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Telecommunications and polarimetry both require the active control of the polarization of light, Currently, this is done by combining intrinsically anisotropic materials with tunable isotropic materials into heterostructures using complicated fabrication techniques due to the lack of scalable materials that possess both properties. Tunable birefringent and dichromic materials are scarce and rarely available in high-quality thin films over wafer scales. In this paper, we report semiconducting, highly aligned, single-walled carbon nanotubes (SWCNTs) over 4" wafers with normalized birefringence and dichroism values 0.09 and 0.58, respectively. The real and imaginary parts of the refractive index of the SWCNT films are tuned by up to 5.9% and 14.3% in the infrared at 2200 nm and 1660 nm, respectively, using electrostatic doping. Our results suggest that aligned SWCNTs are among the most anisotropic and tunable optical materials known and opens new avenues for their application in integrated photonics and telecommunications.

Low power consumption in both static and dynamic modes of operation is a key requirement in modern, highly scaled nanoelectronics. Tunneling field-effect transistors (TFETs) that exploit direct band-to-band tunneling of charges and exhibit steep sub-threshold slope (SS) transfer characteristics are an attractive option in this regard. However, current generation of Si and III-V heterojunction based TFETs while suffer from low ON current density and ON/OFF current ratios for < 60 mV/dec operation. Semiconducting two-dimensional (2D) layers have recently renewed enthusiasm in novel device design for TFETs not only because of their atomically-thin bodies that favor superior electrostatic control but the same feature also favors higher ON current density and consequently high ON/OFF ratio. Here, we demonstrate gate-tunable heterojunction diodes (triodes) fabricated from InSe/Si 2D/3D van der Waals heterostructures, with a minimum subthreshold swing (SS) as low as 6.4 mV/dec and an SS average of 30 mV/dec over 4 decades of current. Further, the devices show a large current on/off ratio of approximately 10^6 and on-state current density of 0.3 uA/um at a drain bias of -1V. Our work opens new avenues for 2D semiconductors for 3D hetero-integration with Si to achieve ultra-low power logic devices., Comment: 5 figures and supporting information

Light-matter coupling in excitonic materials has been the subject of intense investigation due to emergence of new excitonic materials. Two-dimensional layered hybrid organic/inorganic perovskites (2D HOIPs) support strongly bound excitons at room-temperatures with some of the highest oscillator strengths and electric loss tangents among the known excitonic materials. Here, we report strong light-matter coupling in Ruddlesden-Popper phase 2D-HOIPs crystals without the necessity of an external cavity. We report concurrent occurrence of multiple-orders of hybrid light-matter states via both reflectance and luminescence spectroscopy in thick (> 100 nm) crystals and near-unity absorption in thin (< 20 nm) crystals. We observe resonances with quality factors > 250 in hybridized exciton-polaritons and identify a linear correlation between exciton-polariton mode splitting and extinction coefficient of the various 2D-HOIPs. Our work opens the door to studying polariton dynamics in self-hybridized and open cavity systems with broad applications in optoelectronics and photochemistry.

Two-dimensional (2D) MoS$_2$ has been intensively investigated for its use in the fields of microelectronics, nanoelectronics, and optoelectronics. However, intrinsic 2D MoS$_2$ is usually used as the n-type semiconductor due to the unintentional sulphur vacancies and surface gas adsorption.The synthesis and characterization of 2D MoS$_2$ semiconductor of p-type are crucial for the development of relevant p-n junction devices, as well as the practical applications of 2D MoS$_2$ in the next-generation CMOS integrated circuit. Here, we synthesize high-quality, wafer-scale, 2D p-type MoS$_2$ (Mo$_{1-x}$Nb$_x$S$_2$) with various niobium (Nb) mole fractions from 0 to 7.6% by a creative two-step method. The dielectric functions of 2D Mo1-xNbxS2 are accurately determined by spectroscopic ellipsometry. We find that the increasing fraction of Nb dopant in 2D MoS$_2$ can modulate and promote the combination of A and B exciton peaks of 2D MoS$_2$. The direct causes of this impurity-tunable combination are interpreted as the joint influence of decreasing peak A and broadening peak B. We explain the broadening peak B as the multiple transitions from the impurity-induced valance bands to the conductive band minimum at K point of Brillouin zone by comparing and analyzing the simulated electronic structure of intrinsic and 2D Nb-doped MoS$_2$. A p-type FET based on the 2D Nb-doped MoS$_2$ was fabricated for characterization, and its working performance is expected to be adjustable as a function of concentration of Nb dopant according to our theoretical research. Our study is informative for comprehending optical and electronic properties of extrinsic 2D transitional metal dichalcogenides, which is important and imperative for the development and optimization of corresponding photonics and optoelectronics devices.

Two-dimensional (2D) crystals have renewed opportunities in design and assembly of artificial lattices without the constraints of epitaxy. However, the lack of thickness control in exfoliated van der Waals (vdW) layers prevents realization of repeat units with high fidelity. Recent availability of uniform, wafer-scale samples permits engineering of both electronic and optical dispersions in stacks of disparate 2D layers with multiple repeating units. We present optical dispersion engineering in a superlattice structure comprised of alternating layers of 2D excitonic chalcogenides and dielectric insulators. By carefully designing the unit cell parameters, we demonstrate > 90 % narrowband absorption in < 4 nm active layer excitonic absorber medium at room temperature, concurrently with enhanced photoluminescence in cm2 samples. These superlattices show evidence of strong light-matter coupling and exciton-polariton formation with geometry-tunable coupling constants. Our results demonstrate proof of concept structures with engineered optical properties and pave the way for a broad class of scalable, designer optical metamaterials from atomically-thin layers., Comment: 4 figures + supporting

Two-dimensional (2D) hybrid organic inorganic perovskite (HOIP) semiconductors have attracted widespread attention as a platform of next generation optoelectronic devices benefiting from their naturally occurring and tunable multiple quantum-well like (QW) structures, which enable a wide range of physical properties. Determining the intrinsic optical/electronic properties of 2D HOIPs is extremely important for further utility in photonic and optoelectronics devices. Here, we obtain the optical dielectric functions, complex refractive indices, and complex optical conductivities of both Ruddlesden-Popper (RP) and Dion-Jacobsen (DJ) phases of 2D HOIPs as a function of the perovskite QW thickness via spectroscopic ellipsometry over a broad energy range of 0.73 - 3.34 eV. We identify a series of feature peaks in the dielectric functions, and explain the evolution of ground state exciton peak with unit cell thickness and changing excitonic confinement. We observe extraordinary values of optical extinction and electric loss tangents at the primary excitonic resonances and provide their detailed comparison with other known excitonic materials. Our study is expected to lay foundation for understanding optical properties of pure phase 2D HOIPs, which will be helpful for the accurate modelling of their photonics and optoelectronic devices.

Electrically-tunable optical properties in materials are desirable for many applications ranging from displays to lasing and optical communication. In most two-dimensional thin-films and other quantum confined materials, these constants have been measured accurately. However, the optical constants of single wall nanotubes (SWCNT) as a function of electrostatic tuning are yet to be measured due to lack of electronic purity and spatial homogeneity over large areas. Here, we measure the basic optical constants of ultrathin high-purity (>99%) semiconducting single wall carbon nanotube (s-SWCNT) films with spectroscopic ellipsometry. We extract the gate-tunable complex refractive index of s-SWCNT films and observe giant modulation of the real refractive index (~11.2% or an absolute value of >0.2) and extinction coefficient (~11.6%) in the near-infrared (IR) region (1.3-1.55 {\mu}m) induced by the applied electric field significantly higher than all existing electro-optic semiconductors in this wavelength range. We further design a multilayer IR reflection phase modulator stack by combining s-SWCNT and monolayer MoS2 heterostructures that can attain >45{\deg} reflection phase modulation at 1600 nm wavelength for < 200 nm total stack thickness. Our results highlight s-SWCNT as a promising material system for infrared photonics and electro-optics in telecommunication applications.

Van der Waals (vdW) semiconductors are attractive for highly scaled devices and heterogeneous integration since they can be isolated into self-passivated, two-dimensional (2D) layers that enable superior electrostatic control. These attributes have led to numerous demonstrations of field-effect devices ranging from transistors to triodes. By exploiting the controlled, substitutional doping schemes in covalently-bonded, three-dimensional (3D) semiconductors and the passivated surfaces of 2D semiconductors, one can construct devices that can exceed performance metrics of 'all-2D' vdW heterojunctions. Here, we demonstrate, 2D/3D semiconductor heterojunctions using MoS2 as the prototypical 2D semiconductor laid upon Si and GaN as the 3D semiconductor layers. By tuning the Fermi levels in MoS2, we demonstrate devices that concurrently exhibit over seven orders of magnitude modulation in rectification ratios and conductance. Our results further suggest that the interface quality does not necessarily affect Fermi-level tuning at the junction opening up possibilities for novel 2D/3D heterojunction device architectures., Comment: Manuscript plus supporting information, 4 Figures