Research

Current Research during Ph.D.

Research Area: "Photon-Magnon Coupled Hybrid System for quantum information processing and spintronic technologies".

Supervisor - Dr. Biswanath Bhoi and Dr. Rajeev Singh

"To investigate the emergent properties due to light-matter interaction for Next-Generation Information Processing Technology".

1. Hybrid Photon-Magnon Systems: Exploring the Purcell Effect

We present a novel approach to observing the Purcell effect in a photon-magnon coupled (PMC) hybrid system consisting of a yttrium iron garnet (YIG) thin film and a hexagonal ring resonator (HRR) arranged in a planar geometry. This hybrid system has been designed and simulated using the commercial electromagnetic full-wave simulator CST Microwave Studio for various values of damping constant (α) of the YIG film while keeping the HRR properties constant. Our results reveal that as the magnon damping increases, the anti-crossing behavior between photon and magnon modes in the transmission spectra diminishes, transitioning the coupled modes into the Purcell regime. This transition is attributed to an enhanced spontaneous emission rate of microwave photons when coupled strongly to lossy magnons, driving the PMC system into the Purcell regime. To elucidate this behavior, we developed a comprehensive theoretical framework based on a quantum model, which accurately describes the observed Purcell phenomena and provides estimations of the PMC strength (g/2π). Notably, by tuning α from 1.4 × 10−5 to 2.8 × 10−2, we achieved precise control over (g/2π) ranging from 63 MHz to 127 MHz. This study highlights the Purcell effect's role in enhancing photon decay rates and establish a clear relationship with PMC strength. Our work offers a comprehensive method for controlling photon resonance dissipation, opening new avenues for exploring the Purcell effect and its applications in on-chip functional devices leveraging magnon–photon interactions for quantum technologies.

Fig. 1. (a) Schematic diagram of the Purcell effect, illustrating the enhancement of the spontaneous emission rate of the quantum source mediated by the atom-field interaction, (b) Simulation setup consisting of a YIG film (green) with dimensions 3 mm×3 mm×20 μm, a microstrip line (dark yellow) with dimensions 30 mm×0.57 mm×35 μm, a dielectric material (gray) with dimensions 30 mm×20 mm×0.64mm, and an HRR (with a = 4 mm, b = 3.4 mm) positioned near the microstrip line. (c) Schematic of the complex photon-magnon coupled hybrid system.

Fig. 2. The variation of coupling strength (g/2π), and the damping constant of the photon mode (β), both as functions of magnon damping (α).

2. Unlocking Photon-Magnon Interplay via Saturation Magnetization

In this study, we investigate the control of photon–magnon coupling (PMC) strength through systematic variation of the saturation magnetization (𝑀𝑠) in a planar hexagonal-ring resonator (HRR) integrated with a yttrium iron garnet (YIG) thin film configuration. Using full-wave numerical simulations in CST Microwave Studio, we demonstrate that tuning the 𝑀𝑠 of the YIG film from 1750 Oe to 900 Oe enables systematic control over the coupling strength across the 127–51 MHz range at room temperature. To explain the observed PMC dynamics, we develop a semiclassical analytical model based on electromagnetic theory that accurately reproduces the observed coupling behavior, revealing the key role of spin density in mediating the light–matter interaction. The model is further extended to include the effects of variable magnon damping across different 𝑀𝑠 values, enabling broader frequency control. These findings establish 𝑀𝑠 as a key tuning parameter for tailoring PMC, with direct implications for the design of tunable hybrid systems for reconfigurable quantum devices.

Fig. 1. Schematic representation of the simulation setup for a magnon–photon hybrid system comprising a YIG film (green), a microstrip line (dark yellow), a dielectric material (navy blue), and an HRR (dark yellow). The YIG film is positioned at the edge of the HRR, which is placed adjacent to the microstrip line. An external magnetic field (H) is applied uniformly in-plane along the y-axis.

Fig. 2. Scanned [f, H] dispersion of microwave |S₂₁| power plotted for different saturation magnetization (MS) values: (a–d) simulation results, and (e–h) theoretical calculations based on an electrodynamic model. The white lines indicate the shift of the coupling center as MS decreases from 175 mT to 90 mT.

Fig. 3. Coupling strength (Δ) as a function of (a) magnon damping (α) for different values of saturation magnetization (Ms), and (b) saturation magnetization (Ms) for different values of magnon damping (α).

Fig. 4. (a) Analytically calculated phase diagram illustrating different types of anti-crossing dispersion on the (MS-α) plane for an HRR damping value of β = 4.69 × 10−3. The color gradient represents the variation in coupling strength (Δ), while the solid black line denotes the contour of constant coupling strength, separating the regions of strong and weak coupling. (b) Contours of constant coupling strength plotted for different values of 𝛽 (0.001, 0.00469, 0.008, 0.01, 0.02, 0.03), highlighting the effect of HRR damping on the strong–weak coupling boundary.

3. Control of Photon-Magnon Coupling in a Planar Hybrid System

Photon-magnon coupling (PMC) integrates microwave or optical photons with magnons, aiming to exploit their distinct strengths in a unified hybrid quantum system, for potential applications in quantum information science and technology. By utilizing numerical simulations, we design a planar hybrid system comprising a hexagonal-ring resonator (HRR) and yttrium iron garnet (YIG) thin film to explore the interaction between microwave photons and magnons. The anti-crossing effects between the hexagonal-ring resonator (HRR)'s photon mode and the yttrium iron garnet (YIG)'s magnon modes were observed in |S21|-frequency plots under various externally applied magnetic fields.

Fig. 1. Schematic illustrations of the sample structure and simulation setup geometry. (a) The sample consists of a YIG film (green), a microstrip line (dark yellow), a dielectric material (gray), and a single HRR positioned near the microstrip line. A uniform external static magnetic field (H) is applied along the y-direction to bias the YIG thin film. (b) The schematic figure depicts the six different positions of YIG film on HRR edges, denoted by distinct angles (φ) ranging from 0º to 360º in a clockwise rotation with a 60º difference with respect to the microstrip line.

Fig. 2. |S21| transmission spectra shown in Fig. 3 represented by the |S21| power on the plane of microwave frequency and magnetic field plane (f-H plane) (a) only for the HRR, (b) for YIG film only and (c) for the planer hybrid system of HRR and YIG thin film.

4. Studies of magnetic and microwave properties of sputter deposited polycrystalline YIG thin films

Yttrium iron garnet (YIG) is an ideal magnetic material with potential applications in microwave and spintronic devices. A key prerequisite for seamless integration into current semiconductor electronics is the growth of high-quality YIG films on substrates beyond isostructural Gadolinium gallium garnet. In this context, we present the successful fabrication of YIG thin films with varying thicknesses (70 ≤ t ≤ 380) on fused quartz substrates utilizing radio-frequency (rf) magnetron sputtering. The Rietveld refinement of the X-ray diffraction data uncovers the formation of body-centered cubic single-phase polycrystalline YIG with the space group of Ia-3d. The saturation magnetization (4πM) and coercivity (HC), as determined by the physical property measurement system (PPMS), exhibit a dependence on the film’s thickness (t). Remarkably, the film with t = 380 nm shows a 4πM value of 1775, closely resembling the bulk YIG value, with an exceptionally low coercivity (HC < 5 Oe). From ferromagnetic resonance (FMR) measurements, the estimated effective saturation magnetization (4πMef) is found to be very much different from the 4πM obtained from PPMS and is attributed to the presence of stressed-induced magnetic anisotropy (HK) in YIG films. The FMR linewidth (ΔH) of the YIG films is found to be quite sensitive to HK and the minimum ΔH value of 80 Oe is observed in the film with the lowest HK. The findings indicate that YIG films deposited on quartz substrates have potential advantages for their application in semiconductor-integrated devices.

Fig. 1. Rietveld-refined XRD patterns of YIG thin films of different thicknesses. The black circles and the red solid line are the experimental and calculated XRD patterns respectively.

Fig. 2. Three-dimensional AFM image for 70 nm, 250 nm, 345 nm and 380 nm thick YIG flm

Fig. 3. a Room-temperature M–H loop of YIG thin flms of varying thickness. Inset: variation of coercivity (Hc) as a function of grain size.

Fig. 4. Parallel (red line) and perpendicular (blue line) FMR spectra of YIG thin films of different thicknesses measured at room temperature

Previous Research (During M.Sc.)

M.Sc. Project: "Phase Ordering Kinetics in fluids: A Langevin Dynamics Study"

Supervisor - Dr. Awaneesh Singh

The main objective of this M.Sc. project was to obtain a better understanding of the molecular dynamics for L-J potential though the computation of the Potential Energy, Kinetic Energy, Total Energy, Radial Distribution Function (RDF) and Temperature for different systems in different ensembles.

In this project, has been computed first the MD simulation for the microcanonical ensemble. It has been found that the microcanonical studied by molecular dynamics simulations in 3D systems were systems of a few thousand of atoms. Therefore, it has been reached that there is fluctuations in potential energy and in kinetic energy but total energy is constant with respect to time, there is no fluctuations in total energy. And then computed the MD simulation for the canonical ensemble using the Anderson method. It has been found that the canonical ensemble studied by molecular dynamics simulations in 3D systems were systems of a few thousand of atoms. Therefore, it has been reached that there is fluctuations in potential energy and in total energy but kinetic energy is constant with respect to time, there is no fluctuations in kinetic energy.

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