We demonstrate tuning of a metamaterial device that incorporates a form of spatial gradient control. Electrical tuning of the metamaterial is achieved through a vanadium dioxide layer which interacts with an array of split ring resonators. We achieved a spatial gradient in the magnitude of permittivity, writeable using a single transient electrical pulse. This induced gradient in our device is observed on spatial scales on the order of one wavelength at 1 THz. Thus we show the viability of elements for use in future devices with potential applications in beamforming and communications.Metamaterials have progressed from academic curiosities 1,2 to candidates for real-world applications. 3,4 Emerging metamaterial applications range from radio frequency (RF) 5 communications to millimeter radar. 6 One key technique which promises to further the applicability of metamaterials is tunability. Operational frequency tuning has been presented as one solution to the narrow bandwidth often present in metamaterial devices. 7,8 Frequency-agile metamaterial designs have been demonstrated across a wide spectrum from microwave 9 to near-visible frequencies. 10 To date, tuning has generally been homogeneously implemented across the entire device as a whole. Developing techniques for controllable spatially variable tuning will present the possibility of devices with a reconfigurable Gradient Index of Refraction (GRIN). GRIN devices have already proven an attractive area for metamaterials, 11 as the metamaterial design process naturally allows for the control needed to fabricate GRIN structures. Additionally, use of spatially nonuniform tuning can leverage the narrow bandwidth of metamaterials in a unique way. For narrow-band operation, minor adjustments in the resonance frequency of a metamaterial can translate to large changes in the index of refraction at that frequency. Overall, spatial control of resonance tuning allows for post-fabrication modification of the index of refraction and therefore the creation of a reconfigurable gradient.In this work, we demonstrate a spatially reconfigurable THz hybrid metamaterial with vanadium dioxide (VO 2 ) and split ring resonators (SRRs) as constituent elements. The SRR has been the "fruit fly" of metamaterials research, allowing for convenient implementation of optical characteristics which are unattainable without a) mgoldfla@physics.ucsd.edu metamaterials. 1,12,13 Our device is composed of an array of 100 nm thick gold SRRs (dimensions shown in figure 2) lithographically fabricated on 90 nm thick VO 2 grown on a sapphire substrate. VO 2 undergoes an insulator to metal transition 14 which can be triggered thermally electrically 15 or optically. 16 The phase transition is hysteretic, and therefore, changes in the conductivity of VO 2 generally persist, provided the device temperature is maintained. Hybrid metamaterial-VO 2 devices benefit from this memory, 17 and from the large tuning dynamic range achievable with VO 2 . Persistent tuning uses this memory to eliminate the need for continu...
We investigate near-field infrared spectroscopy and superfluid polariton imaging experiments on conventional and unconventional superconductors. Our modeling shows that near-field spectroscopy can measure the magnitude of the superconducting energy gap in Bardeen-Cooper-Schrieffer-like superconductors with nanoscale spatial resolution. We demonstrate how the same technique can measure the c-axis plasma frequency, and thus the c-axis superfluid density, of layered unconventional superconductors with a similar spatial resolution. Our modeling also shows that near-field techniques can image superfluid surface mode interference patterns near physical and electronic boundaries. We describe how these images can be used to extract the collective mode dispersion of anisotropic superconductors with sub-diffractional spatial resolution.
We report infrared studies of the insulator-to-metal transition (IMT) in GaAs doped with either magnetic (Mn) or non-magnetic acceptors (Be). We observe a resonance with a natural assignment to impurity states in the insulating regime of Ga1−xMnxAs, which persists across the IMT to the highest doping (16%). Beyond the IMT boundary, behavior combining insulating and metallic trends also persists to the highest Mn doping. Be doped samples however, display conventional metallicity just above the critical IMT concentration, with features indicative of transport within the host valence band.The insulator-to-metal transition (IMT) becomes exceptionally complex when magnetism is involved, as proven in materials such as mixed-valence manganites, rare-earth chalcogenides, and Mn-doped III-V compounds [1,2]. In all these systems, the electronic and magnetic properties are typically interconnected, creating an entising challenge to understand how magnetism affects the IMT physics. A promising route to isolate differences attributable to the presence of magnetism on the IMT physics is to investigate either magnetic or non-magnetic dopants in the same host. p-doped GaAs is well suited for the task since metallicity in this material can be initiated by non-magnetic (Zn, Be, C) and magnetic (Mn) acceptors. Infrared (IR) experiments reported here for Ga 1−x Be x As and Ga 1−x Mn x As monitor the charge dynamics and electronic structure in the course of the IMT. Our results establish that the onset arXiv:1109.0310v1 [cond-mat.mtrl-sci] 1 Sep 2011
We demonstrate an electrolyte-based voltage tunable vanadium dioxide (VO2) memory metasurface. Large spatial scale, low voltage, non-volatile switching of terahertz (THz) metasurface resonances is achieved through voltage application using an ionic gel to drive the insulator-to-metal transition in an underlying VO2 layer. Positive and negative voltage application can selectively tune the metasurface resonance into the “off” or “on” state by pushing the VO2 into a more conductive or insulating regime respectively. Compared to graphene based control devices, the relatively long saturation time of resonance modification in VO2 based devices suggests that this voltage-induced switching originates primarily from electrochemical effects related to oxygen migration across the electrolyte–VO2 interface.
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