Recent Research on Optical Properties of Metals

Why in News?

Researchers from the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bengaluru, have demonstrated for the 1^st time that the optical properties of metals can be actively modified using mechanical strain.

• Plasmon Resonance - Metals have a unique ability to trap and condense light waves into volumes significantly smaller than the wavelength of the light itself.
• The Driver- It is dictated by the material's plasma frequency, which has traditionally been considered unchangeable because it depends entirely on the metal's native free-electron concentration.
• Applications - This property underpins sub-wavelength photonic circuits, ultrasensitive biosensors for cancer diagnostics, and optical metasurfaces. 
• Recent Experiment - To isolate the exact impact of mechanical deformation on light interaction, the team grew 2 identical 10-nanometer-thick films of Titanium Nitride (TiN).
• It mimics the optical behavior of gold but stands out because it is fully compatible with Complementary Metal-Oxide-Semiconductor (CMOS) chip fabrication.
  • Film A (Control) - Grown completely strain-free on a Magnesium Oxide (MgO) substrate.
  • Film B (Strained) - Subjected to controlled, in-plane tensile stretch by growing it on top of an Aluminium Scandium Nitride (Al0.3Sc0.7N) buffer layer, which features a wider crystal lattice space.
• Using Electron Energy Loss Spectroscopy (EELS) inside a highly specialized scanning transmission electron microscope, the researchers mapped the plasmon energy levels at near-atomic resolution across the films.

• The Discovery Mechanism - The mechanically stretched (strained) TiN film exhibited a pronounced blue shift of 0.30 to 0.45 electron volts (eV) in its plasmon resonance relative to the unstrained baseline.
• First-principles Density Functional Theory (DFT) calculations revealed exactly why this happens:
• Tensile Strain Application - Pulling the crystal lattice mechanically lowers the energy barrier required to form nitrogen vacancies within the TiN structure.
• Vacancies as Donors - These newly formed nitrogen vacancies act as intrinsic electron donors.
• Frequency Shift - The sudden surge in free-electron concentration elevates the metal's core plasma frequency, causing the observed blue shift toward higher energy levels.

Reference

PIB | Optical properties of metals