Optically Gated Electrostatic Field-Effect Thermal Transistor.

Dynamic tuning of thermal transport in solids is scientifically intriguing with wide applications for thermal transport control in electronic devices. In this work, we demonstrate a thermal transistor, a device in which heat flow can be regulated using external control, realized in a topological insulator (TI) through the topological surface states. The tuning of thermal transport is achieved by using optical gating of a thin dielectric layer deposited on the TI film. The gate-dependent thermal conductivity is measured using micro-Raman thermometry. The transistor has a large ON/OFF ratio of 2.8 at room temperature and can be continuously and repetitively switched in tens of seconds by optical gating and potentially much faster by electrical gating. Such thermal transistors with a large ON/OFF ratio and fast switching times offer the possibilities of smart thermal devices for active thermal management and control in future electronic systems.

Tunable Circular Photogalvanic and Photovoltaic Effect in 2D Tellurium with Different Chirality.

Chirality arises from the asymmetry of materials, where two counterparts are the mirror image of each other. The interaction between circular-polarized light and quantum materials is enhanced in chiral space groups due to the structural chirality. Tellurium (Te) possesses the simplest chiral crystal structure, with Te atoms covalently bonded into a spiral atomic chain (left- or right-handed) with a periodicity of 3. Here, we investigate the tunable circular photoelectric responses in 2D Te field-effect transistors with different chirality, including the longitudinal circular photogalvanic effect induced by the radial spin texture (electron-spin polarization parallel to the electron momentum direction) and the circular photovoltaic effect induced by the chiral crystal structure (helical Te atomic chains). Our work demonstrates the controllable manipulation of the chirality degree of freedom in materials.

Model for polymerization and self-deactivation in two-photon nanolithography.

A mathematical model is developed to describe the photochemical processes in two-photon nanolithography, including two-step absorption leading to initiation and self-deactivation of the photoinitiator by laser irradiance, polymer chain propagation, termination, inhibition, and inhibitor and photoinitiator diffusion. This model is solved numerically to obtain the concentrations of the reaction species as a function of time and space as a laser beam is scanned through a volume of photoresist, from which a voxel size or linewidth is determined. The most impactful process parameters are determined by fitting the model to experimentally measured linewidths for a range of laser powers and scanning speeds, while also obtaining effective nonlinearities that are similar to previously measured values. The effects and sensitivities of the different process parameters are examined. It is shown that the photopolymerization process is dominated by diffusion of photoinitiators and oxygen inhibitors, and that self-deactivation can lead to higher effective nonlinearities in two-photon nanolithography.

High Accuracy Ultrafast Spatiotemporal Pump-Probe Measurement of Electrical Thermal Transport in Thin Film Gold.

A high resolution spatiotemporal ultrafast pump-probe system is developed to examine the interactions and transport phenomena between the electrical and the lattice thermal subsystems during ultrafast laser-matter interactions. This system incorporates an ultrafast pump-probe scheme with a stationary probe beam that interrogates the response to a spatial scanning pump beam, providing a full spatiotemporal mapping of a material's response due to an ultrafast pump excitation. The material's response, which is highly sensitive to its transport properties, is measured with a high spatial accuracy of up to ±10 nm and subpicosecond time resolution. Details of achieving this high spatial accuracy are described, and a study of the ultrafast transport processes in thin film gold is demonstrated. With the aid of transport and optical response models, the electrical thermal transport properties of gold and the electron-lattice coupling constant are simultaneously determined.

Thermoelectric Performance of Lead-Free Two-Dimensional Halide Perovskites Featuring Conjugated Ligands.

Sn-based halide perovskites are promising for thermoelectric (TE) device applications because of their high electrical conductivity as well as the low thermal conductivity associated with their soft lattices. However, conventional three-dimensional Sn-based perovskites are not stable under typical TE device operating conditions. Here, we report a stable two-dimensional Sn-based perovskite for thermoelectric energy conversion by incorporating bulky conjugated ligands. We demonstrate a thin film with a large power factor of 5.42 ± 3.07 (average) and 7.07 (champion) µW m-1 K-2 at 343 K with an electrical conductivity of 5.07 S cm-1 and a Seebeck coefficient of 118.1 µV K-1. Importantly, these thin films show excellent operational stability (i.e., for over 100 h) at 313 K. This work suggests that the novel hybrid two-dimensional perovskites are a promising platform for thermoelectric energy conversion applications.

Chiro-optical response of a wafer scale metamaterial with ellipsoidal metal nanoparticles.

We report a large chiro-optical response from a nanostructured film of aperiodic dielectric helices decorated with ellipsoidal metal nanoparticles. The influence of the inherent fabrication variation on the chiro-optical response of the wafer-scalable nanostructured film is investigated using a computational model which closely mimics the material system. From the computational approach, we found that the chiro-optical signal is strongly dependent on the ellipticities of the metal nanoparticles and the developed computational model can account for all the variations caused by the fabrication process. We report the experimentally realized dissymmetry factor ∼1.6, which is the largest reported for wafer scalable chiro-plasmonic samples till now. The calculations incorporate strong multipolar contributions of the plasmonic interactions to the chiro-optical response from the tightly confined ellipsoidal nanoparticles, improving upon the previous studies carried in the coupled dipole approximation regime. Our analyzes confirm the large chiro-optical response in these films developed by a scalable and simple fabrication technique, indicating their applicability pertaining to manipulation of optical polarization, enantiomer selective identification and enhanced sensing and detection of chiral molecules.

Near-Field Thermal Radiation between Two Plates with Sub-10 nm Vacuum Separation.

Radiation greatly exceeding blackbody between two objects separated by microscale distances has attracted great interest. However, challenges in reaching such a small separation between two plates have so far prevented studies below a separation distance of about 25 nm. Here, we report a study of radiation enhancement in the near-field regime of less than 10 nm between two parallel plates. We make use of bulk, rigid plates to approach small separation distances without the adverse snap-in effect, develop embedded temperature sensors to allow near-zero separation, and employ advanced sensing method to level the plates and approach and maintain small separations. Our findings agree with theoretical predictions between parallel surfaces with separations down to 7 nm where an 18000 times enhancement in radiation between two quartz plates is observed. Our method can also be used to explore heat transfer between other materials and can possibly be extended to smaller separation gaps.

Thermoelectric Performance of 2D Tellurium with Accumulation Contacts.

Tellurium (Te) is an intrinsically p-type-doped narrow-band gap semiconductor with an excellent electrical conductivity and low thermal conductivity. Bulk trigonal Te has been theoretically predicted and experimentally demonstrated to be an outstanding thermoelectric material with a high value of thermoelectric figure-of-merit ZT. In view of the recent progress in developing the synthesis route of 2D tellurium thin films as well as the growing trend of exploiting nanostructures as thermoelectric devices, here for the first time, we report the excellent thermoelectric performance of tellurium nanofilms, with a room-temperature power factor of 31.7 µW/cm K2 and ZT value of 0.63. To further enhance the efficiency of harvesting thermoelectric power in nanofilm devices, thermoelectrical current mapping was performed with a laser as a heating source, and we found that high work function metals such as palladium can form rare accumulation-type metal-to-semiconductor contacts to Te, which allows thermoelectrically generated carriers to be collected more efficiently. High-performance thermoelectric Te devices have broad applications as energy harvesting devices or nanoscale Peltier coolers in microsystems.

Split ring resonator as a nanoscale optical transducer for heat-assisted magnetic recording.

Split ring resonators (SRR) are optical nanostructures that have received a lot of attention for their ability to support magnetic resonance and for their potential use as materials with negative dielectric constant. In this work, we design SRRs as near-field transducers (NFT) for generating a nanoscale hotspot in heat-assisted magnetic recording (HAMR), which is considered a candidate for the next-generation data storage technology. The underlying mechanisms for the generation of hotspot and the dependence on wavelength and geometry of the SRR structure are studied. Optical and thermal performance of SRRs functioning as NFTs in a HAMR device are evaluated. These structures were fabricated using focused ion beam milling. The focusing capability of the SRR is experimentally demonstrated using a scattering near field scanning optical microscope.

Demonstration of Enhanced Optical Pressure on a Structured Surface.

The interaction of electromagnetic waves with condensed matter and the resultant force is fundamental in the physical sciences. The maximum pressure on a planar surface is understood to be twice the incident wave power density normalized by the background velocity. We demonstrate for the first time that this pressure can be exceeded by a substantial factor by structuring a surface. Experimental results for direct optomechanical deflection of a nanostructured gold film on a silicon nitride membrane illuminated by a laser beam are shown to significantly exceed those for the planar surface. This enhanced pressure can be understood as being associated with an asymmetric optical cavity array realized in the membrane film. The possible enhancement depends on the material properties and the geometrical parameters of the structured material. Such control and increase of optical pressure with nanostructured material should impact applications across the physical sciences.

Ultrafast Surface State Spin-Carrier Dynamics in the Topological Insulator Bi_{2}Te_{2}Se.

Topological insulators are promising candidates for optically driven spintronic devices, because photoexcitation of spin polarized surface states is governed by angular momentum selection rules. We carry out femtosecond midinfrared spectroscopy on thin films of the topological insulator Bi_{2}Te_{2}Se, which has a higher surface state conductivity compared to conventionally studied Bi_{2}Se_{3} and Bi_{2}Te_{3}. Both charge and spin dynamics are probed utilizing circularly polarized light. With a sub-band-gap excitation, clear helicity-dependent dynamics is observed only in thin (<20 nm) flakes. On the other hand, such dependence is observed for both thin and thick flakes with above-band-gap excitation. The helicity dependence is attributed to asymmetric excitation of the Dirac-like surface states. The observed long-lasting asymmetry over 10 ps even at room temperature indicates low backscattering of surface state carriers which can be exploited for spintronic devices.

One-Dimensional van der Waals Material Tellurium: Raman Spectroscopy under Strain and Magneto-Transport.

Experimental demonstrations of one-dimensional (1D) van der Waals material tellurium (Te) have been presented by Raman spectroscopy under strain and magneto-transport. Raman spectroscopy measurements have been performed under strains along different principle axes. Pronounced strain response along the c-axis is observed due to the strong intrachain covalent bonds, while no strain response is obtained along the a-axis due to the weak interchain van der Waals interaction. Magneto-transport results further verify its anisotropic property, which results in dramatically distinct magneto-resistance behaviors in terms of three different magnetic field directions. Specifically, phase coherence length extracted from weak antilocalization effect, Lϕ ≈ T-0.5, claims its two-dimensional (2D) transport characteristics when an applied magnetic field is perpendicular to the thin film. In contrast, Lϕ ≈ T-0.33 is obtained from universal conductance fluctuations once the magnetic field is along the c-axis of Te, which indicates its nature of 1D transport along the helical atomic chains. Our studies, which are obtained on high quality single crystal Te thin film, appear to serve as strong evidence of its 1D van der Waals structure from experimental perspectives. It is the aim of this paper to address this special concept that differs from the previous well-studied 1D nanowires or 2D van der Waals materials.

Fabrication of silver nanostructures using femtosecond laser-induced photoreduction.

Silver nanostructures were fabricated by femtosecond laser-induced reduction of silver ions and the impact of solution chemistry on the fabricated structures was evaluated. By investigating the exact photochemistry of the nanofabrication solutions, which contained varying amounts of diamine silver ions, trisodium citrate, and n-lauroylsarcosine sodium, and optimizing the laser processing parameters, we fabricated two-dimensional silver pads with surface roughness values of 7 nm and stable 2.5-dimensional shell structures with heights up to 10 µm and aspect ratios of 20 in a ready manner. Moreover, thermal annealing of these structures afforded materials where the average resistivity value was only a factor of 4 greater than that of bulk silver. In this way, the work presented here provides for a methodology that can be used for laser direct fabrication of metal nanostructures for applications in plasmonics and micro- and nano-electronics.

16 nm-resolution lithography using ultra-small-gap bowtie apertures.

Photolithography has long been a critical technology for nanoscale manufacturing, especially in the semiconductor industry. However, the diffractive nature of light has limited the continuous advance of optical lithography resolution. To overcome this obstacle, near-field scanning optical lithography (NSOL) is an alternative low-cost technique, whose resolution is determined by the near-field localization that can be achieved. Here, we apply the newly-developed backside milling method to fabricate bowtie apertures with a sub-15 nm gap, which can substantially improve the resolution of NSOL.  A highly confined electric near field is produced by localized surface plasmon excitation and nanofocusing of the closely-tapered gap. We show contact lithography results with a record 16 nm resolution (FWHM). This photolithography scheme promises potential applications in data storage, high-speed computation, energy harvesting, and other nanotechnology areas.

Auxetic Black Phosphorus: A 2D Material with Negative Poisson's Ratio.

The Poisson's ratio of a material characterizes its response to uniaxial strain. Materials normally possess a positive Poisson's ratio - they contract laterally when stretched, and expand laterally when compressed. A negative Poisson's ratio is theoretically permissible but has not, with few exceptions of man-made bulk structures, been experimentally observed in any natural materials. Here, we show that the negative Poisson's ratio exists in the low-dimensional natural material black phosphorus and that our experimental observations are consistent with first-principles simulations. Through applying uniaxial strain along armchair direction, we have succeeded in demonstrating a cross-plane interlayer negative Poisson's ratio on black phosphorus for the first time. Meanwhile, our results support the existence of a cross-plane intralayer negative Poisson's ratio in the constituent phosphorene layers under uniaxial deformation along the zigzag axis, which is in line with a previous theoretical prediction. The phenomenon originates from the puckered structure of its in-plane lattice, together with coupled hinge-like bonding configurations.

Observation of Optical and Electrical In-Plane Anisotropy in High-Mobility Few-Layer ZrTe5.

Transition metal pentatelluride ZrTe5 is a versatile material in condensed-matter physics and has been intensively studied since the 1980s. The most fascinating feature of ZrTe5 is that it is a 3D Dirac semimetal which has linear energy dispersion in all three dimensions in momentum space. Structure-wise, ZrTe5 is a layered material held together by weak interlayer van der Waals force. The combination of its unique band structure and 2D atomic structure provides a fertile ground for more potential exotic physical phenomena in ZrTe5 related to 3D Dirac semimentals. However, the physical properties of its few-layer form have yet to be thoroughly explored. Here we report strong optical and electrical in-plane anisotropy of mechanically exfoliated few-layer ZrTe5. Raman spectroscopy shows a significant intensity change with sample orientations, and the behavior of angle-resolved phonon modes at the Γ point is explained by theoretical calculations. DC conductance measurement indicates a 50% of difference along different in-plane directions. The diminishing of resistivity anomaly in few-layer samples indicates the evolution of band structure with a reduced thickness. A low-temperature Hall experiment sheds light on more intrinsic anisotropic electrical transport, with a hole mobility of 3000 and 1500 cm2/V·s along the a-axis and c-axis, respectively. Pronounced quantum oscillations in magnetoresistance are observed at low temperatures with the highest electron mobility up to 44 000 cm2/V·s.

Subdiffraction light focusing using a cross sectional ridge waveguide nanoscale aperture.

We report a new type of plasmonic nanoscale ridge aperture and its fabrication process which is based on layer-by-layer planar lithography. This new fabrication method allows us to create desired nanoscale features of a plasmonic ridge waveguide nanoscale aperture, which helps to confine a near-field spot to sub-wavelength dimensions. Numerical simulations using Finite Element Method (FEM) are performed to calculate the near-field distribution around the exit of the aperture. Measurements using scattering near-field scanning optical microscopy (s-NSOM) confirm the design and demonstrate that the aperture is capable of producing focused spots in the ridge gap at the exit of the aperture. The planar lithography process is a step toward mass production of such plasmonic structures for applications including heat-assisted magnetic recording (HAMR).

Laser direct writing of modulation-doped nanowire p/n junctions.

We demonstrate a single-step, laser-based technique to fabricate axial modulation-doped silicon nanowires. Our method is based on laser-direct-write chemical vapor deposition and has the capability to fabricate nanowires as small as 60 nm, which is far below the diffraction limit of the laser wavelength of 395 nm, with precise control of nanowire position, length, and orientation. By switching dopant gases during nanowire writing, p-n junction nanowires are produced. The p-n junction nanowires are fabricated into multifinger devices with parallel metal contacts and electrically tested to demonstrate diode characteristics.

Three-dimensional mapping of optical near field with scattering SNOM.

Scattering-type scanning near-field optical microscopy (s-SNOM) is applied to investigate three-dimensional optical near field distribution, including both amplitude and phase information. A method analogous to the force volume mode of the atomic force microscopy (AFM) technique is adapted for the measurement. The results show high lateral resolution of tens of nanometers, and even higher vertical resolution of only a few nanometers. The experiment results provide a straightforward illustration of the optical near fields in the space above the sample surface. Numerical computations support the experimental data. The ability to map the 3D optical near filed helps to reveal the factors that influence the performance of the designed near-field structures.

Fabrication of bowtie aperture antennas for producing sub-20 nm optical spots.

Bowtie aperture antennas are known to generate sub-diffraction limited optical spots in the visible and near-infrared frequencies, which can be applied to many areas. Regular bowtie apertures fabricated by FIB suffer from tapered sidewall and rounded corner, which degrade its optical enhancement and localization. In this work, a new fabrication method is demonstrated to manufacture bowtie aperture antennas which can produce optical spots with lateral size smaller than 20 nm. We also employ numerical simulations to compute the near-field distribution on the surface of the bowtie aperture with topography extracted from the fabrication antennas. The near-field distribution measured by s-NSOM agrees well with the simulation and confirms the improved near-field localization of our bowtie aperture. This new fabrication method can be applied to other types of ridged apertures, which promises wide applications of deep sub-diffraction limited optical spots in many areas.