Time-Domain Terahertz Spectroscopy in High Magnetic Fields

There are a variety of elementary and collective terahertz-frequency excitations in condensed matter whose magnetic field dependence contains significant insight into the states and dynamics of the electrons involved. Often, determining the frequency, temperature, and magnetic field dependence of the optical conductivity tensor, especially in high magnetic fields, can clarify the microscopic physics behind complex many-body behaviors of solids. While there are advanced terahertz spectroscopy techniques as well as high magnetic field generation techniques available, combination of the two has only been realized relatively recently. Here, we review the current state of terahertz time-domain spectroscopy experiments in high magnetic fields. We start with an overview of time-domain terahertz detection schemes with a special focus on how they have been incorporated into optically accessible high-field magnets. Advantages and disadvantages of different types of magnets in performing terahertz time-domain spectroscopy experiments are also discussed. Finally, we highlight some of the new fascinating physical phenomena that have been revealed by terahertz time-domain spectroscopy in high magnetic fields

Terahertz spin dynamics in rare-earth orthoferrites

Recent interest in developing fast spintronic devices and laser-controllable magnetic solids has sparked tremendous experimental and theoretical efforts to understand and manipulate ultrafast dynamics in materials. Studies of spin dynamics in the terahertz (THz) frequency range are particularly important for elucidating microscopic pathways toward novel device functionalities. Here, we review THz phenomena related to spin dynamics in rare-earth orthoferrites, a class of materials promising for antiferromagnetic spintronics. We expand this topic into a description of four key elements. (1) We start by describing THz spectroscopy of spin excitations for probing magnetic phase transitions in thermal equilibrium. While acoustic magnons are useful indicators of spin reorientation transitions, electromagnons that arise from dynamic magnetoelectric couplings serve as a signature of inversion-symmetry-breaking phases at low temperatures. (2) We then review the strong laser driving scenario, where the system is excited far from equilibrium and thereby subject to modifications to the free energy landscape. Microscopic pathways for ultrafast laser manipulation of magnetic order are discussed. (3) Furthermore, we review a variety of protocols to manipulate coherent THz magnons in time and space, which are useful capabilities for antiferromagnetic spintronic applications. (4) Finally, new insights on the connection between dynamic magnetic coupling in condensed matter and the Dicke superradiant phase transition in quantum optics are provided. By presenting a review on an array of THz spin phenomena occurring in a single class of materials, we hope to trigger interdisciplinary efforts that actively seek connections between subfields of spintronics, which will facilitate the invention of new protocols of active spin control and quantum phase engineering

Additional file 1: of Destabilization of Surfactant-Dispersed Carbon Nanotubes by Anions

Experimental Results. (DOCX 636 kb

Magnonic Superradiant Phase Transition

We show that the low-temperature phase transition in ErFeO3 that occurs at a critical temperature of ~ 4 K can be described as a magnonic version of the superradiant phase transition (SRPT). The role of photons in the quantum-optical SRPT is played by Fe magnons, while that of two-level atoms is played by Er spins. Our spin model, which is reduced to an extended Dicke model, takes into account the short-range, direct exchange interactions between Er spins in addition to the long-range Er-Er interactions mediated by Fe magnons. By using realistic parameters determined by recent terahertz magnetospectroscopy and magnetization experiments, we demonstrate that it is the cooperative, ultrastrong coupling between Er spins and Fe magnons that causes the phase transition. This work thus proves ErFeO3 to be a unique system that exhibits a SRPT in thermal equilibrium, in contrast to previous observations of laser-driven non-equilibrium SRPTs

Macroscopically Aligned Carbon Nanotubes as a Refractory Platform for Hyperbolic Thermal Emitters

Nanophotonic thermal emitters with large photonic density of states (PDOS) have the potential to significantly enhance the efficiency of radiative cooling and waste heat recovery. Because of their nearly infinite PDOS, refractory hyperbolic materials make a promising material platform for thermal emitters. However, it is challenging to achieve a prominent PDOS in existing refractory hyperbolic materials, especially in a broad bandwidth. Here, we demonstrate macroscopically aligned carbon nanotubes as an excellent refractory material platform for hyperbolic nanophotonic devices. Aligned carbon nanotubes are thermally stable up to 1600 °C and exhibit extreme anisotropy: metallic in one direction and insulating in the other two directions. Such extreme anisotropy results in an exceptionally large PDOS over a broadband spectrum range (longer than 4.3 μm) in the mid-infrared, manifesting as strong resonances in deeply subwavelength-sized cavities. We demonstrate polarized, spectrally selective, thermal emission from aligned carbon nanotube films and indefinite cavities of volume as small as ∼λ3/700 operating at 700 °C. These experiments suggest that aligned carbon nanotubes enhance PDOS and hence also thermal photon density by over 2 orders of magnitude, making them a promising refractory nanophotonics platform

High-Speed Modulation of Polarized Thermal Radiation from an On-Chip Aligned Carbon Nanotube Film

Spectroscopic analysis with polarized light has been widely used to investigate molecular structure and material behavior. A broadband polarized light source that can be switched on and off at a high speed is indispensable for reading faint signals, but such a source has not been developed. Here, using aligned carbon nanotube (CNT) films, we have developed broadband thermal emitters of polarized infrared radiation with switching speeds of ≲20 MHz. We found that the switching speed depends on whether the electrical current is parallel or perpendicular to the CNT alignment direction with a significantly higher speed achieved in the parallel case. Together with detailed theoretical simulations, our experimental results demonstrate that the contact thermal conductance to the substrate and the conductance to the electrodes are important factors that determine the switching speed. These emitters can lead to advanced spectroscopic analysis techniques with polarized radiation

Nematic-Like Alignment in SWNT Thin Films from Aqueous Colloidal Suspensions

We present a modification of the vacuum filtration technique for fabricating transparent conductive SWNT thin films with local nematic-like orientational ordering. Dilute SWNT surfactant dispersions are filtered through a vacuum filtration setup in a slow and controlled fashion. The slow filtration creates a region of high SWNT concentration close to the filter membrane. While slowly moving through this region, SWNTs interact and align with each other, resulting in the formation of thin films with local nematic ordering. Scanning electron microscopy and image analysis revealed a local scalar order parameter (S2D) of 0.7–0.8 for slow filtration, three times higher than those produced from “fast filtration” (S2D ≈ 0.24). Orientational ordering is demonstrated with different stabilizing surfactants, as well as with dispersions enriched in metallic SWNTs, produced by density-gradient ultracentrifugation. Simple estimates of relative convective versus diffusive transport highlight the main differences between slow versus fast filtration and the resulting SWNT concentration profiles. Comparisons with previous studies on three stages of liquid-crystal phase transition provide insight into the spontaneous ordering process, indicating the lack of a “healing stage”, which results in a microstructure consisting of staggered domains in our SWNT films

Nematic-Like Alignment in SWNT Thin Films from Aqueous Colloidal Suspensions

We present a modification of the vacuum filtration technique for fabricating transparent conductive SWNT thin films with local nematic-like orientational ordering. Dilute SWNT surfactant dispersions are filtered through a vacuum filtration setup in a slow and controlled fashion. The slow filtration creates a region of high SWNT concentration close to the filter membrane. While slowly moving through this region, SWNTs interact and align with each other, resulting in the formation of thin films with local nematic ordering. Scanning electron microscopy and image analysis revealed a local scalar order parameter (<i>S</i>_2D) of 0.7–0.8 for slow filtration, three times higher than those produced from “fast filtration” (<i>S</i>_2D ≈ 0.24). Orientational ordering is demonstrated with different stabilizing surfactants, as well as with dispersions enriched in metallic SWNTs, produced by density-gradient ultracentrifugation. Simple estimates of relative convective versus diffusive transport highlight the main differences between slow versus fast filtration and the resulting SWNT concentration profiles. Comparisons with previous studies on three stages of liquid-crystal phase transition provide insight into the spontaneous ordering process, indicating the lack of a “healing stage”, which results in a microstructure consisting of staggered domains in our SWNT films

Electrical Generation of Polarized Broadband Radiation from an On-Chip Aligned Carbon Nanotube Film

Microsized light sources with polarized or broadband emission can be used for a variety of applications. However, the system directly generating polarized and broad-spectrum light without using polarizers has not been established. Here, we found that a nano-device of densely packed and highly aligned carbon nanotube (CNT) films on silicon chips can emit polarized light with a broad spectrum. We observed spatial emission patterns that are dependent on the angle between the electrical bias and the CNT alignment direction, which are caused not only by the large thermal conductivity anisotropy of the film but also by finite thermal conductance in the CNT-electrode contact. Utilizing the thermal and electrical anisotropy, strongly localized emission was achieved from a narrow (∼1 μm) strip of aligned CNTs connecting edges of two displaced electrodes. This device’s unique ability to directly generate polarized ultrabroadband radiation may greatly expand the range of applications of microsized light sources

Generation of Terahertz Radiation by Optical Excitation of Aligned Carbon Nanotubes

We have generated coherent pulses of terahertz radiation from macroscopic arrays of aligned single-wall carbon nanotubes (SWCNTs) excited by femtosecond optical pulses without externally applied bias. The generated terahertz radiation is polarized along the SWCNT alignment direction. We propose that top-bottom asymmetry in the SWCNT arrays produces a built-in electric field in semiconducting SWCNTs, which enables generation of polarized terahertz radiation by a transient photocurrent surge directed along the nanotube axis