Abstract-Switchable generation of azimuthally- and radially-polarized terahertz beams from a spintronic terahertz emitter

 

Hiroaki Niwa, Naotaka Yoshikawa, Masashi Kawaguchi, Masamitsu Hayashi, and Ryo Shimano

(a) THz generation from a spintronic THz emitter. Transient spin current 𝐣s${\mathbf{j}}_{\mathrm{s}}$ generated upon photoexcitation converts into charge current 𝐣c${\mathbf{j}}_{\mathrm{c}}$ via the inverse spin Hall effect, which radiates THz electric field with the polarization perpendicular to the magnetization 𝐦̂ $\hat{\mathbf{m}}$. (b) Schematic of the experimental setup. (c) and (d) Schematic presentation of generating azimuthal and radial polarization by converting HE21${\text{HE}}_{\text{21}}$ mode with different orientations, respectively.

https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-29-9-13331&id=450178

We propose and demonstrate a method of generating two fundamental terahertz cylindrical vector beams (THz-CVBs), namely the azimuthally- and radially-polarized THz pulses, from a spintronic THz emitter. We begin by presenting that the spintronic emitter generates the HE21 mode, a quadrupole like polarization distribution, when placed between two magnets with opposing polarity. By providing an appropriate mode conversion using a triangular Si prism, we show both from experiment and numerical calculation that we obtain azimuthal and radial THz vector beams. The proposed method facilitates the access of CVBs and paves the way toward sophisticated polarization control in the THz regime.

© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Charge carrier dynamics of FeSe thin film investigated by terahertz magneto-optical spectroscopy

Naotaka Yoshikawa, Masayuki Takayama, Naoki Shikama, Tomoya Ishikawa, Fuyuki Nabeshima, Atsutaka Maeda, and Ryo Shimano

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.100.035110

We performed terahertz magneto-optical spectroscopy of FeSe thin film to elucidate the charge carrier dynamics. The measured diagonal (longitudinal) and off-diagonal (Hall) conductivity spectra are well reproduced by a two-carrier Drude model, from which the carrier densities, scattering times, and effective masses of electron and hole carriers are determined in a wide range of temperature. The hole density decreases below the structural transition temperature while electron density increases, which is attributed to the band-structure modification in the electronic nematic phase. The scattering time of the hole carrier becomes substantially longer than that of the electron at lower temperature, which accounts for the increase of the positive dc Hall coefficient at low temperature.

• 
• 
• 
• 
• 
• 

Abstract-Ultrafast terahertz nonlinear optics of Landau level transitions in a monolayer graphene

Go Yumoto, Ryusuke Matsunaga, Hiroki Hibino, and Ryo Shimano

https://journals.aps.org/prl/accepted/85071Yb3X8a14b5b084a77887f1fd7112a6574506


We investigated the ultrafast terahertz (THz) nonlinearity in a monolayer graphene under the strong magnetic field using THz pump-THz probe spectroscopy. An ultrafast suppression of the Faraday rotation associated with inter-Landau level (LL) transitions is observed, reflecting the Dirac electron character of non-equidistant LLs with large transition dipole moments. A drastic modulation of electron distribution in LLs is induced by far off-resonant THz pulse excitation in the transparent region. Numerical simulation based on the density matrix formalism without rotating wave approximation reproduces the experimental results, indicating that the strong light-matter coupling regime where the Rabi frequency exceeds the carrier wave frequency and even the relevant energy scale of the inter-LLs transition is realized.

;Abstract-Higgs mode in the d-wave superconductor Bi2Sr2CaCu2O8+x driven by an intense terahertz pulse

Kota Katsumi, Naoto Tsuji, Yuki I. Hamada, Ryusuke Matsunaga, John Schneeloch, Ruidan D. Zhong, Genda. D. Gu, Hideo Aoki, Yann Gallais, Ryo Shimano

https://arxiv.org/abs/1711.04923

We investigated the terahertz (THz)-pulse driven nonlinear response in the d-wave cuprate superconductor Bi2Sr2CaCu2O8+x (Bi2212) using a THz pump near-infrared probe scheme in the time domain. We have observed an oscillatory behavior of the optical reflectivity that follows the THz electric field squared and is strongly enhanced below Tc. The corresponding third-order nonlinear effect exhibits both A1g and B1g symmetry components, which are decomposed from polarization-resolved measurements. Comparison with a BCS calculation of the nonlinear susceptibility indicates that the A1g component is associated with the Higgs mode of the d-wave order parameter.

Abstract-Polarization-resolved terahertz third-harmonic generation in a single-crystal superconductor NbN: Dominance of the Higgs mode beyond the BCS approximation

Ryusuke Matsunaga, Naoto Tsuji, Kazumasa Makise, Hirotaka Terai, Hideo Aoki, and Ryo Shimano

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.020505

Recent advances in time-domain terahertz (THz) spectroscopy have unveiled that resonantly enhanced strong THz third-harmonic generation (THG) mediated by the collective Higgs amplitude mode occurs in $s$-wave superconductors, where charge-density fluctuations (CDFs) have been shown to also contribute to the nonlinear third-order susceptibility. It has been theoretically proposed that the nonlinear responses of Higgs and CDF exhibit essentially different polarization dependences. Here we experimentally discriminate the two contributions by polarization-resolved intense THz transmission spectroscopy for a single-crystal NbN film. The result shows that the resonant THG in the transmitted light always appears in the polarization parallel to that of the incident light with no appreciable polarization-angle dependence relative to the crystal axis. When we compare this with the theoretical calculation here with the BCS approximation and the dynamical mean-field theory for a model of NbN constructed from first principles, the experimental result strongly indicates that the Higgs mode rather than the CDF dominates the THG resonance in NbN. A possible mechanism for this is the retardation effect in the phonon-mediated pairing interaction beyond BCS.

• 
• 
• 

Abstract-Polarization-resolved terahertz third-harmonic generation in a single-crystal NbB superconductor: Dominance of the Higgs mode beyond the BCS approximation

Ryusuke Matsunaga, Naoto Tsuji, Kazumasa Makise, Hirotaka Terai, Hideo Aoki, and Ryo Shimano

https://journals.aps.org/prb/accepted/06073Oa1P3bEc21dc38d310718d5f15931783f2ce

Recent advances in time-domain terahertz (THz) spectroscopy have unveiled that resonantly-enhanced strong THz third-harmonic generation (THG) mediated by the collective Higgs amplitude mode occurs in }{{s}}{-wave superconductors, where charge-density fluctuations (CDF) have been shown to also contribute to the nonlinear third-order susceptibility. It has been theoretically proposed that the nonlinear responses of Higgs and CDF exhibit essentially different polarization dependences. Here we experimentally discriminate the two contributions by polarization-resolved intense THz transmission spectroscopy for a single-crystal NbN film. The result shows that the resonant THG in the transmitted light always appears in the polarization parallel to that of the incident light with no appreciable polarization-angle dependence relative to the crystal axis. When we compare this with the theoretical calculation here with the BCS approximation and the dynamical mean-field theory for a model of NbN constructed from first principles, the experimental result strongly indicates that the Higgs mode rather than the CDF dominates the THG resonance in NbN. A possible mechanism for this is the retardation effect in the phonon-mediated pairing interaction beyond BCS.

Abstract-Polarization-resolved terahertz third-harmonic generation in a superconductor NbN: dominance of Higgs mode beyond the BCS approximation

Ryusuke Matsunaga, Naoto Tsuji, Kazumasa Makise, Hirotaka Terai, Hideo Aoki, Ryo Shimano

(Submitted on 8 Mar 2017 (v1), last revised 9 Mar 2017 (this version, v2))

https://arxiv.org/abs/1703.02815

Recent advances in time-domain terahertz (THz) spectroscopy have unveiled that resonantly-enhanced strong THz third-harmonic generation (THG) mediated by the collective Higgs amplitude mode occurs in s-wave superconductors, where charge-density fluctuations (CDF) have also been shown to contribute to the nonlinear third-order susceptibility. It has been theoretically proposed that the nonlinear responses of Higgs and CDF exhibit essentially different polarization dependences. Here we experimentally discriminate the two contributions by polarization-resolved intense THz transmission spectroscopy for a single-crystal NbN film. The result shows that the resonant THG in the transmitted light always appears in the polarization parallel to that of the incident light with no appreciable crystal axis dependence. When we compare this with the theoretical calculation here with the BCS approximation and the dynamical mean-field theory for a model of NbN constructed from first principles, the experimental result strongly indicates that the Higgs mode rather than the CDF dominates the THG resonance in NbN. A possible mechanism for this is discussed such as the retardation effect in the phonon-mediated pairing interaction beyond BCS.

Abstract-Higgs mode excitation in superconductors by intense terahertz pulse

Ryusuke Matsunaga, Ryo Shimano

The Univ. of Tokyo (Japan)

Proc. SPIE 9835, Ultrafast Bandgap Photonics, 98351G (May 13, 2016); doi:10.1117/12.2223010

http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=2523983

Recent development of intense terahertz (THz) pulse generation technique has offered novel opportunities to reveal ultrafast phenomena in a variety of materials on tabletop experiments and provided a new pathway toward ultrafast control of quantum phases. Here we present our recent study of nonequilibrium dynamics in metallic superconductors NbN excited by intense THz pulse. Since the superconducting gap energy is located in the THz frequency range, the intense THz pulse excitation makes it possible to instantaneously excite high-density quasiparticles at the gap edge without injecting excess energies. It has also become possible to coherently drive the superconducting ground state without exciting incoherent quasiparticles by tuning the pump frequency below the gap energy. The ultrafast dynamics of the order parameter induced by such an intense low energy excitation is directly probed, and the nature of a collective excitation, namely the Higgs amplitude mode, is revealed. Efficient THz higher-harmonic generation from a superconductor is discovered, manifesting the nonlinear coupling between the THz wave and the Higgs mode. We also report the experimental results in a multi-gap superconductor MgB2. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Abstract-Excitonic correlation in the Mott crossover regime in Ge

Fumiya Sekiguchi and Ryo Shimano

Phys. Rev. B 91, 155202 – Published 6 April 2015

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.155202

Exciton Mott transition (EMT) in Ge was investigated by using optical-pump and terahertz-probe spectroscopy. From the quantitative analysis of optical conductivity and dielectric function, we evaluated the densities of unbound electron-hole pairs and excitons after the photoexcitation, from which we determined the ionization ratio of excitons α. The Mott crossover density region in Ge was elucidated from the density dependence of α in the temperature range above the critical temperature of electron-hole droplets. The 1s−2p excitonic transition energy hardly shifted with increasing density toward the EMT. Combined with the similar results recently observed in bulk Si, we suggest that the robustness of excitonic correlation against the Coulomb screening is a universal feature in bulk semiconductors in the Mott crossover regime.

• 
• 
• 
• 
• 
• 

Higgs boson analog in superconductors revealed by terahertz spectroscopy

Ryusuke Matsunaga and Ryo Shimano

Using terahertz pump–terahertz probe spectroscopy in conventional superconductors enables the observation of the Higgs mode, revealing its resonant nonlinear coupling to the light field.

30 March 2015, SPIE Newsroom. DOI: 10.1117/2.1201503.005778

http://spie.org/x113052.xml

The discovery of the Higgs particle at CERN in 2012 represented exciting progress in the field of particle physics, providing strong proof for spontaneous symmetry breaking (SSB) and the existence of a Higgs field. The concept of SSB in particle physics, developed by Yoichiro Nambu, was partially triggered by the Bardeen-Cooper-Schrieffer (BCS) theory for superconductivity. This theory is based on an effect that arises when an interaction links pairs of electrons into bosonic states called Cooper pairs. The existence of a condensed matter analog of the Higgs boson (a collective amplitude mode that arises from oscillations of the superconducting order parameter: see Figure 1) was proposed by Philip W. Anderson nearly half a century ago,1 prior to the prediction of the Higgs boson in particle physics.2 This collective amplitude mode, which has only recently been referred to as the Higgs mode, has attracted much attention3from a fundamental point of view (e.g., for the study of quantum phase transitions, nonequilibrium dynamics, and novel nonlinear phenomena, all of which can be investigated in tabletop experiments). Using external fields to artificially control the order parameter of superconducting materials represents another highly intriguing prospect. Clarifying the behavior of coherent light–matter interaction in superconductors would open up a new avenue for the study of unconventional superconductors, for photo-control of superconductivity, and potentially for photoinduced superconductivity.

Figure 1. A schematic picture of the Higgs mode (red arrow) on the free energy potential in the plane of the complex order parameter (Ψ). Re: Real. Im: Imaginary.

The Higgs mode in superconductors has, however, long evaded experimental detection. The essential difficulty lies in the fact that the Higgs mode itself does not directly couple with electromagnetic waves (light) in the linear response regime, due to the absence of electric or magnetic polarization. So far, the Higgs mode has only been observed in the superconductor niobium selenide (NbSe2), via Raman scattering. This observation is possible because the charge-density wave (CDW) in NbSe2makes the Higgs mode Raman active.3, 4 For decades, it has remained unclear whether the Higgs mode can be observed in conventional superconductors without CDW.

We have observed the Higgs mode in the conventional superconductor niobium titanium nitride (Nb1−xTixN) using state-of-art terahertz (THz) pump–THz probe spectroscopy.5, 6 Although the Higgs mode does not couple directly to the radiation field, it can be excited by THz light. We irradiated an intense monocycle THz pulse, generated by optical rectification in a lithium niobate (LiNbO3) crystal,7 onto Nb1−xTixN. Cooper pairs are instantaneously broken by the intense THz pulse. Such a sudden (nonadiabatic) perturbation of the superconducting ground state induces a fluctuation of the order parameter amplitude.8, 9

The emergence of a superfluid density, which gives rise to the superconducting order parameter (Δ), appears in the optical conductivity spectra at a photon energy around the superconducting gap, 2Δ (i.e., in the THz frequency range). We are therefore able to probe the temporal evolution of this order parameter by using another probe THz pulse with sub-picosecond temporal resolution. Figure 2 shows the measured pump-probe signal as a function of pump-probe delay, clearly indicating that the order parameter oscillates after the THz pulse excitation.5 The oscillation frequency coincides with 2Δ (twice the magnitude of the superconducting order parameter after excitation), strongly reflecting a signature of the Higgs mode with frequency 2Δ.8

 

Figure 2. Temporal evolution of the pump-probe signal showing the oscillatory behavior of the order parameter. Oscillation frequency decreases as the terahertz (THz) pump pulse intensity increases, reflecting a reduction of the order parameter after the pumping. δEprobe: Change of probe electric field (i.e., the pump-probe signal reflecting the change of order parameter). Arb.: Arbitrary.

In contrast to the above nonadiabatic excitation scheme of the Higgs mode with monocycle THz pulses, the Higgs mode oscillation can also be induced through coherent nonlinear coupling between the Higgs mode and narrow-band multicycle THz pulses with a frequency tuned below the gap energy (ω< 2Δ). Although this sub-gap pump THz pulse is not able to break Cooper pairs, we found that the order parameter coherently oscillates with frequency 2ω (twice the pump frequency) during the THz pulse irradiation. We also found that this forced oscillation of the order parameter leads to a strong third-order harmonic generation (THG), as shown in Figure 3. The most striking result is that this forced oscillation of the order parameter and the THG are strongly enhanced when 2ω coincides with 2Δ (i.e., twice the pump pulse frequency is equal to the Higgs mode frequency).6 This result unveils resonance between the Higgs mode and electromagnetic waves in the nonlinear-response regime. This nonlinear coupling between the strong light field and the Higgs mode is described by the collective precession of Anderson's pseudospins.10

Figure 3. Power spectra of the transmitted pump THz pulse below and above the superconducting critical temperature (15K). The center frequency of the incident pump THz pulse is ω=0.6THz. Third-harmonic generation (THG) is observed at 3ω=1.8THz, below the critical temperature.

In summary, we have confirmed the existence of a Higgs mode in superconductors nearly half a century after its initial prediction. As well as prompting further study of the Higgs mode in unconventional superconductors, our results unveil a new type of nonlinear light–matter interaction associated with cooperative phenomena in correlated quantum systems. This new optical phenomenon shows promise for application in nonlinear THz photonics and provides a new approach for the study of superconductivity by optical means. In future work, we intend to use this scheme to study the behavior of unconventional superconductors.
___________________

Ryusuke Matsunaga, Ryo Shimano

Department of Physics
The University of Tokyo

Tokyo, Japan

---

References:

1. P. W. Anderson, Coherent excited states in the theory of superconductivity: gauge invariance and the Meissner effect, Phys. Rev. 110, p. 827, 1958.

2. P. W. Higgs, Broken symmetries, massless particles and gauge fields, Phys. Lett. 12(2), p. 132-133, 1964.

3. D. Pekker, C. M. Varma, Amplitude/Higgs modes in condensed matter physics, Annu. Rev. Condens. Matter Phys. 6, 2015.

4. M.-A. Méasson, Y. Gallais, M. Cazayous, B. Clair, P. Rodière, L. Cairo, A. Sacuto, Amplitude Higgs mode in the 2H-NbSe2 superconductor, Phys. Rev. B 89, p. 060503(R), 2014.

5. R. Matsunaga, Y. I. Hamada, K. Makise, Y. Uzawa, H. Terai, Z. Wang, R. Shimano, Higgs amplitude mode in the BCS superconductors Nb1 - xTixN induced by terahertz pulse excitation, Phys. Rev. Lett. 111, p. 057002, 2013.

6. R. Matsunaga, N. Tsuji, H. Fujita, A. Sugioka, K. Makise, Y. Uzawa, H. Terai, Z. Wang, H. Aoki, R. Shimano, Light-induced collective pseudospin precession resonating with Higgs mode in a superconductor, Science 345(6201), p. 1145-1149, 2014.

7. J. Fülöp, L. Pálfalvi, G. Almási, J. Hebling, High energy THz pulse generation by tilted pulse front excitation and its nonlinear optical applications, J. Infrared Millim. Terahertz Waves 32(5), p. 553-561, 2011.

8. A. F. Volkov, Sh. M. Kogan, Collisionless relaxation of the energy gap in superconductors,Sov. Phys. JETP 38(5), p. 1018-1021, 1974.

9. R. A. Barankov, L. S. Levitov, B. Z. Spivak, Collective Rabi oscillations and solitons in a time-dependent BCS pairing problem, Phys. Rev. Lett. 93, p. 160401, 2004.

10. N. Tsuji, H. Aoki, Theory of Anderson pseudospin resonance with Higgs mode in superconductors, arXiv:1404.2711, 2014.