RESEARCH

Investigating light-matter interaction with THz-MIR light

Over the past few decades, highly-intriguing ultrafast phenomena in condensed matters have been discovered via light-matter interaction, owing to the remarkable developments of material science and laser science. Our research group is interested in (i) investigating fundamental properties of novel materials by using light, (ii) controlling the material properties by using light, and (ii) the control of light by using materials.

Especially low-frequency laser pulses in terahertz (THz) frequency or mid-infrared (MIR) region is essentially important because various kinds of electromagnetic responses in condensed matters exist in this energy scale (~1-100 meV). Their short pulse duration less than 1 picosecond also allows us to track ultrafast dynamics in nonequilibrium materials by time-resolved spectroscopy. We are developing THz and MIR light sources and laser techniques with nonlinear optics, and also investigating light-matter interaction in various kinds of novel materials.

Novel electromagnetic responses in quantum materials

The properties of solids are determined by vast degrees of freedoms that are composed of charge, lattice distortion, and spins. Many-body interactions between them gives rise to highly-intriguing phases of matter, such as structural phase transition, metal-insulator transition, superconductivity, or magnetism. These phase transitions are often characterized by spontaneous breaking of “symmetry”, and the symmetry-broken ground states can exhibit remarkable responses to external field, like infinite conductivity in superconductors. In addition, recent interests of condensed matter physics have been also focused on “topology” for classification of materials since nontrivial electronic states appear as represented by relativistic massless fermions. Novel response functions closely related to the geometric (Berry) phase can also manifest itself in topological materials. Our research group is interested in revealing the novel electromagnetic responses in quantum materials and finding their unprecedented functionalities by means of state-of-the-art ultrafast laser technique.

Efficient THz harmonic generation in Dirac semimetal Cd3As2

B. Cheng, N. Kanda, RM et al., Phys. Rev. Lett. 124, 117402 (2020).

High-harmonic generation, i.e., the production of coherent high-energy photons, has been developed in gaseous media for attosecond science or high-resolution ARPES. Recently it has been also reported in solids with near-infrared or mid-infrared pulses, which has opened a new research field for nonperturbative nonlinear light-matter interaction. If the high-harmonic generation is also realized in THz frequency at room temperature, it would be a key technology for frequency conversion and mixing in high-speed electronics and in sensitive detection of cosmic microwave background.

We discovered the extremely efficient THz harmonic generation in 3-dimensional Dirac semimetal Cd3As2 at room temperature. Furthermore, by subcycle time-resolved study of THz pump-THz probe spectroscopy, we also revealed that the origin of the remarkable nonlinearity arises from coherent intraband acceleration of massless Dirac fermions across the Dirac node. It has been theoretically anticipated for over 10 years but has been not demonstrated so far. Our results provide clear insights for extremely nonlinear current of Dirac electrons driven by THz field under an influence of scattering and will facilitate designing novel functional devices for high-speed electronics and photonics based on topological semimetals.

THz anomalous Hall effect in Weyl antiferromagnet Mn3Sn

T. Matsuda, RM et al., Nature Commun. 11, 909 (2020).

Control of magnetism has been a key issue for spintronic devices. From the viewpoint of manipulation speed, antiferromagnets are promising materials since spin precession motions occur typically at THz frequencies, a few orders of magnitude higher than that in ferromagnets. However, readout of the magnetization information in antiferromagnets is still difficult since they are not sensitive to external field, which has made their practical application challenging. Recently, large anomalous Hall effect (AHE) in a noncollinear antiferromagnet Mn3Sn comparable to ferromagnets has been discovered in spite of the vanishingly-small net magnetization. Therefore, deep understanding of the dynamical properties of AHE in Mn3Sn in the THz frequency is important to develop novel devices based on fast motion of antiferromagnetic spins and the large response to external field.

We observed the AHE in Mn3Sn by broadband polarization-resolved THz spectroscopy. Large anomalous Hall conductivity σxy ~ 20 Ω-1 cm-1 at THz frequencies was clearly observed as polarization rotation of light in a non-contact way. A peculiar temperature dependence corresponding to the breaking/recovery of symmetry in the spin texture was also identified. Observation of the THz AHE at room temperature demonstrates the ultrafast readout for the antiferromagnetic spintronics. Furthermore, Mn3Sn has been also interested as a possible candidate for Weyl (antiferro)magnets, which host massless Weyl fermions with broken time-reversal symmetry. Our study will lead to the study of nonequilibrium dynamics in Weyl antiferromagnets by ultrafast detection of AHE.

Collective Higgs amplitude mode in superconductors

RM and R. Shimano, Phys. Rev. Lett. 109, 187002 (2012).

RM et al., Phys. Rev. Lett. 111, 057002 (2013).

RM et al., Science 354, 1145 (2014).

RM et al., Phys. Rev. B 96, 020505(R) (2017).

K. Katsumi, RM et al., Phys. Rev. Lett. 120, 117001 (2018).

S. Nakamura, RM et al., Phys. Rev. Lett. 122, 257001 (2019).

These experiments have been conducted in Shimano group.

Development of light souce and optical technique

Time-domain spectroscopy with phase-stable THz-to-MIR pulses

Under construction

Intense THz pulse generation and subcycle time-resolved study

THz spectroscopy has been developed in the past few decades. In particular, the intense THz pulse generation technique has been remarkably developed and the peak field strength up to 1 MV/cm has been realized at around 1 THz frequency. Such an intense THz field is now available on tabletop experiments and has led to the studies of nonlinear light-matter interaction for control of matters by intense low-frequency THz field.

We have developed a pump-probe spectroscopy system with intense THz pulse generation based on highly-stable Yb-based regenerative amplified laser. It has realized nonlinear transmission spectroscopy to observe THz high-harmonic generation, as well as THz pump-THz probe spectroscopy to observe the carrier dynamics induced by the THz pump. In particular, the sub-cycle time-resolved measurement during the phase-locked THz pump pulse irradiation is essential for ultrafast dynamics in matter driven by the light wave.

See also: B. Cheng, N. Kanda, RM et al., Phys. Rev. Lett. 124, 117402 (2020).

Polarization-resolved THz time-domain spectroscopy

The quantum Hall effect and the anomalous Hall effect have been ones of the central issues in modern condensed matter physics with their topological aspects characterized by the Berry phase. The Hall conductivity is usually measured with DC (static) electric transport, but it can be also measured in non-contact optical means because the Hall conductivity is directly related with the polarization angle of light. When the Hall conductivity is nonzero, the polarization of incident light slightly rotates, which is also known as the Faraday effect. Especially the Faraday rotation in the low-frequency THz regime can be directly connected with the DC Hall effect and also with optical excitation. Therefore, the polarimetry with THz pulses can play important roles for investigating the frequency dependence of the Hall effect, cyclotron resonance, or Landau-level transition with sub-picosecond time resolution.

We have developed polarization-resolved THz time-domain spectroscopy system with high precision of several tens of μrad in the frequency range from 0.5 to 2.0 THz (2 to 8 meV) with 20-min accumulation time. Broadband THz polarimetry up to 6 THz (25 meV) has been also realized in a regenerative amplified laser system for time-resolved study of the THz Hall effect.

See also: T. Matsuda, RM et al., Nature Commun. 11, 909 (2020).