ECE PhD Prospectus Defense: Jiaze Yin

Title: High-speed Mid-infrared Photothermal Microscope for Dynamic and Spectroscopic Imaging

Presenter: Jiaze Yin

Advisor: Professor Ji-Xin Cheng

Chair: Professor Lei Tian

Committee: Professor Irving Bigio, Professor Darren Roblyer, Professor Lei Tian

Abstract: Mid-infrared spectroscopic imaging using inherent vibrational contrast has been broadly used as a powerful analytical tool for sample identification and characterization. The low spatial resolution and large water absorption associated with the long IR wavelengths hinder its applications to study subcellular features in living systems. Recently developed mid-infrared photothermal (MIP) microscopy overcomes these limitations by probing the IR absorption–induced photothermal effect using visible light. MIP microscopy yields sub-micrometer spatial resolution with and reduced water background. However, subject to the sensitivity of measuring small modulation over the probe laser background, the imaging speed of current MIP microscope is limited to tens of seconds per frame. The low imaging throughput makes it hard to visualize the living dynamics. In addition, rich molecular information beneath the spectroscopic domain is unveiled due to the slow acquisition process. In this prospectus, we explored the solutions for improving the imaging speed and the spectral throughput. In the first aim, we invent a video-rate MIP microscope for visualizing dynamics happening in the living system. This design employs a lock-in free wide-band detection scheme for reaching single pulse sensitivity, enabling imaging line rate over 1kHz. By using a quantum cascade laser (QCL) with fast wavelength tuning speed, we propose a hyperspectral MIP imaging method and demonstrate the compositional analysis in complex systems. In the second aim, we further explore the benefits of wide-band photothermal detection and demonstrate a new approach for acquiring broad high-resolution spectra in microseconds scale. By using the fast-tuning QCLs array, a sequential laser pulses composed of 32 distinctive colors excite the sample within 4 microseconds. Absorbed energy of different colors can be evaluated from the recorded heating curve by calculating its first derivative over time. The absorption spectrum is then computed by normalizing the deposit energy with IR pulse energy, providing a new scheme for ultrafast spectroscopic photothermal imaging. The system will allow hyperspectral imaging in a window of 150 cm-1 with 5 cm-1 spectrum resolution in one second, allowing for multiplexed molecular analysis in real-time.