Techniques

Watch a video tour of our lab, narrated by "David Attenborough":

Time- & Angle-Resolved Photoemission Spectroscopy

We explore unoccupied electronic bands and light-induced phases of quantum materials with time- & angle- resolved photoemission spectroscopy (tr-ARPES). We first use a short (<200 fs) "pump" pulse from a laser (Pharos, Light Conversion) to excite a sample. A short delay later, we use a UV "probe" pulse to photoemit electrons from the sample. We measure these photoelectrons' energies and momenta with a hemispherical analyzer (DA30, Scienta Omicron), and thus map electronic band dispersion and evaluate many-body interactions in the sample.

A closed-cycle He compressor can cool the sample to 6 K, allowing us to observe macroscopic quantum behavior ordinarily suppressed by thermal fluctuations. An optical parametric amplifier tunes the pump pulse from the laser between 400 nm and 20,000 nm; we use this to excite specific electronic transitions and phonon modes, and to manipulate materials via Floquet engineering.

2e Angle-Resolved Photoemission Spectroscopy

We probe strong electron correlations in quantum materials using two electron angle-resolved photoemission spectroscopy (2e-ARPES). This measurement technique is sensitive to 𝛾 → 𝑒− + 𝑒− processes, which are only allowed when electronic interactions are present. Such detection events provide insight into complex phases of matter (e.g. superconductivity, strange metallicity, and charge density waves).

This setup is laser-based (Carbide, Light Conversion), with a fundamental wavelength of 1030 nm. Using a hollow-core fiber, we implement high-harmonic generation (XUUS, KMLabs) for our photon source (20-45 eV). Detection is facilitated by two spatially separated time-of-flight analyzers (ARTOF2, Scienta Omicron), which are synchronized using a timing circuit (TDC8HP, RoentDek). Furthermore, our sample manipulator (Prevac) provides fine control over all 6 positional degrees of freedom in addition to precise temperature control (8-350 K) via a closed-cycle He compressor (Cryoarc).

Terahertz Time-Domain Spectroscopy and Polarimetry

Terahertz (THz) is part of the far infrared electromagnetic radiation with energy scale of few to tens of meV. Many interesting collective excitations and features in condensed matter systems, such as antiferromagnetic resonance and superconducting energy gaps, lie within this energy range.

We generate single-cycled THz pulses by directing short pulses (~160 fs) from a laser onto an organic N-benzyl-2-methyl-4-nitroaniline (BNA) crystal. The emitted THz pulses interact with our sample through a transmission geometry, and are subsequently focused onto a CdTe detection crystal. The THz pulses' electric fields are directly read out using electro-optic sampling. We then directly extract the THz complex transmission and complex conductivity of our sample.

The setup can also perform THz emission spectroscopy, where the sample itself serves as a THz emitter. In this measurement scheme, we relate the emitted THz signal to the photo-induced transient charge current inside the sample. A closed-cycle helium cryostat holds the sample between 4 K and 325 K.

We additionally probe nonequilibrium electrodynamics by shining visible to mid-infrared pump pulses onto the sample prior to the THz pulse. The pump photo-excites the sample above its ground state, altering its optical properties such as refractive index and complex conductivity. By varying the time delay between pump pulse and THz pulse, we take “snapshots” of sample’s electrodynamic response to the optical pump at subsequent times with a time resolution of under 200 fs. This lets us study the excitation and relaxation of quasiparticles and light-induced phases of matter.

Material systems with non-zero Hall conductivity alter the polarization and ellipticity of the transmitted light, known as Faraday rotation (or Kerr rotation in reflection geometry). With our THz polarimetry setup, we access the THz signal's polarization with sub-mrad precision.

Terahertz Pump / Magneto-Optical Kerr Effect (MOKE) Probe

In this setup, we generate strong THz pulses on the order of 100 kV/cm using optical rectification of tilted wave-front laser pulses in a LiNbO₃ crystal. This THz pulse is focused onto a sample to resonantly excite collective modes and alter the magnetization of a sample. The resulting transient change in magnetic order can then be probed by measuring the polarization rotation (Faraday or Kerr) of a time delayed IR pulse. This setup can also be adapted for non-linear and multidimensional THz spectroscopy to measure the second and third order THz conductivity and susceptibility of a sample down to a temperature of 4 K.

Terahertz Time-Domain Spectroscopy in Strong Magnetic Fields

We study collective excitations and topology-related nonlinear optical properties in complex materials using an OptiCool system. This is built around an optical cryostat that can apply strong magnetic fields of up to ±7 T perpendicular to the optical table, and access temperatures between 1.7 K and 350 K. The eight optical access ports, including one at the top, offer a high degree of freedom in experimental designs, including optical pump-optical probe measurements, THz transmission, and THz emission, all under an external magnetic field. OptiCool helps us explore the uncharted physics related to time reversal symmetry breaking within quantum materials.

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