Session J27: Experimental optical spectroscopic measurements of 2D materials

Giant nonlieaner optical modulation based on layered materials beyond EFISH

The efficient modulation of optical signals is central to communication and computing, the key in emerging data science revolution. Towards the realization of on-chip nonlinear optical applications such as on-demand nonlinear optical sources and neutral network, electrical-field-induced second harmonic generation (EFISH) dominates the development because of its ubiquitous presence in different material systems. However, its third order nature grinds itself to be a weak and inefficient process. A new mechanism that directly controlling second-order electric susceptibility to realize efficient nonlinear tunability is on demand. We report an electrically controlled new strategy to achieve giant and broadband modulation of second-order optical nonlinearities with an ultra-large modulation strength as well as efficiency. This demonstration will have a profound impact to optical communication and photonic computation.   

Probing the Influence of Dielectric Environment upon Volume-confined Hyperbolic phonon Polaritons

The unique ability of hyperbolic phonon polaritons (HPhPs) to confine long-wavelength light to nanoscale volumes in low-loss, naturally hyperbolic materials – such as hexagonal boron nitride (hBN) – has generated immense interest. We employ nano-scale imaging and spectroscopy techniques to elucidate the characteristics of HPhPs to elucidate polariton characteristics as a function of the complex refractive index of the substrate, including the impact of both the real and imaginary contributions. Although higher-order polariton modes exhibit wavelengths weakly sensitive to the environment, the principal mode strongly depends on the substrate dielectric constant. Furthermore, we demonstrate a reconfigurable hyperbolic metasurface comprised of a heterostructure of hBN in direct contact with the phase-change material (PCM) single-crystal vanadium dioxide (VO2). Metallic and dielectric domains in VO2 provide spatially localized changes in the local dielectric environment, enabling launching, reflection, and transmission of hyperbolic phonon polaritons (HPhPs) at the PCM domain boundaries, and tuning the wavelength of HPhPs propagating in hBN over these domains.
  

Polarized Raman spectroscopy in monolayer ReSe2

Raman spectroscopy is a powerful tool to study two-dimensional compounds and has been widely used to obtain important information of their electronic and vibrational structures. In the case of graphene and MoS2-type compounds, the Raman spectrum is isotropic when the light polarization lies in the layer plane. However, for low-symmetry materials such as black phosphorus and triclinic transition metal dichalcogenides, the spectra are polarized dependent and polarized Raman spectroscopy should be used. By changing the angle between the light polarization and the crystallographic axes, the elements of the Raman tensors for the different phonon modes can be determined. Previous studies in black phosphorus showed that Raman tensor elements are complex numbers, but the physical origin of the phase differences are not yet well understood. In this work, polarized Raman spectroscopy is used to investigate the anisotropic behavior in monolayer ReSe2. The angular dependence of the polarized Raman spectra using different polarization configurations is obtained for the 18 Raman active modes as well as the Raman tensor elements for each mode. It was also observed that the principal axes of those Raman tensors are not along the crystallographic axes.   

Langmuir-Blodgett of Black Phosphorus Thin Films and Photodiodes

Black phosphorus (BP) emerges as a promising optoelectronic material because of its outstanding electrical and optical properties. [ Proc. Natl. Acad. Sci. USA 2015, 112 (15), 4523-30.], Like graphite, BP has a layered structure and can be exfoliated into nanosheets from its bulk form through mechanical and liquid exfoliation. Mechanical exfoliation (the scotch-tape method) has been predominantly used for proof-of-concept devices [Nat. Nanotechnol. 2014, 9 (5), 372-7], but it is inherently unscalable and typically produces BP sheets with lateral size below ten micrometers. For practical applications, scalable approaches for the fabrication of BP large-area films are essential. We will present our results on the development of a Langmuir-Blodgett (LB) protocol which is well suitable for assembling BP nanosheets into thin films. Through functionalization, we achieve large-area (centimeters), homogenous, and smooth BP thin films. With ZnO as an electron transport layer and 1,1-Bis[(di-4-tolylamino) phenyl] cyclohexane (TAPC) as a hole transport layer, the BP films are fabricated into photodiodes, which show responsivity from visible to infrared region. Our work highlights the great potential of BP for applications in near- and mid-infrared detection and imaging.   

Photocurrent Spectroscopy of Titanium Trisulfide Nanoflakes

Titanium trisulfide (TiS3) is an emerging 2D semiconductor which possesses a direct bandgap in the near-infrared regime, along with strong polarization-dependent electronic and optical properties, making it a promising material for various optoelectronic applications. However, constrained by the infrared response, it is challenging to characterize the bandstructure of TiS3 via optical spectroscopy, especially for devices based on thin layer TiS3, which often exist in the form of ribbon due to their quasi-1D atomic structure. In this work, we carry out low-temperature polarization-dependent photocurrent spectroscopy characterization of TiS3 ribbons. We clearly resolve the bandgap of TiS3 for both thick and thin flakes, and the results are consistent with the theoretical expectation. Our findings are crucial to the understanding of the electronic structure of TiS3 and lay the foundation for future device applications.   

Substrate-Mediated Hyperbolic Phonon Polaritons in MoO3

Hyperbolic phonon polaritons (HPhPs) are hybrid excitations of light and coherent charge oscillations that exist in strongly optically anisotropic 2D materials (e.g., MoO₃). These polaritons propagate through the material’s volume with long lifetimes, enabling novel mid-infrared nanophotonic applications by compressing light to sub-diffraction dimensions. Here, the dispersion relations and HPhP lifetimes (up to ≈ 2 ps) in single-crystal α-MoO₃ are determined by Fourier analysis of real-space, nanoscale-resolution polariton images obtained with the photothermal induced resonance (PTIR) technique. Measurements of MoO₃ crystals deposited on periodic gratings showed longer HPhPs propagation lengths (≈ 2 ×) and lower optical compressions in suspended regions compared to regions in direct contact with the substrate. Additionally, PTIR data reveal polymeric contaminants, resulting from sample preparation, localized under parts of the MoO₃ crystals. This work enhances the ability to engineer nanophotonic devices by leveraging substrate morphology to control polariton propagation.   

Giant second harmonic generation from polar van der Waals Bismuth tellurohalide semiconductors

Rashba materials from the Bismuth tellurohalide family of polar layered van der Waals (vdW) semiconductors hold great promise for nonlinear optical (NLO) applications, since their broken spatial inversion symmetry leads to a large second-order nonlinear optical polarizability, χ⁽²⁾. Here, we report the first second harmonic generation (SHG) studies of the bulk polar semiconductors BiTeBr and BiTeI. Our results reveal that BiTeBr, in particular, hosts a large SHG response, comparable to that of archetypal semiconductors and larger than other vdW materials. In contrast to BiTeBr, the response of BiTeI is substantially smaller, suggesting that their relative halide polarity plays a key role in the dramatically different nonlinear optical response. Furthermore, we compared our results to the nonlinear optical response of the Weyl semimetal TaAs, which has previously been shown to produce extremely large SHG, under identical conditions and observe that BiTeBr has nearly half the nonlinear conversion efficiency of TaAs, despite the absence of any known topological properties. This suggests that the BiTeX family of compounds, particularly the BiTeBr compound, are ideal candidates for NLO applications.   

Nanoscale imaging and spectroscopy of charge carrier distribution with terahertz and mid infrared near-field nanoscopy

We perform terahertz (THz) and mid infrared nanoscopy to probe and quantify charge carriers in doped semiconductor surfaces and doped Si nanowires at the nanoscale. We introduce hyperspectral THz nano-imaging by combining scattering-type scanning near-field optical microscopy (s-SNOM) with THz time-domain spectroscopy (TDS). We describe the technical implementations that enabled this achievement and demonstrate its performance. Combination of nanoscale spectroscopy and Drude model allows for measuring—noninvasively and without the need for Ohmic contacts—the local mobile carrier concentration of the differently doped semiconductor areas.   

Raman Studies on Polytypism in Layered Gallium Selenide

Gallium selenide (GaSe) is one of layered group-III metal monochalcogenides, which has an indirect bandgap of 3.0 eV in monolayer and a direct bandgap of 2.0 eV in bulk phase unlike other conventional transition metal dichalcogenides (TMDs) such as MoX₂ and WX₂ (X=S and Se). Since GaSe has high photo-responsivity and external quantum efficiency (EQE) in the UV-range, it can be used as a photodevice such as a photodetector [1]. In bulk phase, four polytypes designated as β-, ε-, γ-, and δ-GaSe have been reported. Since different polytypes result in different optical and electrical properties even for the same thickness, identifying the polytype is essential in utilizing this material for various optoelectronic applications. We found different ultra-low-frequency Raman spectra of inter-layer vibrational modes even for the same thickness due to GaSe polytypism. By comparing the ultra-low-frequency Raman spectra with theoretical calculations and high-resolution electron microscopy measurements, we established the correlation between the ultra-low-frequency Raman spectra and the polytypes for trilayer GaSe. We further found that the AB-type stacking is more stable than the AA'-type stacking in GaSe.

[1] PingAn Hu et al., ACS nano 6 5988 (2012).   

Ultrafast Carrier Recombination from Quantum Pillars in Black Silicon

We report on ultrafast photoluminescence(PL) phenomena from black silicon produced by ion etching using chlorine plasma. An ultrafast blue PL component competing with non-radiative recombination at surface defects was quantified as originating from the no-phonon transition. This component involves two decay processes with a peak energy at 480 nm: a fast component of 6 ps followed by a component of 48 ps decay time constant. It exhibits also a slow component in the red spectral region with a time constant of about 2.5 ns. When it is oxidized, slow band at around 600 nm is enhanced in intensity to the detriment of blue emission band. This process results in a much slower sates assuming 3-components exponential decay. Ultrafast PL decay leads to transfer of carriers to long-lived defect states as evidenced from red emission at around 2 eV. Time-correlated single photon counting revealed a life-time of about 5 ns for these states. The results are discussed in terms of electronic band structure modification at reduced sizes and surface point defects.   

Anisotropic optical and structural properties of hexagonal boron nitride epilayers probed by optical ellipsometry

Hexagonal boron nitride (h-BN) is a 2D layered material that has gained increasing attention in recent years due to its unique physical properties that make h-BN a promising material for DUV optoelectronics and solid-state neutron detectors. There are many material properties still uncertain, including h-BN’s optical constants. The anisotropic index of refraction of h-BN free-standing epilayers grown by metal organic chemical vapor deposition were measured using spectroscopic ellipsometry in the UV spectral range. It was found that the index of refraction for E⊥c-axis (ordinary) is much higher than E||c-axis (extraordinary). The scattered data for the index of refraction of h-BN reported earlier for various h-BN materials can be explained by including the inclination of turbostratic (t-) phase layers or the change of the c-plane orientation of individual layers. Our results show that ellipsometry can be used to characterize the crystalline quality of h-BN epilayers by determining the average inclined angle of t-phase layers within the sample while being non-invasive and highly sensitive.   

Unraveling nonlinear formation and relaxation of excitons in atomically thin 2D semiconductors

Transition metal dichalcogenide semiconductors are layered van der Walls materials that exhibit exceptional optoelectronic properties in monolayer form. Their atomically thin nature and reduced long-range dielectric screening make them ideal systems in which to study many-body electronic states. Here, the dynamics of several higher-order exciton states in monolayer-WSe₂ are probed using temperature-, energy-, and power-dependent time-resolved optical spectroscopy. These studies reveal a complex interplay between multiexciton states and single-exciton states in 2D materials that depends on both the density and excitation energy of the initial exciton population. In addition, the presence of defect-bound excitons is found to drastically alter the formation of multiexciton states. This competition between exciton trapping and multiexciton formation highlights the need for high-quality materials to enhance multiexciton physics. Understanding these formation and relaxation dynamics of the rich manifold of exciton states is critical for leveraging this new class of 2D semiconductors for advanced technologies.   

Dipole Orientation Shift of Ga2Se2 by Quantum Confinement

In the family of III-VI monochalcogenides M₂X₂ (M = Gallium, Indium; X = Sulfur, Selenide, etc), the interlayer interaction and the electronic band edges share the contribution of the same chalcognide atomic orbits. This makes quantum confinement and interlayer interaction play a subtle role in 2 dimensional (2D) monochalcogenides crystals. In this report we study the direction-resolved photoluminescence of 2D Ga₂Se₂ at various thickness. We observe that the in-plane dipole radiation survives but out-of-plane dipole radiation fades at 2D limit.
  

Valley Phonons and Exciton Complexes in a Monolayer Semiconductor

The coupling between spin, charge, and lattice degrees of freedom plays an important role in a wide range of fundamental phenomena. Monolayer semiconducting transitional metal dichalcogenides have emerged as an outstanding platform for studying these coupling effects because they possess unique spin-valley locking physics for hosting rich excitonic species and the reduced screening for strong Coulomb interactions. Here, we report the observation of multiple valley phonons – phonons with momentum vectors pointing to the corners of the hexagonal Brillouin zone, in monolayer WSe₂. We find that these valley phonons lead to efficient intervalley scattering of quasi particles in both exciton formation and relaxation. This leads to a series of photoluminescence peaks as valley phonon replicas of dark trions, and the identification of intervalley exciton near charge neutrality.   

Microcavity Organic Light Emitting Diodes with Higher Order Resonance Modes

Organic light emitting diodes (OLED) have a characteristic broad spectral width. The output emission characteristic of the light source can be controlled by the use of a microcavity. A microcavity OLED has metal electrodes encasing the organic layer which allow for the unique bandwidth narrowing and angle resolved spectral emission. We investigate the emission spectrum through the fundamental and higher order resonant modes using a single organic emitter molecule, just by designing the microcavity structure alone. We use Tris(8-hydroxyquinolinato)aluminium, Alq3, as the emissive layer (EML) with the electron transport layers (ETL) and hole transport layers (HTL) added in between the metal electrodes. The output characteristic (spectral bandwidth and viewing angle) of the microcavity is affected by its optical path length. The desired design is achieved by varying the thickness of the ETL, EML, and HTL layers which make up the microcavity. We generate the full visible spectrum at the forward emission and the angle resolved emission spectrum for various order resonance modes.