Ishaan Lohia

United States of America

University of Cambridge



Mid-infrared (MIR) light is a band of the electromagnetic spectrum from ~2 − 20 𝜇𝑚 wavelengths. Many molecular vibrations can be triggered or excited by MIR light, which is useful for identifying specific molecules. Because of this, MIR light has applications ranging from medical analysis to astronomy. Current MIR detectors require energy-intensive and costly cryogenic cooling to operate. My research aims to overcome this hurdle by converting MIR light to visible light, which is easier to detect at room temperature. My project demonstrates this MIR-to-visible frequency upconversion using 4-nitrothiophenol (4NTP) molecules in tailored gold nanostructures. These nanostructures, in which a single layer of 4NTP molecules are sandwiched between a gold microresonator (~6 𝜇𝑚 diameter disk) and a gold nanoparticle (80 𝑛𝑚 diameter sphere), confine MIR and visible light into a small ~1 𝑛𝑚 gap. Using a MIR laser to excite the -𝑁𝑂! vibration in 4NTP in the nanoparticle-on-microresonators increases the intensity of specific Raman peaks, demonstrating upconversion. These Raman peaks are visible-wavelength signals that correspond to specific molecular vibrations. Ultrafast MIR-Raman experiments are used to estimate how long the 4NTP’s -𝑁𝑂! group vibrates, known as decay time, found to be ~0.8 picoseconds. We report a MIR-to-visible upconversion quantum efficiency of ~5 ∙ 10″#, meaning for every 50 million MIR photons that enter the system, only 1 successfully upconverted visible photon is detected. This research is a first step to understanding how molecules can be used to detect MIR light through upconversion, and future experiments will attempt to improve this system’s quantum
efficiency and detect low-power MIR sources.