Abstract
Nonlinear optical materials possess wide applications, ranging from terahertz and mid-infrared detection to energy harvesting. Recently, the correlations between nonlinear optical responses ...and certain topological properties, such as the Berry curvature and the quantum metric tensor, have attracted considerable interest. Here, we report giant room-temperature nonlinearities in non-centrosymmetric two-dimensional topological materials—the Janus transition metal dichalcogenides in the 1
T’
phase, synthesized by an advanced atomic-layer substitution method. High harmonic generation, terahertz emission spectroscopy, and second harmonic generation measurements consistently show orders-of-the-magnitude enhancement in terahertz-frequency nonlinearities in 1
T’
MoSSe (e.g., > 50 times higher than 2
H
MoS
2
for 18
th
order harmonic generation; > 20 times higher than 2
H
MoS
2
for terahertz emission). We link this giant nonlinear optical response to topological band mixing and strong inversion symmetry breaking due to the Janus structure. Our work defines general protocols for designing materials with large nonlinearities and heralds the applications of topological materials in optoelectronics down to the monolayer limit.
•Harmonic generation from fundamental modes in plate is numerically investigated.•Role of material and geometric nonlinearities in harmonic generation is assessed.•It was found that group velocity ...matching is not necessary for harmonic generation.•Phenomenon of mode-interaction is demonstrated.
Harmonic generation from non-cumulative fundamental symmetric (S0) and antisymmetric (A0) modes in plate is studied from a numerical standpoint. The contribution to harmonic generation from material nonlinearity is shown to be larger than that from geometric nonlinearity. Also, increasing the magnitude of the higher order elastic constants increases the amplitude of second harmonics. Second harmonic generation from non-phase-matched modes illustrates that group velocity matching is not a necessary condition for harmonic generation. Additionally, harmonic generation from primary mode is continuous and once generated, higher harmonics propagate independently. Lastly, the phenomenon of mode-interaction to generate sum and difference frequencies is demonstrated.
Achieving on-chip optical signal isolation is a fundamental difficulty in integrated photonics. The need to overcome this difficulty is becoming increasingly urgent, especially with the emergence of ...silicon nano-photonics, which promises to create on-chip optical systems at an unprecedented scale of integration. Until now, there have been no techniques that provide complete on-chip signal isolation using materials or processes that are fundamentally compatible with silicon CMOS processes. Based on the effects of photonic transitions, we show here that a linear, broadband and non-reciprocal isolation can be accomplished by spatial-temporal refractive index modulations that simultaneously impart frequency and wavevector shifts during the photonic transition process. We further show that a non-reciprocal effect can be accomplished in dynamically modulated micrometre-scale ring-resonator structures. This work demonstrates that on-chip isolation can be accomplished with dynamic photonic structures in standard material systems that are widely used for integrated optoelectronic applications.
A widely tunable third-harmonic generation (THG) in a mid-infrared step-index tellurite optical fiber was investigated in this work. By pumping a 3 cm-long tellurite optical fiber using an optical ...parametric oscillator with the pump wavelength of 1180~3500 nm, a tunable THG from 393 to 1167 nm was achieved. To the best of our knowledge, this is the broadest THG in the optical fiber. The THG conversion efficiency with different fiber lengths was investigated, which showed that the THG conversion efficiency decreased with the fiber length increasing. In addition, the third-harmonic (TH) signal pattern showed that it did not arise from phase matching between the fundamental mode of the pump and the higher-order mode of the TH signal, but rather resulted from the tellurite optical fiber's high nonlinearity. It provides a unique pathway for the development of new light sources.
Optoelectronically active hybrid lead halide perovskites dissociate in water. To prevent this dissociation, here, we introduce long‐range intermolecular cation‐π interactions between A‐site cations ...of hybrid perovskites. An aromatic diamine like 4,4′‐trimethylenedipyridine, if protonated, can show a long‐range cation‐π stacking, and therefore, serves as our A‐site cation. Consequently, 4,4′‐trimethylenedipyridinium lead bromide (4,4′‐TMDP)Pb2Br6, a one‐dimensional hybrid perovskite, remains completely stable after continuous water treatment for six months. Mechanistic insights about the cation‐π interactions are obtained by single‐crystal X‐ray diffraction and nuclear magnetic resonance spectroscopy. The concept of long‐range cation‐π interaction is further extended to another A‐site cation 4,4′‐ethylenedipyridinium ion (4,4′‐EDP), forming water‐stable (4,4′‐EDP)Pb2Br6 perovskite. These water‐stable perovskites are then used to fabricate white light‐emitting diode and for light up‐conversion through tunable third‐harmonic generation. Note that the achieved water stability is the intrinsic stability of perovskite composition, unlike the prior approach of encapsulating the unstable perovskite material (or device) by water‐resistant materials. The introduced cation‐π interactions can be a breakthrough strategy in designing many more compositions of water‐stable low‐dimensional hybrid perovskites.
Lead halide perovskites are unstable in water due to the high water solubility of the A‐site cations present in them. We introduce the intermolecular cation‐π interactions between the A‐site organic cations in 1D hybrid lead bromide perovskites. The cation‐π interactions make these 1D perovskites completely stable in water.
Bismuth oxyselenide (Bi2O2Se), a new type of 2D material, has recently attracted increased attention due to its robust bandgap, stability under ambient conditions, and ultrahigh electron mobility. In ...such complex oxides, fine structural distortion tends to play a decisive role in determining the unique physical properties, such as the ferrorotational order, ferroelectricity, and magnetoelasticity. Therefore, an in‐depth investigation of the fine structural symmetry of Bi2O2Se is necessary to exploit its potential applications. However, conventional techniques are either time consuming or requiring tedious sample treatment. Herein, a noninvasive and high‐throughput approach is reported for characterizing the fine structural distortion in 2D centrosymmetric Bi2O2Se by polarization‐dependent third‐harmonic generation (THG). Unprecedentedly, the divergence between the experimental results and the theoretical prediction of the perpendicular component of polarization‐dependent THG indicates a fine structural distortion, namely, a <1.4° rotation of the oxygen square in the tetragonal (Bi2O2) layers. This rotation breaks the intrinsic mirror symmetry of 2D Bi2O2Se, eventually reducing the symmetry from the D4h to the C4h point group. The results demonstrate that THG is highly sensitive to even fine symmetry variations, thereby showing its potential to uncover hidden phase transitions and interacting polarized sublattices in novel 2D material systems.
Unveiling fine structural distortions, especially the oxygen rotation/tilt, is important to in‐depth understanding of the physical properties of complex oxides. However, most available techniques are either time consuming or requiring tedious sample treatment. A noninvasive and highly sensitive method based on third‐harmonic generation is reported, wherein oxygen square rotation <1.4° in 2D centrosymmetric Bi2O2Se is successfully revealed.
We demonstrate the generation of phase-coherent frequency combs in the vacuum utraviolet spectral region. The output from a mode-locked laser is stabilized to a femtosecond enhancement cavity with a ...gas jet at the intracavity focus. The resulting high-peak power of the intracavity pulse enables efficient high-harmonic generation by utilizing the full repetition rate of the laser. Optical-heterodyne-based measurements reveal that the coherent frequency comb structure of the original laser is fully preserved in the high-harmonic generation process. These results open the door for precision frequency metrology at extreme ultraviolet wavelengths and permit the efficient generation of phase-coherent high-order harmonics using only a standard laser oscillator without active amplification of single pulses.
Quantitatively mapping and monitoring the strain distribution in 2D materials is essential for their physical understanding and function engineering. Optical characterization methods are always ...appealing due to unique noninvasion and high‐throughput advantages. However, all currently available optical spectroscopic techniques have application limitation, e.g., photoluminescence spectroscopy is for direct‐bandgap semiconducting materials, Raman spectroscopy is for ones with Raman‐active and strain‐sensitive phonon modes, and second‐harmonic generation spectroscopy is only for noncentrosymmetric ones. Here, a universal methodology to measure the full strain tensor in any 2D crystalline material by polarization‐dependent third‐harmonic generation is reported. This technique utilizes the third‐order nonlinear optical response being a universal property in 2D crystals and the nonlinear susceptibility has a one‐to‐one correspondence to strain tensor via a photoelastic tensor. The photoelastic tensor of both a noncentrosymmetric D3h WS2 monolayer and a centrosymmetric D3d WS2 bilayer is successfully determined, and the strain tensor distribution in homogenously strained and randomly strained monolayer WS2 is further mapped. In addition, an atlas of photoelastic tensors to monitor the strain distribution in 2D materials belonging to all 32 crystallographic point groups is provided. This universal characterization on strain tensor should facilitate new functionality designs and accelerate device applications in 2D‐materials‐based electronic, optoelectronic, and photovoltaic devices.
Quantitatively mapping the strain distribution in strain‐engineered 2D materials is the cornerstone of desirable functionalized nanodevices, but all available noninvasive optical spectroscopic techniques have application limitations. This work reports a universal methodology based on optical third‐harmonic generation to measure the full strain tensor in any 2D crystalline material regardless of its band structure, crystal symmetry, and phonon modes.
Ultrathin metasurfaces with record‐high nonlinear optical response of 1.2 × 106 pm V−1 for second harmonic generation are experimentally demonstrated in the mid‐infrared spectral range. A second ...harmonic power conversion efficiency of 0.075% is achieved in a 400‐nm‐thick (λ/25) metasurface at a pump intensity of only 15 kW cm−2.
Recent studies have reported electromagnetic ion cyclotron (EMIC) waves with nonlinear second harmonics (SHs) whose frequency and wave number are twice those of the fundamental waves (FWs). In this ...paper, we provide detailed analyses of the cross‐band SH generation of EMIC waves in a multi‐ion plasma. In the presence of heavy ions, EMIC wave modes can be divided into distinct wave bands according to heavy ion gyrofrequencies, and the SH can exist in a wave band different from that of the FW. The SH can be generated around or even exactly on the normal modes of the plasma system, leading to a more efficient energy transfer than that in H+‐only plasmas. The detailed parametric analysis also manifests the vital role of heavy ions in promoting SH's amplitude. Further, multiple 1‐D hybrid simulations are performed to investigate the cross‐band SH generation and validate the theoretical results. The SHs are well reproduced in simulations and the quantitative relation of frequency and wave number between the FW and the SH is well satisfied. The variations of the amplitude ratios (the SH to the FW) with fundamental frequencies are well consistent with those obtained from theoretical calculations in magnitude. Therefore, our results demonstrate the significant roles of heavy ions in the SH generation.
Plain Language Summary
Electromagnetic ion cyclotron (EMIC) waves have recently been observed to have nonlinear second harmonics (SHs) with frequencies twice those of the fundamental waves (FWs). With the existence of heavy ions (He+ and O+), EMIC waves are categorized into three distinct wave bands in terms of heavy ion gyrofrequencies, i.e., H+ band, He+ band, and O+ band. In this situation, the FW in the He+ band could generate the SH in the H+ band. The cross‐band energy transfer would be expected to be more efficient than that in a singular H+ plasma, leading to larger amplitudes of the SH. But such effects of heavy ions on the SH generation have not been studied and verified in detail yet. In this letter, we provide theoretical analyses of the EMIC wave cross‐band energy transfer and utilize hybrid simulations to reproduce the SH generation and verify the theoretical results. Heavy ions are found to play significant roles in the SH excitation.
Key Points
The theoretical amplitude ratio of the fundamental to the second harmonic (SH) for electromagnetic ion cyclotron (EMIC) waves is obtained in cold multi‐ion plasmas
The cross‐band SHs of EMIC waves in multi‐ion plasmas are successfully reproduced by 1‐D hybrid simulations
Heavy ions can promote the cross‐band energy transfer efficiency from the fundamental waves to the SHs
