Electronic states and their dynamics are of critical importance for electronic and optoelectronic applications. Here, various relevant electronic states in monolayer MoS2, such as multiple excitonic ...Rydberg states and free‐particle energy bands are probed with a high relative contrast of up to ≥200 via broadband (from ≈1.79 to 3.10 eV) static third‐harmonic spectroscopy (THS), which is further supported by theoretical calculations. Moreover, transient THS is introduced to demonstrate that third‐harmonic generation can be all‐optically modulated with a modulation depth exceeding ≈94% at ≈2.18 eV, providing direct evidence of dominant carrier relaxation processes associated with carrier–exciton and carrier–phonon interactions. The results indicate that static and transient THS are not only promising techniques for the characterization of monolayer semiconductors and their heterostructures, but also a potential platform for disruptive photonic and optoelectronic applications, including all‐optical modulation and imaging.
An advanced optical technique is introduced: broadband static and transient third‐harmonic spectroscopies capture clear fingerprints of relevant electronic states in monolayer semiconductors with an unprecedentedly high contrast ratio up to 200 and further offer a dramatic advance in understanding of exciton dynamics with an ultrafast speed of ≈1 ps and high modulation depth up to 94%.
Out-of-plane ferroelectricity with a high transition temperature in ultrathin films is important for the exploration of new domain physics and scaling down of memory devices. However, depolarizing ...electrostatic fields and interfacial chemical bonds can destroy this long-range polar order at two-dimensional (2D) limit. Here we report the experimental discovery of the locking between out-of-plane dipoles and in-plane lattice asymmetry in atomically thin In_{2}Se_{3} crystals, a new stabilization mechanism leading to our observation of intrinsic 2D out-of-plane ferroelectricity. Through second harmonic generation spectroscopy and piezoresponse force microscopy, we found switching of out-of-plane electric polarization requires a flip of nonlinear optical polarization that corresponds to the inversion of in-plane lattice orientation. The polar order shows a very high transition temperature (∼700 K) without the assistance of extrinsic screening. This finding of intrinsic 2D ferroelectricity resulting from dipole locking opens up possibilities to explore 2D multiferroic physics and develop ultrahigh density memory devices.
Optical interrogation of cellular electrical activity has proven itself essential for understanding cellular function and communication in complex networks. Voltage‐sensitive dyes are important tools ...for assessing excitability but these highly lipophilic sensors may affect cellular function. Label‐free techniques offer a major advantage as they eliminate the need for these external probes. In this work, it is shown that endogenous second‐harmonic generation (SHG) from live cells is highly sensitive to changes in transmembrane potential (TMP). Simultaneous electrophysiological control of a living human embryonic kidney (HEK293T) cell, through a whole‐cell voltage‐clamp reveals a linear relation between the SHG intensity and membrane voltage. The results suggest that due to the high ionic strengths and fast optical response of biofluids, membrane hydration is not the main contributor to the observed field sensitivity. A conceptual framework is further provided that indicates that the SHG voltage sensitivity reflects the electric field within the biological asymmetric lipid bilayer owing to a nonzero χeff(2) tensor. Changing the TMP without surface modifications such as electrolyte screening offers high optical sensitivity to membrane voltage (≈40% per 100 mV), indicating the power of SHG for label‐free read‐out. These results hold promise for the design of a non‐invasive label‐free read‐out tool for electrogenic cells.
The transmembrane potential (TMP) of a living cell is controlled using a patch‐clamp. Using a pulsed laser, second‐harmonic generation (SHG) and two‐photon fluorescence can be generated from the cell membrane. Due to the strong interfacial fields of biological cell membranes, and the DC‐field sensitivity of SHG, the changes in cellular activity can be optically monitored.
Polarization‐dependent second‐harmonic generation (P‐SHG) microscopy is used to characterize molecular nonlinear optical properties of collagen and determine a three‐dimensional (3D) orientation map ...of collagen fibers within a pig tendon. C6 symmetry is used to determine the nonlinear susceptibility tensor components ratios in the molecular frame of reference χzzz2/χzxx2 and χxyz2/χzxx2, where the latter is a newly extracted parameter from the P‐SHG images and is related to the chiral structure of collagen. The χxyz2/χzxx2 is observed for collagen fibers tilted out of the image plane, and can have positive or negative values, revealing the relative polarity of collagen fibers within the tissue. The P‐SHG imaging was performed using a linear polarization‐in polarization‐out (PIPO) method on thin sections of pig tendon cut at different angles. The nonlinear chiral properties of collagen can be used to construct the 3D organization of collagen in the tissue and determine the orientation‐independent molecular susceptibility ratios of collagen fibers in the molecular frame of reference.
Chirality of collagen is studied in a tendon tissue using a polarimetric second‐harmonic generation microscopy technique. A three‐dimensional (3D) orientation and the relative polarity of neighboring fibers are revealed. This information can be utilized for 3D reconstruction of collagen organization, which aid scientists in better understanding of the architecture and function of the collagen‐based tissues.
Third‐order harmonic generation (THG) plays a vital role in microscopy, optical communications, etc. Conventional methods of obtaining efficient THG in macroscopic crystals is already mature; ...however, they will finally limit the miniaturization and integration of on‐chip laser sources. To date, THG from either photonic crystals or metamaterials provides compact photonic platforms; however, selection of materials remains elusive. Herein, a giant enhancement of THG efficiency from an air/high index Ge2Sb2Te5 (GST)/gold multi‐layered Fabry–Perot cavity is experimentally demonstrated. At cavity resonant wavelength in near‐infrared regime, the efficiency of THG from a 50‐nm thick amorphous GST planar film is boosted 422 times compared to that of nonresonant conditions. Interestingly, the THG efficiency has a dramatic decrease of three orders when the structural state of GST is transformed from amorphous to crystalline. These findings have a potential for achieving an ultracompact nonlinear optical source with high efficiency and switchable functionality.
A giant enhancement of third‐harmonic generation (THG) efficiency from an air/Ge2Sb2Te5 (GST)/gold multi‐layered Fabry–Perot cavity is presented. Interestingly, the THG efficiency has a dramatic decrease of three orders when the structural state of GST transforms from amorphous to crystalline. These findings have a potential for achieving an ultracompact nonlinear optical source with high efficiency and switchable functionality.
In this work, optical properties of a tuned quantum dot/ring system with considering the electron–phonon (e-p) interaction are theoretically investigated. In particular, the second harmonic ...generation (SHG) and third harmonic generation (THG) of the system under e-p effect are determined. Here, we consider three phonon modes, longitudinal optical (LO), the surface optical (SO), and LO+SO phonons. First, the energy levels and the wave functions of the system are evaluated without e-p effect. Then, the SHG and THG are investigated with considering the LO e-p, SO e-p, and LO+SO e-p interactions. We have noticed that the e-p interaction has a great effect on the SHG and THG of our system. The effect of SO phonons on the obtained optical properties is more than the LO phonons. With considering the LO+SO-phonons, we obtain the largest influence on the optical properties.
•Electron–phonon interaction effect on optical properties of a tuned quantum dot/ring system.•The second and third harmonic generation of the system under e-p effect are determined.•Longitudinal optical, the surface optical, and LO+SO phonons are considered.•Effect of SO phonons on the obtained optical properties is more than the LO phonons.
An ultrafast spectral imaging optical system featuring adaptive precompensation for group delay dispersion, automated tuning and collimation, rapid power stabilization, and rapid spectral imaging ...switching is developed. In this system, real‐time visualization ensures that the excited Gaussian beam remains focused on the nanoarray of the electron‐beam lithographed metasurface, as well as maximum gain of the nonlinear effect information. Based on the system, two surface‐lattice resonance metasurfaces under broadly tunable excitation (800–1300 nm) are investigated. The results demonstrated a 16‐fold enhancement in the third harmonic generation intensity and a 168‐fold enhancement in the second harmonic generation intensity at a 1200 nm excitation compared to the unpatterned films, with a narrow resonant bandwidth of 8 nm. This can be attributed to the frequency‐matching lattice‐enhanced localized surface plasmon resonance, which results in effective and vast excitation of plasmons that produce numerous energetic carriers. The method offers the possibility to reveal various ultrafast dynamics in nonlinear plasmonic metasurfaces.
This study presents an ultrafast spectral imaging optical system, featuring precompensation for group delay dispersion and autoregulations, to investigate two surface‐lattice resonance metasurfaces under near infrared (NIR) I and II excitation. A 16‐fold enhancement in the third harmonic generation intensity and a 168‐fold enhancement in the second harmonic generation intensity at a 1200 nm excitation are demonstrated as compared to unpatterned films.
A new SHG material, namely, Pb2(BO3)(NO3), which contains parallel π‐conjugated nitrate and borate anions, was obtained through a facile hydrothermal reaction by using Pb(NO3)2 and Mg(BO2)2⋅H2O as ...starting materials. Its structure contains honeycomb Pb2(BO3)∞ layers with noncoordination NO3− anions located at the interlayer space. Pb2(BO3)(NO3) shows a remarkable strong SHG response of approximately 9.0 times that of potassium dihydrogen phosphate (KDP) and the material is also phase‐matchable. The large SHG coefficient of Pb2(BO3)(NO3) arises from the synergistic effect of the stereoactive lone pairs on Pb2+ cations and parallel alignment of π‐conjugated BO3 and NO3 units. Based on its unique properties, Pb2(BO3)(NO3) may have great potential as a high performance NLO material in photonic applications.
Pb2(BO3)(NO3) is a new second‐harmonic generation (SHG) material that contains parallel π‐conjugated nitrate and borate anions. It was obtained through a facile hydrothermal reaction, shows a remarkable strong SHG response of about 9.0 times that of potassium dihydrogen phosphate (KDP), and is phase‐matchable.
We perform a perturbative calculation of the third order optical conductivities of doped graphene, using approximations valid around the Dirac points and neglecting effects due to scattering and ...electron-electron interactions. In this limit analytic formulas can be constructed for the conductivities. We discuss in detail the results for third harmonic generation, the Kerr effect and two-photon carrier injection, parametric frequency conversion, and two-color coherent current injection. We find a complicated dependence on the chemical potential and photon energies. The linear dispersion causes resonances over a wide range of photon energies, and it is possible to obtain large optical nonlinearities by tuning the chemical potential.
Optical frequency conversion via the nonlinear effect of third harmonic generation is shown to be resonantly enhanced in few‐layer black phosphorus. This feature is believed to be a consequence of ...exciton‐related resonance, as the enhancement is strongly correlated with the observation of exciton‐recombination photoluminescence. Few‐layer thicknesses are obtained both via mechanical exfoliation and laser thinning.
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