Two-color 3D printing for reduction in femtosecond laser printing power.

Two-photon polymerization (TPP) has emerged as a favored advanced manufacturing tool for creating complex 3D structures in the sub-micron regime. However, the widescale implementation of this technique is limited partly due to the cost of a high-power femtosecond laser. In this work, a method is proposed to reduce the femtosecond laser 3D printing power by as much as 50% using a combination of two-photon absorption from an 800 nm femtosecond laser and single photon absorption from a 532 nm nanosecond laser.

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.

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.

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.

Rapid, continuous projection multi-photon 3D printing enabled by spatiotemporal focusing of femtosecond pulses.

There is demand for scaling up 3D printing throughput, especially for the multi-photon 3D printing process that provides sub-micrometer structuring capabilities required in diverse fields. In this work, high-speed projection multi-photon printing is combined with spatiotemporal focusing for fabrication of 3D structures in a rapid, layer-by-layer, and continuous manner. Spatiotemporal focusing confines printing to thin layers, thereby achieving print thicknesses on the micron and sub-micron scale.

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.

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.

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.

Deep tuning of photo-thermoelectricity in topological surface states.

Three-dimensional topological insulators have been demonstrated in recent years, which possess intriguing gapless, spin-polarized Dirac states with linear dispersion only on the surface. The spin polarization of the topological surface states is also locked to its momentum, which allows controlling motion of electrons using optical helicity, i.e.

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.

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.

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.

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.

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.

Plasmonic Resonance Enhanced Polarization-Sensitive Photodetection by Black Phosphorus in Near Infrared.

Black phosphorus, a recently intensely investigated two-dimensional material, is promising for electronic and optoelectronic applications due to its higher mobility and thickness-dependent direct band gap. With its low direct band gap and anisotropic properties in nature, black phosphorus is also suitable for near-infrared polarization-sensitive photodetection. To enhance photoresponsivity of a black phosphorus based photodetector, we demonstrate two designs of plasmonic structures.

Large Enhancement of Thermal Conductivity and Lorenz Number in Topological Insulator Thin Films.

Topological insulators (TI) have attracted extensive research effort due to their insulating bulk states but conducting surface states. However, investigation and understanding of thermal transport in topological insulators, particularly the effect of surface states, are lacking. In this work, we studied thickness-dependent in-plane thermal and electrical conductivity of BiTeSe TI thin films.

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.

Controlled Growth of a Large-Size 2D Selenium Nanosheet and Its Electronic and Optoelectronic Applications.

Selenium has attracted intensive attention as a promising material candidate for future optoelectronic applications. However, selenium has a strong tendency to grow into nanowire forms due to its anisotropic atomic structure, which has largely hindered the exploration of its potential applications. In this work, using a physical vapor deposition method, we have demonstrated the synthesis of large-size, high-quality 2D selenium nanosheets, the minimum thickness of which could be as thin as 5 nm.

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.

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.

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

Transition metal pentatelluride ZrTe is a versatile material in condensed-matter physics and has been intensively studied since the 1980s. The most fascinating feature of ZrTe is that it is a 3D Dirac semimetal which has linear energy dispersion in all three dimensions in momentum space. Structure-wise, ZrTe is a layered material held together by weak interlayer van der Waals force.

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.

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.

Sub-Diffraction Limited Writing based on Laser Induced Periodic Surface Structures (LIPSS).

Controlled fabrication of single and multiple nanostructures far below the diffraction limit using a method based on laser induced periodic surface structure (LIPSS) is presented. In typical LIPSS, multiple lines with a certain spatial periodicity, but often not well-aligned, were produced. In this work, well-controlled and aligned nanowires and nanogrooves with widths as small as 40 nm and 60 nm with desired orientation and length are fabricated.