High-power narrow spectrum GaSb-based DBR lasers emitting near 2.1 µm.

Stable high-power narrow-linewidth operation of the 2.05-2.1 µm GaSb-based diode lasers was achieved by utilizing the sixth-order surface-etched distributed Bragg reflector (DBR) mirrors. The DBR multimode devices with 100 µm wide ridge waveguides generated ∼850mW in the continuous wave (CW) regime at 20°C. The device CW output power was limited by thermal rollover. The laser emission spectrum was defined by Bragg reflector reflectivity at all operating currents in a wide temperature range. The devices operated at DBR line with detuning from gain peak exceeding 10 meV.

Nanoimprinted Chiral Plasmonic Substrates with Three-Dimensional Nanostructures.

We report a large-area fabrication method to prepare chiral substrates patterned with arrays of multilayer, three-dimensional nanostructures using a combination of nanoimprint lithography and glancing angle deposition. Several structures are successfully fabricated using this method, including L-shaped, twisted arc and trilayer twisted Au nanorod structures, demonstrating its generality. As one typical example, arrays of L-shaped nanostructures, consisting of two layers of orthogonally oriented Au nanorods separated by a Ge dielectric layer in the thickness direction, exhibit giant optical chirality in the infrared region with an experimentally achieved g-factor as high as 0.38. Electromagnetic simulations show that the optical chirality results from plasmon hybridization between the two orthogonal Au segments. To demonstrate scalability, a 1 cm2 chiral substrate is fabricated with uniform chiral optical property. This method combines both high throughput and precise geometrical control and is therefore promising for applications of chiral metamaterials.

External cavity type-I quantum well cascade diode lasers with a tuning range of 440 nm near 3 µm.

A three-stage cascade GaSb-based diode laser heterostructure with an enhanced optical gain spectral bandwidth was designed and fabricated. The gain broadening was achieved by varying the thickness of the type-I quantum wells in different stages of the cascade active region from 10 to 14 nm. The structures were processed into bent ridge gain chips with virtually eliminated feedback from the anti-reflection-coated angled facet. The external cavity devices based on a novel gain chip design demonstrated a record wide tuning range from 2.79 to 3.23 µm in a Littrow cavity configuration at 20°C.

Ultrahigh Elastic Strain Energy Storage in Metal-Oxide-Infiltrated Patterned Hybrid Polymer Nanocomposites.

Modulus of resilience, the measure of a material's ability to store and release elastic strain energy, is critical for realizing advanced mechanical actuation technologies in micro/nanoelectromechanical systems. In general, engineering the modulus of resilience is difficult because it requires asymmetrically increasing yield strength and Young's modulus against their mutual scaling behavior. This task becomes further challenging if it needs to be carried out at the nanometer scale. Here, we demonstrate organic-inorganic hybrid composite nanopillars with one of the highest modulus of resilience per density by utilizing vapor-phase aluminum oxide infiltration in lithographically patterned negative photoresist SU-8. In situ nanomechanical measurements reveal a metal-like high yield strength (∼500 MPa) with an unusually low, foam-like Young's modulus (∼7 GPa), a unique pairing that yields ultrahigh modulus of resilience, reaching up to ∼24 MJ/m3 as well as exceptional modulus of resilience per density of ∼13.4 kJ/kg, surpassing those of most engineering materials. The hybrid polymer nanocomposite features lightweight, ultrahigh tunable modulus of resilience and versatile nanoscale lithographic patternability with potential for application as nanomechanical components which require ultrahigh mechanical resilience and strength.

Aberration-Corrected Electron Beam Lithography at the One Nanometer Length Scale.

Patterning materials efficiently at the smallest length scales is a longstanding challenge in nanotechnology. Electron-beam lithography (EBL) is the primary method for patterning arbitrary features, but EBL has not reliably provided sub-4 nm patterns. The few competing techniques that have achieved this resolution are orders of magnitude slower than EBL. In this work, we employed an aberration-corrected scanning transmission electron microscope for lithography to achieve unprecedented resolution. Here we show aberration-corrected EBL at the one nanometer length scale using poly(methyl methacrylate) (PMMA) and have produced both the smallest isolated feature in any conventional resist (1.7 ± 0.5 nm) and the highest density patterns in PMMA (10.7 nm pitch for negative-tone and 17.5 nm pitch for positive-tone PMMA). We also demonstrate pattern transfer from the resist to semiconductor and metallic materials at the sub-5 nm scale. These results indicate that polymer-based nanofabrication can achieve feature sizes comparable to the Kuhn length of PMMA and ten times smaller than its radius of gyration. Use of aberration-corrected EBL will increase the resolution, speed, and complexity in nanomaterial fabrication.

Ultra-short beam expander with segmented curvature control: the emergence of a semi-lens.

We introduce direct curvature control in designing a segmented beam expander, and explore novel design possibilities for ultra-compact beam expanders. Assisted by the particle swarm optimization algorithm, we search for an optimal curvature-controlled multi-segment taper that maintains width continuity. Counterintuitively, the optimization yields a structure with abrupt width discontinuity and width compression features. Through spatial phase and parameterized analysis, a semi-lens feature is revealed that helps to flatten the wavefront at the output end for higher coupling efficiency. Such functionality cannot be achieved by normal tapers in a short distance. The structure is fabricated and characterized experimentally. By a figure of merit that accounts for expansion ratio, length, and efficiency, this structure outperforms an adiabatic taper by 9 times.

Laterally coupled distributed feedback type-I quantum well cascade diode lasers emitting near 3.22 µm.

The laterally coupled distributed feedback (LC-DFB) GaSb-based type-I quantum well cascade diode lasers using the second- and the sixth-order gratings to stabilize the output spectrum near 3.22 µm were designed and fabricated. The laser heterostructure contained three cascades. The devices were manufactured using a single dry etching step defining the ∼5-µm-wide ridge with ∼5-µm-wide gratings sections adjacent to the ridge sides. The grating coupling coefficients were estimated to be about 1 cm-1. The stability of the single-frequency operation was ensured by alignment of the DFB mode to the relatively wide gain peak. The 2-mm-long second-order LC-DFB lasers generated above 10 mW of continuous-wave (CW) output power at 20°C in epi-side-up configuration and demonstrated power conversion efficiency above 2%. The sixth-order LC-DFB lasers showed lower efficiency but still generated several milliwatts of CW output power. The devices demonstrated a CW current tuning range of about 3.5 nm at the temperature of 20°C.

Generation of ensembles of individually resolvable nitrogen vacancies using nanometer-scale apertures in ultrahigh-aspect ratio planar implantation masks.

A central challenge in developing magnetically coupled quantum registers in diamond is the fabrication of nitrogen vacancy (NV) centers with localization below ∼20 nm to enable fast dipolar interaction compared to the NV decoherence rate. Here, we demonstrate the targeted, high throughput formation of NV centers using masks with a thickness of 270 nm and feature sizes down to ∼1 nm. Super-resolution imaging resolves NVs with a full-width maximum distribution of 26 ± 7 nm and a distribution of NV-NV separations of 16 ± 5 nm.

Metal-semiconductor-metal ion-implanted Si waveguide photodetectors for C-band operation.

Metal-semiconductor-metal Si waveguide photodetectors are demonstrated with responsivities of greater than 0.5 A/W at a wavelength of 1550 nm for a device length of 1mm. Sub-bandgap absorption in the Si waveguide is achieved by creating divacancy lattice defects via Si(+) ion implantation. The modal absorption coefficient of the ion-implanted Si waveguide is measured to be ≈ 185 dB/cm, resulting in a detector responsivity of ≈ 0.51 A/W at a 50 V bias. The frequency response of a typical 1mm-length detector is measured to be 2.6 GHz, with simulations showing that a frequency response of 9.8 GHz is achievable with an optimized contact configuration and bias voltage of 15 V. Due to the ease with which these devices can be fabricated, and their potential for high performance, these detectors are suitable for various applications in Si-based photonic integrated circuits.

A 60 Gb/s MDM-WDM Si photonic link with < 0.7 dB power penalty per channel.

Mode-division-multiplexing (MDM) and wavelength-division-multiplexing (WDM) are employed simultaneously in a multimode silicon waveguide to realize on-chip MDM and MDM-WDM transmission. Asymmetric Y-junction MDM multiplexers and demultiplexers are utilized for low coherently suppressed demultiplexed crosstalk at the receiver. We demonstrate aggregate bandwidths of 20 Gb/s and 60 Gb/s for MDM and MDM-WDM on-chip links, respectively, with measured 10(-9) BER power penalties between 0.1 dB and 0.7 dB per channel.

Si⁺-implanted Si-wire waveguide photodetectors for the mid-infrared.

CMOS-compatible Si⁺-implanted Si-waveguide p-i-n photodetectors operating at room temperature and at mid-infrared wavelengths from 2.2 to 2.3 µm are demonstrated. Responsivities of 9.9 ± 2.0 mA/W are measured at a 5 V reverse bias with an estimated internal quantum efficiency of 2.7 - 4.5%. The dark current is found to vary from a few microamps down to less than a nanoamp after a post-implantation annealing of 350°C. The measured photocurrent dependence on input power shows a linear correspondence over more than three decades, and the frequency response of a 250 µm-length p-i-n device is measured to be ~1.7 GHz for a wavelength of λ = 2.2 µm, thus potentially opening up new communication bands for photonic integrated circuits.

Mitigation of X-ray damage in macromolecular crystallography by submicrometre line focusing.

Reported here are measurements of the penetration depth and spatial distribution of photoelectron (PE) damage excited by 18.6 keV X-ray photons in a lysozyme crystal with a vertical submicrometre line-focus beam of 0.7 µm full-width half-maximum (FWHM). The experimental results determined that the penetration depth of PEs is 5 ± 0.5 µm with a monotonically decreasing spatial distribution shape, resulting in mitigation of diffraction signal damage. This does not agree with previous theoretical predication that the mitigation of damage requires a peak of damage outside the focus. A new improved calculation provides some qualitative agreement with the experimental results, but significant errors still remain. The mitigation of radiation damage by line focusing was measured experimentally by comparing the damage in the X-ray-irradiated regions of the submicrometre focus with the large-beam case under conditions of equal exposure and equal volumes of the protein crystal, and a mitigation factor of 4.4 ± 0.4 was determined. The mitigation of radiation damage is caused by spatial separation of the dominant PE radiation-damage component from the crystal region of the line-focus beam that contributes the diffraction signal. The diffraction signal is generated by coherent scattering of incident X-rays (which introduces no damage), while the overwhelming proportion of damage is caused by PE emission as X-ray photons are absorbed.

Large-aperture refractive lenses for momentum-resolved spectroscopy with hard X-rays.

One-dimensional kinoform and prism refractive lenses with large aperture and high transmittance at 22 keV have been investigated. A 12.0 µm focus size (full width at half-maximum) and an effective aperture of 0.85 mm, at a focal length of 705 mm and 21.747 keV, were achieved.

Autonomous discovery of emergent morphologies in directed self-assembly of block copolymer blends.

The directed self-assembly (DSA) of block copolymers (BCPs) is a powerful approach to fabricate complex nanostructure arrays, but finding morphologies that emerge with changes in polymer architecture, composition, or assembly constraints remains daunting because of the increased dimensionality of the DSA design space. Here, we demonstrate machine-guided discovery of emergent morphologies from a cylinder/lamellae BCP blend directed by a chemical grating template, conducted without direct human intervention on a synchrotron x-ray scattering beamline. This approach maps the morphology-template phase space in a fraction of the time required by manual characterization and highlights regions deserving more detailed investigation. These studies reveal localized, template-directed partitioning of coexisting lamella- and cylinder-like subdomains at the template period length scale, manifesting as previously unknown morphologies such as aligned alternating subdomains, bilayers, or a "ladder" morphology. This work underscores the pivotal role that autonomous characterization can play in advancing the paradigm of DSA.

Open and Close-Packed, Shape-Engineered Polygonal Nanoparticle Metamolecules with Tailorable Fano Resonances.

A top-down lithographic patterning and deposition process is reported for producing nanoparticles (NPs) with well-defined sizes, shapes, and compositions that are often not accessible by wet-chemical synthetic methods. These NPs are ligated and harvested from the substrate surface to prepare colloidal NP dispersions. Using a template-assisted assembly technique, fabricated NPs are driven by capillary forces to assemble into size- and shape-engineered templates and organize into open or close-packed multi-NP structures or NP metamolecules. The sizes and shapes of the NPs and of the templates control the NP number, coordination, interparticle gap size, disorder, and location of defects such as voids in the NP metamolecules. The plasmonic resonances of polygonal-shaped Au NPs are exploited to correlate the structure and optical properties of assembled NP metamolecules. Comparing open and close-packed architectures highlights that introduction of a center NP to form close-packed assemblies supports collective interactions, altering magnetic optical modes and multipolar interactions in Fano resonances. Decreasing the distance between NPs strengthens the plasmonic coupling, and the structural symmetries of the NP metamolecules determine the orientation-dependent scattering response.

High-purity transmission of a slow light odd mode in a photonic crystal waveguide.

We demonstrate a novel scheme to control the excitation symmetry for an odd mode in a photonic crystal waveguide and investigate the spectral signature of this slow light mode. An odd-mode Mach-Zehnder coupler is introduced to transform mode symmetry and excite a high-purity odd mode with 20 dB signal contrast over the background. Assisted by a mixed-mode Mach-Zehnder coupler, slow light mode beating can be observed and is utilized to determine the group index of this odd mode. With slow light enhancement, this odd mode can help enable novel miniaturized devices such as one-way waveguides.

The ripple effect: How leader workplace anxiety shape follower job performance.

Although the dominant view in the literature suggests that work-related anxiety experienced by employees affects their behavior and performance, little research has focused on how and when leaders' workplace anxiety affects their followers' job performance. Drawing from Emotions as Social Information (EASI) theory, we propose dual mechanisms of cognitive interference and emotional exhaustion to explain the relationship between leader workplace anxiety and subordinate job performance. Specifically, cognitive interference is the mechanism that best explains the link between leader workplace anxiety and follower task performance, while emotional exhaustion is the mechanism that best explains the link between leader workplace anxiety and follower contextual performance. Additionally, we examine how follower epistemic motivation serves as a boundary condition for the effect of leader anxiety on follower performance outcomes. Results from a 2-wave study of 228 leader-follower dyads in a high-tech company mostly supported our theoretical model. We conclude the study with a discussion of the theoretical and practical implications of our findings.

Spatial dependence and mitigation of radiation damage by a line-focus mini-beam.

Recently, strategies to reduce primary radiation damage have been proposed which depend on focusing X-rays to dimensions smaller than the penetration depth of excited photoelectrons. For a line focus as used here the penetration depth is the maximum distance from the irradiated region along the X-ray polarization direction that the photoelectrons penetrate. Reported here are measurements of the penetration depth and distribution of photoelectron damage excited by 18.6 keV photons in a lysozyme crystal. The experimental results showed that the penetration depth of ~17.35 keV photoelectrons is 1.5 ± 0.2 µm, which is well below previous theoretical estimates of 2.8 µm. Such a small penetration depth raises challenging technical issues in mitigating damage by line-focus mini-beams. The optimum requirements to reduce damage in large crystals by a factor of 2.0-2.5 are Gaussian line-focus mini-beams with a root-mean-square width of 0.2 µm and a distance between lines of 2.0 µm. The use of higher energy X-rays (> 26 keV) would help to alleviate some of these requirements by more than doubling the penetration depth. It was found that the X-ray dose has a significant contribution from the crystal's solvent, which initially contained 9.0%(w/v) NaCl. The 15.8 keV photoelectrons of the Cl atoms and their accompanying 2.8 keV local dose from the decay of the resulting excited atoms more than doubles the dose deposited in the X-ray-irradiated region because of the much greater cross-section and higher energy of the excited atom, degrading the mitigation of radiation damage from 2.5 to 2.0. Eliminating heavier atoms from the solvent and data collection far from heavy-atom absorption edges will significantly improve the mitigation of damage by line-focus mini-beams.

Demonstration of a hitless bypass switch using nanomechanical perturbation for high-bitrate transparent networks.

We demonstrate an optical hitless bypass switch based on nanomechanical proximity perturbation for high-bitrate transparent networks. Embedded in a single-level pi-imbalanced Mach-Zehnder interferometer, the two nanomechanical-based Deltabeta-directional couplers permit broadband signal rerouting on-chip, while the selected wavelength remains unaffected at all times for optical filter reconfiguration. The optical hitless switch is implemented in the silicon nanophotonics platform, with experimental measurements matching well with numerical and theoretical modeling.