TECHNIQUES
REFLECTIVITY & PHOTOLUMINESCENCE
These experiments use light to probe the states present in the material under study. By shining a broadband light source at a sample and measuring the reflected spectrum, we can gain information about the spectrum of energies where states absorb the light. On the other hand, exciting the sample with a high-energy narrowband laser and observing what light is emitted can reveal relaxation pathways, scattering mechanisms, and excitonic physics.
REFLECTIVE MAGNETIC CIRCULAR DICHROISM & MAGNETO-OPTICAL KERR EFFECT
These techniques allow researchers to use light to measure the magnetic ordering of a material under study by considering the interaction of the lights polarization and the orientation of spins in the sample. In both measurements, the data provides information about the magnetization in the direction of the propagation of the incoming light and allows researchers to measure ferromagnetic hystersis loops, magnetic transitions under applied fields, and even ferrimagnetic order.
SECOND HARMONIC GENERATION
Second harmonic generation (SHG) is an optical technique that employs ultrafast laser pulses to drive a material response into the second order nonlinear regime. In this regime, an optical response is forbidden along axes of symmetry in the crystal, making SHG a powerful tool for identifying crystal axes or measuring symmetry breaking, such as the time-reversal symmetry breaking seen in ferromagnets.
TIME-RESOLVED REFLECTIVITY, MAGNETO-OPTICAL KERR EFFECT, & SECOND HARMONIC GENERATION
Time resolved spectroscopies such as these employ sequences of ultrafast pulses with precise control over their relative arrival time. In this way, we can use the initial pulse to excite the system nearly instantaneously and the following pulse can be used to measure changes in the system response over time. By combining this idea with measurements that probe optical states (reflectivity), structure (second harmonic generation), or magnetic ordering (magneto-optical Kerr effect), we can build up the dynamics of myriad material properties in response to optical excitations.
MULTIDIMENSIONAL COHERENT SPECTROSCOPY
An extension to the time-resolved techniques described above, multidimensional coherent spectroscopy (MDCS) leverages multiple pulse sequences to access higher-order nonlinear responses in the material under study. By controlling the relative phases of the pulses and their separation in time, the experiment allows us to build maps that correlate the energies at which where photons where absorbed and emitted. This powerful feature provides access to the entire series of quantum states that an electron occupied over a given time frame, allowing for direct observation of relaxation pathways, coherent coupling processes, and more.