Localized coherent phonon generation in monolayer MoSe2 from ultrafast exciton trapping at shallow traps.

We report spectroscopic evidence for the ultrafast trapping of band edge excitons at defects and the subsequent generation of defect-localized coherent phonons (CPs) in monolayer MoSe2. While the photoluminescence measurement provides signals of exciton recombination at both shallow and deep traps, our time-resolved pump-probe spectroscopy on the sub-picosecond time scale detects localized CPs only from the ultrafast exciton trapping at shallow traps. Based on occupation-constrained density functional calculations, we identify the Se vacancy and the oxygen molecule adsorbed on a Se vacancy as the atomistic origins of deep and shallow traps, respectively. Establishing the correlations between the defect-induced ultrafast exciton trapping and the generation of defect-localized CPs, our work could open up new avenues to engineer photoexcited carriers through lattice defects in two-dimensional materials.

Dynamics of surface-plasmon lasing in planar metal gratings on semiconductor.

We investigate the dynamics of surface plasmon (SP) lasing in Au gratings fabricated on InGaAs with a period of around 400 nm, which locates the SP resonance near the semiconductor energy gap and facilitates efficient energy transfer. By optically pumping the InGaAs to reach the population inversion required for the amplification and the lasing, we observe SP lasing at specific wavelengths that satisfy the SPR condition depending on the grating period. The carrier dynamics in semiconductor and the photon density in the SP cavity was investigated from the time-resolved pump-probe measurement and the time resolved photoluminescence spectroscopy, respectively. Our results reveal that the photon dynamics is strongly correlated with the carrier dynamics and the lasing build-up is accelerated as the initial gain proportional to the pumping power increases, and this trend is satisfactorily explained using the rate equation model.

Exciton Transfer at Heterointerfaces of MoS2 Monolayers and Fluorescent Molecular Aggregates.

Integration of distinct materials to form heterostructures enables the proposal of new functional devices based on emergent physical phenomena beyond the properties of the constituent materials. The optical responses and electrical transport characteristics of heterostructures depend on the charge and exciton transfer (CT and ET) at the interfaces, determined by the interfacial energy level alignment. In this work, heterostructures consisting of aggregates of fluorescent molecules (DY1) and 2D semiconductor MoS2 monolayers are fabricated. Photoluminescence spectra of DY1/MoS2 show quenching of the DY1 emission and enhancement of the MoS2 emission, indicating a strong electronic interaction between these two materials. Nanoscopic mappings of the light-induced contact potential difference changes rule out the CT process at the interface. Using femtosecond transient absorption spectroscopy, the rapid interfacial ET process from DY1 aggregates to MoS2 and a fourfold extension of the exciton lifetime in MoS2 are elucidated. These results suggest that the integration of 2D inorganic semiconductors with fluorescent molecules can provide versatile approaches to engineer the physical characteristics of materials for both fundamental studies and novel optoelectronic device applications.

Anisotropic Generation and Detection of Coherent Ag Phonons in Black Phosphorus.

Black phosphorus (BP) has attracted great attention due to its layer-tuned direct bandgap, in-plane anisotropic properties, and novel optoelectronic applications. In this work, the anisotropic characteristics of BP crystal in terms of the Raman tensor and birefringence are studied by investigating polarization dependence in both the generation and detection of Ag mode coherent phonons. While the generated coherent phonons exhibit the typical linear dichroism of BP crystal, the detection process is found here to be influenced by anisotropic multiple thin film interference, showing wavelength and sample thickness sensitive behaviors. We additionally find that the Ag1 and Ag2 optical phonons decay into lower frequency acoustic phonons through the temperature-dependent anharmonic process.

Enhanced optical absorption in conformally grown MoS2 layers on SiO2/Si substrates with SiO2 nanopillars with a height of 50 nm.

The integration of transition metal dichalcogenide (TMDC) layers on nanostructures has attracted growing attention as a means to improve the physical properties of the ultrathin TMDC materials. In this work, the influence of SiO2 nanopillars (NPs) with a height of 50 nm on the optical characteristics of MoS2 layers is investigated. Using a metal organic chemical vapor deposition technique, a few layers of MoS2 were conformally grown on the NP-patterned SiO2/Si substrates without notable strain. The photoluminescence and Raman intensities of the MoS2 layers on the SiO2 NPs were larger than those observed from a flat SiO2 surface. For 100 nm-SiO2/Si wafers, the 50 nm-NP patterning enabled improved absorption in the MoS2 layers over the whole visible wavelength range. Optical simulations showed that a strong electric-field could be formed at the NP surface, which led to the enhanced absorption in the MoS2 layers. These results suggest a versatile strategy to realize high-efficiency TMDC-based optoelectronic devices.

Valley depolarization in monolayer transition-metal dichalcogenides with zone-corner acoustic phonons.

Although single-layer transition-metal dichalcogenides with novel valley functionalities are a promising candidate to realize valleytronic devices, the essential understanding of valley depolarization mechanisms is still incomplete. Based on pump-probe experiments performed for MoSe2 and WSe2 monolayers and corroborating analysis from density functional calculations, we demonstrate that coherent phonons at the K-point of the Brillouin zone can effectively mediate the valley transfer of electron carriers. In the MoSe2 monolayer case, we identify this mode as the flexural acoustic ZA(K) mode, which has broken inversion symmetry and thus can enable electron spin-flip during valley transfer. On the other hand, in the monolayer WSe2 case where spin-preserving inter-valley relaxations are preferred, coherent LA(K) phonons with even inversion symmetry are efficiently generated. These findings establish that while the specifics of inter-valley relaxations depend on the spin alignments of energy bands, the K-point phonons should be taken into account as an effective valley depolarization pathway in transition metal dichalcogenide monolayers.

Polarization-Dependent Light Emission and Charge Creation in MoS2 Monolayers on Plasmonic Au Nanogratings.

We fabricated plasmonic hybrid nanostructures consisting of MoS2 monolayer flakes and Au nanogratings with a period of 500 nm. The angle-resolved reflectance and photoluminescence spectra of the hybrid nanostructures clearly indicated a coupling between surface plasmon polaritons (SPPs) and incoming photons. The surface photovoltage (SPV) maps could visualize the spatial distribution of net charges while shining light on the sample. Considerable polarization and wavelength dependence of the SPV signals suggested that the SPP mode enhanced the light-matter interaction and resulting exciton generation in the MoS2 monolayer. From the photoluminescence spectra and the morphology of the suspended MoS2 region, it could be noted that light irradiation did not much raise the temperature of the MoS2 monolayers on the nanogratings. Nanoscopic SPV and surface topography measurements could reveal the local optoelectronic and mechanical properties of MoS2 monolayers. This work provided us insights into the proposal of a high-performance MoS2/metal optoelectronic devices, based on the understanding of the SPP-photon and SPP-exciton coupling.

Effective Photoluminescence Imaging of Bubbles in hBN-Encapsulated WSe2 Monolayer.

Interfacial bubbles are unintentionally created during the transfer of atomically thin 2D layers, a required process in the fabrication of van der Waals heterostructures. By encapsulating a WSe2 monolayer in hBN, we study the differing photoluminescence (PL) properties of the structure resulting from bubble formation. Based on the differentiated absorption probabilities at the bubbles compared to the pristine areas, we demonstrate that the visibility of the bubbles in PL mapping is enhanced when the photoexcitation wavelength lies between the n = 1 and n = 2 resonances of the A-exciton. An appropriate choice of detection window, which includes localized exciton emission but excludes free exciton emission, further improves bubble imaging capability. The interfacial position dependence of the bubbles, whether they are located above or below the WSe2 monolayer, gives rise to measurable consequences in the PL shape.

Spectroscopic studies of atomic defects and bandgap renormalization in semiconducting monolayer transition metal dichalcogenides.

Assessing atomic defect states and their ramifications on the electronic properties of two-dimensional van der Waals semiconducting transition metal dichalcogenides (SC-TMDs) is the primary task to expedite multi-disciplinary efforts in the promotion of next-generation electrical and optical device applications utilizing these low-dimensional materials. Here, with electron tunneling and optical spectroscopy measurements with density functional theory, we spectroscopically locate the mid-gap states from chalcogen-atom vacancies in four representative monolayer SC-TMDs-WS2, MoS2, WSe2, and MoSe2-, and carefully analyze the similarities and dissimilarities of the atomic defects in four distinctive materials regarding the physical origins of the missing chalcogen atoms and the implications to SC-mTMD properties. In addition, we address both quasiparticle and optical energy gaps of the SC-mTMD films and find out many-body interactions significantly enlarge the quasiparticle energy gaps and excitonic binding energies, when the semiconducting monolayers are encapsulated by non-interacting hexagonal boron nitride layers.

Broadband tuning of ring-type cavity-dumped femtosecond optical parametric oscillator in the near-infrared.

We report a cavity-dumped optical parametric oscillator (OPO) with a ring-type cavity configuration, which is based on periodically poled lithium niobate gain synchronously pumped by a mode-locked Ti:sapphire laser. Because of reduced cavity loss and group velocity dispersion inherent to ring-cavity employment, a wide wavelength tuning capability from 1.02 to 1.65 µm was achieved by the simple displacement of a cavity mirror. At a wavelength of 1.28 µm, the cavity-dumped system provides femtosecond pulses with 42 nJ energy and 50% dumping efficiency. The group delay dispersion (GDD) of the OPO cavity could be characterized through the wavelength tuning behavior with cavity displacement, and its validity was confirmed by the numerical GDD calculation of each optical component within the cavity.

Self-consistent dielectric constant determination for monolayer WSe2.

Frequency-dependent dielectric constant dispersion of monolayer WSe2, ε(ω)=ε1(ω)+i Îµ2(ω), was obtained from simultaneously measured transmittance and reflectance spectra. Optical transitions of the trion as well as A-, B-, and C-excitons are clearly resolved in the  Îµ2 spectrum. A consistent Kramers-Kronig transformation between the ε1 and  Îµ2 spectra support the validity of the applied analysis. It is found that the A- and B-exciton splitting in the case of the double-layer WSe2 can be attributed to the spin-orbit coupling, which is larger than that in the monolayer WSe2. In addition, the temperature-induced evolution of the A-exciton energy and its width are explained by model equations with electron-phonon interactions.

Mott transition in chain structure of strained VO2 films revealed by coherent phonons.

The characteristic of strongly correlated materials is the Mott transition between metal and insulator (MIT or IMT) in the same crystalline structure, indicating the presence of a gap formed by the Coulomb interaction between carriers. The physics of the transition needs to be revealed. Using VO2, as a model material, we observe the emergence of a metallic chain in the intermediate insulating monoclinic structure (M2 phase) of epitaxial strained films, proving the Mott transition involving the breakdown of the critical Coulomb interaction. It is revealed by measuring the temperature dynamics of coherent optical phonons with separated vibrational modes originated from two substructures in M2: one is the charge-density-wave, formed by electron-phonon (e-ph) interaction, and the other is the equally spaced insulator-chain with electron-electron (e-e) correlations.

Broadband Surface Plasmon Lasing in One-dimensional Metallic Gratings on Semiconductor.

We report surface plasmon (SP) lasing in metal/semiconductor nanostructures, where one-dimensional periodic silver slit gratings are placed on top of an InGaAsP layer. The SP nature of the lasing is confirmed from the emission wavelength governed by the grating period, polarization analysis, spatial coherence, and comparison with the linear transmission. The excellent performance of the device as an SP source is demonstrated by its tunable emission in the 400-nm-wide telecom wavelength band at room temperature. We show that the stimulated emission enhanced by the Purcell effect enables successful SP lasing at high energies above the gap energy of the gain. We also discuss the dependence of the lasing efficiency on temperature, grating dimension, and type of metal.

Coherent Lattice Vibrations in Mono- and Few-Layer WSe2.

We report the observation of coherent lattice vibrations in mono- and few-layer WSe2 in the time domain, which were obtained by performing time-resolved transmission measurements. Upon the excitation of ultrashort pulses with the energy resonant to that of A excitons, coherent oscillations of the A1g optical phonon and longitudinal acoustic phonon at the M point of the Brillouin zone (LA(M)) were impulsively generated in monolayer WSe2. In multilayer WSe2 flakes, the interlayer breathing mode (B1) is found to be sensitive to the number of layers, demonstrating its usefulness in characterizing layered transition metal dichalcogenide materials. On the basis of temperature-dependent measurements, we find that the A1g optical phonon mode decays into two acoustic phonons through the anharmonic decay process.

Resonance effects in thickness-dependent ultrafast carrier and phonon dynamics of topological insulator Bi2Se3.

Resonance effects in the thickness-dependent ultrafast carrier and phonon dynamics of topological insulator Bi2Se3 are found irrespective of the kind of substrate by measuring thickness-dependent abrupt changes of pump-probe differential-reflectivity signals (ΔR/R) from Bi2Se3 thin films on four different substrates of poly- and single-crystalline (sc-) ZnO, sc-GaN and SiO2. The absolute peak intensity of the ΔR/R is maximized at ∼t C (6 ∼ 9 quintuple layers), which is not directly related to but is very close to the critical thickness below which the energy gap opens. The intensities of the two phonon modes deduced from the oscillatory behaviors superimposed on the ΔR/R profiles are also peaked at ∼t C for the four kinds of substrates, consistent with the thickness-dependent Raman-scattering behaviors. These resonant effects and others are discussed based on possible physical mechanisms including the effects of three-dimensional carrier depletion and intersurface coupling.

Visible-pulse generation in gain crystal of near-infrared femtosecond optical parametric oscillator.

An optical parametric oscillator (OPO) based on magnesium-oxide-doped periodically poled lithium niobate (MgO:PPLN) is demonstrated to deliver visible femtosecond pulses, which were created through the intra-cavity nonlinear interactions within the PPLN itself. The signal from the OPO produces femtosecond pulses in the near-infrared region tunable from 1050 to 1600 nm. Visible femtosecond pulses in the range of 522-800 nm and those of 455-540 nm, respectively, were generated via second-harmonic generation (SHG) of signal photons and through sum-frequency generation (SFG) of pump and signal photons. Maximum output efficiencies of 9.2% at 614 nm and 8.0% at 522 nm for the SHG and SFG are attained, respectively, where the efficient visible pulse generation relies on the quasi-phase matching with the aid of the higher-order grating momentum.

Strong optical modulation of surface plasmon polaritons in metal/semiconductor nanostructures.

We demonstrate strong modulation of the transmission around the surface plasmon polariton (SPP) resonance in metal/semiconductor hybrid nanostructures based on Ag film on top of InGaAs. The change in the real and imaginary parts of the refractive index due to photoexcited carriers in InGaAs generates a shift in the SPP resonance and enhanced transmission near the SPP resonance. Temporal evolution of the complex refractive index was traced by comparing the transient transmission with finite-difference time-domain (FDTD) simulations.

Composite dielectric metasurfaces for phase control of vector field.

We designed, fabricated, and characterized a dielectric metamaterial lens created by varying the density of subwavelength low refractive index nanoholes in a high refractive index substrate, resulting in a locally variable effective refraction index. It is shown that a constructed graded index lens can overcome diffraction effects even when the aperture/wavelength (D/λ) ratio is smaller than 40. In addition to the conventional design of a polarization insensitive lens, we also show that a polarization diversity lens (f(o)≠f(e)) can be realized by arranging nanoholes in patterns with variable density in different transverse directions. Such a anisotropic microlens demonstrates polarization dependent focal lengths of 32 and 22 µm for linearly x- and y-polarized light, respectively, operating at a wavelength of λ=1550 nm. We also show numerically and demonstrate experimentally achromatic performance of the devices operating in the wavelength range of 1500-1900 nm with full width at half-maximum (FWHM) of the focal spots of about 4 µm.

Electron and phonon coupling dynamics in low-gap semiconductor: quantum versus classical scale.

We have studied the characteristics of longitudinal-optical-phonon-plasmon coupled (LOPC) mode by using the ultrashort pulsed laser with 45 THz bandwidth as a function of thickness in InAs epilayers, ranging from 10 to 900 nm. We have observed the LOPC modes split into the upper (L(+) mode) and the lower (L(-) mode) branches only in the classical scale, but the longitudinal-optical (LO) phonon peak was persistently observed. The shorter decay time of the plasmon-like L(+) modes rather than the phonon-like L(-) modes should be associated with carrier-carrier scattering which is further considered with diffusion properties in the low-gap semiconductors. This result leads to that the absence of the LOPC modes in a scale less than exciton Bohr radius manifests the role of electron diffusion rather than the carrier screening via drift motion in surface depletion region.

Hot carrier-driven catalytic reactions on Pt-CdSe-Pt nanodumbbells and Pt/GaN under light irradiation.

Hybrid nanocatalysts consisting of metal nanoparticle-semiconductor junctions offer an interesting platform to study the role of metal-oxide interfaces and hot electron flows in heterogeneous catalysis. Here, we report that hot carriers generated upon photon absorption significantly impact the catalytic activity of CO oxidation. We found that Pt-CdSe-Pt nanodumbbells exhibit a higher turnover frequency by a factor of 2 during irradiation by light with energy higher than the bandgap of CdSe, while the turnover rate on bare Pt nanoparticles did not depend on light irradiation. We found that Pt nanoparticles deposited on a GaN substrate under light irradiation exhibit changes in catalytic activity of CO oxidation that depends on the type of doping of the GaN. We suppose that hot electrons are generated upon the absorption of photons by the semiconducting nanorods or substrates, whereafter the hot electrons are injected into the Pt nanoparticles, resulting in the change in catalytic activity. The results imply that hot carrier flows generated during light irradiation significantly influence the catalytic activity of CO oxidation, leading to potential applications as a hot electron-based catalytic actuator.