I was pursuing this research with Professor Feng Wang in Physics Department of UC Berkeley. I benefited significantly from this station of my life. Here is my Thesis titled "Optical Spectroscopy at the Nanoscale". Alghouth most of my results are about discoverying new properties of nano-materials, I think of my own contributions are more in developing the necessary yet unavailable optical tools to enable that goal. Here are what I have done and achieved.
1. Construct a continuous wave Mid-IR laser from scratch. We had to built it because no such laser was available for sale. It is constructed as a optical paramatric oscillator that turns a high energy photon into two smaller energy photons via nonlinear interactions. It features a wide tunability, large range of mode-hop-free fine tuning and stable performance. Combining with a scanning tunelling microscopy, we used this laser to study the diamondoids (nano-sized diamonds) and their interactions with metals. (Optics Letters 2012, PRL 2013)
2. Construct a new type of optical microscope that could see individual nanotubes (1nm in diameter). This microscope enabled the first direct color imaging of individual carbon nanotubes on a camera. It uses the interference principle called optical theorem. For that reason, we call this type of microscope an optical homodyne microscope. One has to be very careful with a few technical details to successfully construct this microscope, such as separate controls of numerical apertures for excitation and collection, polarization maintainance throughout the optical path, considuration of field of view and selection of the right components. With this new type of microscope, we were able to see the many different kinds of nanotubes with different colors that are only nanoscale. (Nature Nanotechnology 2013, PNAS 2014, Nature Photonics 2015)
3. Construct a highly-integrated optical pump-probe system based on a femtosecond laser. We were able to generate pulses with wavelengths that cover from UV to 10 μm via nonlinear conversions. We also integrated microscopy systems, Raman scattering systems, absorptions spectroscopy systems and cryostat systems with this pump-probe setup (see the "spectral" photo on top of this page, credit also goes to Jonghwan Kim and Long Ju). Using this integrated system, we probed the optical properties of single layer 2D semiconductor materials and their heterostructures. (Science 2014, Nature Nanotechnology 2014)