured using a setup described in more detail elsewhere [12]. In brief, the light beam from a stabilized halogen lamp passed through a spatial filter and a polarizer, and then was focused on the MPC surface into a spot of approximately 50 μm in diameter. The transmission spectra were normalized to the transmission spectra of the bare BIG film. In order to provide the excitation of SPPs and WG modes, the samples were placed in such a way that Au stripes were perpendicular to the plane of incidence of the probe beam. Magnetic field of 3.0 kOe was applied in the transversal geometry, i.e. parallel to the stripes. The MPC sample was placed on a rotation stage.

Figure 2 shows transmission spectra versus both the angle of incidence and the wavelength for the samples with various spatial periods d and slit widths l measured for p-polarized incident light. A number of distinct spectral features can be seen. First, two types of SPPs excited on Au/air and Au/BIG interfaces are observed, indicated by dashed and solid lines in Fig. 2, respectively. These lines correspond to theoretical calculations, where typical values for the dielectric permittivities of gold and BIG [15, 16] were used. According to our estimations, the peculiarities shown in Fig. 2 correspond to SPPs of orders m=±1 for Au/air interface and m=±2, m=±3 for Au/BIG interface, the tilt angle of the corresponding linear (in the small-angle approximation) dependencies is governed by the refractive indexes of the adjacent layers, MPC period and the SPP mode number.

 

Fig. 2 Transmission versus the wavelength and the angle of incidence for MPCs with various periods and stripe widths l: d=660 nm, l=100 nm; d=690 nm, l=90 nm; d=720 nm, l=90 nm; d=790 nm, l=110 nm; d=810 nm, l=165 nm; d=820 nm, l=100 nm. Lines correspond to the Au/air SPP dispersion curves for m=±1 (dashed lines) and Au/BIG SPP of the orders m=±2 and m=±3 (solid lines).

Download Full Size | PDF

Second, a series of WG modes appear as minima in the transmission spectra, spatial intervals between them depend on the thickness and refractive index of BIG layer, as was discussed in [12]. Interestingly, the effect of the WG modes on the MPC transmission is the highest for the largest ratio l/d, when d = 810 nm and l = 165 nm. Moreover, the WG modes turn out to be SPP-assisted. The coupling of the Au/BIG SPPs and the WG modes can be clearly seen in Fig. 3, where transmission spectra of the MPC with d = 720 nm for p- and s-polarized light are compared. In the first case the TM-polarized SPPs are excited together with TM-polarized WG modes, and the latter ones strongly modify the spectrum. In the second case only TE-polarized WG modes are excited without SPP assistance, which results in low efficiency of the excitations. Features in Fig. 3b that do not depend on the angle of incidence are related to the interference in the BIG film.

 

Fig. 3 Transmission versus the wavelength and the angle of incidence for the structure with d = 720 and incident (a) p-polarized light, (b) s-polarized light.

Download Full Size | PDF

The third feature to be noted is the presence of an area of high transmittance at normal incidence, which is observed at the intersection of the Au/BIG SPPs with m = ±2 for all samples. Anomalous transmission was observed for hole arrays and gratings [17, 18] and was explained as coupling of plasmons on different sides of the metal film. In our case the dispersion relations for SPPs on two sides of gold grating are different, therefore this effect can not be easily explained as the coupling of two types of SPPs.

We have also studied the intensity magneto-optical effect in transmission through the MPCs and bare BIG film for the transversal magnetic field. Odd magneto-optical intensity effects in transmission for a transverse magnetization can be observed in a continuous magnetic film if it has different interfaces on both sides [19] and this effect is usually less than 1 × 10⁻³. However, no modulation in the transmitted intensity was observed under the variation of the external magnetic field for a bare BIG film.

At the same time, as was shown in [12], magnetization-induced modulation of the dielectric permittivity leads to the change in the resonance conditions for the SPP and WG mode excitation, so that the magnetic contrast in the intensity of the transmitted light may appear for the MPC. Commonly this effect is described by the magnetic contrast ρ = (I(+M) −I(−M))/I(0), where I(±M) is the intensity of the transmitted radiation measured for the opposite directions of magnetization and I(0) = (I(+M) + I(−M))/2. Figure 4 shows the spectra of the transversal MO intensity effect for two MPCs with the periods 720 nm and 820 nm, which reveal the non-zero magnetic contrast in the spectral regions of Au/BIG SPP excitations. The maximal contrast values are approximately 3×10⁻³, which is typical for such types of structures. The WG modes contribute much less to the magnetic contrast, but still the effect is clearly visible and reaches 1×10⁻³.

 

Fig. 4 Magnetic contrast ρ versus the wavelength and the angle of incidence for MPCs with the periods of (a) 720 nm and (b) 820 nm. Lines correspond to the dispersion curves of Au/BIG SPPs of different orders.

Download Full Size | PDF

One can see that the sign of the magnetic contrast is opposite for the counter-propagating SPPs, both for m = ±2 and m = ±3. This is consistent with the main mechanism of the MO effect, which is the magnetization-induced spectral shift of the SPP resonances. It is important to note that ρ also vanishes at the edges of the Brillouin zones, where counter-propagating Au/BIG SPPs are excited simultaneously. SPPs degenerate into standing waves with zero group velocity and the discussed magneto-optical effect can not be observed.

3. Conclusion

Summing up, we present a promising technique for the fabrication of magnetoplasmonic crystals composed of an Au grating formed on top of a bismuth iron garnet film. We demonstrate that by combining a high-energy focused ion-beam etching, used on the initial stage of MPC fabrication, with a low energy and broad ion-beam etching, one can produce high-quality and defectless MPC. Optical and magneto-optical spectroscopy of planar 1D magnetoplasmonic crystals shows that their spectral characteristics are governed by the MPC design, i.e. by the period of the structure and the width of the MPC slits. For all the studied structures, anomalous transmission is observed at normal incidence in the spectral region, where SPPs are excited at the Au/BIG interface. We show that the most significant magneto-optical intensity effect is observed close to the resonant SPP excitation at Au/BIG interface, the sign of the magnetic contrast keeps constant for the SPPs of different orders propagating in the same direction.