Split ring resonator as a nanoscale optical transducer for heat-assisted magnetic recording

Split ring resonators (SRR) are optical nanostructures that have received a lot of attention for their ability to support magnetic resonance and for their potential use as materials with negative dielectric constant. In this work, we design SRRs as near-field transducers (NFT) for generating a nanoscale hotspot in heat-assisted magnetic recording (HAMR), which is considered a candidate for the next-generation data storage technology. The underlying mechanisms for the generation of hotspot and the dependence on wavelength and geometry of the SRR structure are studied. Optical and thermal performance of SRRs functioning as NFTs in a HAMR device are evaluated. These structures were fabricated using focused ion beam milling. The focusing capability of the SRR is experimentally demonstrated using a scattering near field scanning optical microscope. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
Generation of a focused optical spot beyond the diffraction limit has received a whole lot of attention due to its wide potential applications in the field of nanophotonics [1,2]. Heat assisted magnetic recording (HAMR) is one such area of commercial importance where nanoscale hotspot plays a significant role in developing the next generation hard disk drives [3][4][5][6]. Contemporary hard disk drives based on perpendicular magnetic recording technology and shingled magnetic recording have reached the practical limits of data storage density due to the physical constraint imposed by the superparamagnetic limit of the recording medium [7]. Further novelties in the recording medium technology to achieve higher density data storage necessitates the use of a medium with higher coercivity, which helps in stabilizing the orientations of the magnetic bits at room temperature. But in order to write data into the high coercivity medium, its temperature needs to be raised temporarily to lower the medium coercivity [8]. HAMR technology promises to address this challenge by locally heating the recording medium over a tiny area of tens of nanometers through the use of a plasmonic antenna, also known as near field transducer (NFT). This localized heating of the recording medium allows the magnetic writing to occur only at the high temperature region. Hence, the NFT design needs to be able to generate a tiny hotspot for the proper functioning of a HAMR device.
Producing a sub diffraction limited hotspot generally requires appropriately designed subwavelength structures which can be optically excited by properly polarized incident light. Several different types of antenna-based and aperture based designs have been proposed as NFTs and their optical and thermal performance have been studied for their usage in a HAMR device [5, 9,10]. NFT designs are geometrically optimized to support localized surface plasmon resonance at a specific operating wavelength which helps in enhancing the intensity of the near field spot. Typically, an NFT design contains some sharp features such as notches and ridges which help in concentrating the electric field through the lightning rod effect for producing a very tiny spot size in the near field [11][12][13][14]. Also, tiny gaps in a metallic structure like of the inciden In this wo of generating capability of negative diele size at the res tiny gap resu magnetic writ performance waveguides [ SRR is influe mode. In the n for the genera study incorpo recording med also performe For feasibility should be n lithographic f characterizatio optical micros capability of t

Descripti
SRRs are ant double loop. between the e either of a p understood by capacitance a portion of the inductive por representation  [15]. ork, we explore g a nanoscale generating m ectric constant sonance condit lting in a very te pole in the v of an SRR s 12, [21][22][23][24]. Lik enced by the d next section, th ation of a hotsp orating the geo dium through e ed to predict th y studies, such oted that for fabrication tech on of the near scope (s-NSOM the SRR beyon on of the SR tenna-based op The single loo ends of the ring plasmonic orig y drawing an an and inductance e resonant circ rtion of the cir n is shown in F . (a) Schematic of t, (c) double loop nal ridge apertu e the use of a n hot spot. SRR magnetic resona t [16][17][18][19]. SRR tion [20] and t y small optical vicinity of the N structure resem ke these struct dimensions of he SRR structu pot at different ometric variatio electromagneti he temperature h structures w bulk produc hniques combi r field spot is M). A localize nd the diffractio RR structure ptical nanostru op SRR consist g as shown in gin or an LCnalogy to the r e. In the case cuit with a cap rcuit with an i Fig. 1

Design of
The potential consider both be more suita E field can be the electric e system has b her hand, a do the two arms a 1(d) has two i cuit is inverse of the structur the loops of th e gap of the SR th the effect of pot is formed in m the LC-circ velengths depe onances are si he SRR desig be used for ge periments on onances and n [17], and sub ven in the visi directions of th vectors for exc s for the excita perpendicular t the propagating way to excite is pointed acr introduces an the SRR canno modes dependin detail in the nex

Resonant modes of a single loop SRR
We consider a single loop SRR where, the gap, g is taken as 20 nm, the width of the arms of the SRR, w is 30 nm, the length of each of the arms, l x and l y is 90 nm, and the thickness of the gold film is taken to be 60 nm. A fillet of radius 5 nm is assumed at the sharp corners of the arms of the SRR. The dimensions of the SRR are chosen such that two resonance peaks are observed in the range between 600 nm and 1700 nm. For the dimensions described above, two resonance peaks, one near 800 nm and other near 1550 nm are observed as shown in Fig.  4. The two peaks correspond to two different resonance modes supported by the NFT. The origin of the resonance modes can be explained from the fields at different wavelengths.  Figure 6(a) s of w for l x = nm, and 250 he different ex 0 nm and w = 3 modes are pre ier and peak (2 the resonance w, there is a de cy. On the oth es. It is known length of the g. 6  ons, (b) Coupling s of w and the arm n in Fig. 7(b), ce current mov ly. The surfac and then they e out-of-plane f the surface cu The maximum b). Due to the SRR is about t on, the surfac field plot are response of th 8(f) and the m rns as seen in SRR, we find h regime while ngths and high the single loop g m is the LC vement at ce current converge magnetic urrent are coupling effect of twice that ce current shown in he out-ofmultipolar Fig. 8(e). that with e with the her order p SRR, a

Therma
In order to inv loop SRR des near IR wave HAMR devic support mostl resonances ar in HAMR de these SRR str unlike plasmo secondary hot mode in the lo in the NFT. T [29]. Thus, ut lower temper focus on wa implications.
First, a sin which has the Fig. 6(b) Fig. 9(b). The of 1 ns while t recording medium ode (1), (c) Out-of ng medium corres for mode (2) and e resonance peaks mance of SRR, mized to have t 00 nm) since his wavelength can be seen f r wavelength re ant benefits fro e of resonance from the hotsp of the structur so ensures that g absorptivity nances in the l e remaining p to 900 nm ra osen having dim of 3.1% at 75 were done to n input power hown in Fig. 9 Fig. 11. pography of the tip centration d the spot ize of the has larger observed ffects and sk size of 5 nm x 54 is close to that the m, l y = 237 re slightly m and gap near field

Conclusio
In this study, HAMR devic modes can be serve as a g Through the functionalities 7.2% was fou order to chara fabricated and of a focused n would include on the overall serve as alter efficiencies an