Over the past few decades, time-resolved ultrafast spectroscopy measurements have emerged as new frontiers of condensed matter physics via manipulating and detecting different orders of quantum materials.
By combining some traditional experimental techniques with ultrafast pulse lasers, such as angle-resolved photoemission spectroscopy (ARPES), X-ray diffraction (XRD) and optical spectroscopy, ultrafast pump-probe experiments can provide new insights into the electronic nature of quantum materials. This is done by selectively exciting certain quasiparticles or different degree of freedoms with ”pump” pulses, and subsequently tracking their decay pathways back to the equilibrium state with the ”probe” beam, i.e. time-resolved ARPES/XRD/optical spectroscopy.
For strongly correlated materials, whose electronic, spin and orbital degrees of freedom are usually coupled and acting on multiple energy scales, infrared optical spectroscopy is a traditional and powerful tool for exploring the electronic properties, e.g. band gaps, single particle and collective excitations. Using commercial ultrafast lasers, which are often designed to generate narrowband pulses in NIR range (∼800 nm, ∼1.5 eV), an ultrafast 800 nm optical pump- 800 nm optical probe spectroscopy system can be constructed.
However, the energy of NIR pulses is completely overwhelmed, for the low-energy excitations in strongly correlated materials, such as superconducting energy gap, Josephson plasma resonances, and specific lattice vibrations, always extend from gigahertz to mid-infrared (MIR) frequencies. So if one wants to selectively control the low-energy excitations in strongly correlated materials without deliver- ing excess energy to other excitation pathways, it is a must to find a way to lower the energy scale of pump pulses. Thanks to recent developments in nonlinear optics, ultrafast laser pulses ranging from MIR to terahertz (THz) frequencies can be generated using non-linear materials, such as ZnTe, GaSe, and LiNbO3 crystals.
Researchers at Peking University recently constructed a tunable ultrafast broadband optical (wavelengths of 0.4μm–15μm, ∼80meV–3eV ) pump, THz (∼0.25–2.5 THz, ∼1meV–10meV) probe spectroscopy system in reflection geometry, which can provide a powerful tool for manipulating and detecting different orders in strongly correlated materials.
A two-output optical parametric amplifier (OPA), pumped with an amplified Ti:sapphire laser system producing 800 nm, 35 fs pulses at a 1 kHz repetition rate, is used for pump pulse generation. The signal and idler beams of the OPA can be used as NIR pump pulses directly. To obtain the MIR pump pulses, two signal beams are used for difference frequency generation (DFG) collinearly on a 1-mm-thick z-cut GaSe crystal. By tuning the frequency of the two signal beams and the orientation of the GaSe crystal to meet the phase-matching conditions, MIR pulses with tunable polarization ranging from 3 to 15 μm can be generated. The THz probe pulses are generated by 800 nm pulses using a 1-mm-thick (110) ZnTe crystal and the THz profile was detected via electro-optic sampling. Samples sit at the end of a cold finger in a helium continuous-flow cryostat, which is capable of reaching a base pressure as low as 2×10−5 Pa and a temperature of 4 K.
As an application of the ultrafast spectroscopy system, they performed near and mid-infrared pump, c-axis terahertz probe measurement on a superconducting single crystal La1.905Ba0.095CuO4 with Tc=32 K. A very sharp Josephson plasma edge develops near 18 cm−1 (∼0.54 THz) can be clearly seen in the reflectivity along c-axis below Tc, which indicates a Josephson plasma resonance mode. Using the tunable ultrafast broadband optical pump THz probe spectroscopy system, they observe the redshift of the original Josephson plasma edge and the emergence of a new light-induced edge at a higher energy within a very short time after excited by the strong NIR/MIR pulses (∼1.5 ps). The results imply that the light can induce new Josephson plasmon modes with different coupling strengths below Tc.
The construction of the ultrafast spectroscopy system is described in the article entitled Tunable near- to mid-infrared pump terahertz probe spectroscopy in reflection geometry, published in the journal Frontiers of Physics. This work was led by Nan-Lin Wang from Peking University.