Abdelaziz Boulesbaa

Nonlinear Fano-Resonant Dielectric Metasurfaces

Strong nonlinear light-matter interaction is highly sought-after for a variety of applications including lasing and all-optical light modulation. Recently, resonant plasmonic structures have been considered promising candidates for enhancing nonlinear optical processes due to their ability to greatly enhance the optical near-field; however, their small mode volumes prevent the inherently large nonlinear susceptibility of the metal from being efficiently exploited. Here, we present an alternative approach that utilizes a Fano-resonant silicon metasurface. The metasurface results in strong near-field enhancement within the volume of the silicon resonator while minimizing two photon absorption. We measure a third harmonic generation enhancement factor of 1.5 × 10(5) with respect to an unpatterned silicon film and an absolute conversion efficiency of 1.2 × 10(-6) with a peak pump intensity of 3.2 GW cm(-2). The enhanced nonlinearity, combined with a sharp linear transmittance spectrum, results in transmission modulation with a modulation depth of 36%. The modulation mechanism is studied by pump-probe experiments.

Patterned arrays of lateral heterojunctions within monolayer two-dimensional semiconductors

The formation of semiconductor heterojunctions and their high-density integration are foundations of modern electronics and optoelectronics. To enable two-dimensional crystalline semiconductors as building blocks in next-generation electronics, developing methods to deterministically form lateral heterojunctions is crucial. Here we demonstrate an approach for the formation of lithographically patterned arrays of lateral semiconducting heterojunctions within a single two-dimensional crystal. Electron beam lithography is used to pattern MoSe2 monolayer crystals with SiO2, and the exposed locations are selectively and totally converted to MoS2 using pulsed laser vaporization of sulfur to form MoSe2/MoS2 heterojunctions in predefined patterns. The junctions and conversion process are studied by Raman and photoluminescence spectroscopy, atomically resolved scanning transmission electron microscopy and device characterization. This demonstration of lateral heterojunction arrays within a monolayer crystal is an essential step for the integration of two-dimensional semiconductor building blocks with different electronic and optoelectronic properties for high-density, ultrathin devices.

Interlayer Coupling in Twisted WSe2/WS2 Bilayer Heterostructures Revealed by Optical Spectroscopy

van der Waals (vdW) heterostructures are promising building blocks for future ultrathin electronics. Fabricating vdW heterostructures by stamping monolayers at arbitrary angles provides an additional range of flexibility to tailor the resulting properties than could be expected by direct growth. Here, we report fabrication and comprehensive characterizations of WSe2/WS2 bilayer heterojunctions with various twist angles that were synthesized by artificially stacking monolayers of WS2 and WSe2 grown by chemical vapor deposition. After annealing the WSe2/WS2 bilayers, Raman spectroscopy reveals interlayer coupling with the appearance of a mode at 309.4 cm(-1) that is sensitive to the number of WSe2 layers. This interlayer coupling is associated with substantial quenching of the intralayer photoluminescence. In addition, microabsorption spectroscopy of WSe2/WS2 bilayers revealed spectral broadening and shifts as well as a net ∼10% enhancement in integrated absorption strength across the visible spectrum with respect to the sum of the individual monolayer spectra. The observed broadening of the WSe2 A exciton absorption band in the bilayers suggests fast charge separation between the layers, which was supported by direct femtosecond pump-probe spectroscopy. Density functional calculations of the band structures of the bilayers at different twist angles and interlayer distances found robust type II heterojunctions at all twist angles, and predicted variations in band gap for particular atomistic arrangements. Although interlayer excitons were indicated using femtosecond pump-probe spectroscopy, photoluminescence and absorption spectroscopies did not show any evidence of them, suggesting that the interlayer exciton transition is very weak. However, the interlayer coupling for the WSe2/WS2 bilayer heterojunctions indicated by substantial PL quenching, enhanced absorption, and rapid charge transfer was found to be insensitive to the relative twist angle, indicating that stamping provides a robust approach to realize reliable optoelectronics.

Tailoring Vacancies Far Beyond Intrinsic Levels Changes the Carrier Type and Optical Response in Monolayer MoSe2−x Crystals

Defect engineering has been a critical step in controlling the transport characteristics of electronic devices, and the ability to create, tune, and annihilate defects is essential to enable the range of next-generation devices. Whereas defect formation has been well-demonstrated in three-dimensional semiconductors, similar exploration of the heterogeneity in atomically thin two-dimensional semiconductors and the link between their atomic structures, defects, and properties has not yet been extensively studied. Here, we demonstrate the growth of MoSe2-x single crystals with selenium (Se) vacancies far beyond intrinsic levels, up to ∼20%, that exhibit a remarkable transition in electrical transport properties from n- to p-type character with increasing Se vacancy concentration. A new defect-activated phonon band at ∼250 cm(-1) appears, and the A1g Raman characteristic mode at 240 cm(-1) softens toward ∼230 cm(-1) which serves as a fingerprint of vacancy concentration in the crystals. We show that post-selenization using pulsed laser evaporated Se atoms can repair Se-vacant sites to nearly recover the properties of the pristine crystals. First-principles calculations reveal the underlying mechanisms for the corresponding vacancy-induced electrical and optical transitions.

Digital Transfer Growth of Patterned 2D Metal Chalcogenides by Confined Nanoparticle Evaporation

Developing methods for the facile synthesis of two-dimensional (2D) metal chalcogenides and other layered materials is crucial for emerging applications in functional devices. Controlling the stoichiometry, number of the layers, crystallite size, growth location, and areal uniformity is challenging in conventional vapor-phase synthesis. Here, we demonstrate a method to control these parameters in the growth of metal chalcogenide (GaSe) and dichalcogenide (MoSe2) 2D crystals by precisely defining the mass and location of the source materials in a confined transfer growth system. A uniform and precise amount of stoichiometric nanoparticles are first synthesized and deposited onto a substrate by pulsed laser deposition (PLD) at room temperature. This source substrate is then covered with a receiver substrate to form a confined vapor transport growth (VTG) system. By simply heating the source substrate in an inert background gas, a natural temperature gradient is formed that evaporates the confined nanoparticles to grow large, crystalline 2D nanosheets on the cooler receiver substrate, the temperature of which is controlled by the background gas pressure. Large monolayer crystalline domains (∼100 μm lateral sizes) of GaSe and MoSe2 are demonstrated, as well as continuous monolayer films through the deposition of additional precursor materials. This PLD-VTG synthesis and processing method offers a unique approach for the controlled growth of large-area metal chalcogenides with a controlled number of layers in patterned growth locations for optoelectronics and energy related applications.

Vibrational Dynamics of Interfacial Water by Free Induction Decay Sum Frequency Generation (FID-SFG) at the Al2O3(1120)/H2O Interface

The dephasing dynamics of a vibrational coherence may reveal the interactions of chemical functional groups with their environment. To investigate this process at a surface, we employ free induction decay sum frequency generation (FID-SFG) to measure the time that it takes for free OH stretch oscillators at the charged (pH ≈ 13, KOH) interface of alumina/water (Al2O3/H2O) to lose their collective coherence. By employing noncollinear optical parametric amplification (NOPA) technology and nonlinear vibrational spectroscopy, we showed that the single free OH peak actually corresponds to two distinct oscillators oriented opposite to each other and measured the total dephasing time, T2, of the free OH stretch modes at the Al2O3/H2O interface with a sub-40 fs temporal resolution. Our results suggested that the free OH oscillators associated with interfacial water dephase on the time scale of 89.4 ± 6.9 fs, whereas the homogeneous dephasing of interfacial alumina hydroxyls is an order of magnitude slower.

Generation of sub-30-fs microjoule mid-infrared pulses for ultrafast vibrational dynamics at solid/liquid interfaces.

We describe temporal compression of ultrabroadband, few microjoule mid-infrared (mid-IR) pulses from a noncollinear optical parametric amplifier (NOPA) employed in a sum-frequency generation (SFG) vibrational spectroscopic system, operating in total-internal-reflection geometry. The propagation of the mid-IR beam through optical materials results in a significant temporal chirp at the probed interface, which is analyzed and corrected by properly managing the total dispersion of materials introduced into the mid-IR beam path. By employing the simultaneous spatial and temporal focusing of the broadband infrared pulses at the probed interface, we achieve a sub-50-fs full width at half-maximum (FWHM) for the instrument response function, measured via SFG cross correlation of the ultrashort mid-IR pulses with an ultrashort (~30 fs) near-IR pulse from a synchronized, independently tunable NOPA. From the SFG cross-correlation FWHM, we extract a sub-30-fs mid-IR pulse duration, making it a suitable SFG spectroscopic system to investigate vibrational dynamics in hydrogen-bonded systems at interfaces.

Ultrafast charge and energy exchanges at hybrid interfaces involving 2D semiconductors (Conference Presentation)

Two-dimensional transition metal dichalcogenide (2D-TMD) semiconductors are new class of functional materials with a great promise for optoelectronics. Despite their atomic thickness, they strongly interact with light. This allows 2D-TMDs to become suitable converters of photons into useful electric charges in heterostructures involving 2D-TMDs and metallic nano-plasmonics or semiconductor quantum dots (QDs). In this talk, I will illustrate how femtosecond pump-probe spectroscopy can reveal a sub-45 fs charge transfer at a 2D/QDs heterostructure composed of tungsten disulfide monolayers (2D-WS2) and a single layer of cadmium selenide (CdSe)/zinc sulfide (ZnS) core/shell 0D-QDs. In another heterostructure involving 2D-TMDs and plasmonics, I will describe how plasmons of an array of aluminum (Al) nanoantennas are excited indirectly via energy transfer from photoexcited exciton of 2D-WS2 semiconductor. In particular, femtosecond spectroscopy measurements indicated that the lifetime of the resulting plasmon-induced hot electrons in the Al array continue as long as that of the 2D-WS2 excitons. Conversely, the presence of these excited plasmons almost triples the lifetime of the 2D-WS2 excitons from ~15 to ~44 ps. This exciton-plasmon coupling enabled by such hybrid nanostructures may open new opportunities for optoelectronic applications. This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. Synthesis of the two-dimensional materials was supported by the Materials Science and Engineering Division, Office of Basic Energy Sciences, U.S. Department of Energy.

Correlating the optical properties of WS2 monolayers grown by CVD with isoelectronic Mo doping level(Conference Presentation)

Incorporating dopants in monolayer transition metal dichalcogenides (TMD) can enable manipulations of their electrical and optical properties. Previous attempts in amphoteric doping in monolayer TMDs have proven to be challenging. Here we report the incorporation of molybdenum (Mo) atoms in monolayer WS2 during growth by chemical vapor deposition, and correlate the distribution of Mo atoms with the optical properties including photoluminescence and ultrafast transient absorption dynamics. Dark field scanning transmission electron microscopy imaging quantified the isoelectronic doping of Mo in WS2 and revealed its gradual distribution along a triangular WS2 monolayer crystal, increasing from 0% at the edge to 2% in the center of the triangular WS2 triangular crystals. This agrees well with the Raman spectra data that showed two obvious modes between 360 cm-1 and 400 cm-1 that corresponded to MoS2 in the center. This in-plane gradual distribution of Mo in WS2 was found to account for the spatial variations in photoluminescence intensity and emission energy. Transition absorption spectroscopy further indicated that the incorporation of Mo in WS2 regulate the amplitude ratio of XA and XB of WS2. The effect of Mo incorporation on the electronic structure of WS2 was further elucidated by density functional theory. Finally, we compared the electrical properties of Mo incorporated and pristine WS2 monolayers by fabricating field-effect transistors. The isoelectronic doping of Mo in WS2 provides an alternative approach to engineer the bandgap and also enriches our understanding the influence of the doping on the excitonic dynamics.