Shining light on quantum matter
Our goal is to deepen our understanding of quantum materials through their interaction with light, using wavelengths from the far-infrared through the visible regions of the spectrum to probe their structure and dynamics. We use a broad range of optical techniques, including:
Time-domain terahertz emission spectroscopy
Time-resolved magneto-optic Kerr effect
Time-resolved magneto-optic microscopy
In our lab, these tools are integrated with microscopes that enable spatially-resolved mapping of optical response on a sub-micron length scale. The aims of our measurements can be divided into two broad classes: (1) characterization of symmetry breaking phase transitions in the ground state, and (2) probing the dynamics of the excitations above the equilibrium state.
Tools that probe symmetries
This is our basic tool for exploring symmetry breaking. Polarimetry is the measurement of the polarization state of light, which may change upon reflection from or transmission through a medium under study. Specific changes in polarization are the unique signatures of different forms of symmetry breaking. For example, rotation of polarization on reflection only occur with the breaking of time-reversal symmetry. We perform spatially-resolved measurements of local symmetry breaking by scanning samples under a laser beam focused to sub-micron dimensions using high-power microscope objectives. From these scans we obtain maps of domains of magnetic, nematic, and superconducting order. Our microscopy set-ups are interfaced with cryostats with base temperature of 2 K and applied magnetic fields to 7 Tesla. Our 4 K system has an in situ device for application of uniaxial pressure.
The apparatus for polarimetric detection is illustrated below. Rotation of polarization on reflection unbalances the two outputs of the Wollaston prism, resulting in a voltage difference between the two photodetectors. The tricky part of the measurement is that polarization rotation can derive from either birefringence (which implies breaking of rotational symmetry) or the Kerr effect (which implies breaking of time-reversal). In principle, it is straightforward to distinguish them by rotating the sample: birefringence yields a detector output proportional to cos2θ whereas as the Kerr effect is independent of θ. However, in practice, rotating the sample is highly inconvenient when it sits inside of a magneto-optical cryostat. To circumvent this problem, we use a pair of λ/2 waveplates, one before the sample and one after, that rotate the plane of polarization of the light. Co-rotating the waveplates synchronously simulates rotating the sample. To actually get this scheme to work required a lot of tricks to compensate for unavoidable birefringence in the optical setup. This basic setup is now one of the most important tools in the toolbox.
To enhance signal-to-noise ratio, we typically modulate the temperature of sample at frequencies near 1 KHz using a chopped CW laser. Recently we installed miniature coils inside our cryostats to enable modulation of the optical properties by an ac magnetic field. We used these devices to measure domain wall mobility in Co3Sn2S2 and to discover the linear magneto-optic birefringence in Eu2CdP2, which is a new probe of time-reversal symmetry breaking.
Terahertz emission spectroscopy
Terahertz emission spectroscopy is our tool for measuring the photogalvanic effects (PGE), which is the name given to rectified photocurrents that arise in media that lack inversion symmetry. This distinguishes PGEs from photoconductive or photovoltaic effects, where inversion symmetry is broken by the application of an external electric field or inhomogeneous chemical doping. A further distinctionis that PGE-driven currents typically depend on the polarization state of the photoexcitation in a way that reflects the point–group symmetry of the medium. The earliest measurements of PGEs were conducted using leads deposited directly onto the sample to measure the current generated from a continuous-wave laser source.
Measurements without the need for contacts are performed in our lab with the use of picosecond and femtosecond lasers. Photocurrents generated in a sample by nonlinear interaction with the pump laser radiate THz pulses into free space that are collected by off-axis paraboloids and imaged onto a ZnTe crystal for time-resolved electro-optic detection. By detecting the photocurrent optically, we eliminate artifacts from contacts and enable continuous readout of the photocurrent direction using THz polarizers.
Tools that probe dynamics
Pump Probe Transmission and Reflection
Pump-probe measurements of transient absorption and reflection are the time-honored, and most straightforward, methods of exploring excited state dynamics. Exciting a material with laser pulses populates quantum states above the ground state, changing its optical response, as characterized by the index of refraction for example. This change is then detected as a change in transmission or reflection of a time-delayed probe pulse.
Time-resolved magneto-optic Kerr effect (TR-MOKE)
In this method, a short duration (∼100 fs) laser pulse induces a transient change in the magnetic anisotropy. The ensuing precession of the magnetization, M, about the new preferred axis leads to oscillations of its component parallel to the optic axis, Mz, which are detected via the polar Kerr effect. An example from measurements on a spiral antiferromagnet are shown below.
A TR-MOKE microscope
In recent work we extended TR-MOKE to the spatial domain. A simplified layout of the setup for TR-MOKE microscopy is shown in below. The 4f optical system equipped with 2-axis galvo-driven mirrors enables continuous 2D scanning of the pump focus while the location of the probe is fixed . Spin waves photoexcited in one location can be probed remotely at a later time, enabling an all-optical ultrafast investigation of spin wave transport with micron-scale spatial resolution and sub-microradian polarization sensitivity.