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Register a new userSecond-Harmonic Young Interference in Atom-Thin Heterocrystals
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Optical second-harmonic generation (SHG) is a nonlinear parametric process that doubles the frequency of incoming light. Only allowed in non-centrosymmetric materials, it has been widely used in frequency modulation of lasers, surface scientific investigation, and label-free imaging in biological and medical sciences. Two-dimensional crystals are ideal SHG-materials not only for their strong light-matter interaction and atomic thickness defying the phase-matching requirement but also for their stackability into customized hetero-crystals with high angular precision and material diversity. Here we directly show that SHG in hetero-bilayers of transition metal dichalcogenides (TMDs) is governed by optical interference between two coherent SH fields with material-dependent phase delays using spectral phase interferometry. We also quantify the frequency-dependent phase difference between MoS2 and WS2, which also agrees with polarization-resolved data and first-principles calculations on complex susceptibility. The second-harmonic analogue of Young double-slit interference shown in this work demonstrates the potential of custom-designed parametric generation by atom-thick nonlinear optical materials.
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We show that the lack of inversion symmetry in monolayer MoS2 allows strong optical second harmonic generation. Second harmonic of an 810-nm pulse is generated in a mechanically exfoliated monolayer, with a nonlinear susceptibility on the order of 1E-7 m/V. The susceptibility reduces by a factor of seven in trilayers, and by about two orders of magnitude in even layers. A proof-of-principle second harmonic microscopy measurement is performed on samples grown by chemical vapor deposition, which illustrates potential applications of this effect in fast and non-invasive detection of crystalline orientation, thickness uniformity, layer stacking, and single-crystal domain size of atomically thin films of MoS2 and similar materials.
Recent experiments in the topological Weyl semimetal TaAs have observed record-breaking second-harmonic generation, a non-linear optical response at $2omega$ generated by an incoming light source at $omega$. However, whether second-harmonic generation is enhanced in topological semimetals in general is a challenging open question because their band structure entangles the contributions arising from trivial bands and topological band crossings. In this work, we circumvent this problem by studying RhSi, a chiral topological semimetal with a simple band structure with topological multifold fermions close to the Fermi energy. We measure second-harmonic generation (SHG) in a wide frequency window, $omegain [0.27,1.5]$eV and, using first principle calculations, we establish that, due to their linear dispersion, the contribution of multifold fermions to SHG is subdominant as compared with other regions in the Brillouin zone. Our calculations suggest that parts of the bands where the dispersion is relatively flat contribute significantly to SHG. As a whole, our results suggest avenues to enhance SHG responses.
Recently, atomically thin PdSe$_2$ semiconductors with rare pentagonal Se-Pd-Se monolayers were synthesized and were also found to possess superior properties such as ultrahigh air stability, and high carrier mobility, thus offering a new family of two-dimensional (2D) materials for exploration of 2D semiconductor physics and for applications in advanced opto-electronic and nonlinear photonic devices. In this work, we systematically study the nonlinear optical (NLO) responses [namely, bulk photovoltaic effect (BPVE), second-harmonic generation (SHG) and linear electric-optic (LEO) effect] of noncentrosymmetric bilayer (BL) and four-layer (FL) PdS$_2$ and PdSe$_2$ by applying the first-principles density functional theory with the generalized gradient approximation plus scissors-correction. First of all, the shift current conductivity is in the order of 130 $mu$A/V$^2$, being very high compared to known BPVE materials. Similarly, their injection current susceptibilities are in the order of 100$times$10$^8$A/V$^2$s, again being large. Secondly, the SHG coefficients ($chi^{(2)}$) of these materials are also large, being one order higher than that of the best-known few-layer group 6B transition metal dichalcogenides. For example, the maximum magnitude of $chi^{(2)}$ can reach 1.4$times$10$^3$ pm/V for BL PdSe$_2$ at 1.9 eV and 1.2$times$10$^3$ pm/V at 3.1 eV for BL PdS$_2$. Thirdly we find significant LEO coefficients for these structures in the low photon energy. All these indicate that 2D PdX$_2$ semiconductors will find promising NLO applications. Fourthly, we find that the large BPVE and SHG of the few-layer PdX$_2$ structures are due to strong intralayer directional covalent bonding and also 2D quantum confinement.
A hallmark of wave-matter duality is the emergence of quantum-interference phenomena when an electronic transition follows different trajectories. Such interference results in asymmetric absorption lines such as Fano resonances, and gives rise to secondary effects like electromagnetically induced transparency (EIT) when multiple optical transitions are pumped. Few solid-state systems show quantum interference and EIT, with quantum-well intersubband transitions in the IR offering the most promising avenue to date to devices exploiting optical gain without inversion. Quantum interference is usually hampered by inhomogeneous broadening of electronic transitions, making it challenging to achieve in solids at visible wavelengths and elevated temperatures. However, disorder effects can be mitigated by raising the oscillator strength of atom-like electronic transitions - excitons - which arise in monolayers of transition-metal dichalcogenides (TMDCs). Quantum interference, probed by second-harmonic generation (SHG), emerges in monolayer WSe2, without a cavity, splitting the SHG spectrum. The splitting exhibits spectral anticrossing behaviour, and is related to the number of Rabi flops the strongly driven system undergoes. The SHG power-law exponent deviates strongly from the canonical value of 2, showing a Fano-like wavelength dependence which is retained at room temperature. The work opens opportunities in solid-state quantum-nonlinear optics for optical mixing, gain without inversion and quantum-information processing.
We report on the observation of metallic behavior in thin films of oxygen-deficient SrTiO$_3$ - down to 9 unit cells - when coherently strained on (001) SrTiO$_3$ or DyScO$_3$-buffered (001) SrTiO$_3$ substrates. These films have carrier concentrations of up to 2$times10^{22}$ cm$^{-3}$ and mobilities of up to 19,000 cm$^2$/V-s at 2 K. There exists a non-conducting layer in our SrTiO$_{3-delta}$ films that is larger in films with lower carrier concentrations. This non-conducting layer can be attributed to a surface depletion layer due to a Fermi level pinning potential. The depletion width, transport, and structural properties are not greatly affected by the insertion of a DyScO$_3$ buffer between the SrTiO$_3$ film and SrTiO$_3$ substrate.
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