fs-Laser Spectroscopy of Quantum Materials
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Research
My group has long term goal to search exotic light-matter interaction and photoinduced intriguing phenomena in emergent materials. To understand light-matter interactions, we develop ultrafast optical techniques to study, control, and manipulate the function of condensed matters with novel properties.
Our research on novel phenomena in quantum materials covers
(i) Spin control and magnetism: application of M-RAM, quantum information, spintronics, and quantum magnet
(ii) Phase transition: application of phase change memory
(iii) Metal-insulator transition: application of device switching
(iv) Ferroelectricity and multiferroics: application of ferroelectric random access memory (FE-RAM) and quantum electromagnet
(v) Correlated electron systems and their nanostructures: developing fundamental understanding of physics
To study the above topics, we develop optical techniques of
(I) Pump and probe,
(II) Nonlinear optics,
(III) Faraday or Kerr rotation,
(IV) Pulse shaping, and
(V) Confocal scanning microscopy
with cryogenic capability (<10 K) in a superconducting magnet (up to 5 Tesla).
Some examples of our research topics include:
(a) Spin flip dynamics and spin controls:
We design time-resolved spectroscopy including Faraday and Kerr probes. We study the photoinduced spin dynamics with the aim to control spin precession and spin flip on ultrafast timescale of fs-ps. We can use photon to manipulate spin dynamics in various condensed matters (Fig. 1). We develop and shape the optical pulse to achieve the spin control.
(Fig.1)
(b) Photoinduced phase transition, hidden states, (de)magnetization, (de)polarization,
and metal-insulator transition : (applicable for M-RAM, FE-RAM or memory devices)
We study order parameters of strongly correlated electron systems, such as spin and electric dipole orders, under ultrafast laser illumination, aiming to find functionalities resulting from large photoinduced effects. These include ultrafast (de)magnetization, (de)polarization, metal-insulator transitions and how these dynamics couple to other process when size or substrate plays an important role (Fig.2). In some exotic materials, we study highly nonequilibrium system and look for the existence of hidden states that cannot be reached through thermal dynamic process. Such hidden states do not exist during or after the material growth, instead, they only can be found through certain extreme environments or special electronic heterostructures (Fig. 3) , and exhibit very unusual physical properties with limited lifetime (Fig. 4-5).
(Fig.2)
(Fig.3)
(Fig.4)
(Fig.5)
(c) Coupling mechanisms after light-matter interaction:
We study fundamental couplings in condensed matter physics (Fig. 6). Using pump beams to create an optically perturbed system, we study relaxations and coupling of electron-phonon, spin-lattice, magnetoelectric multiferroics and so on. The relaxation process can be measured through different optical probes like (I)-(III).
(Fig.6)
(Fig.7)
(Fig.8)
(d) Application of nonlinear optics:
We use nonlinear optical processes to study and analyze crystal or surface structure (Fig. 7). Polar or ferroelectric materials without an inversion center allow the generation of odd harmonics, which is a great tool for understanding polar properties (Fig. 8).
(e) Coherent control (applicable for quantum informaiton):
Similar to spin control, we can also control the lattice vibration in novel materials (Fig. 1), and we study the resulting impact on other properties which usually has strong correlation with lattice distortions. We use light polarization or pulse shaping techniques to control and generate high frequency (THz) optical phonons.
