Reconfigurable Photonic and Microwave Devices Based on Phase Change Materials (PhD viva voce)

Abstract: Chalcogenide phase-change materials (PCMs) have emerged as a versatile class of functional materials due to their ability to reversibly switch between amorphous and crystalline phases, accompanied by large contrasts in optical and electrical properties. These unique characteristics, combined with non-volatility and fast switching dynamics, make PCMs highly attractive for applications spanning a broad electromagnetic spectrum, from microwave to optical frequencies. In this work, we explore the potential of chalcogenide PCMs for tunable and reconfigurable devices, demonstrating their effectiveness in both optical and microwave regimes. In the optical domain, this work presents a reflective optical modulator based on the phase-change material Ge2Sb1Te4, designed to operate at the telecommunications wavelength of 1550 nm. The material exhibits a large refractive index contrast (Δ𝑛 ≈ 3.44 and Δ𝑘 ≈1.3),along with low absorption in the amorphous state(𝑘 ≈ 0.03), enabling efficient modulation. By integrating the PCM into a planar cavity structure capped with SiN𝑥, and without relying on meta-surfaces or plasmonic architectures, we achieve an exceptionally high modulation depth (Δ𝑅) of ∼ 87% at near-normal incidence. Furthermore, controlled optical excitation enables the realization of 64 distinct intermediate reflectance states, corresponding to 6-bit multilevel encoding for high-density optical data transmission.

To further enhance performance, in this work we propose an all-dielectric, metal-free reflective optical modulator that integrates Ge2Sb1Te4 with a Si/SiO2 distributed Bragg reflector (DBR). The DBR provides near-unity reflectivity at 1550 nm, while a SiN𝑥 capping layer suppresses parasitic reflections. Owing to the large optical contrast of the PCM, the device exhibits a transition from a highly reflective (∼ 96%) amorphous state to a strongly absorptive (∼ 7%) crystalline state, corresponding to a modulation depth of ∼ 90%. The design maintains a contrast exceeding 80% over a wide angular range (0◦–60◦) and a broad wavelength window (1400–1650 nm). Electro-thermal simulations further confirm reliable electrical switching using a doped-silicon heater, demonstrating the feasibility of practical device integration.

In addition to optical modulation, this work extends the application of PCMs to the microwave regime by demonstrating an amplitude-tunable absorber based on a 50 nm thick GeTe layer. The device operates in the Ka-band (30–40 GHz) and exploits the large electrical contrast between the amorphous and crystalline phases of GeTe to achieve dynamic absorption tuning without continuous biasing. In the crystalline state (sheet resistance ∼ 80 Ω/sq.), a peak absorption of 89% is achieved at 39.1 GHz, whereas in the amorphous state (conductivity ∼ 0.04 S/m), the absorption reduces to 30%, resulting in a modulation depth of 60%. The structure exhibits an ultrathin profile with a total thickness of only 0.066𝜆 at 40 GHz. Experimental results show strong agreement with analytical transmission-line models and full-wave simulations, confirming impedance matching as the underlying mechanism. Overall, this work demonstrates the versatility of phase-change materials in enabling high performance, non-volatile, and broadband tunable devices across optical and microwave frequencies. The proposed designs provide a simple, scalable, and energy-efficient platform for next-generation free-space optical communication, integrated photonics, and reconfigurable electromagnetic systems.

Event Details
Title: Reconfigurable Photonic and Microwave Devices Based on Phase Change Materials (PhD viva voce)
Date: June 22, 2026 at 04:00 PM
Venue: Google Meet (https://meet.google.com/oja-dwmk-ndj)
Speaker: Mr. Atchyut Phalgun (EE20D430)
Guide: Dr. Anbarasu Manivannan
Type: PHD seminar