Ultrastrong light–matter coupling and cavity vacuum-driven materials

Light and matter can strongly mix to form hybrid particles called polaritons. In recent years, polaritons in the so-called ultrastrong coupling (USC) regime have attracted much attention both from fundamental and applied points of view. A variety of nonintuitive phenomena and novel ground states with exotic properties have been predicted for systems in the USC regime, some of which have been experimentally realized. Moreover, USC regime is promising for applications in quantum computing and quantum information processing. [Read more]

Engineering quantum vacuum around matter inside a cavity can also lead to modifications of macroscopic properties and new ground states. Contrary to laser induced transient and metastable phases of matter, coupling to quantum vacuum fluctuations can result in novel phases of matter in a dark cavity.

Physical phenomena at high magnetic fields

There are a variety of elementary and collective terahertz-frequency excitations in condensed matter, whose magnetic field dependence contains significant insight into the states and dynamics of the electrons involved. Often, determining the frequency, temperature, and magnetic field dependence of the optical conductivity tensor, especially in high magnetic fields, can clarify the microscopic physics behind complex many-body behaviors of solids.

To carry out this research direction, I use RAMBO – Rice Advanced Magnet with Broadband Optics. This facility houses an ultracompact pulsed magnet capable of producing a peak field of 30 Tesla, combined with an arsenal of state-of-the-art instruments for modern materials research. A unique feature of this system is the marriage of a strong magnetic field and ultrashort laser pulses from nearly “DC to daylight.” This unconventional coupling of two extreme conditions will likely lead to new scientific knowledge, advancing the frontier of materials research.

Terahertz and ultrafast spectroscopy of quantum materials

Quantum materials involve many materials classes with peculiar behavior. Ultrafast spectroscopic techniques allow disentangling different degrees of freedom to understand their behavior and investigate novel transient states of quantum matter. On the other hand, quantum materials can be used to control light propagation due to its unique light matter interactions.

In pump probe spectroscopy, an ultrafast laser pulse is split into two: a strong pump pulse and a weaker probe pulse. The pump beam is used to excite various degrees of freedom in a material and, therefore, drive the system out of equilibrium, while the probe is used to monitor the rise and decay of this transient state as a function of pump-probe delay. Thus, it can reveal various dissipation mechanisms and even discover new forms of excitations.

Terahertz time-domain spectroscopy (THz-TDS) has proven to be an invaluable tool to study low-energy excitations in condensed matter systems including intraband transitions, superconducting gap excitations, phonons, magnons, and plasmons. THz-TDS allows measuring both magnitude and phase of the electric field. On the other hand, search for materials and development of new devices to manipulate polarization, amplitude, or phase of sub-THz electromagnetic waves is necessary for faster wireless communications.

Coherent phonons / picosecond ultrasonics

Time domain Brillouin scattering known as picosecond ultrasonics or coherent acoustic phonon (CAP) spectroscopy is a subset of ultrafast pump-probe spectroscopy, which relies on generation of coherent acoustic phonons. When a strong ultrafast (<1 ps) optical pump pulse is incident on a surface of opaque material, it can generate a traveling CAP wave (or strain wave) due to inhomogeneous pump light absorption. The CAP wave locally perturbs the refractive index as it traverses the material. Thus, when an optical probe pulse is incident on a sample at a delayed time its reflection or transmission is modulated by the CAP wave. Interference of probe light waves reflected from the surface of the material and the traveling CAP wave results in an oscillatory time dependent reflectivity/transmissivity signal (Brillouin oscillations). Amplitude, decay, and frequency of Brillouin oscillations are highly sensitive to optical and elastic material properties. This technique finds many applications in materials characterization and their control by transient strain.