Based on transformation optics, we introduce another set of generalized laws of reflection and refraction (differs from that of [Science 334, 333 (2011)]), through which a transformation media slab is derived as a meta-surface, producing anomalous reflection and refraction for all polarizations of incident light.

Generalized laws of reflection and refraction from transformation optics

Semiconductor nanowires have attracted extensive interest as one of the best-defined classes of nanoscale building blocks for the bottom-up assembly of functional electronic and optoelectronic devices over the past two decades. The article provides a comprehensive review of the continuing efforts in exploring semiconductor nanowires for the assembly of functional nanoscale electronics and macroelectronics. Specifically, we start with a brief overview of the synthetic control of various semiconductor nanowires and nanowire heterostructures with precisely controlled physical dimension, chemical composition, heterostructure interface, and electronic properties to define the material foundation for nanowire electronics. We then summarize a series of assembly strategies developed for creating well-ordered nanowire arrays with controlled spatial position, orientation, and density, which are essential for constructing increasingly complex electronic devices and circuits from synthetic semiconductor nanowires. Next, we review the fundamental electronic properties and various single nanowire transistor concepts. Combining the designable electronic properties and controllable assembly approaches, we then discuss a series of nanoscale devices and integrated circuits assembled from nanowire building blocks, as well as a unique design of solution-processable nanowire thin-film transistors for high-performance large-area flexible electronics. Last, we conclude with a brief perspective on the standing challenges and future opportunities.

Nanowire Electronics: From Nanoscale to Macroscale

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) and graphene compose a new family of crystalline materials with atomic thicknesses and exotic mechanical, electronic, and optical properties. Due to their inherent exceptional mechanical flexibility and strength, these 2D materials provide an ideal platform for strain engineering, enabling versatile modulation and significant enhancement of their optical properties. For instance, recent theoretical and experimental investigations have demonstrated flexible control over their electronic states via application of external strains, such as uniaxial strain and biaxial strain. Meanwhile, many nondestructive optical measurement methods, typically including absorption, reflectance, photoluminescence, and Raman spectroscopies, can be readily exploited to quantitatively determine strain-engineered optical properties. This review begins with an introduction to the macroscopic theory of crystal elasticity and microscopic effective low-energy Hamiltonians coupled with strain fields, and then summarizes recent advances in strain-induced optical responses of 2D TMDCs and graphene, followed by the strain engineering techniques. It concludes with exciting applications associated with strained 2D materials, discussions on existing open questions, and an outlook on this intriguing emerging field.

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Strain engineering of 2D semiconductors and graphene: from strain fields to band-structure tuning and photonic applications.

Strained GeSn alloys are promising for realizing light emitters based entirely on group IV elements. Here, we report GeSn microdisk lasers encapsulated with a SiNx stressor layer to produce tensile strain. A 300 nm-thick GeSn layer with 5.4 at% Sn, which is an indirect-bandgap semiconductor as-grown, is transformed via tensile strain engineering into a direct-bandgap semiconductor that supports lasing. In this approach, the low Sn concentration enables improved defect engineering and the tensile strain delivers a low density of states at the valence band edge, which is the light hole band. We observe ultra-low-threshold continuous-wave and pulsed lasing at temperatures up to 70 K and 100 K, respectively. Lasers operating at a wavelength of 2.5 μm have thresholds of 0.8 kW cm−2 for nanosecond pulsed optical excitation and 1.1 kW cm−2 under continuous-wave optical excitation. The results offer a path towards monolithically integrated group IV laser sources on a Si photonics platform. Continuous-wave lasing in strained GeSn alloys is reported at temperatures of up to 100 K. The approach offers a route towards a group-IV-on-silicon laser.

/pdf/ultra-low-threshold-continuous-wave-and-pulsed-lasing-in-1ui13e6nyb.pdf

Ultra-low-threshold continuous-wave and pulsed lasing in tensile-strained GeSn alloys

Germanium (Ge) has played a key role in silicon photonics as an enabling material for datacom applications. Indeed, the unique properties of Ge have been leveraged to develop high performance integrated photodectors, which are now mature devices. Ge is also very useful for the achievement of compact modulators and monolithically integrated laser sources on silicon. Interestingly, research efforts in these domains also put forward the current revolution of mid-IR photonics. Ge and Ge-based alloys also present strong advantages for mid-infrared photonic platform such as the extension of the transparency window for these materials, which can operate at wavelengths beyond 8 μm. Different platforms have been proposed to take benefit from the broad transparency of Ge up to 15 μm, and the main passive building blocks are now being developed. In this review, we will present the most relevant Ge-based platforms reported so far that have led to the demonstration of several passive and active building blocks for mid-IR photonics. Seminal works on mid-IR optical sensing using integrated platforms will also be reviewed.

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Germanium-based integrated photonics from near- to mid-infrared applications

The integration of efficient, miniaturized group IV lasers into CMOS architecture holds the key to the realization of fully functional photonic-integrated circuits. Despite several years of progress, however, all group IV lasers reported to date exhibit impractically high thresholds owing to their unfavourable bandstructures. Highly strained germanium with its fundamentally altered bandstructure has emerged as a potential low-threshold gain medium, but there has yet to be a successful demonstration of lasing from this seemingly promising material system. Here we demonstrate a low-threshold, compact group IV laser that employs a germanium nanowire under a 1.6% uniaxial tensile strain as the gain medium. The amplified material gain in strained germanium can sufficiently overcome optical losses at 83 K, thus allowing the observation of multimode lasing with an optical pumping threshold density of ~3.0 kW cm−2. Our demonstration opens new possibilities for group IV lasers for photonic-integrated circuits. Integrating group IV lasing devices into technologically relevant CMOS architectures has proven challenging. Here, the authors demonstrate low-threshold lasing, which is important for potential electronic and photonic circuits, using strained germanium nanowires as the gain material.

/pdf/low-threshold-optically-pumped-lasing-in-highly-strained-2rx3072c4k.pdf

Low-threshold optically pumped lasing in highly strained germanium nanowires.

The integration of efficient, miniaturized group IV lasers into CMOS architecture holds the key to the realization of fully functional photonic-integrated circuits. Despite several years of progress, however, all group IV lasers reported to date exhibit impractically high thresholds owing to their unfavorable bandstructures. Highly strained germanium with its fundamentally altered bandstructure has emerged as a potential low-threshold gain medium, but there has yet to be any successful demonstration of lasing from this seemingly promising material system. Here, we demonstrate a low-threshold, compact group IV laser that employs germanium nanowire under a 1.6% uniaxial tensile strain as the gain medium. The amplified material gain in strained germanium can sufficiently surmount optical losses at 83 K, thus allowing the first observation of multimode lasing with an optical pumping threshold density of ~3.0 kW cm^-^2. Our demonstration opens up a new horizon of group IV lasers for photonic-integrated circuits.

/pdf/low-threshold-optically-pumped-lasing-in-highly-strained-ge-1si26phgn6.pdf

Low-threshold optically pumped lasing in highly strained Ge nanowires

Low-threshold optically pumped lasing in highly strained germanium nanowires

Nonlinear metasurfaces are advancing into a new paradigm of "flat nonlinear optics" owing to the ability to engineer local nonlinear responses in subwavelength-thin films. Recently, attempts have been made to expand the design space of nonlinear metasurfaces through nonlinear chiral responses. However, the development of metasurfaces that display both giant nonlinear circular dichroism and significantly large nonlinear optical response is still an unresolved challenge. Herein, we propose a method that induces giant nonlinear responses with near-unity circular dichroism using polaritonic metasurfaces with optical modes in chiral plasmonic nanocavities coupled with intersubband transitions in semiconductor heterostructures designed to have giant second and third order nonlinear responses. A stark contrast between effective nonlinear susceptibility elements for the two spin states of circularly polarized pump beams was seen in the hybrid structure. Experimentally, near-unity nonlinear circular dichroism and conversion efficiencies beyond 10-4% for second- and third-harmonic generation were achieved simultaneously in a single chip.

Daeik Kim

Papers

Low-threshold optically pumped lasing in highly strained germanium nanowires.

Low-threshold optically pumped lasing in highly strained Ge nanowires

Low-threshold optically pumped lasing in highly strained germanium nanowires

Giant Nonlinear Circular Dichroism from Intersubband Polaritonic Metasurfaces.

Third‐Harmonic Generation from Plasmonic Metasurfaces Coupled to Intersubband Transitions