32 research outputs found
Parametric Nonlinear Optics with Layered Materials and Related Heterostructures
Nonlinear optics is of crucial importance in several fields of science and technology with applications in frequency conversion, entangledâphoton generation, selfâreferencing of frequency combs, crystal characterization, sensing, and ultraâshort light pulse generation and characterization. In recent years, layered materials and related heterostructures have attracted huge attention in this field, due to their huge nonlinear optical susceptibilities, their ease of integration on photonic platforms, and their 2D nature which relaxes the phaseâmatching constraints and thus offers a practically unlimited bandwidth for parametric nonlinear processes. In this review the most recent advances in this field, highlighting their importance and impact both for fundamental and technological aspects, are reported and explained, and an outlook on future research directions for nonlinear optics with atomically thin materials is provided
Hyperspectral microscopy of two-dimensional semiconductors
Here we present an interferometric wide field hyperspectral microscope based on a common-path birefringent interferometer with translating wedges, to measure photoluminescence emission from two-dimensional semiconductors. We show diffraction-limited hyperspectral photoluminescence microscopy from two-dimensional materials across millimeter areas, proving that our hyperspectral microscope is a compact, stable and fast tool to characterize the optical properties and the morphology of 2D materials across ultralarge areas
Towards compact phase-matched and waveguided nonlinear optics in atomically layered semiconductors
Nonlinear frequency conversion provides essential tools for light generation,
photon entanglement, and manipulation. Transition metal dichalcogenides (TMDs)
possess huge nonlinear susceptibilities and 3R-stacked TMD crystals further
combine broken inversion symmetry and aligned layering, representing ideal
candidates to boost the nonlinear optical gain with minimal footprint. Here, we
report on the efficient frequency conversion of 3R-MoS2, revealing the
evolution of its exceptional second-order nonlinear processes along the
ordinary (in-plane) and extraordinary (out-of-plane) directions. By measuring
second harmonic generation (SHG) of 3R-MoS2 with various thickness - from
monolayer (~0.65 nm) to bulk (~1 {\mu}m) - we present the first measurement of
the in-plane SHG coherence length (~530 nm) at 1520 nm and achieve record
nonlinear optical enhancement from a van der Waals material, >10^4 stronger
than a monolayer. It is found that 3R-MoS2 slabs exhibit similar conversion
efficiencies of lithium niobate, but within propagation lengths >100-fold
shorter at telecom wavelengths. Furthermore, along the extraordinary axis, we
achieve broadly tunable SHG from 3R-MoS2 in a waveguide geometry, revealing the
coherence length in such structure for the first time. We characterize the full
refractive index spectrum and quantify both birefringence components in
anisotropic 3R-MoS2 crystals with near-field nano-imaging. Empowered with these
data we assess the intrinsic limits of the conversion efficiency and nonlinear
optical processes in 3R-MoS2 attainable in waveguide geometries. Our analysis
highlights the potential of 3R-stacked TMDs for integrated photonics, providing
critical parameters for designing highly efficient on-chip nonlinear optical
devices including periodically poled structures, resonators, compact optical
parametric oscillators and amplifiers, and optical quantum circuits
Exciton-phonon coupling strength in single-layer MoSe2 at room temperature
Single-layer transition metal dichalcogenides are at the center of an ever
increasing research effort both in terms of fundamental physics and
applications. Exciton-phonon coupling plays a key role in determining the
(opto)electronic properties of these materials. However, the exciton-phonon
coupling strength has not been measured at room temperature. Here, we develop
two-dimensional micro-spectroscopy to determine exciton-phonon coupling of
single-layer MoSe2. We detect beating signals as a function of waiting time T,
induced by the coupling between the A exciton and the A'1 optical phonon.
Analysis of two-dimensional beating maps combined with simulations provides the
exciton-phonon coupling. The Huang-Rhys factor of ~1 is larger than in most
other inorganic semiconductor nanostructures. Our technique offers a unique
tool to measure exciton-phonon coupling also in other heterogeneous
semiconducting systems with a spatial resolution ~260 nm, and will provide
design-relevant parameters for the development of optoelectronic devices
Time-Dependent Screening Explains the Ultrafast Excitonic Signal Rise in 2D Semiconductors
We calculate the time evolution of the transient reflection signal in an MoS2 monolayer on a SiO2/Si substrate using first-principles out-of-equilibrium real-time methods. Our simulations provide a simple and intuitive physical picture for the delayed, yet ultrafast, evolution of the signal whose rise time depends on the excess energy of the pump laser: at laser energies above the A- and B-exciton, the pump pulse excites electrons and holes far away from the K valleys in the first Brillouin zone. Electronâphonon and holeâphonon scattering lead to a gradual relaxation of the carriers toward small Active Excitonic Regions around K, enhancing the dielectric screening. The accompanying time-dependent band gap renormalization dominates over Pauli blocking and the excitonic binding energy renormalization. This explains the delayed buildup of the transient reflection signal of the probe pulse, in excellent agreement with recent experimental data. Our results show that the observed delay is not a unique signature of an exciton formation process but rather caused by coordinated carrier dynamics and its influence on the screening
Strong Coupling of Coherent Phonons to Excitons in Semiconducting Monolayer MoTe
The coupling of the electron system to lattice vibrations and their
time-dependent control and detection provides unique insight into the
non-equilibrium physics of semiconductors. Here, we investigate the ultrafast
transient response of semiconducting monolayer 2-MoTe encapsulated with
BN using broadband optical pump-probe microscopy. The sub-40-fs pump pulse
triggers extremely intense and long-lived coherent oscillations in the spectral
region of the A' and B' exciton resonances, up to 20% of the maximum
transient signal, due to the displacive excitation of the out-of-plane
phonon. Ab-initio calculations reveal a dramatic rearrangement of the optical
absorption of monolayer MoTe induced by an out-of-plane stretching and
compression of the crystal lattice, consistent with an -type
oscillation. Our results highlight the extreme sensitivity of the optical
properties of monolayer TMDs to small structural modifications and their
manipulation with light.Comment: 27 pages, 4 figures, supporting informatio
Broadband nonlinear optical response of monolayer MoSe2under ultrafast excitation
Due to their strong light-matter interaction, monolayer transition metal dichalcogenides (TMDs) have proven to be promising candidates for nonlinear optics and optoelectronics. Here, we characterize the nonlinear absorption of chemical vapour deposition (CVD)-grown monolayer MoSe2in the 720-810 nm wavelength range. Surprisingly, despite the presence of strong exciton resonances, monolayer MoSe2exhibits a uniform modulation depth of âŒ80 ± 3% and a saturation intensity of âŒ2.5 ± 0.4 MW/cm2. In addition, pump-probe spectroscopy is performed to confirm the saturable absorption and reveal the photocarrier relaxation dynamics over hundreds of picoseconds. Our results unravel the unique broadband nonlinear absorptive behavior of monolayer MoSe2under ultrafast excitation and highlight the potential of using monolayer TMDs as broadband ultrafast optical switches with customizable saturable absorption characteristics
Excitonâphonon coupling strength in single-layer MoSe 2 at room temperature
Funder: EC | EC Seventh Framework Programm | FP7 Ideas: European Research Council (FP7-IDEAS-ERC - Specific Programme: "Ideas" Implementing the Seventh Framework Programme of the European Community for Research, Technological Development and Demonstration Activities (2007 to 2013)); doi: https://doi.org/10.13039/100011199; Grant(s): 319277Abstract: Single-layer transition metal dichalcogenides are at the center of an ever increasing research effort both in terms of fundamental physics and applications. Excitonâphonon coupling plays a key role in determining the (opto)electronic properties of these materials. However, the excitonâphonon coupling strength has not been measured at room temperature. Here, we use two-dimensional micro-spectroscopy to determine excitonâphonon coupling of single-layer MoSe2. We detect beating signals as a function of waiting time induced by the coupling between A excitons and AâČ1 optical phonons. Analysis of beating maps combined with simulations provides the excitonâphonon coupling. We get a HuangâRhys factor ~1, larger than in most other inorganic semiconductor nanostructures. Our technique offers a unique tool to measure excitonâphonon coupling also in other heterogeneous semiconducting systems, with a spatial resolution ~260 nm, and provides design-relevant parameters for the development of optoelectronic devices
Nonlinear interactions of dipolar excitons and polaritons in MoS2 bilayers
Nonlinear interactions between excitons strongly coupled to light are key for
accessing quantum many-body phenomena in polariton systems. Atomically-thin
two-dimensional semiconductors provide an attractive platform for strong
light-matter coupling owing to many controllable excitonic degrees of freedom.
Among these, the recently emerged exciton hybridization opens access to
unexplored excitonic species, with a promise of enhanced interactions. Here, we
employ hybridized interlayer excitons (hIX) in bilayer MoS2 to achieve highly
nonlinear excitonic and polaritonic effects. Such interlayer excitons possess
an out-of-plane electric dipole as well as an unusually large oscillator
strength allowing observation of dipolar polaritons(dipolaritons) in bilayers
in optical microcavities. Compared to excitons and polaritons in MoS2
monolayers, both hIX and dipolaritons exhibit about 8 times higher
nonlinearity, which is further strongly enhanced when hIX and intralayer
excitons, sharing the same valence band, are excited simultaneously. This gives
rise to a highly nonlinear regime which we describe theoretically by
introducing a concept of hole crowding. The presented insight into many-body
interactions provides new tools for accessing few-polariton quantum
correlations