Finding the metal-insulator transition of ultra-thin silver films in real time and in situ

-Newly graduated (June 2020) Dr. Fu Liu demonstrated in this APL Photonics paper (APL Photon. 5, 076101 (2020)) that a tilted FBG could detect in real time the exact point at which an ultrathin silver films ceases to conduct electricity when being slowly etched, i.e. at the so-called “percolation threshold” of the metal. 

Experimental validation was performed by simultaneous measurements of the TFBG spectrum and the sheet resistance of the silver film, showing a sudden decrease of the conductivity that was simultaneous with the total attenuation of the resonances in the TFBG spectrum when the film thickness decreased to 18 nm.  Simulations and other experiments showed that at percolation, the ultrathin metal coating on the surface of the standard single mode optical fiber acts as an anti-reflection coating for light at wavelengths near 1550 nm.

This work was performed in collaboration with Prof. Tuan Guo (a former postdoctoral researcher in our group) and Dr. Xuejun Zhang from Jinan University in Guangzhou (China).

Killing cancer cells in tumors selectively with a light-activated and optically thermostatted optical fiber probe

-Also recently graduated Dr. Sondos Alqarni published the main results of her PhD program on a tilted FBG device designed to deliver controlled amounts of heat to destroy cancerous cells while preserving the surrounding healthy tissue. The results were impressive enough for Sondos’ paper to be selected as an “Editor’s pick” in the March 10, 2021 ssue of Applied Optics.  The paper can be found here: Applied Optics 60, 2400-2411 (2021).

In this paper, it is shown that heat generated by a pump laser emitting near infrared light in the fiber core is redirected towards the fiber surface by the tilted grating. Upon reaching the surface the light is absorbed by a thin coating that heats up instantly as a result of the absorption.  What is important in this process is to control the local temperature to circumscribe the heat-affected zone to the tissue affected by the cancer.  In order to do this simultaneously, the grating transmission spectrum is measured simultaneously by another broadband light signal propagating in the same fiber and reflected back towards a spectrum analyzer which measures the local temperature from the wavelength shifts of the grating resonances.   With real-time feedback of the local temperature vs pump power, and adjusting the duration of the heating process, the size of the “death zone” of the cells can be controlled accurately.

The whole process has been modeled using a multiphysics simulation tool and several experiments demonstrate the accuracy of the model predictions.

This work was the result of a multidisciplinary effort with Sondos’ supervisor, Prof. Chris Smelser (COIL group) from the Dept. of Electronics and Prof. Bill Willmore (Willmore Lab), from the Department of Biology.