Harmonic to anharmonic tuning of moiré potential leading to unconventional Stark effect and giant dipolar repulsion in WS2/WSe2 heterobilayer.

Excitonic states trapped in harmonic moiré wells of twisted heterobilayers is an intriguing testbed for exploring many-body physics. However, the moiré potential is primarily governed by the twist angle, and its dynamic tuning remains a challenge. Here we demonstrate anharmonic tuning of moiré potential in a WS2/WSe2 heterobilayer through gate voltage and optical power. A gate voltage can result in a local in-plane perturbing field with odd parity around the high-symmetry points. This allows us to simultaneously observe the first (linear) and second (parabolic) order Stark shift for the ground state and first excited state, respectively, of the moiré trapped exciton - an effect opposite to conventional quantum-confined Stark shift. Depending on the degree of confinement, these excitons exhibit up to twenty-fold gate-tunability in the lifetime (100 to 5 ns). Also, exciton localization dependent dipolar repulsion leads to an optical power-induced blueshift of ~ 1 meV/µW - a five-fold enhancement over previous reports.

Pulsed Carrier Gas Assisted High-Quality Synthetic 3R-Phase Sword-like MoS2: A Versatile Optoelectronic Material.

Synthesizing a material with the desired polymorphic phase in a chemical vapor deposition (CVD) process requires a delicate balance among various thermodynamic variables. Here, we present a methodology to synthesize rhombohedral (3R)-phase MoS2 in a well-defined sword-like geometry having lengths up to 120 µm, uniform width of 2-3 µm and thickness of 3-7 nm by controlling the carrier gas flow dynamics from continuous mode to pulsed mode during the CVD growth process. Characteristic signatures such as high degree of circular dichroism (∼58% at 100 K), distinct evolution of low-frequency Raman peaks and increasing intensity of second harmonic signals with increasing number of layers conclusively establish the 3R-phase of the material. A high value (∼844 pm/V) of second-order susceptibility for few-layer-thick MoS2 swords signifies the potential of MoS2 to serve as an atomically thin nonlinear medium. A field effect mobility of 40 cm2/V-s and Ion/Ioff ratio of ∼106 further confirm the electronic-grade standard of this 3R-phase MoS2. These findings are significant for the development of emerging quantum electronic devices utilizing valley-based physics and nonlinear optical phenomena in layered materials.

Focus-engineered sub-diffraction imaging in infrared-sensitive third-order sum frequency generation microscope.

We experimentally demonstrate sub-diffraction imaging in infrared-sensitive third-order sum frequency generation (TSFG) microscope using focal-field engineering technique. The TSFG interaction studied here makes use of two mid infrared photons and a single 1040 nm pump photon to generate up-converted visible photons. Focal field engineering scheme is implemented using a Toraldo-style single annular phase mask imprinted on the 1040 nm beam using a spatial light modulator. The effect of focal field engineered excitation beam on the non-resonant-TSFG process is studied by imaging isolated silicon sub-micron disks and periodic grating structures. Maximum reduction in the measured TSFG central-lobe size by ∼43% with energy in the central lobe of 35% is observed in the presence of phase mask. Maximum contrast improvement of 30% is observed for periodic grating structures. Furthermore, to validate the infrared sensitivity of the focus engineered TSFG microscope, we demonstrate imaging of amorphous Germanium-based guided-mode resonance structures, and polystyrene latex beads probed near the O-H vibrational region. We also demonstrate the utility of the focus engineered TSFG microscope for high resolution imaging of two-dimensional layered material. Focus-engineered TSFG process is a promising imaging modality that combines infrared selectivity with improved resolution and contrast, making it suitable for nanostructure and surface layer imaging.

Strong near band-edge excited second-harmonic generation from multilayer 2H Tin diselenide.

We report strong second-harmonic generation (SHG) from 2H polytype of multilayer Tin diselenide (SnSe2) for fundamental excitation close to the indirect band-edge in the absence of excitonic resonances. Comparison of SHG and Raman spectra from exfoliated SnSe2 flakes of different polytypes shows strong (negligible) SHG and Raman Eg mode at 109 cm-1 (119 cm-1), consistent with 2H (1T) polytypes. The difference between the A1g-Eg Raman peak positions is found to exhibit significant thickness dependent for the 1T form, which is found to be absent for the 2H form. The observed thickness dependence of SHG with rapid oscillations in signal strength for small changes in flake thickness are in good agreement with a nonlinear wave propagation model considering nonlinear polarization with alternating sign from each monolayer. The nonlinear optical susceptibility extracted from SHG signal comparison with standard quartz samples for 1040 nm excitation is found to be more than 4-times higher than that at 1550 nm. This enhanced nonlinear response at 1040 nm is attributed to the enhanced nonlinear optical response for fundamental excitation close to the indirect band-edge. We also study SHG from heterostructures of monolayer MoS2/multilayer SnSe2 which allows us to unambiguously compare the nonlinear optical response of SnSe2 with MoS2. We find the SHG signal and any interference effect in the overlap region to be dominated by the SnSe2 layer for the excitation wavelengths considered. The comparison of SHG from SnSe2 and MoS2 underscores that the choice of the 2D material for a particular nonlinear optical application is contextual on the wavelength range of interest and its optical properties at those wavelengths. The present works further highlights the usefulness of near band-edge enhancement of nonlinear processes in emerging 2D materials towards realizing useful nanophotonic devices.

A simple fabrication strategy for orientationally accurate twisted heterostructures.

Van der Waals (vdW) heterostructure is a type of metamaterial where multiple layers of 2D materials are vertically aligned at controlled misorientation. The relative rotation in between the adjacent layers, or the twist angle between them plays a crucial role in changing the electronic band structure of the superlattice. The assembly of multi-layers of precisely twisted two dimensional layered materials requires knowledge of the atomic structure at the edge of the flake. It may be artificially created by the 'tear and stack' process. Otherwise, the crystallographic orientation needs to be determined through invasive processes such as transmission electron microscopy or scanning tunneling microscopy, and via second-harmonic generation (SHG). Here, we demonstrate a simple and elegant transfer protocol using only an optical microscope as a edge identifier tool through which, controlled transfer of twisted homobilayer and heterobilayer transition metal dichalcogenides is performed with close to 100% yield. The fabricated twisted vdW heterostructures have been characterized by SHG, Raman spectroscopy and photoluminiscence spectroscopy, confirming the desired twist angle within ∼0.5° accuracy. The presented method is reliable, quick and prevents the use of invasive tools which is desirable for reproducible device functionalities.

Beam steering in a narrow-beam phosphor down-converted white light visible light communication link using transmitter lens decentering.

Indoor visible light communication (VLC) systems with narrow beams can achieve practical few-meters-long wireless optical links. Such links can operate at low power levels and high data rate for supporting point-to-point or multipoint communication. The narrow-beam VLC links can, however, benefit from beam steering to support mobility of user equipment and cater to multiple users. Simple beam-steering techniques with minimal changes to the existing optical hardware are required to enable widespread adoption of beam steering in VLC links. We study the performance of a simple transmitter-lens-decenter-based beam-steering scheme in a VLC link, utilizing a phosphor down-converted blue laser transmitter. The beam-steering angle and hence the receiver coverage depend on the transmitter lens decenter and the choice of the transmitter and receiver lens's focal lengths. Optical ray tracing is used to quantify the collection efficiency achievable with beam steering, choose a suitable receiver lens, and understand the role of off-axis aberration in the system performance. In our experimental implementation, the transmitter lens decentering technique results in a maximum steering angle of 7.1°. This corresponds to a receiver coverage of 30 cm per cm of transmitter lens decenter for a fixed link length of 300 cm. The measured on-axis white light color coordinates of (0.286, 0.253) is found to shift toward warmer white colors with beam steering. The on-axis illuminance level of ∼19lux decreases slightly with beam steering and is found to be below the maximum permissible exposure limit for indoor illumination. We also quantify the data communication performance as a function of beam steering using on-off modulated data. Bit-error rates below the forward error correction limit are obtained for receiver coverage diameter of 75 cm and 60 cm for 1.25 Gbps and 1.5 Gbps data rates, respectively.

Quantitative probe for in-plane piezoelectric coupling in 2D materials.

Piezoelectric response in two-dimensional (2D) materials has evoked immense interest in using them for various applications involving electromechanical coupling. In most of the 2D materials, piezoelectricity is coupled along the in-plane direction. Here, we propose a technique to probe the in-plane piezoelectric coupling strength in layered nanomaterials quantitively. The method involves a novel approach for in-plane field excitation in lateral Piezoresponse force microscopy (PFM) for 2D materials. Operating near contact resonance has enabled the measurement of the piezoelectric coupling coefficients in the sub pm/V range. Detailed methodology for the signal calibration and the background subtraction when PFM is operated near the contact resonance of the cantilever is also provided. The technique is verified by estimating the in-plane piezoelectric coupling coefficients (d11) for freely suspended MoS2 of one to five atomic layers. For 2D-MoS2 with the odd number of atomic layers, which are non-centrosymmetric, finite d11 is measured. The measurements also indicate that the coupling strength decreases with an increase in the number of layers. The techniques presented would be an effective tool to study the in-plane piezoelectricity quantitatively in various materials along with emerging 2D-materials.

Polarization independent enhancement of zeroth order diffracted second harmonic from multilayer gallium selenide on a silicon resonant metasurface.

We demonstrate polarization-independent resonant-enhancement of second harmonic generation (SHG) from multilayer Gallium Selenide (GaSe) on a silicon-based resonant metasurface. Two-dimensional hexagonal photonic lattice with circularly symmetric silicon meta-atoms are designed to achieve resonant field enhancement at the fundamental wavelength independent of the incident polarization direction. Such structures are however found to exhibit strong resonant field depolarization effects at the fundamental excitation fields resulting in modified nonlinear polarization components when compared to the native GaSe layer. Furthermore, the sub-wavelength metasurface designed to have resonances at the fundamental wavelengths act as a higher order diffraction grating at the second harmonic wavelength. Nonlinear wave propagation simulations show that the higher order diffracted SHG exhibit strong polarization dependent enhancement with characteristics very different from the native GaSe layer. In this context, polarization independent enhancement of the second harmonic signal is achieved only for the zeroth order diffracted component. Experimental study of second harmonic generation from the GaSe layer integrated with the silicon metasurface shows maximum nonlinear signal enhancement on-resonance with polarization dependence identical to the native GaSe layer by selectively detecting the zeroth-order diffracted component. This work shows that it is not sufficient to use symmetric meta-atoms in such 2D material integrated resonant metasurfaces for achieving polarization independent nonlinear optical enhancement. Depolarization of the resonant fields and higher-order diffraction at the nonlinear signal wavelength need to be considered as well.

Evolution of high-frequency Raman modes and their doping dependence in twisted bilayer MoS2.

Twisted van der Waals heterostructures provide a new platform for studying strongly correlated quantum phases. The interlayer coupling in these heterostructures is sensitive to the twist angle (θ) and key to controllably tuning several interesting properties. Here, we demonstrate the systematic evolution of the interlayer coupling strength with twist angle in bilayer MoS2 using a combination of Raman spectroscopy and classical simulations. At zero doping, we observe a monotonic increase in the separation between the A1g and E2g1 mode frequencies as θ decreases from 10°â†’ 1°, and the separation approaches that of a bilayer at small twist angles. Furthermore, using doping dependent Raman spectroscopy, we reveal the θ dependent softening and broadening of the A1g mode, whereas the E2g1 mode remains unaffected. Using first principles based simulations, we demonstrate large (weak) electron-phonon coupling for the A1g (E2g1) mode, which explains the experimentally observed trends. Our study provides a non-destructive way to characterize the twist angle and the interlayer coupling and establishes the manipulation of phonons in twisted bilayer MoS2 (twistnonics).

Nonlinear Optics in Dielectric Guided-Mode Resonant Structures and Resonant Metasurfaces.

Nonlinear optics is an important area of photonics research for realizing active optical functionalities such as light emission, frequency conversion, and ultrafast optical switching for applications in optical communication, material processing, precision measurements, spectroscopic sensing and label-free biological imaging. An emerging topic in nonlinear optics research is to realize high efficiency optical functionalities in ultra-small, sub-wavelength length scale structures by leveraging interesting optical resonances in surface relief metasurfaces. Such artificial surfaces can be engineered to support high quality factor resonances for enhanced nonlinear optical interaction by leveraging interesting physical mechanisms. The aim of this review article is to give an overview of the emerging field of nonlinear optics in dielectric based sub-wavelength periodic structures to realize efficient harmonic generators, wavelength mixers, optical switches etc. Dielectric metasurfaces support the realization of high quality-factor resonances with electric field concentrated either inside or in the vicinity of the dielectric media, while at the same time operate at high optical intensities without damage. The periodic dielectric structures considered here are broadly classified into guided-mode resonant structures and resonant metasurfaces. The basic physical mechanisms behind guided-mode resonances, electromagnetically-induced transparency like resonances and bound-states in continuum resonances in periodic photonic structures are discussed. Various nonlinear optical processes studied in such structures with example implementations are also reviewed. Finally, some future directions of interest in terms of realizing large-area metasurfaces, techniques for enhancing the efficiency of the nonlinear processes, heterogenous integration, and extension to non-conventional wavelength ranges in the ultra-violet and infrared region are discussed.

Third-harmonic generation in multilayer Tin Diselenide under the influence of Fabry-Perot interference effects.

Two-dimensional layered materials are in general known to exhibit strong layer dependent nonlinear optical response owing to the crystal symmetry and associated phase matching considerations. Here we report up-conversion of 1550 nm incident light using third-harmonic generation (THG) in multilayered tin di-selenide (SnSe2) and study its thickness dependence by simultaneously acquiring spatially-resolved images in the forward and backward propagation direction. We find good agreement between the experimental measurements and a coupled-wave equation model we have developed when including the effect of Fabry-Perot interference between the SnSe2 layer and the surrounding medium. We extract the magnitude of the third order electronic nonlinear optical susceptibility of SnSe2, for the first time to our knowledge, by comparing its nonlinear response with a glass substrate and find this to be ∼1500 times higher than that of glass. We also study the polarization dependence and find good agreement with the expected angular dependence of nonlinear polarization considering the crystal symmetry of SnSe2. The large nonlinear optical susceptibility of multi-layer SnSe2 makes it a promising material for studying nonlinear optical effects. This work demonstrates that in addition to the large inherent nonlinear optical susceptibility, the high refractive index of these materials and optical absorption above the bandgap strongly influence the overall nonlinear optical response and its thickness dependence characteristics.

Strong Single- and Two-Photon Luminescence Enhancement by Nonradiative Energy Transfer across Layered Heterostructure.

The strong light-matter interaction in monolayer transition metal dichalcogenides (TMDs) is promising for nanoscale optoelectronics with their direct band gap nature and the ultrafast radiative decay of the strongly bound excitons these materials host. However, the impeded amount of light absorption imposed by the ultrathin nature of the monolayers impairs their viability in photonic applications. Using a layered heterostructure of a monolayer TMD stacked on top of strongly absorbing, nonluminescent, multilayer SnSe2, we show that both single-photon and two-photon luminescence from the TMD monolayer can be enhanced by a factor of 14 and 7.5, respectively. This is enabled through interlayer dipole-dipole coupling induced nonradiative Förster resonance energy transfer (FRET) from SnSe2 underneath, which acts as a scavenger of the light unabsorbed by the monolayer TMD. The design strategy exploits the near-resonance between the direct energy gap of SnSe2 and the excitonic gap of monolayer TMD, the smallest possible separation between donor and acceptor facilitated by van der Waals heterojunction, and the in-plane orientation of dipoles in these layered materials. The FRET-driven uniform single- and two-photon luminescence enhancement over the entire junction area is advantageous over the local enhancement in quantum dot or plasmonic structure integrated 2D layers and is promising for improving quantum efficiency in imaging, optoelectronic, and photonic applications.

Microscopic study of resonant third-harmonic generation from amorphous silicon nanodisk arrays.

A detailed microscopic study of third-harmonic generation (THG) from two-dimensional arrays of sub-wavelength spaced amorphous silicon nanodisks is reported. The arrays are designed to support broadband, minimally angle-sensitive resonances for the fundamental excitation wavelength in the 1500 nm region. This results in resonantly enhanced visible THG in the green spectral range with ∼500-fold enhancement on-resonance, compared to the unpatterned a-Si thin-film. THG multispectral microscopic imaging reveals individual nanodisks with enhanced nonlinear signal on-resonance. For increasing pump intensities, spatially dependent saturation effects are observed for the first time, to the best of our knowledge, in such dielectric nanostructure arrays with THG images showing a reversal of contrast.