Three dimensional topological insulators, as a new phase of quantum matters, are characterized by an insulating gap in the bulk and a metallic state on the surface. Particularly, most of the topological insulators have narrow band gaps, and hence have promising applications in the area of terahertz optoelectronics. In this work, we experimentally demonstrate an electronically-tunable terahertz intensity modulator based on Bi1:5Sb0:5Te1:8Se1:2 single crystal, one of the most insulating topological insulators. A relative frequency-independent modulation depth of ~62% over a wide frequency range from 0.3 to 1.4 THz has been achieved at room temperature, by applying a bias current of 100 mA. The modulation in the low current regime can be further enhanced at low temperature. We propose that the extraordinarily large modulation is a consequence of thermally-activated carrier absorption in the semiconducting bulk states. Our work provides a new application of topological insulators for terahertz technology.

Topological-insulator-based terahertz modulator

The ultrafast relaxation and recombination dynamics of photogenerated electrons and holes in epitaxial graphene are studied using optical-pump Terahertz-probe spectroscopy. The conductivity in graphene at Terahertz frequencies depends on the carrier concentration as well as the carrier distribution in energy. Time-resolved studies of the conductivity can therefore be used to probe the dynamics associated with carrier intraband relaxation and interband recombination. We report the electron-hole recombination times in epitaxial graphene for the first time. Our results show that carrier cooling occurs on sub-picosecond time scales and that interband recombination times are carrier density dependent.

Ultrafast Optical-Pump Terahertz-Probe Spectroscopy of the Carrier Relaxation and Recombination Dynamics in Epitaxial Graphene

Orbital occupancy control in correlated oxides allows the realization of new electronic phases and collective state switching under external stimuli. The resultant structural and electronic phase transitions provide an elegant way to encode, store, and process information. In this review, we examine the utilization of Mott metal-to-insulator transitions, for memory and neuromorphic devices. We emphasize the overarching electron–phonon coupling and electron–electron interaction-driven transition mechanisms and kinetics, which renders a general description of Mott memories from aspects such as nonvolatility, sensing scheme, read/write speed, and switching energy. Various memory and neuromorphic device architectures incorporating phase transition elements are reviewed, focusing on their operational principles. The role of Peierls distortions and crystal symmetry changes during phase change is discussed. Prospects for such orbitronic devices as hardware components for information technologies are summarized.

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Mott Memory and Neuromorphic Devices

Hybrid metamaterials that exhibit reconfigurable responses under external stimulus, such as electric fields and light radiation, have only recently been demonstrated by combining active media with patterned metallic structures. Nevertheless, hybrid terahertz (THz) metamaterials whose spectral performance can be dynamically tuned over a large scale remain rare. Compared with most active media (for instance, silicon) that provide limited activity, vanadium dioxide (VO2), which exhibits an insulator-to-metal transition, has been recently explored to facilitate dynamically tunable metamaterials. More importantly, the phase transition yields a three orders of magnitude increase in THz electrical conductivity, which suggests the potential for creating VO2 based hybrid resonators that operate at THz frequencies. Here, we show that an integration of VO2 structures and conventional metallic resonating components can enable a class of highly tunable THz metamaterials. Considering the widely studied phase-transition dynamics in VO2, the proposed hybrid metamaterials are capable of offering ultrafast modulation of THz radiation.

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Hybrid Resonators and Highly Tunable Terahertz Metamaterials Enabled by Vanadium Dioxide (VO 2 )

The past few decades have witnessed a substantial increase in terahertz (THz) research. Utilizing THz waves to transmit communication and imaging data has created a high demand for phase and amplitude modulation. However, current active THz devices, including modulators and switches, still cannot meet THz system demands. Double-channel heterostructures, an alternative semiconductor system, can support nanoscale two-dimensional electron gases (2DEGs) with high carrier concentration and mobility and provide a new way to develop active THz devices. In this Letter, we present a composite metamaterial structure that combines an equivalent collective dipolar array with a double-channel heterostructure to obtain an effective, ultrafast, and all-electronic grid-controlled THz modulator. Electrical control allows for resonant mode conversion between two different dipolar resonances in the active device, which significantly improves the modulation speed and depth. This THz modulator is the first to achieve a 1 GHz ...

Gbps terahertz external modulator based on a composite metamaterial with a double-channel heterostructure.

We demonstrate an active metamaterial device that allows to electrically control terahertz transmission over more than one order of magnitude. Our device consists of a lithographically defined gold nano antenna array fabricated on a thin film of vanadium dioxide (VO2), a material that possesses an insulator to metal transition. The nano antennas let terahertz (THz) radiation funnel through when the VO2 film is in the insulating state. By applying a dc-bias voltage through our device, the VO2 becomes metallic. This electrically shorts the antennas and therefore switches off the transmission in two distinct regimes: reversible and irreversible switching.

Electrical control of terahertz nano antennas on VO2 thin film.

We report on an order of magnitude enhanced nonlinear response of vanadium dioxide thin film patterned with nanoresonators--nano slot antennas fabricated on the gold film. Transmission of terahertz radiation, little affected by an optical pumping for the case of bulk thin film, can now be completely switched-off: DeltaT/T approximately -0.9999 by the same optical pumping power. This unprecedentedly large optical pump-terahertz probe nonlinearity originates from the insulator-to-metal phase transition drastically reducing the antenna cross sections of the nanoresonators. Our scheme enables nanoscale-thin film technology to be used for all-optical switching of long wavelength light.

Giant nonlinear response of terahertz nanoresonators on VO2 thin film.

Purpose: For cardiovascular patients, family caregivers play a vital role in daily nursing and cardiac emergencies. This study aimed to evaluate the effect of patient-centered CPR education (PCE) for family caregivers of patients with cardiovascular diseases. Methods: Fifty-four participants were randomly assigned to the PCE or control group. The PCE group received tailored counseling on overall cardiovascular disease information and CPR followed by interactive instructor-guided CPR training and re-education follow-up by telephone 2 weeks later. The control group received only video-based CPR self-education and booklets. Cardiovascular disease and CPR knowledge and self-efficacy were measured before (pre-test), immediately after (post-test 1), and 4 weeks after the PCE (post-test 2). CPR skills and performance were measured pre-test and at post-test1. Results: The PCE group demonstrated significant improvements in knowledge (F=91.09, p <.001), self-efficacy (F=15.19, p <.001) and CPR skills and performance (F=8.10, p =.008), as well as significant differences over time (knowledge: F=364.25, p <.001; self-efficacy: F=1162.28, p <.001; CPR skills and performance: F=1798.81, p <.001). There were significant group-by-time interactions for knowledge (F=8.10, p =.001), self-efficacy (F=4.30, p =.019) and CPR skills and performance (F=4.81, p =.036) by repeated measures ANOVA. Conclusion: This is the first study to demonstrate the effects of a patient-centered intervention with CPR education tailored for patients’ and family caregivers’ preferences, needs, and lifestyles. The results of this study encourage the use of tailored, patient-centered interventions in cardiovascular nursing practice.

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Hyun Sun Kim

Papers

Electrical control of terahertz nano antennas on VO2 thin film.

Giant nonlinear response of terahertz nanoresonators on VO2 thin film.

The Effect of Patient-centered CPR Education for Family Caregivers of Patients with Cardiovascular Diseases