Applications of plasmonics in two dimensional materials & thin films

The demand for the faster information transport and better computational abilities is ever increasing. In the last few decades, the electronic industry has met this requirement by increasing the number of transistors per square inch. This lead to the scaling of devices to tens of nm. However, the speed of the electronics is limited to few GHz. Using light, the operating speed of photonic devices can be much larger than GHz. But the photonic devices are diffraction limited and hence the size of photonic device is much larger than the electronic components. Plasmonics is an emerging field with light-induced surface excitations, and can manipulate the light at nanoscale. It can bridge the gap between electronics and photonics.

With the present scaling of devices to few nm, the scientific community is looking for alternatives for continued progress. This has opened up several promising routes recently, including two-dimensional materials, quantum computing, topological computing, spintronics and valleytronics. The discovery of graphene has led to the immense interest in the field of two-dimensional materials. Two dimensional-materials have extraordinary properties compared to its bulk. This work discusses the applications of plasmonics in this emerging field of two-dimensional materials and for heat assisted magnetic recording.

Black phosphorus is an emerging low-direct bandgap two-dimensional semiconductor, with anisotropic optical and electronic properties. It has high mobility and is promising for photo detection at infrared wavelengths due to its low band gap. We demonstrate two different plasmonic designs to enhance the photo responsivity of black phosphours by localized surface plasmons. We use bowtie antenna and bowtie apertures to increase the absorption and polarization selectivity respectively. Plasmonic structures are designed by numerical electromagnetic simulations, and are fabricated to experimentally demonstrate the enhanced photo responsivity of black phosphorus.

Next, we look at another emerging two-dimensional material, bismuth telluride selenide (Bi₂Te₂Se). It is a topological insulator with an insulating bulk but conducting electronic surface states. These surface states are Dirac like, similar to graphene and can lead to exotic plasmonic phenomena. We investigated the optical properties of Bi₂Te₂Se and found that the bulk is plasmonic below 650 nm wavelength. We study the distinct surface plasmons arising from the bulk and surface state of the topological insulator, Bi₂Te₂Se. The propagating surface plasmons at a nanoscale slit in Bi₂Te₂Se are imaged using near-field scanning optical microscopy. The surface state plasmons are studied with a below band gap excitation of 10.6 µm wavelength and the surface plasmons of the bulk are studied with a visible wavelength of 633 nm. The surface state plasmon wavelength is 100 times shorter than the incident wavelength in sharp contrast to the plasmon wavelength of the bulk.

Next, we look at the application of plasmonics in heat assisted magnetic recording (HAMR). HAMR is one of the next generation data storage technology that can increase the areal density to beyond 1 Tb/in². Near-field transducer (NFT) is a key component of the HAMR system that locally heats the recording medium by concentrating light below the diffraction limit using surface plasmons. In this work, we use density-based topology optimization for inverse design of NFT for a desired temperature profile in the recording medium. We first perform an inverse thermal calculation to obtain the required volumetric heat generation (electric field) for a desired temperature profile. Then an inverse electromagnetic design of NFT is performed for achieving the desired electric field. NFT designs for both generating a small heated spot size and a heated spot with desired aspect ratio in recording medium are demonstrated. The effect of waveguide, write pole and moving recording medium on the heated spot size is also investigated.