Nonlinear 2D layered crystals provide ideal platforms for applications and fundamental studies in ultrathin nonlinear optical (NLO) devices. However, the NLO frequency conversion efficiency ...constrained by lattice symmetry is still limited by layer numbers of 2D crystals. In this work, 3R MoS2 with broken inversion symmetry structure are grown and proved to be excellent NLO 2D crystals from monolayer (0.65 nm) toward bulk‐like (300 nm) dimension. Thickness and wavelength‐dependent second harmonic generation spectra offer the selection rules of appropriate working conditions. A model comprising of bulk nonlinear contribution and interface interaction is proposed to interpret the observed nonlinear behavior. Polarization enhancement with two petals along staggered stacking direction appears in 3R MoS2 is first observed and the robust polarization of 3R MoS2 crystal is caused by the retained broken inversion symmetry. The results provide a new arena for realizing ultrathin NLO devices for 2D layered materials.
A 2D van der Waals crystal (3R MoS2) can serve as a promising ultrathin nanoscale frequency doubling crystal under phase matching conditions. Thickness and wavelength‐dependent second harmonic generation spectrum of 3R MoS2 offers the selection rules of appropriate working conditions. Polarization enhancement with two petals along staggered stacking direction appears in 3R MoS2 is observed.
A big challenge for nonlinear optical (NLO) materials is the application in high power lasers, which needs the simultaneous occurrence of large second harmonic generation (SHG) and high laser induced ...damage threshold (LIDT). Herein we report the preparation of a new Ga2Se3 phase, which shows the SHG intensities of around 2.3 times and the LIDT of around 16.7 times those of AgGaS2 (AGS), respectively. In addition, its IR transparent window ca. 0.59–25 μm is also significantly wider than that of AGS (ca. 0.48–≈11.4 μm). The occurrence of the strong SHG responses and good phase‐matching indicate that the structure of the new Ga2Se3 phase can only be non‐centrosymmetric and have a lower symmetry than the cubic γ‐phase. The observed excellent SHG and phase‐matching properties are consistent with our diffraction experiments and can be well explained by using the orthorhombic models obtained through our high throughput simulations.
A simple but perfect case: A new phase of gallium selenides (δ‐Ga2Se3) has been obtained via solid‐state reactions. δ‐Ga2Se3 is phase‐matchable with large second harmonic generation (SHG) responses, high laser‐induced damage thresholds (LIDTs), and wide transparent range of 0.59–25 μm. These properties are all required for the application of NLO materials with high‐power lasers.
Carbon nanotubes (CNTs) possess extremely anisotropic electronic, thermal, and optical properties owing to their 1D character. While their linear optical properties have been extensively studied, ...nonlinear optical processes, such as harmonic generation for frequency conversion, remain largely unexplored in CNTs, particularly in macroscopic CNT assemblies. In this work, macroscopic films of aligned and type‐separated (semiconducting and metallic) CNTs are synthesized and polarization‐dependent third‐harmonic generation (THG) from the films with fundamental wavelengths ranging from 1.5 to 2.5 µm is studied. Both films exhibited strongly wavelength‐dependent, intense THG signals, enhanced through exciton resonances, and third‐order nonlinear optical susceptibilities of 2.50 × 10−19 m2 V−2 (semiconducting CNTs) and 1.23 × 10−19 m2 V−2 (metallic CNTs), respectively are found, for 1.8 µm excitation. Further, through systematic polarization‐dependent THG measurements, the values of all elements of the susceptibility tensor are determined, verifying the macroscopically 1D nature of the films. Finally, polarized THG imaging is performed to demonstrate the nonlinear anisotropy in the large‐size CNT film with good alignment. These findings promise applications of aligned CNT films in mid‐infrared frequency conversion, nonlinear optical switching, polarized pulsed lasers, polarized long‐wave detection, and high‐performance anisotropic nonlinear photonic devices.
Macroscopic films of aligned and type‐separated (semiconducting and metallic) carbon nanotubes are synthesized, and third‐harmonic generation (THG) processes with extreme polarization anisotropy in the broad spectral range are studied, due to the 1D character of CNTs. The THG processes are enhanced by exciton resonance (S11, S22, and M11 excitons) in both films, revealing coupling between the excitons and nonlinear effects.
It is a great challenge to develop UV nonlinear optical (NLO) material due to the demanding conditions of strong second harmonic generation (SHG) intensity and wide band gap. The first ultraviolet ...NLO selenite material, Y3F(SeO3)4, has been obtained by control of the fluorine content in a centrosymmetric CaYF(SeO3)2. The two new compounds represent similar 3D structures composed of 3D yttrium open frameworks strengthened by selenite groups. CaYF(SeO3)2 has a large birefringence (0.138@532 nm and 0.127@1064 nm) and a wide optical band gap (5.06 eV). The non‐centrosymmetric Y3F(SeO3)4 can exhibit strong SHG intensity (5.5×KDP@1064 nm), wide band gap (5.03 eV), short UV cut‐off edge (204 nm) and high thermal stability (690 °C). So, Y3F(SeO3)4 is a new UV NLO material with excellent comprehensive properties. Our work shows that it is an effective method to develop new UV NLO selenite material by fluorination control of the centrosymmetric compounds.
The non‐centrosymmetric hydrogen‐free selenite Y3F(SeO3)4 was synthesized from the centrosymmetric CaYF(SeO3)2 by controlling the fluorine content. Y3F(SeO3)4 features large SHG intensity, wide band gap, short UV cut‐off edge and high thermal stability.
Multiphoton microscopy has recently passed the milestone of its first 30 years of activity in biomedical research. The growing interest around this approach has led to a variety of applications from ...basic research to clinical practice. Moreover, this technique offers the advantage of label-free multiphoton imaging to analyze samples without staining processes and the need for a dedicated system. Here, we review the state of the art of label-free techniques; then, we focus on two-photon autofluorescence as well as second and third harmonic generation, describing physical and technical characteristics. We summarize some successful applications to a plethora of biomedical research fields and samples, underlying the versatility of this technique. A paragraph is dedicated to an overview of sample preparation, which is a crucial step in every microscopy experiment. Afterwards, we provide a detailed review analysis of the main quantitative methods to extract important information and parameters from acquired images using second harmonic generation. Lastly, we discuss advantages, limitations, and future perspectives in label-free multiphoton microscopy.
The attosecond time-scale electron-recollision process that underlies high harmonic generation has uncovered extremely rapid electronic dynamics in atoms and diatomics. We showed that high harmonic ...generation can reveal coupled electronic and nuclear dynamics in polyatomic molecules. By exciting large amplitude vibrations in dinitrogen tetraoxide, we showed that tunnel ionization accesses the ground state of the ion at the outer turning point of the vibration but populates the first excited state at the inner turning point. This state-switching mechanism is manifested as bursts of high harmonic light that is emitted mostly at the outer turning point. Theoretical calculations attribute the large modulation to suppressed emission from the first excited state of the ion. More broadly, these results show that high harmonic generation and strong-field ionization in polyatomic molecules undergoing bonding or configurational changes involve the participation of multiple molecular orbitals.
Polarization-sensitive second harmonic generation (SHG) microscopy is an established imaging technique able to provide information related to specific molecular structures including collagen. In this ...investigation, polarization-sensitive SHG microscopy was used to investigate changes in the collagen ultrastructure between histopathology slides of normal and diseased human thyroid tissues including follicular nodular disease, Grave's disease, follicular variant of papillary thyroid carcinoma, classical papillary thyroid carcinoma, insular or poorly differentiated carcinoma, and anaplastic or undifferentiated carcinoma ex vivo. The second-order nonlinear optical susceptibility tensor component ratios, χ(2)zzz′/χ(2)zxx′ and χ(2)xyz′/χ(2)zxx′, were obtained, where χ(2)zzz′/χ(2)zxx′ is a structural parameter and χ(2)xyz′/χ(2)zxx′ is a measure of the chirality of the collagen fibers. Furthermore, the degree of linear polarization (DOLP) of the SHG signal was measured. A statistically significant increase in χ(2)zzz′/χ(2)zxx′ values for all the diseased tissues except insular carcinoma and a statistically significant decrease in DOLP for all the diseased tissues were observed compared to normal thyroid. This finding indicates a higher ultrastructural disorder in diseased collagen and provides an innovative approach to discriminate between normal and diseased thyroid tissues that is complementary to standard histopathology.
Polarization-second harmonic microscopy was utilized to investigate whether collagen ultrastructure in thyroid due to four carcinoma types and Graves' disease could be differentiated in human histopathology samples. Three parameters were extracted, revealing that the degree of linear polarization and χ(2)zzz/χ(2)zxx were effective in differentiating some diseases, while the parameter χ(2)xyz/χ(2)zxx was less effective.
In this review, the authors discuss the underlying principles of SHG and specific factors that affect the generation properties and describe the essential components of a SHG instrument. In addition, ...results on the recent progress and impact SHG microscopy has made in different areas of biology and medicine are presented. In particular, the authors focus on disease diagnosis and basic research associated with connected tissues, musculo‐skeletal disorders, and epithelial cancers. The presentation is concluded by offering a perspective on the future technical development of SHG microscopy and additional forefronts to be addressed.
In this review, the underlying principles of second harmonic generation (SHG) are discussed and specific factors that affect the generation properties and describe the essential components of a SHG instrument. In addition, results on the recent progress and impact SHG microscopy has made in different areas of biology and medicine are presented. In particular, a focus is set on disease diagnosis and basic research associated with connected tissues, musculo‐skeletal disorders, and epithelial cancers.
High‐index dielectric subwavelength structures and metasurfaces are capable of enhancing light‐matter interaction by orders of magnitude via geometry‐dependent optical resonances. This enhancement, ...however, comes with a fundamental limitation of a narrow spectral range of operation in the vicinity of one or few resonant frequencies. Here, this limitation is tackled by introducing an innovative and practical approach to achieve spectrally tunable enhancement of light‐matter interaction with resonant metasurfaces. Resonance‐gradient metasurfaces are designed and fabricated with varying geometrical parameters that translate into resonant frequencies dependence on one of the coordinates of the metasurface. The metasurfaces are composed of bone‐like nanoresonators, which are made of germanium and support high‐quality optical resonances in the mid‐IR spectral range. The concept is applied to observe the resonant enhancement of the third and fifth harmonics generated from the gradient metasurfaces being used in conjunction with a tunable excitation laser to provide a wide spectral coverage of resonantly‐enhanced tunable generation of multiple optical harmonics.
Novel resonance‐gradient metasurfaces are designed and fabricated with varying geometrical parameters that translate into resonant frequencies dependence on one of the coordinates of the metasurface. This general concept is applied to observe the resonant enhancement of the third and fifth harmonics generated from the gradient metasurfaces being used in conjunction with a tunable excitation laser to provide a wide spectral coverage of resonantly‐enhanced tunable generation of multiple optical harmonics.
Being motivated by the recent prediction of high-Q modes in subwavelength dielectric resonators inspired by bound states in the continuum (BIC), we study the second-harmonic generation from isolated ...subwavelength AlGaAs nanoantennas. We reveal that nonlinear effects at the nanoscale can be enhanced dramatically provided the resonator parameters are tuned to the BIC regime. We predict a record-high conversion efficiency for nanoscale resonators that exceeds by 2 orders of magnitude the conversion efficiency observed at the magnetic dipole Mie resonance, thus opening the way for highly efficient nonlinear metasurfaces and metadevices.
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