Chiral Second Harmonic Generation Research Articles

Electrically switchable chiral nonlinear optics in an achiral ferroelectric 2D van der Waals halide perovskite.

Two-dimensional (2D) van der Waals (vdW) semiconductors play a key role in developing nanoscale nonlinear optical devices. 2D Ruddlesden-Popper lead halide perovskites (RPPs) expand the potential of using 2D vdW semiconductors in nonlinear optical applications because they exhibit electrically switchable and chiral second-order optical nonlinearity originating from the emergence of ferroelectricity and chirality. However, electrically switchable chiral nonlinear optics has not yet been realized because of the difficulty in electrically manipulating chiral structures. Here, we demonstrate that chiral second-harmonic generation (SHG) can be electrically induced and switched in an achiral biaxial ferroelectric 2D RPP. We observe reversible and continuous electrical switching of SHG circular dichroism and large nonlinear chiroptical activity. Polarization-resolved SHG imaging reveals that electrical poling induces the ferroelectric multidomain structure arising from the biaxial nature of the material, and the planar chirality appears. Our findings show a simple electrical control of the nonlinear chiroptical responses and establish chiral nonlinear optics based on ferroelectric 2D RPPs.

In situ nonlinear optical spectroscopic study of the structural chirality in DPPC Langmuir monolayers at the air/water interface.

We conduct a molecular study on the structural chirality in Langmuir monolayers composed of dipalmitoylphosphatidylcholine (DPPC) using in situ nonlinear optical spectroscopies, including second harmonic generation (SHG) and sum frequency generation (SFG). Chiral SHG response is observed from L-DPPC monolayers at moderate surface pressures and almost vanishes at a high surface pressure. SFG spectra of L-DPPC monolayers show chiral features that can be assigned to the terminal CH3 groups and the CH2 groups attached to the chiral center atom. This means that these achiral moieties form chiral superstructures at the interface. Along with increasing surface pressure, the structural chirality of CH3 groups shows a similar trend as that of chiral SHG, but CH2 chirality increases monotonically. Furthermore, in a racemic DPPC monolayer with a moderate surface pressure, both chiral SHG and chiral SFG of CH3 groups are absent, whereas chiral SFG of CH2 groups is clearly present, indicating that L- and D-DPPC are diastereomers at the air/water interface and interfacial CH2 prefers a certain orientation regardless of the molecular handedness. A molecular mechanism is proposed to explain the origin of the structural chirality in DPPC monolayers.

High spectral contrast chiral second-harmonic generation from multilayer gallium selenide coupled to silicon holey disk metasurfaces

We present electromagnetic design and analysis of hybrid metasurfaces composed of multilayer gallium selenide (GaSe) coupled to silicon holey disk arrays for achieving high spectral contrast chiral second-harmonic generation (SHG). The silicon holey disk structures are designed to support electromagnetically induced transparency (EIT)-like optical resonances in the 1.5–1.8 µm wavelength range in the vicinity of electric and toroidal dipole scattering modes. The fundamental electric field enhancement above the silicon structures shows right-circularly polarized (RCP)- and left-circularly polarized (LCP)-like characteristics at distinct wavelengths resulting in two prominent peaks for the LCP and RCP resolved SHG from the GaSe layer above the holey disks. This results in high contrast SHG degree of circular polarization spanning − 1 to + 1 over the fundamental excitation spectral range considered. The chiral SHG response is also found to be strongly influenced by the unit-cell lattice arrangement with square or hexagonal lattice exhibiting very different chiral SHG response. This observation is consistent with the circular polarization state selection rules for the harmonic generation process considering the rotational symmetry of the underlying metasurface lattice arrangement. Two-dimensional (2D) material–dielectric resonant metasurface hybrid systems exhibit nonlinear optical polarization selection rules, which differ from the native 2D material, making them an interesting platform for realizing engineered chiral nonlinear photonic devices.

Chiral Second-Harmonic Generation from Monolayer WS2/Aluminum Plasmonic Vortex Metalens.

Two-dimensional spiral plasmonic structures have emerged as a versatile approach to generate near-field vortex fields with tunable topological charges. We demonstrate here a far-field approach to observe the chiral second-harmonic generation (SHG) at designated visible wavelengths from a single plasmonic vortex metalens. This metalens comprises an Archimedean spiral slit fabricated on atomically flat aluminum epitaxial film, which allows for precise tuning of plasmonic resonances and subsequent transfer of two-dimensional materials on top of the spiral slit. The nonlinear optical measurements show a giant SHG circular dichroism. Furthermore, we have achieved an enhanced chiral SHG conversion efficiency (about an order of magnitude greater than the bare aluminum lens) from monolayer tungsten disulfide (WS2)/aluminum metalens, which is designed at the C-exciton resonance of WS2. Since the C-exciton is not a valley exciton, the enhanced chiral SHG in this hybrid system originates from the plasmonic vortex field-enhanced SHG under the optical spin-orbit interaction.

Significant Chiral Signal Amplification of Langmuir Monolayers Probed by Second Harmonic Generation.

With the development of the nonlinear optical technique such as SHG (second harmonic generation), the in situ measurements of the chirality in the monolayers at the air/water interface have become possible. However, when performing the SHG measurement of the chirality in a monolayer, it is still a great challenge to obtain the chiral signals with a good S/N (signal-to-noise) ratio. In this Letter, interfacial assemblies with induced supramolecular chirality were used to amplify the weak chiral SHG signals from the monolayers at the air/water interface. Tetrakis(4-sulfonatophenyl) porphyrin (TPPS) J aggregates were used as the subphase, and when chiral amphiphilic molecules were spread on it, chiral domains of the amphiphile/TPPS J aggregates were formed and then significantly amplified chiral signals that otherwise could not be detected. Moreover, the sign of the DCE (degree of chiral excess) changed with the chirality of the amphiphilic molecules, thus providing a possible way to obtain the absolute chiral information in situ in the monolayers.

Probing chiral monolayers of cysteine on Au(110) using reflection anisotropy spectroscopy and second-harmonic generation

Chiral synthesis is an important route to biologically useful chemicals. Linear optical techniques like circular dichroism are too weak to detect chirality in monolayers of small molecules at surfaces, unless the surface area is very high and, in addition, the chiral response is annulled in reflection geometry. Chiral second-harmonic generation (SHG) from monolayer quantities of larger, more polarizable organic molecules has been detected at silica surfaces, where the presence of electronic resonances enhances the signal. Non-resonant chiral SHG has also been reported from multilayers of cysteine adsorbed on silica and silicon surfaces, where the strength of the response indicated potential monolayer sensitivity. Here, the chiral response of cysteine monolayers adsorbed on atomically clean, well-ordered Au(110) under ultra-high vacuum conditions is explored. Reproducible differences in the linear and nonlinear optical response from (R)-(+)-cysteine and (S)-(−)-cysteine monolayers are observed. Possible origins of the differences are discussed. Reflection anisotropy spectroscopy and SHG, in principle, provide complementary information on chiral structures at surfaces and interfaces and have potential technological application in the area of heterogeneous enantioselective catalysis.

Chiral second‐harmonic generation from small organic molecules at surfaces

AbstractThe molecules of life are chiral. Advanced materials for heterogeneous enantioselective catalysis and biomolecular sensing and recognition will need nanoscale chiral structures, and methods of characterizing them. Optical circular dichroism (CD) is the main technique for probing chiral molecules in solution. Amino acids are levorotatory, but CD cannot detect chirality in ultrathin films of these small molecules at surfaces, as the response is too weak. Chiral second‐harmonic generation (SHG) from ultrathin layers of larger, more polarizable organic molecules has been detected at silica surfaces, where the presence of electronic resonances enhanced the signal. Here chiral SHG is reported from ultrathin films of 1,1′‐binaphthalene‐2,2′‐diol (binol) and cysteine. The SH response from the different chiral enantiomers was measured away from resonance, using unamplified femtosecond excitation at 800 nm, by rotating a quarter‐wave plate polarizer, and detecting the p‐polarized SH output. The SH response from the molecules is measurable and differs significantly between the enantiomers. Simultaneous fitting of the polarizer rotation plots, using a simple phenomenological model, produces reasonable agreement with the experimental results. This exploratory work shows that SHG has sufficient sensitivity to detect the chirality of small molecules adsorbed on surfaces.

Using the Intrinsic Chirality of a Molecule as a Label-Free Probe to Detect Molecular Adsorption to a Surface by Second Harmonic Generation

Chiral second harmonic generation (C-SHG) has been used for the label-free detection of (R)-(+)-1,1'-bi-2-naphthol (RBN) and (S)-(+)-1,1'-bi-2-naphthol (SBN) binding to planar-supported lipid bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphotidylcholine (POPC) based on the intrinsic chirality of the molecules. C-SHG adsorption isotherms of RBN and SBN reveal Langmuir adsorption behavior with binding constants of 2.7 +/- 0.2 x 10(5) M(-1) and 3.0 +/- 0.1 x 10(5) M(-1), respectively. The kinetics of RBN binding to a POPC bilayer was also measured. It was determined that the adsorption rate for RBN was 5.7 +/- 0.4 x 10(3) s(-1)M(-1) and the desorption rate was 2.1 +/- 0.8 x 10(-2) s(-1). From the kinetic data a binding constant of 2.7 +/- 1.0 x 10(5) M(-1) was calculated, which agrees well with the thermodynamic measurement. The C-SHG technique was correlated with surface tension measurements in order to determine the RBN surface excess within the POPC membrane. The maximum surface excess of RBN in a monolayer of POPC was 4.3 +/- 0.5 x 10(-11) mol cm2. Using the maximum surface excess in conjunction with the C-SHG binding data a lower limit of detection of 1.5 +/- 0.1 x 10(-13) mols cm(-2) was calculated. The results of these studies show that C-SHG is a powerful tool for the study of chiral molecular interactions at surfaces.

Imaging Chirality with Surface Second Harmonic Generation Microscopy

Chirality is a fundamental construct in nature which arises from an antisymmetric arrangement of atoms, molecules, or larger structures, resulting in the formation of nonsuperimposable mirror images. Bulk chiral effects can easily be measured using circular dichroism (CD) or optical rotary dispersion (ORD). However, the imaging of chirality originating from molecular surface films cannot be obtained with these linear optical methods. By using chiral second harmonic generation (C-SHG), with its inherent surface sensitivity and ability to discriminate between the symmetry of surface adsorbed species in combination with a counter-propagating optical geometry, we have developed the first nonlinear chiral microscope. In the study presented here, the intrinsic chirality of R- and S-(+)-1,1'-bi-2-naphthol (RBN, SBN) has been used to image a patterned planar supported lipid bilayer (PSLB) of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) using C-SHG. Spatial resolution of the patterned PSLB is visible when either RBN or SBN is intercalated into the membrane. No image is observed when a racemic mixture of RBN and SBN is present. The C-SHG images are compared with those obtained from fluorescence microscopy to verify the C-SHG imaging technique. The results presented here demonstrate that C-SHG possesses the requisite surface selectivity and sensitivity to detect interfacial chirality and provides a direct route for the visualization of chirality originating from molecular surface films.

Counterpropagating second-harmonic generation: a new technique for the investigation of molecular chirality at surfaces

The theoretical and experimental investigation of a new chiral second-harmonic generation technique that utilizes a counterpropagating optical geometry was conducted. The counterpropagating optical geometry employed here can effectively separate the chiral and achiral contributions to the SH emission, which cannot be easily accomplished under a copropagating geometry. The technique was applied to an experimental investigation of the molecular adsorption of (R)-(+)-1, 1′-bi-2-naphthol to a planar-supported lipid bilayer of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphotidylcholine. A strong chiral second-harmonic generation response was observed when a single enantiomer intercalated into the membrane, but showed no chiral response when equal concentrations of the enantiomers were present in the bilayer.

Label-free chiral detection of melittin binding to a membrane.

The study presented here describes an innovative approach for the detection of surface-confined proteins using chiral second harmonic generation (C-SHG). A unique optical geometry has been employed which allows for the separation of the chiral and achiral nonlinear response. By utilizing this optical arrangement, the detection of chirality originating from melittin adsorbed to a planar supported lipid bilayer has been performed for the first time by C-SHG. Melittin binding to the membrane was monitored as a function of bulk concentration through detection of the C-SHG signal. Analysis of the C-SHG adsorption isotherms reveals Frumkin adsorption behavior with a positive interaction energy. The binding constant (Ka) obtained was determined to be (8.3 +/- 1.0) x 105 M-1. The results of these studies have far-reaching implication in the use of C-SHG for the label-free detection of protein association to surfaces and in the analysis of protein interfacial phenomena.