MSE PhD Prospectus Defense of Alket Mertiri

10:00 am on Monday, November 26, 2012
12:00 pm on Monday, November 26, 2012
8 Saint Mary's Street, Rm 940 (enter through Photonics Main Office)
TITLE: Mid-infrared Photothermal Heterodyne Spectromicroscopy

ABSTRACT: Photothermal spectroscopy (PTS) has rapidly emerged as one of the most sensitive label-free optical Spectroscopic methods, rivaling even well established methods such as fluorescence spectroscopy. Within the past few years, many innovative observations have been demonstrated in the visible regime. It is a label-free method that does not suffer from photobleaching or blinking and it has found great applications in biology and in chemical analysis. PTS has shown the ability to measure the absorption cross section of individual nanoparticles. The method has been used to image single nonfluorescent azo dye molecules in room temperature and to directly
characterize biological features such as mitochondria and red blood cells. Despite great breakthroughs in the visible regime, the method has not been explored in the infrared regime where most of the biological molecules have characteristic vibrational modes.

This thesis mainly focuses on demonstrating a new technique to measure the mid-infrared photothermal response induced by tunable high
power lasers such as Quantum Cascade Lasers (QCLs). We are utilizing heterodyne detection using a visible laser to directly detect weak mid-infrared normal modes absorption using ultrasensitive photodetectors. Tunable lasers that can access still stronger modes will facilitate photothermal heterodyne mid-infrared vibrational spectroscopy. Our mid-infrared photothermal spectroscopy (MIPS) will be used for biosensing applications and it has shown promises to replace current methods such as Fourier transform infrared spectroscopy (FTIR).

To further increase the sensitivity of our MIPS method we will utilize enhancement of light though localized surface plasmons. Plasmonic Metamaterials will be engineered for functional studies on monolayers of proteins and other biomolecules. Informed by extensive numerical simulations, electron beam lithography is used to fabricate nanostructures that enhance selected vibrational infrared “fingerprint” modes
of biomolecules. We will take advantage of the strong interaction between light and matter and will investigate properties of the material that are difficult to detect through conventional spectroscopic methods. Fourier Transform Infrared Microscopy using a broadband 1200
K Globar source shows sensitivity at the attomole level. Tunable Infrared QCLs has a spectral brightness more than 105 greater than the Globar blackbody source and >102 greater than mid-infrared synchrotron radiation. It promises to provide unprecedented sensitivity when
combined with engineered plasmonic metamaterials. A home-built IR microscope is used in combination with a tunable QCL, a plasmonic substrate and lock-in detection to measure the sensitivity and specificity of performing vibrational infrared spectroscopy on biomolecules.

Using high power laser sources we will develop a high-resolution nonlinear absorption spectromicroscopy method that may increase both the sensitivity and the spectral resolution for identifying weakly absorbing molecules. Two-photon pump-probe excitation technique is used
to investigate ultrasharp resonances by the photothermal signal in the nonlinear regime. Liquid crystal sample is used as a proof of concepts sample in the smectic to nematic phase transition to demonstrate the spectral sharpening effect. Future projects include studying the mid infrared photothermal method using our ultrafast femtosecond laser system. We intend to build an infrared microscope for pump-probe photothermal microscopy and we expect to achieve high spectral resolution and increase the sensitivity of mid-infrared photothermal signal.

COMMITTEE: Professor Shyamsunder Erramilli, MSE/CAS, Physics; Professor Hatice Altug, MSE/ECE; Professor Jerome C. Mertz, BME; Professor David Bishop, MSE/ECE/CAS, Physics