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

Sub-picosecond all-optical switching in a hybrid VO2:silicon waveguide at 1550 nm

Achieving ultrafast all-optical switching in a silicon waveguide geometry is a key milestone on the way to an integrated platform capable of handling the increasing demands for higher speed and higher capacity for information transfer. Given the weak electro-optic and thermo-optic effects in silicon, there has been intense interest in hybrid structures in which that switching could be accomplished by integrating another material into the waveguide, including the phase-changing material, vanadium dioxide (VO2). It has long been known that the phase transition in VO2 can be triggered by ultrafast laser pulses, and that pump-laser fluence is a critical parameter governing the recovery time of thin films irradiated by femtosecond laser pulses near 800 nm. However, thin-film experiments are not a priori reliable guides to using VO2 for all-optical switching in on-chip silicon photonics because of the large changes in VO2 optical constants in the telecommunications band, the requirement of low insertion loss, and the limits on switching energy permissible in integrated photonic systems. Here we report the first measurements to show that the reversible, ultrafast photo-induced phase transition in VO2 can be harnessed to achieve sub-picosecond switching when small VO2 volumes are integrated in a silicon waveguide as a modulating element. Switching energies above threshold are of order 600 fJ/switch. These results suggest that VO2 can now be pursued as a strong candidate for all-optical switching with sub-picosecond on-off times

Giant Terahertz Polarization Rotation in Ultrathin Films of Aligned Carbon Nanotubes

For facile manipulation of polarization states of light for applications in communications, imaging, and information processing, an efficient mechanism is desired for rotating light polarization with a minimum interaction length. Here, we report giant polarization rotations for terahertz (THz) electromagnetic waves in ultrathin (~45 nm), high-density films of aligned carbon nanotubes. We observed polarization rotations of up to ~20{\deg} and ~110{\deg} for transmitted and reflected THz pulses, respectively. The amount of polarization rotation was a sensitive function of the angle between the incident THz polarization and the nanotube alignment direction, exhibiting a `magic' angle at which the total rotation through transmission and reflection becomes exactly 90{\deg}. Our model quantitatively explains these giant rotations as a result of extremely anisotropic optical constants, demonstrating that aligned carbon nanotubes promise ultrathin, broadband, and tunable THz polarization devices

Graphene Terahertz Technology

During the past two decades, a variety of linear and nonlinear dynamical phenomena, especially in the terahertz (THz) frequency range, have been revealed for charge carriers in graphene, which can be utilized to develop THz devices - i.e., emitters, receivers, and modulators of THz electromagnetic waves. This review discusses various applications of graphene in THz technology, including sensing, spectroscopy, photonics, and communications. First, the basic physics and techniques of THz-wave absorption processes in graphene are discussed. THz wave absorption in graphene can occur both through interband and intraband absorption processes. Such absorption can be readily modulated by an applied gate voltage and can be enhanced by utilizing parallel-plate waveguides or a total internal reflection geometry. Next, the effect of adsorbed molecules on THz emission from graphene is described. This phenomenon can be used to construct a metamaterial-free THz sensor for biointerfaces. The manipulation of THz waves through thermal annealing is also discussed, as well as their enhancement in a graphene-based THz modulator employing metallic ring apertures. Lastly, the review highlights the excitation of propagating surface plasmon polaritons in graphene at THz frequencies, which can play important roles in THz devices for communications, nanophotonics, and imaging

Supplementary document for Observation of Colossal Terahertz Magnetoresistance and Magnetocapacitance in a Perovskite Manganite - 6434884.pdf

Supplementary Material

Observation of Colossal Terahertz Magnetoresistance and Magnetocapacitance in a Perovskite Manganite

We have studied the terahertz response of a bulk single crystal of La$_{0.875}$Sr$_{0.125}$MnO$_3$ at around its Curie temperature, observing large changes in the real and imaginary parts of the optical conductivity as a function of magnetic field. The terahertz resistance and capacitance extracted from the optical conductivity rapidly increased with increasing magnetic field and did not show any sign of saturation up to 6 T, reaching 60% and 15%, respectively, at 180 K. The observed terahertz colossal magnetoresistance and magnetocapacitance effects can be qualitatively explained by using a two-component model that assumes the coexistence of two phases with vastly different conductivities. These results demonstrate the potential use of perovskite manganites for developing efficient terahertz devices based on magnetic modulations of the amplitude and phase of terahertz waves

Terahertz cavity magnon polaritons

Hybrid light-matter coupled states, or polaritons, in magnetic materials have attracted significant attention due to their potential for enabling novel applications in spintronics and quantum information processing. However, most studies to date have been carried out for ferromagnetic materials with magnon excitations at gigahertz frequencies. Here, we have investigated strong resonant photon-magnon coupling at frequencies above 1 terahertz for the first time in a prototypical room-temperature antiferromagnetic insulator, NiO, inside a Fabry-P\'erot cavity. The cavity was formed by the crystal itself when it was thinned down to an optimized thickness. By using terahertz time-domain spectroscopy in high magnetic fields up to 25 T, we swept the magnon frequency through Fabry-P\'erot cavity modes and observed photon-magnon anticrossing behavior, demonstrating clear vacuum Rabi splittings exceeding the polariton linewidths. These results show that NiO is a promising platform for exploring antiferromagnetic spintronics and cavity magnonics in the terahertz frequency range

Magnetically Tuned Continuous Transition from Weak to Strong Coupling in Terahertz Magnon Polaritons

Depending on the relative rates of coupling and dissipation, a light-matter coupled system is either in the weak- or strong-coupling regime. Here, we present a unique system where the coupling rate continuously increases with an externally applied magnetic field while the dissipation rate remains constant, allowing us to monitor a weak-to-strong coupling transition as a function of magnetic field. We observed a Rabi splitting of a terahertz magnon mode in yttrium orthoferrite above a threshold magnetic field of ~14 T. Based on a microscopic theoretical model, we show that with increasing magnetic field the magnons transition into magnon polaritons through an exceptional point, which will open up new opportunities for in situ control of non-Hermitian systems

Controlled synthetic chirality in macroscopic assemblies of carbon nanotubes

There is an emerging recognition that successful utilization of chiral degrees of freedom can bring new scientific and technological opportunities to diverse research areas. Hence, methods are being sought for creating artificial matter with controllable chirality in an uncomplicated and reproducible manner. Here, we report the development of two straightforward methods for fabricating wafer-scale chiral architectures of ordered carbon nanotubes (CNTs) with tunable and giant circular dichroism (CD). Both methods employ simple approaches, (i) mechanical rotation and (ii) twist-stacking, based on controlled vacuum filtration and do not involve any sophisticated nanofabrication processes. We used a racemic mixture of CNTs as the starting material, so the intrinsic chirality of chiral CNTs is not responsible for the observed chirality. In particular, by controlling the stacking angle and handedness in (ii), we were able to maximize the CD response and achieve a record-high deep-ultraviolet ellipticity of 40 $\pm$ 1 mdeg/nm. Our theoretical simulations using the transfer matrix method reproduce the salient features of the experimentally observed CD spectra and further predict that a film of twist-stacked CNTs with an optimized thickness will exhibit an ellipticity as high as 150 mdeg/nm. The created wafer-scale objects represent a new class of synthetic chiral matter consisting of ordered quantum wires whose macroscopic properties are governed by nanoscopic electronic signatures such as van Hove singularities. These artificial structures with engineered chirality will not only provide playgrounds for uncovering new chiral phenomena but also open up new opportunities for developing high-performance chiral photonic and optoelectronic devices

Tunable Plasmonic Ultrastrong Coupling: Emulating Dicke Physics at Room Temperature

A system of N two-level atoms cooperatively interacting with a photonic field can be described as a single giant atom coupled to the field with interaction strength ~N^0.5. This enhancement, known as Dicke cooperativity in quantum optics, has recently become an indispensable element in quantum information technology based on strong light-matter coupling. Here, we extend the coupling beyond the standard light-matter interaction paradigm, emulating Dicke cooperativity in a terahertz metasurface with N meta-atoms. Cooperative enhancement manifested in the form of matter-matter coupling, through the hybridization of localized surface plasmon resonance in individual meta-atoms and surface lattice resonance due to the periodic array of the meta-atoms. By varying the lattice constant of the array, we observe a clear anticrossing behavior, a signature of strong coupling. Furthermore, through engineering of the capacitive split-gap in the meta-atoms, the coupling rate was cooperatively enhanced into the ultrastrong coupling regime by a factor of N^0.5. This room-temperature technology serves as a convenient quantum emulator of the dynamics of a qubit with a giant dipole moment coherently driven by a single bosonic field