Transition metal nitrides are promising alternative plasmonic materials to noble metals for data storage applications as they exhibit localized surface plasmon resonances and have high melting temperatures. Here, angle dependent spectral measurements of the plasmonic resonances of nanodisk arrays made from titanium nitride are examined. Polarized light is used to excite the quadrupole and higher order resonance plasmonic modes which are required in the state-of-the-art designs of near-field transducers used in plasmonic enhanced magnetic recording. Numerical simulations compare the energy distribution and absorption efficiencies for different sized Au and Ti nanodisks. A high electric field enhancement is calculated at the termination of a lollipop plasmonic transducer made of titanium nitride which is shifted to longer wavelengths when compared with an Au transducer of the same dimensions. This, together with its outstanding material properties makes TiN a favourable material for data storage applications.
© 2017 Optical Society of America
Plasmonics with its capability of sub-wavelength confinement of electromagnetic energy has become a driving force for progress in the area of nanophotonics. The phenomenon originates from strong coupling of the photon energy with free-electrons in the metal that supports a wave of charge density fluctuations on the surface of the metal to create a subwavelength oscillating mode called a surface plasmon1. This strong light-matter interaction leads to strong confinement of light [1,2] and high electromagnetic field enhancement [2–4]. The requirement for an abundance of free electrons limits the material choice to the metallic components that provide a negative real permittivity. The noble metals which are commonly used in plasmonics [5,6] suffer from excessive losses, lack of tunability and incompatibility with standard CMOS technology which have been major limiting factors for the uptake of plasmonics . The undesired properties of the noble metals have been addressed through design. Thus, for example, the propagation losses in surface plasmon polariton (SPP) waveguides can be reduced by designing dielectrically loaded SPP waveguides  which is at the cost of mode confinement. In terms of localized surface plasmon resonances (LSPR) , the undesired temperature increase in nanoantennae as a result of the high imaginary part of metal permittivity was addressed through the incorporation of a heat sink to prevent the material reaching its melting temperature . It has to be noted there is no universally perfect plasmonic material as the ideal properties depend on the particular structure and applications . Thus, for each specific application the specific material properties need to be considered. Recent progress in material science has opened new possibilities to develop the optimum materials for individual applications and functional in the specific spectral range. New plasmonic materials that possess extraordinary tuning and modulation capabilities such as transparent conducting oxides (TCO) [11–13], transition metal nitrides (TMN) [14–20], 2D materials [20–23], and many others [24,25] are being considered to replace the noble metals. TMN are one of the best candidates due to the similarities of their optical properties with gold, CMOS compatibility and high melting temperature.
One such application where TMN can be employed is in heat-assisted magnetic recording (HAMR) which is a large volume application for plasmonics [26–29]. HAMR utilizes the concept of a plasmonic antenna to shrink the optical focused spot to sizes of tens of nanometres to overcome the limits of conventional magnetic recording and to reach areal density >1 Tb/in2 [26,27]. The near-field transducer (NFT) acts as an antenna to locally heat and lower the coercivity of the magnetic recording media needed to maintain data stability at nanometre dimensions. The NFT design is based on excitation of surface plasmons on a metal structure, which transfers power to the recording media with a sub-diffraction limited light spot (Fig. 1(a)) [28,29]. Gold, which to date has been used in HAMR applications, suffers from its relatively low bulk melting temperature, low thermal stability and lack of compatibility with standard CMOS processing techniques [18,19]. It has been previously shown that the mechanical properties of Au degrade at temperatures around 100 °C [18,19] which is much below the estimated operational temperature for HAMR of around 400 °C [9,28,29].
Here, we studied the plasmonic properties of TiN nanodisks ranging in diameter from 150 nm to 300 nm which were deposited at low temperature. The dipole mode along with quadrupole and higher order resonances [29,31] needed for HAMR transducers [9,26,30,31] were measured by transmission in the wavelength range of 600 – 1600 nm. The transmission properties of the 20 nm thick TiN nanodisk arrays showed stable resonances properties over 6 months. The simulated absorption characteristics and field enhancements for TiN nanodisks were compared with those made from Au. Complex mode interactions were seen to influence the absorption resonances. The TiN nanodisks exhibit a broader resonance peak compared to Au nanodisks  though with a reduced absorption efficiency.
2. Angle dependent TiN nanodisk simulations
To compare TiN with Au for local heating applications the absorption efficiencies of nanodisks, defined as the absorption cross-section normalized by the geometrical cross-section area of the nanodisk under illumination , with varying diameters were calculated using finite element method (FEM) based commercial software (Comsol). The permittivity of Au was taken from the literature  while the dielectric constants of TiN were obtained from ellipsometry measurements (J. A. Woollam M2000) of a 20 nm thick TiN film deposited on a SiO2 substrate at a temperature of 150 °C. The permittivity measurements show similar properties to the results found in the literature  with the real part of the permittivity close to Au despite being deposited at a low temperature. TiN films deposited at higher temperatures are generally more metallic (larger magnitude of the real part of permittivity) [17–19] while films deposited at lower temperatures show lower oxidation leading to stable characteristics with time.
The calculated absorption efficiencies for perpendicular and angle dependent illumination are plotted in Fig. 2 and Fig. 3 for Au and Ti respectively. Angle dependent illumination is essential for the excitation of the quadrupole and higher order modes in NFTs used in HAMR [26,28,30,31] which require the electric field of light to be polarized along the side of the NFT. For Au nanodisks with a radius ~50 nm and perpendicular illumination, the LSPR corresponding to a dipole antenna is located around 610 nm with a peak absorption efficiency of ~4 . The resonance is narrow which requires a precise control of the nanodisk size. This is a very important factor for data storage applications as a small deviation in the transducer size or a small shift in operating wavelength can strongly influence the performance of the transducer. For angle dependent coupling, two components of the electric field should be considered. The first one, associated with the component of the electric field oriented along the nanodisk surface, results in the excitation of the dipole antenna mode, while the second component, associated with the electric field being oriented along the side of the nanodisk, is capable of exciting the quadrupole and higher order modes. Here, the calculations were performed for a coupling angle of α = 80° (Fig. 2(a) and Fig. 2(e)). For small nanodisk sizes it is observed that only the dipole mode associated with the perpendicular coupling of light is supported (Fig. 2(e)). When the nanodisk radius increases to around 100 nm, the higher order quadrupole mode appears at the wavelength of 600 nm. The maximum absorption efficiency of the quadrupole mode is found at wavelength of ~680 nm for a nanodisk radius of 125 nm. Further increase of the nanodisk size to r = 175 nm causes the quadrupole mode to shifts to longer wavelengths of 830 nm (Fig. 2(c)) but, simultaneously, the higher order hexapolar mode (Fig. 2(b)) appears at a wavelength of ~620 nm (Fig. 2(e)).
The corresponding calculations for TiN nanodisks (Fig. 3) show, for perpendicular illumination, a maximum absorption efficiency of around 1.8 for a nanodisk with radius of 75 nm at a wavelength of 830 nm (Fig. 3(b)). The dipole mode that is associated with such a coupling arrangement redshifts for a larger nanodisk (r = 125 nm) to a wavelength of a 960 nm with the absorption efficiency reduced to 0.86. Further increase of the nanodisk size shifts the maximum absorption efficiency back to shorter wavelengths. An explanation for this is that as the radius of nanodisk increases, higher order modes are supported and interference effects between the multiple modes reduce the absorption efficiency. Those higher order modes correspond to the multiple half-wave dipole antennae . At a coupling angle of α = 80° the dipole resonance dominates for smaller nanodisks with a maximum absorption efficiency corresponding to a nanodisk radius of 75 nm with a resonance wavelength of 830 nm. Increase of the nanodisk to r = 100 nm decreases the absorption efficiency to 0.145 with a simultaneous shift in resonance wavelength to 920 nm. Further increase in nanodisk radius to r = 125 nm shifts the resonance condition back to a shorter wavelength of a 820 nm as a consequence of a higher order quadrupole mode being supported by the nanodisk. As the nanodisk radius increases to r = 150 nm, the resonance conditions for a quadrupole mode shifts to a wavelength of 850 nm (Fig. 3(c)) and to 940 nm for a disk of r = 175 nm. However, a further increase of the disk radius to r = 200 nm blueshifts the resonance wavelength to 900 nm which is associated with the emergence of the hexapolar mode that is expected at larger disk dimensions.
Examination of the dipole (Fig. 3(e)) and quadrupole (Fig. 3(f)) mode excitation lead to the observation that for smaller disk dimensions of 125 nm a quadrupole mode appears at a shorter wavelength of 830 nm compared to the dipole mode at 980 nm. However, for a larger disk of r = 175 nm, the quadrupole mode appears at a wavelength of 940 nm while the dipole mode is at a wavelength of 750 nm. This behaviour is consistent with transmission measurements presented at Fig. 5(c) and 5(d).
3. TiN nanodisks transmission measurements
Thin films of TiN were deposited on 100 mm diameter AF32 glass substrates by DC reactive magnetron sputtering from a 99.99% titanium target in an Argon-Nitrogen environment. The deposition rate and substrate temperature were kept constant at 1.38 nm/min and 150°C respectively. Electron Beam Lithography (EBL) with a Jeol 6000FS was used to write disks of different diameters (150 nm, 200 nm, 250 nm and 300 nm) on 20 nm thick TiN films. Nanometer sized disks (diameter d) are arranged in a square lattice with the nearest neighboring disks spaced by h = 2d, the lattice constant. A chrome hard mask was deposited onto the patterned resist and a lift-off performed. The TiN is then etched from regions not protected by chrome by Reactive Ion Etching using Cl2 gas. The chamber pressure was 10 mTorr, gas flow 30 sccm and platen power of 50 W. The chrome mask is then removed by wet chemical etching leaving TiN disks on AF32 glass. Nanodisk arrays occupied a region of 1 mm x 1 mm to facilitate the transmission measurements.
Variable angle spectroscopic ellipsometry measurements were performed on the TiN films to obtain the thickness and optical constants. The ellipsometry system was equipped with a rotating compensator and a high speed CCD camera. Measurements were performed at room temperature in the spectral range of 400 – 1700 nm. The dielectric function of the TiN material was retrieved from ellipsometric measurements with a Drude-Lorentz fit (Fig. 4).
Polarization controlled, angle dependent spectral transmission measurements were performed for 2D arrays with different disk dimensions (Fig. 5). Unpolarized light from a white light source (Ocean Optic HL 2000) was passed through a polarizer to control the polarization of the incident light. A lens with 20 cm focal length was used to direct the light onto the 1 mm x 1 mm region containing the fabricated disks so the transmitted signal was averaged over entire nanodisk region. The samples were mounted on a rotation stage to obtain the angle dependent measurements. Transmitted light from the sample was coupled to a fiber for detection using two spectrometers one for the visible (Ocean Optic SD2000) and one for the near-infrared (Ocean Optic NIR512). This configuration allow both angle and polarization dependent measurements.
The strength of each resonance, dipole or higher order, depends on the nanodisk size and the coupling angle. For small incident angles we obtain only the dipole resonance independent of the polarization of light (Fig. 5(c) and 5(d)). To excite the quadrupole or higher order modes the electric field components of the incident light must be polarized along the side of the thin nanodisk (Fig. 5(b)). The resonance depends on the nanodisk size and shifts from 980 nm for a disk size of 150 nm to 1010 nm for a nanodisk of 250 nm. When the coupling angle increases to α = 30° the transmission dips (green curves Fig. 5(c) and 5(d)) redshift to a wavelength of 1000 nm for the smaller disk of 150 nm and to 1030 nm for larger disk of 250 nm. For such relatively small angles, the electric field component associated with the perpendicular illumination of light is still dominant, however, the component of the electric field polarized along the side axis (E2││ in Fig. 5(b)) appears leading to a drop in the overall transmission.
When the coupling angle α is increased to 60° the electric field component polarized along the side axis of the nanodisk becomes dominant and the higher order modes appear at the wavelength of 850 nm for small nanodisks (d = 150 nm) (Fig. 5(c)) and shifts to 1200 nm for larger nanodisks (d = 250 nm) (Fig. 5(d)). Those resonances appear as dips in the transmission curves (blue curves in Fig. 5(c) and 5(d)). These are of particular importance for the realization of TiN-based plasmonic transducers for HAMR application where the lollipop shaped transducer requires side illumination in order to excite the electric field polarized along the side axis of the transducer [9,26,30,31].
To confirm that only the electric field polarized along the side axis of the transducer excites the proper higher order transducer mode the transmission for the two different electric field polarization directions were compared. For the s-polarization, the electric field component lies in the direction of the side axis of the disk, while for the p-polarized light, the electric field component is perpendicular to the nanodisk. For an illumination angle of α = 60° it can be observed that only the s-polarized light excites the higher order resonance mode (Fig. 5(e) and 5(f)). No dips in the transmission curves are measured for the p-polarized light that can be associated with higher-order modes. However, it should be remembered that for an illumination angle of α = 60° there remains a small component of the electric field associated with the perpendicular illumination (α = 0°) that is able to excite the dipole mode. The shallow dips in the p-polarized spectra in Fig. 5(e) and 5(f) corresponds exactly to the s-polarized spectra in Fig. 5(c) and 5(d) for perpendicular illumination of light. Thus, the quadrupole and higher order modes can only be excited with the electric field polarized along the side axis of the nanodisk. At the same time it should be notified that very shallow dip in Fig. 5(f) for p-polarized light and associate with a higher order mode can be observed that it associated with slightly different polarization angle from a desired. It can be easily avoided by a precise control of the polarization angle of light.
4. Ceramic transducer for HAMR
The maximum electric field enhancement and working wavelength were simulated for Au and Ti based lollipop transducers. Here, it was assumed that the light is coupled to the transducer by two in-plane beams with a 90° angle and with a π-phase shift between both beams to achieve a charge distribution on the lollipop which results in a maximum electric field at the termination of the peg  (Fig. 6(a), 6(b) and 6(d)). It was assumed that the transducers are embedded in a material with permittivity, εmat = 2. The material between the transducer and image plane was assumed to be εspace = 2. For comparison purposes both transducers were assumed to consist of disks with radius r = 100 nm and peg of length and width of 20 nm. The thickness of the transducer was kept at 20 nm. The distance between the peg end and the image plane was assumed to be 7.5 nm.
The cross-section of the electric field enhancement at a distance of 7 nm from the peg in the plane perpendicular to the transducer and taken at the center of the peg for both transducers (Fig. 6(c)) revealed a very strong wavelength dependence of the field enhancement. For the Au transducer a maximum electric field enhancement of ~25 was calculated for a wavelength of 860 nm which corresponds to the quadrupole mode on the transducer. The maximum electric field enhancement of ~9.5 was calculated for the TiN-based transducer at a wavelength λ = 1160 nm and corresponding to the quadrupole mode. Thus, if only the plasmonic properties of the NFT are considered, the Au based NFT is superior over TiN. This is due to the larger magnitude of the real part of permittivity and lower imaginary part of permittivity for Au compared with TiN. However, the long term stability of the Au NFT poses a problem due to harsh operation conditions – the NFT has to operate at temperature around 400 °C. Compared to Au, TiN is characterized by higher heating efficiencies, higher melting temperature (~3000 °C), chemical stability and compatibility to CMOS technology that makes TiN a very good candidate material to replace Au for HAMR applications. Thus, many different properties should be taken into account when considering a material for specific application. A very good example is silver (Ag) which has superior plasmonic properties to Au, however, it suffers from high chemical instability that degrade the material performance and, consequently, limits its applications.
In conclusion, TiN nanodisk arrays were fabricated at low deposition temperature of 150 °C that show good plasmonic properties over a long period of time. Angle dependent transmission measurements reveal that quadrupole and higher order modes can be excited on the TiN nanodisks when the excitation light is polarized along the side axis of the nanodisk. Such illumination conditions are required to excite the higher order modes of a NFT to achieve high energy transfer to a recording media. The polarization dependent measurements show that only s-polarized light can excite a plasmonic resonance mode. FEM simulations suggest that reasonable field enhancement can be achieved with a TiN based transducer with the resonance wavelength shifted to longer wavelengths compared to an Au transducer. Furthermore, considering the fact that TiN is CMOS compatible, durable material with a high melting temperature, our results suggest that TiN is a promising material for data storage applications where the low melting temperature of Au poses a reliability challenge.
7th Framework EU Program within the Marie Curie Industry-Academia Partnerships and Pathways through the COMPASS Project under Grant 286285 and by Science Foundation Ireland under 12/RC/2276 (IPIC).
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