Abstract: Reflective optical modulators that dynamically control the intensity or phase of light are essential components in modern photonic technologies, including modulating retroreflectors, spatial light modulators, reconfigurable photonic surfaces, and adaptive optical systems. However, achieving large modulation depth while maintaining angle, wavelength, and polarization independence remains a significant challenge. Phase-change materials (PCMs) have recently emerged as a promising platform for such applications because they exhibit large and reversible changes in their optical and electrical properties between amorphous and crystalline phases, enabling efficient and non-volatile modulation. This seminar presents the design, modelling, and experimental investigation of PCM-based reconfigurable devices operating in both optical and microwave regimes. In the first part of the work, reflective optical modulators based on chalcogenide phase-change materials are demonstrated for operation at the telecommunication wavelength of 1550 nm. The large refractive-index contrast of the PCM between its amorphous and crystalline phases enables a strong transition between highly reflective and absorptive optical states, resulting in a modulation depth approaching 90%. By integrating Ge–Sb–Te-based PCMs within planar cavity structures, efficient optical modulation is achieved without relying on plasmonic metasurfaces or complex nanostructures. Furthermore, an all-dielectric reflective optical modulator based on a Si/SiO₂ distributed Bragg reflector (DBR) is designed to eliminate metallic losses while maintaining near-unity reflectivity. In addition, controlled partial phase transitions enable multiple intermediate reflectance states, allowing multilevel optical modulation for high-density optical data encoding. In the second part of the work, the tunable electrical properties of phase-change materials are utilised to realise an ultrathin amplitude-tunable microwave absorber operating in the Ka-band (30–40 GHz). By exploiting the large resistance contrast of GeTe between its amorphous and crystalline phases, the absorber demonstrates significant modulation of absorption without continuous biasing, while maintaining a subwavelength thickness.

Event Details
Title: High-Efficiency Broadband Reflective Optical Modulators at Telecommunication Wavelengths Using Phase-Change Materials
Date: March 25, 2026 at 3:00 PM
Venue: ESB 210B / Google Meet (https://meet.google.com/tao-tctk-xuv)
Speaker: Mr. Atchyut Phalgun M (EE20D430)
Guide: Dr. Anbarasu Manivannan
Type: PHD seminar

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