Tuesday, October 25, 2016

Abstract-Acoustic terahertz graphene plasmons revealed by photocurrent nanoscopy


Terahertz (THz) fields are widely used for sensing, communication and quality control. In future applications, they could be efficiently confined, enhanced and manipulated well below the classical diffraction limit through the excitation of graphene plasmons (GPs). These possibilities emerge from the strongly reduced GP wavelength, λp, compared with the photon wavelength, λ0, which can be controlled by modulating the carrier density of graphene via electrical gating. Recently, GPs in a graphene/insulator/metal configuration have been predicted to exhibit a linear dispersion (thus called acoustic plasmons) and a further reduced wavelength, implying an improved field confinement, analogous to plasmons in two-dimensional electron gases (2DEGs) near conductive substrates. Although infrared GPs have been visualized by scattering-type scanning near-field optical microscopy (s-SNOM), the real-space imaging of strongly confined THz plasmons in graphene and 2DEGs has been elusive so far—only GPs with nearly free-space wavelengths have been observed. Here we demonstrate real-space imaging of acoustic THz plasmons in a graphene photodetector with split-gate architecture. To that end, we introduce nanoscale-resolved THz photocurrent near-field microscopy, where near-field excited GPs are detected thermoelectrically rather than optically. This on-chip detection simplifies GP imaging as sophisticated s-SNOM detection schemes can be avoided. The photocurrent images reveal strongly reduced GP wavelengths (λp ≈ λ0/66), a linear dispersion resulting from the coupling of GPs with the metal gate below the graphene, and that plasmon damping at positive carrier densities is dominated by Coulomb impurity scattering.

Abstract-Effects of plasmonic absorption of terahertz radiation in silica nanocomposite doped with erbium silicate

L. V. Grigoryev, A. A. GorbachevE. A. Sedykh, S. O. Solomin


This paper studies terahertz transmission spectra of silicon and erbium silicides which can be used to create uncooled bolometers for terahertz frequency domain.

Enhanced optical modulation depth of terahertz waves


The terahertz region of the electromagnetic spectrum (covering ~0.1 – 10 THz corresponding to wavelengths from 3 mm to 30 mm) is a hotbed of scientific and technological activity based, in part, on the unique attributes of radiation at these frequencies. This includes spectroscopic imaging with sufficient spectral and spatial resolution through materials that are opaque at other spectral ranges (e.g. microwave, infrared, or visible) and the promise of short-range high-bit-rate data transfer far beyond existing modalities. To advance beyond demonstration towards low-cost real-world applications requires continued development of devices such as modulators and phase shifters to adeptly control terahertz waves. Indeed, groups around the globe are exploring novel device concepts using metamaterials and plasmonics.
ultra-large-area-self-assembled-monolayers-of-gold-nanoparticlesAlong these lines, a particularly intriguing terahertz modulator has been created by Dr. Tianlong Wen, Prof. Qiye Wen and their colleagues. They report on a broadband optically-controlled silicon modulator with impressive amplitude modulation accomplished by depositing a single monolayer of gold nanoparticles on the silicon surface. Crucially, the plasmon resonance of the gold nanoparticles strongly enhances carrier generation in the insulating silicon substrate upon optical excitation. This is important because insulating silicon is transparent to THz radiation (modulo Fresnel reflection losses). With sufficient carrier excitation the reflectivity of the silicon increases, leading to a corresponding decrease in the transmission and thereby modulating the THz beam. The plasmonic layer leads to a dramatic improvement of the modulation depth: for 100 mW of incident optical power, the absolute transmission only changes by ~3% for the bare silicon device in comparison to nearly 30% for the device with a plasmonic layer for an order-of-magnitude improvement. Further, in this elegant approach the THz beam is “blind” to the gold nanoparticle layer, meaning that there is no additional insertion loss.
These results represent an interesting example of a multiscale device where an important performance metric is fruitfully augmented using nanoscience. It will be interesting to follow subsequent developments of this idea to see if the incident optical power could be further reduced to achieve a given modulation amplitude. One could also envision, for example, using metamaterials resonant at THz frequencies in conjunction with gold plasmonic particles to further optimize the modulation response.

Abstract-Manipulation of dual band ultrahigh index metamaterials in the terahertz region

Xufeng Jing, Weimin Wang, Rui Xia, Jingyin Zhao, Ying Tian, and Zhi Hong


By drastically decreasing the diamagnetic effect with a thin metallic structure in the unit cell and increasing the effective permittivity through strong capacitive coupling, we designed the crossed I-shaped metallic patches metamaterial with an extremely high refractive index in the dual band terahertz region. The peak index of refraction of near 80 at about 0.75 THz is predicted, along with another peak index of about 25 at 2.85 THz. Based on the careful analysis of the high index on the dependence of the electric field coupling effect, the magnetic field diamagnetic response, and the geometric parameters in the unit cell, it is found that both of the high index bands respectively correspond to different components in the metamaterial structure. To realize a higher effective refractive index for the second band as well as for the first one, we proposed the triple I-shaped metallic metamaterial structure.
© 2016 Optical Society of America
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Monday, October 24, 2016

Abstract-Photoelectric polarization-sensitive broadband photoresponse from interface junction states in graphene

Nikolai G Kalugin, Lei Jing, Eric Suarez Morell, Gregory C Dyer, Lee Wickey, Mekan Ovezmyradov,Albert D Grine, Michael C Wanke, Eric A Shaner, Chun Ning Lau
Published 24 October 2016 • © 2016 IOP Publishing Ltd 
Graphene has established itself as a promising optoelectronic material. Many details of the photoresponse (PR) mechanisms in graphene in the THz-to-visible range have been revealed, however, new intricacies continue to emerge. Interface junctions, formed at the boundaries between parts of graphene with different number of layers or different stacking orders, and making connection between electrical contacts, provide another peculiar setup to establish PR. Here, we experimentally demonstrate an enhanced polarization sensitive photoelectric PR in graphene sheets containing interface junctions as compared to homogenous graphene sheets in the visible, infrared, and THz spectral regions. Our numerical simulations show that highly localized electronic states are created at the interface junctions, and these states exhibit a unique energy spectrum and enhanced probabilities for optical transitions. The interaction of electrons from interface junction states with electromagnetic fields generates a polarization-sensitive PR that is maximal for the polarization direction perpendicular to the junction interface