Rusen Yan

Broadband graphene terahertz modulators enabled by intraband transitions.

Terahertz technology promises myriad applications including imaging, spectroscopy and communications. However, one major bottleneck at present for advancing this field is the lack of efficient devices to manipulate the terahertz electromagnetic waves. Here we demonstrate that exceptionally efficient broadband modulation of terahertz waves at room temperature can be realized using graphene with extremely low intrinsic signal attenuation. We experimentally achieved more than 2.5 times superior modulation than prior broadband intensity modulators, which is also the first demonstrated graphene-based device enabled solely by intraband transitions. The unique advantages of graphene in comparison to conventional semiconductors are the ease of integration and the extraordinary transport properties of holes, which are as good as those of electrons owing to the symmetric conical band structure of graphene. Given recent progress in graphene-based terahertz emitters and detectors, graphene may offer some interesting solutions for terahertz technologies.

Exciton Dynamics in Suspended Monolayer and Few-Layer MoS2 2D Crystals

Femtosecond transient absorption spectroscopy and microscopy were employed to study exciton dynamics in suspended and Si₃N₄ substrate-supported monolayer and few-layer MoS₂ 2D crystals. Exciton dynamics for the monolayer and few-layer structures were found to be remarkably different from those of thick crystals when probed at energies near that of the lowest energy direct exciton (A exciton). The intraband relaxation rate was enhanced by more than 40 fold in the monolayer in comparison to that observed in the thick crystals, which we attributed to defect assisted scattering. Faster electron-hole recombination was found in monolayer and few-layer structures due to quantum confinement effects that lead to an indirect-direct band gap crossover. Nonradiative rather than radiative relaxation pathways dominate the dynamics in the monolayer and few-layer MoS₂. Fast trapping of excitons by surface trap states was observed in monolayer and few-layer structures, pointing to the importance of controlling surface properties in atomically thin crystals such as MoS₂ along with controlling their dimensions.

Thermal Conductivity of Monolayer Molybdenum Disulfide Obtained from Temperature-Dependent Raman Spectroscopy

Atomically thin molybdenum disulfide (MoS2) offers potential for advanced devices and an alternative to graphene due to its unique electronic and optical properties. The temperature-dependent Raman spectra of exfoliated, monolayer MoS2 in the range of 100-320 K are reported and analyzed. The linear temperature coefficients of the in-plane E2g 1 and the out-of-plane A1g modes for both suspended and substrate-supported monolayer MoS2 are measured. These data, when combined with the first-order coefficients from laser power-dependent studies, enable the thermal conductivity to be extracted. The resulting thermal conductivity κ = (34.5(4) W/mK at room temperature agrees well with the first principles lattice dynamics simulations. However, this value is significantly lower than that of graphene. The results from this work provide important input for the design of MoS2-based devices where thermal management is critical.

Transistors with chemically synthesized layered semiconductor WS2 exhibiting 105 room temperature modulation and ambipolar behavior

We report the realization of field-effect transistors (FETs) made with chemically synthesized multilayer crystal semiconductor WS2. The Schottky-barrier FETs demonstrate ambipolar behavior and a high (∼105×) on/off current ratio at room temperature with current saturation. The behavior is attributed to the presence of an energy bandgap in the ultrathin layered semiconductor crystal material. The FETs also show clear photo response to visible light. The promising electronic and optical characteristics of the devices combined with the chemical synthesis, and flexibility of layered semiconductor crystals such as WS2 make them attractive for future electronic and optical devices.

Unique prospects for graphene-based terahertz modulators

The modulation depth of two-dimensional electron-gas (2DEG) based terahertz (THz) modulators using AlGaAs/GaAs hetero-structures with metal gates is inherently limited to <30%. The metal gate not only attenuates the THz signal but also severely degrades modulation depth. Metal losses can be significantly reduced employing an alternative material with tunable conductivity. Graphene presents a unique solution to this problem due to its symmetric band structure and extraordinarily high hole mobility. In this work, we show that it is possible to achieve a modulation depth of >90% while simultaneously minimizing signal attenuation to <5% by tuning the Fermi level at its Dirac point. VC 2011 American Institute of Physics. [doi:10.1063/1.3636435]

Extraordinary Control of Terahertz Beam Reflectance in Graphene Electro-absorption Modulators

We demonstrate a graphene-based electro-absorption modulator achieving extraordinary control of terahertz reflectance. By concentrating the electric field intensity in an active layer of graphene, an extraordinary modulation depth of 64% is achieved while simultaneously exhibiting low insertion loss (∼2 dB), which is remarkable since the active region of the device is atomically thin. This modulator performance, among the best reported to date, indicates the enormous potential of graphene for terahertz reconfigurable optoelectronic devices.

Esaki Diodes in van der Waals Heterojunctions with Broken-Gap Energy Band Alignment

van der Waals (vdW) heterojunctions composed of two-dimensional (2D) layered materials are emerging as a solid-state materials family that exhibits novel physics phenomena that can power a range of electronic and photonic applications. Here, we present the first demonstration of an important building block in vdW solids: room temperature Esaki tunnel diodes. The Esaki diodes were realized in vdW heterostructures made of black phosphorus (BP) and tin diselenide (SnSe2), two layered semiconductors that possess a broken-gap energy band offset. The presence of a thin insulating barrier between BP and SnSe2 enabled the observation of a prominent negative differential resistance (NDR) region in the forward-bias current-voltage characteristics, with a peak to valley ratio of 1.8 at 300 K and 2.8 at 80 K. A weak temperature dependence of the NDR indicates electron tunneling being the dominant transport mechanism, and a theoretical model shows excellent agreement with the experimental results. Furthermore, the broken-gap band alignment is confirmed by the junction photoresponse, and the phosphorus double planes in a single layer of BP are resolved in transmission electron microscopy (TEM) for the first time. Our results represent a significant advance in the fundamental understanding of vdW heterojunctions and broaden the potential applications of 2D layered materials.

Graphene for Reconfigurable Terahertz Optoelectronics

In this paper, we examine graphene as a material for reconfigurable terahertz (THz) optoelectronics. The ability of electrically tuning its optical properties in a wide range of THz frequencies, together with its 2-D nature and facile integration, leads to unique opportunities for inventing novel THz devices as well as for extending the performance of existing THz technologies. We first review progress in graphene THz active optoelectronic components to date, including large-area graphene, plasmonic, and metamaterial-based devices. Advanced designs and associated challenges are then discussed.

Efficient terahertz electro-absorption modulation employing graphene plasmonic structures

We propose and discuss terahertz (THz) electro-absorption modulators based on graphene plasmonic structures. The active device consists of a self-gated pair of graphene layers, which are patterned to structures supporting THz plasmonic resonances. These structures allow for efficient control of the effective THz optical conductivity, thus absorption, even at frequencies much higher than the Drude roll-off in graphene where most previously proposed graphene-based devices become inefficient. Our analysis shows that reflectance-based device configurations, engineered so that the electric field is enhanced in the active graphene pair, could achieve very high modulation-depth, even ∼100%, over a wide frequency range up to tens of THz.We propose and discuss terahertz (THz) electro-absorption modulators based on graphene plasmonic structures. The active device consists of a self-gated pair of graphene layers, which are patterned to structures supporting THz plasmonic resonances. These structures allow for efficient control of the effective THz optical conductivity, thus absorption, even at frequencies much higher than the Drude roll-off in graphene where most previously proposed graphene-based devices become inefficient. Our analysis shows that reflectance-based device configurations, engineered so that the electric field is enhanced in the active graphene pair, could achieve very high modulation-depth, even ∼100%, over a wide frequency range up to tens of THz.

Terahertz imaging employing graphene modulator arrays

In this paper we propose and experimentally demonstrate arrays of graphene electro-absorption modulators as electrically reconfigurable patterns for terahertz cameras. The active element of these modulators consists of only single-atom-thick graphene, achieving a modulation of the THz wave reflectance > 50% with a potential modulation depth approaching 100%. Although the prototype presented here only contains 4x4 pixels, it reveals the possibility of developing reliable low-cost video-rate THz imaging systems employing single detector.