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The field of nonlinear optics has grown substantially in past decades, leading to tremendous progress in fundamental research and revolutionized applications. Traditionally, the optical nonlinearity for a light wave at frequencies beyond near-infrared is observed with very high peak intensity, as in most materials only the electronic nonlinearity dominates while ionic contribution is negligible. However, it was shown that the ionic contribution to nonlinearity can be much larger than the electronic one in microwave experiments. In the terahertz (THz) regime, phonon polariton may assist to substantially trigger the ionic nonlinearity of the crystals, so as to enhance even more the nonlinear optical susceptibility. Here, we experimentally demonstrate a giant second-order optical nonlinearity at THz frequency, orders of magnitude higher than that in the visible and microwave regimes. Different from previous work, the phonon-light coupling is achieved under a phase-matching setting, and the dynamic process of nonlinear THz generation is directly observed in a thin-film waveguide using a time-resolved imaging technique. Furthermore, a nonlinear modification to the Huang equations is proposed to explain the observed nonlinearity enhancement. This work brings about an effective approach to achieve high nonlinearity in ionic crystals, promising for applications in THz nonlinear technologies.
We report on terahertz emission from a single layer ferromagnet which involves the generation of backflow nonthermal charge current from the ferromagnet/dielectric interface by femtosecond laser excitation and subsequent conversion of the charge current to a transverse transient charge current via the anomalous Hall effect, thereby generating the THz radiation. The THz emission can be either enhanced or suppressed, or even the polarity can be reversed, by introducing a magnetization gradient in the thickness direction of the ferromagnet. Unlike spintronic THz emitters reported previously, it does not require additional non-magnetic layer or Rashba interface.
We report on terahertz emission from a single layer ferromagnet which involves the generation of backflow nonthermal charge current from the ferromagnet/dielectric interface by femtosecond laser excitation and subsequent conversion of the charge current to a transverse transient charge current via the anomalous Hall effect, thereby generating the THz radiation. The THz emission can be either enhanced or suppressed, or even the polarity can be reversed, by introducing a magnetization gradient in the thickness direction of the ferromagnet. Unlike spintronic THz emitters reported previously, it does not require additional non-magnetic layer or Rashba interface.
(a) Schematic of THz detection of an analyte using a microrod array metasurface as an on-chip sensor. A column of y-polarized dipoles located inside the LN waveguide is used to excite THz waves (blue oscillation signal). The thickness of the SiO2 layer is h =2 μm. The inset shows the detailed design parameters: p, a, l and g are 20, 10, 55, and 15 μm, respectively. (b) Enhanced field confined to the surface of the composite structure. (c) and (d) Distribution of the field components Ey and Ez at f =0.529 THz.
Recognizing special molecules is crucial in many biochemical processes, and thus, highly enhanced sensing methods are in high demand. In this work, we designed a microrod array metasurface with a SiO2-loaded subwavelength lithium niobate waveguide as a unique platform for enhanced experimental fingerprint detection of lactose. The metasurface could lead to strong surface wave modes due to the near-field coupling of the spoof localized surface plasmon, which also could provide a stronger interaction length between light and matter. The selectivity was remarkable in the transmission spectrum at an intrinsic characteristic frequency of 0.529 THz with a thin layer of lactose, while it was faint while transmitting terahertz (THz) waves normally through a lactose layer of the same thickness. Together with the ability to freely design the shape of the metasurface and the electromagnetic properties, we believe that this platform can function as an elegant on-chip-scale enhanced THz sensing platform.
Vibrational modes of 2-thiobarbituric acid (TBA) tautomeric polymorphs (form I, II and IV) were characterized by terahertz time-domain spectroscopy (THz-TDS) and Raman spectral techniques. The experimental results indicate that both vibrational spectroscopy techniques could be used to recognize the above TBA three tautomeric forms clearly. Experimental THz spectral results show that each of TBA tautomeric polymorphs has distinctive fingerprint peaks in the terahertz region. Raman spectra also show similar results about differences of TBA tautomeric polymorphs, but not significant as that of terahertz spectra since Raman-active vibrational modes are mostly from intra-molecular interaction of various functional groups within the specific molecule while that of terahertz region is more sensitive to inter-molecular interaction within crystalline unit cells. In addition, density functional theory (DFT) was used to simulate the optimized structures and vibrational modes of these three TBA tautomeric forms. The characteristic vibrational modes of TBA polymorphs are assigned comparing the simulated DFT results with experimental vibrational spectra. The results provide fundamental benchmark for the study of pharmaceutical polymorphism based on both Raman and terahertz vibrational spectroscopic techniques combined with theoretical simulations.
We show that a femtosecond spin-current pulse can generate terahertz (THz) transients at Rashba interfaces between two nonmagnetic materials. Our results unambiguously demonstrate the importance of the interface in this conversion process that we interpret in terms of the inverse Rashba Edelstein effect, in contrast to the THz emission in the bulk conversion process via the inverse spin-Hall effect. Furthermore, we show that at Rashba interfaces the THz-field amplitude can be controlled by the helicity of the light. The optical generation of electric photocurrents by these interfacial effects in the femtosecond regime will open up new opportunities in ultrafast spintronics.
HOUSTON – (April 16, 2018) – A team led by Rice University scientists used a unique combination of techniques to observe, for the first time, a condensed matter phenomenon about which others have only speculated. The research could aid in the development of quantum computers.
An electron (blue sphere) travels in a circular orbit in a DC magnetic field (B_dc). When an incoming light wave (E_ac) interacts with it, there is a component in the light wave whose electric field rotates in the same direction with the electron motion (red arrow on the left-hand side) and thus resonantly interacts with it – that is, the electron rapidly gains energy. The other component of the electric field rotates in the opposite direction with the electron (red arrow on the right-hand side), whose effect is typically negligible. However, when the electron and light wave mix to an extreme degree, the interaction effect can manifest as a Bloch-Siegert shift. Courtesy of Xinwei Li/Kono Lab
The researchers, led by Rice physicist Junichiro Kono and graduate student Xinwei Li, observed and measured what’s known as a Bloch-Siegert shift in strongly coupled light and matter.
Results of the complicated combination of modeling and experimentation are the subject of a paper in Nature Photonics. The technique could lead to a greater understanding of theoretical predictions in quantum phase transitions because the experimental parameters used in the Rice experiments are highly adjustable, according to Kono. Ultimately, he said, it may help in the development of robust quantum bits for advanced computing.
The Bloch-Siegert shift, a theory born in the 1940s, is a quantum interaction in which counter-rotating fields are able to interact. But such interactions have been difficult to detect.
The theory suggested to Kono and Li that it might be possible to detect such a shift when a light field rotating in one direction strongly couples with a matter-bound electron field rotating in the opposite direction. These interactions have proven difficult to create without the unique tools assembled by the Rice-led team.
“Light and matter should not resonate with each other when they are rotating in opposite directions,” Kono said. “However, in our case, we proved they can still strongly couple, or interact, even though they are not resonating with each other.”
A simplified schematic shows the basic idea behind a Rice University experiment to detect a Bloch-Siegert shift in strongly coupled light and matter. In this illustration, a light field rotating in the opposite direction to an orbiting electron still interacts with the electron in a cavity, in this case the empty space between two mirrors. The influence of resonance on the counter-rotating element defines the shift. Illustration by Xinwei Li
Kono and his colleagues created the resonance frequency shift in a two-level electron system induced by coupling with an electromagnetic field inside a cavity even when the electrons and field are rotating in opposite directions – a truly surprising effect that occurs only in a regime where light and matter are mixed together to an extreme degree.
In this case, the levels are those of two-dimensional electrons in solid gallium arsenide in a strong perpendicular magnetic field. They hybridize with the “vacuum” electromagnetic field in the cavity to form quasiparticles known as polaritons. This vacuum-matter hybridization had been expected to lead to a finite frequency shift, a vacuum Bloch-Siegert shift, in optical spectra for circularly polarized light counter-rotating with the electrons. The Rice team can now measure it.
“In condensed matter physics, we often look for new ground states (lowest-energy states). For that purpose, light-matter coupling is usually considered an enemy because light drives matter to an excited (higher-energy) state,” Kono said. “Here we have a unique system that is predicted to go into a new ground state because of strong light-matter coupling. Our technique will help us know when the strength of light-matter coupling exceeds a certain threshold.”
The research builds upon a strong vacuum field-matter coupling in a high-quality-factor cavity the lab first created and reported in 2016. The results at the time only hinted at the presence of a Bloch-Siegert shift. “Experimentally, we just demonstrated the new regime,” Li said. “But here, we have a very deep understanding of the physics involved.”
Kono and Li credited physicist Motoaki Bamba of Osaka University for providing a theoretical basis for the discovery and Katsumasa Yoshioka of Yokohama National University and a former visiting scholar at Rice for providing a device to produce circularly polarized light in the terahertz range of the electromagnetic spectrum.
Xinwei Li, left, and Junichiro Kono. Photo by Jeff Fitlow
The lab used the light to probe the shift in an ultra-high quality, two-dimensional electron gas supplied by Purdue University physicist Michael Manfra and set in a gallium arsenide quantum well (to contain the particles) under the influence of a strong magnetic field and low temperature. A terahertz spectroscope measured activity in the system.
“Linearly polarized light means an alternating current electric field that is always oscillating in one direction,” Kono said. “In circularly polarized light, the electric field is rotating.” That allowed the researchers to distinguish between left- and right-rotating electrons in their vacuum-bound condensed matter in a magnetic field, and from that, measure the shift.
“In this work, both theoretically and experimentally, we demonstrated that even though the electron is rotating this way and the light is rotating (the other) way, they still strongly interact with each other, which leads to a finite frequency shift known as the Bloch-Siegert shift,” Kono said.
Observing the shift is a direct indication that ultra-strong light-matter coupling invalidated the rotating wave approximation, he said. “That approximation is behind almost all light-matter interaction phenomenon, including lasers, nuclear magnetic resonance and quantum computing,” Kono said. “In any resonant light-matter interaction, people are satisfied with this approximation, because the coupling is usually weak. But if the coupling between light and matter is strong, it doesn’t work. That’s clear evidence that we are in the ultra-strong coupling regime.”
Co-authors of the paper are Rice postdoctoral researcher Weilu Gao and graduate student Minhan Lou of Rice, Rice alumnus Qi Zhang of Argonne National Laboratory and graduate student Saeed Fallahi and visiting scholar Geoff Gardner of Purdue. Kono is a professor of electrical and computer engineering, of physics and astronomy, and of materials science and nanoengineering. Manfra is the Bill and Dee O’Brian Chair Professor of Physics and Astronomy at Purdue. Bamba is an associate professor at Osaka. Yoshioka is a teaching assistant at Yokohama.
The National Science Foundation, the Army Research Office, the Department of Energy Office of Basic Energy Sciences, the PRESTO program of the Japan Science and Technology Agency and the ImPACT program of the Government of Japan’s Council for Science, Technology and Innovation supported the research.
We fabricate a Fabry-Perot resonator in a LiNbO3 subwavelength slab and investigate the spatiotemporal evolution of terahertz pulses in the structure via time-resolved imaging system. The wave confinement and standing wave modes are clearly observed.
Highly intense terahertz electromagnetic field and efficiently surface localized terahertz field in subwavelength volumes are of vital importance for terahertz photonics integration, also will greatly accelerate the development for integrated applications in biochemical sensing, imaging, terahertz spectroscopy, enhancement of nonlinear effects and even quantum research. In this paper, we achieved large terahertz field enhancement and surface field localization through depositing a pair of Au composite antennas on a LiNbO3 subwavelength slab waveguide, which can serve as an excellent on-chip platform for terahertz research and application. The antennas consist of two opposing tip-to-tip triangles separated by a gap, and each triangle combines with a strip antenna. Time-resolved imaging and finite-difference time-domain method were used to resolve the characteristics of the designed antennas experimentally and simulatively. Through these methods, we demonstrated outstanding abilities of the platform: leading to a large electric field enhancement, concentrating almost full terahertz energy on the waveguide’s surface when they are resonant with the terahertz waves and tunable resonant frequency. These abilities make the subwavelength waveguide coupling with the composite antennas be able to sever as a good integrated device to identify terahertz-sensitive small objects, or an excellent platform to terahertz spectroscopy and quantum research.
We report for the first time the ability to perform time resolved imaging of terahertz (THz) waves propagating within a Fabry-Perot resonator on a LiNbO3 slab. Electro-optic effect is used to record the full spatiotemporal evolution of THz fields inside the resonator. In addition to revealing the real-space behavior, the data further demonstrate the confinement and the standing wave modes of THz in the cavity in frequency domain. The experimental results are in good agreement with numerical simulations. Using the coherent imaging technique to gain real-time information about a resonator system provides a unique path to study the physics of optical cavity.
Cocrystallization could improve most physicochemical properties of specific active pharmaceutical ingredients, which has great potential in pharmaceutical development. In this study, the cocrystal of nitrofurantoin and 4-aminobenzoic acid was prepared with solid-state (solvent-free or green-chemistry) grinding approach, and the above cocrystal has been characterized by Raman and terahertz vibrational spectroscopic techniques. Spectral results show that the vibrational modes of the cocrystal within the whole spectral region are different from those of the corresponding parent materials. The dynamic process of such pharmaceutical cocrystal formation has also been monitored directly with Raman spectra. These results offer us unique means for characterizing the cocrystal conformation from the molecule-level, and provides us with rich information about the reaction dynamic of cocrystal formation within pharmaceutical fields. View Full-Text
G. Timothy Noe, Ikufumi Katayama, Fumiya Katsutani, James J. Allred, Jeffrey A. Horowitz, David M. Sullivan, Qi Zhang, Fumiya Sekiguchi, Gary L. Woods, Matthias C. Hoffmann, Hiroyuki Nojiri, Jun Takeda, and Junichiro Kono https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-26-30328
We have developed a single-shot terahertz time-domain spectrometer to perform optical-pump/terahertz-probe experiments in pulsed, high magnetic fields up to 30 T. The single-shot detection scheme for measuring a terahertz waveform incorporates a reflective echelon to create time-delayed beamlets across the intensity profile of the optical gate beam before it spatially and temporally overlaps with the terahertz radiation in a ZnTe detection crystal. After imaging the gate beam onto a camera, we can retrieve the terahertz time-domain waveform by analyzing the resulting image. To demonstrate the utility of our technique, we measured cyclotron resonance absorption of optically excited carriers in the terahertz frequency range in intrinsic silicon at high magnetic fields, with results that agree well with published values.
Where light and matter intersect, the world illuminates. Where light and matter interact so strongly that they become one, they illuminate a world of new physics, according to Rice University scientists.
Rice physicists are closing in on a way to create a new condensed matter state in which all the electrons in a material act as one by manipulating them with light and a magnetic field. The effect made possible by a custom-built, finely tuned cavity for terahertz radiation shows one of the strongest light-matter coupling phenomena ever observed.
Rice graduate student Xinwei Li, with physicist Junichiro Kono, prepares a sample for a cavity quantum electrodynamics experiment. They are part of a team probing the boundaries of light-matter interactions as they bridge traditional condensed matter physics and cavity-based quantum optics. Photo by Jeff Fitlow
The work by Rice physicist Junichiro Kono and his colleagues is described in Nature Physics. It could help advance technologies like quantum computers and communications by revealing new phenomena to those who study cavity quantum electrodynamics and condensed matter physics, Kono said.
The Kono team is working toward something similar, but with electrons that are strongly coupled, or “dressed,” with light. Qi Zhang, a former graduate student in Kono’s group and lead author of the paper, designed and constructed an extremely high-quality cavity to contain an ultrathin layer of gallium arsenide, a material they’ve used to study superfluorescence. By tuning the material with a magnetic field to resonate with a certain state of light in the cavity, they prompted the formation of polaritons that act in a collective manner.
“This is a nonlinear optical study of a two-dimensional electronic material,” said Zhang, who based his Ph.D. thesis on the work. “When you use light to probe a material’s electronic structure, you’re usually looking for light absorption or reflection or scattering to see what’s happening in the material. That light is just a weak probe and the process is called linear optics.
A method created at Rice University closes the gap between light and matter and may help advance technologies like quantum computers and communications. The lab designed and built a high-quality cavity to contain an ultrathin layer of gallium arsenide. By tuning the material with a magnetic field to resonate with a certain state of light in the cavity, they prompted the formation of polaritons that act in a collective manner. Click on the image for a larger version. Illustration by Qi Zhang
“Nonlinear optics means light does something to the material,” he said. “Light is not a small perturbation anymore; it couples strongly with the material. As you change the coupling strength, things change in the material. What we’re doing is the extreme case of nonlinear optics, where the light and matter are coupled so strongly that we don’t have light and matter anymore. We have something in between, called a polariton.”
The researchers employed a parameter known as vacuum Rabi splitting to measure the strength of the light-matter coupling. “In more than 99 percent of previous studies of light-matter coupling in cavities, this value is a negligibly small fraction of the photon energy of the light used,” said Xinwei Li, a co-author and graduate student in Kono’s group. “In our study, vacuum Rabi splitting is as large as 10 percent of the photon energy. That puts us in the so-called ultrastrong coupling regime.
“This is an important regime because, eventually, if the vacuum Rabi splitting becomes larger than the photon energy, the matter goes into a new ground state. That means we can induce a phase transition, which is an important element in condensed matter physics,” he said.
Phase transitions are transitions between states of matter, like ice to water to vapor. The specific transition Kono’s team is looking for is the superradiant phase transition in which the polaritons go into an ordered state with macroscopic coherence.
Rice graduate student Xinwei Li, left, and Professor Junichiro Kono. Photo by Jeff Fitlow
Kono said the amount of terahertz light put into the cavity is very weak. “What we depend on is the vacuum fluctuation. Vacuum, in a classical sense, is an empty space. There’s nothing. But in a quantum sense, a vacuum is full of fluctuating photons, having so-called zero-point energy. These vacuum photons are actually what we are using to resonantly excite electrons in our cavity.
“This general subject is what’s known as cavity quantum electrodynamics (QED),” Kono said. “In cavity QED, the cavity enhances the light so that matter in the cavity resonantly interacts with the vacuum field. What is unique about solid-state cavity QED is that the light typically interacts with this huge number of electrons, which behave like a single gigantic atom.”
He said solid-state cavity QED is also key for applications that involve quantum information processing, like quantum computers. “The light-matter interface is important because that’s where so-called light-matter entanglement occurs. That way, the quantum information of matter can be transferred to light and light can be sent somewhere.
“For improving the utility of cavity QED in quantum information, the stronger the light-matter coupling, the better, and it has to use a scalable, solid-state system instead of atomic or molecular systems,” he said. “That’s what we’ve achieved here.”
The high-quality gallium arsenide materials used in the study were synthesized via molecular beam epitaxy by John Reno of Sandia National Laboratories and John Watson and Michael Manfra of Purdue University, all co-authors of the paper. Weil Pan of Sandia National Laboratories and Rice graduate student Minhan Lou, who participated in sample preparation and transport and terahertz measurements, are also co-authors.
Zhang is now the Alexei Abrikosov Postdoctoral Fellow at Argonne National Laboratory. Kono is a Rice professor of electrical and computer engineering, of physics and astronomy and of materials science and nanoengineering. Li received a “Best First-Year Research Award” from Rice’s Department of Electrical and Computer Engineering for his work on the project.
The research was supported by the National Science Foundation, U.S. Department of Energy, Lockheed Martin Corp. and the W.M. Keck Foundation.
- See more at: http://news.rice.edu/2016/08/22/light-and-matter-merge-in-quantum-coupling-2/#sthash.PW4Qrxie.dpuf
The collective interaction of electrons with light in a high-quality-factor cavity is expected to reveal new quantum phenomena and find applications in quantum-enabled technologies. However, combining a long electronic coherence time, a large dipole moment, and a high quality-factor has proved difficult. Here, we achieved these conditions simultaneously in a two-dimensional electron gas in a high-quality-factor terahertz cavity in a magnetic field. The vacuum Rabi splitting of cyclotron resonance exhibited a square-root dependence on the electron density, evidencing collective interaction. This splitting extended even where the detuning is larger than the resonance frequency. Furthermore, we observed a peak shift due to the normally negligible diamagnetic term in the Hamiltonian. Finally, the high-quality-factor cavity suppressed superradiant cyclotron resonance decay, revealing a narrow intrinsic linewidth of 5.6GHz. High-quality-factor terahertz cavities will enable new experiments bridging the traditional disciplines of condensed-matter physics and cavity-based quantum optics.
Achieving strong light-matter interaction in low-dimensional solid state systems is essential for both fundamental studies and device applications of cavity quantum electrodynamics (QED). It is particularly interesting to understand and even control the dynamics of collective excitations in solid states, when they are strongly coupled to cavity photons. A Landau-quantized, high-mobility two-dimensional electron gas (2DEG) provides a uniquely clean and tunable semiconductor system in which to explore strong light-matter interaction with many-electron states. In this talk, I will first show how rapidly a superposition of massively degenerate Landau levels loses its coherence in the free space.
We observed a collective radiative decay, or superradiance, of cyclotron resonance (CR) in 2DEG with time-domain terahertz magneto-spectroscopy. In the second part, I will demonstrate the strong-coupling between the cyclotron resonance and THz cavity photons. We observed Rabi oscillation in time domain, as well as the collective vacuum Rabi splitting. We significantly suppressed the superradiance decay of CR by the high-Q THz cavity, and resolved an ultra-narrow intrinsic CR linewidth (5 GHz). Our method may also apply to various correlated 2D systems with collective THz excitations. It opens an access to the intriguing physics of THz many-body cavity-QED.
