Conventional Second Harmonic Generation Research Articles

Second-Harmonic-Generation Effect and Giant Optical Birefringence in the Weyl Material CaAgAs.

Nonlinear-optical (NLO) materials suitable for the "8-14 μm" atmospheric transparent window are in urgent need. Arsenide is one of the most promising material systems for such an application. However, the second-harmonic-generation (SHG) effect of arsenide is difficult to characterize using the conventional powder SHG technique with a 2 μm fundamental laser. To overcome this problem, we focused on a novel arsenide Weyl material, CaAgAs, with a zero band gap and proposed a modified powder SHG measurement method for narrow-band-gap materials. We successfully observed the SHG signal of CaAgAs, which was approximately 0.7 times that of CdGeAs2. Moreover, on the basis of first-principles calculations, the largest SHG coefficient for CaAgAs is equal to 236 pm/V, and the ionic bonds in the [Ca3As13] motif play an indispensable role for the SHG effect because of the distorted Kagome lattice pattern. CaAgAs is predicted to have strong optical anisotropy with a giant optical birefringence of 0.52.

Superresolved polarization-enhanced second-harmonic generation for direct imaging of nanoscale changes in collagen architecture.

Superresolution (SR) optical microscopy has allowed the investigation of many biological structures below the diffraction limit; however, most of the techniques are hampered by the need for fluorescent labels. Nonlinear label-free techniques such as second-harmonic generation (SHG) provide structurally specific contrast without the addition of exogenous labels, allowing observation of unperturbed biological systems. We use the photonic nanojet (PNJ) phenomena to achieve SR-SHG. A resolution of with respect to the fundamental wavelength, that is, a -fold improvement over conventional or diffraction-limited SHG under the same imaging conditions is achieved. Crucially we find that the polarization properties of excitation are maintained in a PNJ. This is observed in experiment and simulations. This may have widespread implications to increase sensitivity by detection of polarization-resolved SHG by observing anisotropy in signals. These new, to the best of our knowledge, findings allowed us to visualize biological SHG-active structures such as collagen at an unprecedented and previously unresolvable spatial scale. Moreover, we demonstrate that the use of an array of self-assembled high-index spheres overcomes the issue of a limited field of view for such a method, allowing PNJ-assisted SR-SHG to be used over a large area. Dysregulation of collagen at the nanoscale occurs in many diseases and is an underlying cause in diseases such as lung fibrosis. Here we demonstrate that pSR-SHG allows unprecedented observation of changes at the nanoscale that are invisible by conventional diffraction-limited SHG imaging. The ability to nondestructively image SHG-active biological structures without labels at the nanoscale with a relatively simple optical method heralds the promise of a new tool to understand biological phenomena and drive drug discovery.

Second harmonic generation microscopy using pixel reassignment.

Second harmonic generation (SHG) microscopy is expected to be a powerful tool for observing the cellular-level functionality and morphology information of thick tissue owe to its unique imaging properties. However, the maximum attainable resolution obtainable by SHG microscopy is limited by the use of long-wavelength, near-infrared excitation. In this paper, we report the use of pixel reassignment to improve the spatial resolution of SHG microscopy. The SHG signal is imaged onto a position-sensitive camera, instead of a point detector typically used in conventional SHG microscope. The data processing is performed through pixel reassignment and subsequent deblurring operation. We present the basic principle and a rigorous theoretical model for SHG microscopy using pixel reassignment (SHG-PR). And for the first time, the optimal reassignment factor for SHG-PR is derived based on the coherent characteristics and the dependence of wavelength in SHG microscopy. To evaluate the spatial resolution improvement, images of nano-beads separated by different distances and of a microtubule array have been simulated. We gain about a 1.5-fold spatial resolution enhancement compared to conventional SHG microscopy. When a further deblurring operation is implemented, this method allows for a total spatial resolution enhancement of about 1.87. Additionally, we demonstrate the validity of SHG-PR for raw data with noise. LAY DESCRIPTION: Second harmonic generation (SHG) microscopy has emerged as a powerful imaging technique in clinical diagnostics and biological research. SHG microscopy is label-free and provides intrinsic optical sectioning for three-dimensional (3D) imaging. However, a near-infrared excitation wavelength results a restriction in the maximum attainable spatial resolution of SHG microscopy. In this paper, we present a simple resolution-enhanced SHG imaging method, SHG microscopy using pixel reassignment (SHG-PR). We demonstrate a rigorous theoretical model for SHG-PR and derive the optimal reassignment factor. The simulation result shows the clear improvement of the image resolution and contrast in the SHG-PR after deblurring operation. The FWHM value of single microtubule shows that SHG-PR enables a spatial resolution enhancement by a factor of 1.5, compared to conventional SHG microscopy. After a proper deblurring operation, this method allows for a total spatial resolution enhancement of about 1.87. The improvements of spatial resolution and contrast are still valid for raw data with noise. It is expected that this method can contribute towards new insights in unstained tissue morphology, interaction of cells, and diseases diagnosis.

Harmonic optical tomography of nonlinear structures

Second-harmonic generation microscopy is a valuable label-free modality for imaging non-centrosymmetric structures and has important biomedical applications from live-cell imaging to cancer diagnosis. Conventional second-harmonic generation microscopy measures intensity signals that originate from tightly focused laser beams, preventing researchers from solving the scattering inverse problem for second-order nonlinear materials. Here, we present harmonic optical tomography (HOT) as a novel modality for imaging microscopic, nonlinear and inhomogeneous objects. The HOT principle of operation relies on inter-ferometrically measuring the complex harmonic field and using a scattering inverse model to reconstruct the three-dimensional distribution of harmonophores. HOT enables strong axial sectioning via the momentum conservation of spatially and temporally broadband fields. We illustrate the HOT operation with experiments and reconstructions on a beta-barium borate crystal and various biological specimens. Although our results involve second-order nonlinear materials, we show that this approach applies to any coherent nonlinear process.

Phase-Modulated Degenerate Parametric Amplification Microscopy

Second-order nonlinear optical interactions, including second-harmonic generation (SHG) and sum-frequency generation (SFG), can reveal a wealth of information about chemical, electronic, and vibrational dynamics at the nanoscale. Here, we demonstrate a powerful and flexible new approach, called phase-modulated degenerate parametric amplification (DPA). The technique, which allows for facile retrieval of both the amplitude and phase of the second-order nonlinear optical response, has many advantages over conventional or heterodyne-detected SHG, including the flexibility to detect the signal at either the second harmonic or fundamental field wavelength. We demonstrate the capabilities of this approach by imaging multigrain flakes of single-layer MoS2. We identify the absolute crystal orientation of each MoS2 domain and resolve grain boundaries with high signal contrast and sub-diffraction-limited spatial resolution. This robust all-optical method can be used to characterize structure and dynamics in organic and inorganic systems, including biological tissue, soft materials, and metal and semiconductor nanostructures, and is particularly well-suited for imaging in media that are absorptive or highly scattering to visible and ultraviolet light.

Characteristic measurement for femtosecond laser pulses using a GaAs PIN photodiode as a two-photon photovoltaic receiver

Photodiodes that exhibit a two-photon absorption effect within the spectral communication band region can be useful for building an ultra-compact autocorrelator for the characteristic inspection of optical pulses. In this work, we develop an autocorrelator for measuring the temporal profile of pulses at 1550 nm from an erbium-doped fiber laser based on the two-photon photovoltaic (TPP) effect in a GaAs PIN photodiode. The temporal envelope of the autocorrelation function contains two symmetrical temporal side lobes due to the third order dispersion of the laser pulses. Moreover, the joint time-frequency distribution of the dispersive pulses and the dissimilar two-photon response spectrum of GaAs and Si result in different delays for the appearance of the temporal side lobes. Compared with Si, GaAs displays a greater sensitivity for pulse shape reconstruction at 1550 nm, benefiting from the higher signal-to-noise ratio of the side lobes and the more centralized waveform of the autocorrelation trace. We also measure the pulse width using the GaAs PIN photodiode, and the resolution of the measured full width at half maximum of the TPP autocorrelation trace is 0.89 fs, which is consistent with a conventional second-harmonic generation crystal autocorrelator. The GaAs PIN photodiode is shown to be highly suitable for real-time second-order autocorrelation measurements of femtosecond optical pulses. It is used both for the generation and detection of the autocorrelation signal, allowing the construction of a compact and inexpensive intensity autocorrelator.

Electrically controlled second-harmonic generation in silicon-compatible plasmonic slot waveguides: a new modulation scheme.

The possible realization of an active electro-optical control of the nonlinear second-harmonic generation (SHG) mechanism in a plasmonic slot waveguide is theoretically investigated. Both the conventional SHG and the electrically induced SHG are taken into account with a moderate pump power of 40 mW at the fundamental wavelength (1550 nm). The generated power of the second-harmonic frequency can be modulated by the applied voltage in a quadratic and almost linear form for centrosymmetric and noncentrosymmetric nonlinear polymers integrated in the slot, respectively. Converted power up to 140 μW within a short distance of only 16 μm is predicted for a voltage of 10 V. This mechanism may open a new route to realize high-speed advanced modulations or inversely to detect ultrafast electrical signals.

Wavelength conversion of incoherent light by sum-frequency generation.

In this paper, we reveal that some kinds of optical nonlinearities are further enhanced when incoherent light, instead of a laser, is used as a pump light. This idea was confirmed both theoretically and experimentally in the case of sum-frequency generation (SFG) using the optical second nonlinearity. The conversion efficiency of the SFG with incoherent light pumping increased as the bandwidth of the incoherent pump light decreased, finally reaching twice the conversion efficiency of conventional second harmonic generation (SHG) by laser pumping. This method dramatically relaxes the severe requirements of phase matching in the nonlinear optical process. The conversion efficiency became less sensitive to misalignment of the wavelength of pump light and also of device operation temperature when the bandwidth of the incoherent pump light was sufficiently broad, although the improvement of the conversion efficiency had an inverse relationship with the insensitivity to the phase-matching condition. The temperature tuning range was enhanced by more than two orders of magnitude in comparison with the conventional SHG method. As an example of a promising application of this new idea, we performed the generation of quantum entangled photon-pairs using cascaded optical nonlinearities (SFG and the subsequent spontaneous parametric down conversion) in a single periodically poled LiNbO3 waveguide device, in which the incoherent light was used as the pump source for both the parametric processes. We have achieved high fidelity exceeding 99% in quantum-state tomography experiments.

Pulse splitter-based nonlinear microscopy for live-cardiomyocyte imaging.

Second harmonic generation (SHG) microscopy is a new imaging technique used in sarcomeric-addition studies. However, during the early stage of cell culture in which sarcomeric additions occur, the neonatal cardiomyocytes that we have been working with are very sensitive to photodamage, the resulting high rate of cell death prevents systematic study of sarcomeric addition using a conventional SHG system. To address this challenge, we introduced use of the pulse-splitter system developed by Na Ji et al. in our two photon excitation fluorescence (TPEF) and SHG hybrid microscope. The system dramatically reduced photodamage to neonatal cardiomyocytes in early stages of culture, greatly increasing cell viability. Thus continuous imaging of live cardiomyocytes was achieved with a stronger laser and for a longer period than has been reported in the literature. The pulse splitter-based TPEF-SHG microscope constructed in this study was demonstrated to be an ideal imaging system for sarcomeric addition-related investigations of neonatal cardiomyocytes in early stages of culture.

Low power autocorrelation technique based on the degree-of-polarization measurement

Based on the degree-of-polarization (DOP) measurement, an autocorrelation technique to measure the pulse width of chirp-free optical short pulse with a very low optical power (<−60 dBm) is demonstrated. The impacts of chirp and misalignment of the input light are also discussed in detail. The pulse width measurement results are consistent with the conventional second-harmonic generation (SHG) autocorrelation, while our autocorrelation technique does not require high input power or rigid optical alignment.

Nondestructive quality evaluation of periodically poled lithium niobate crystals by diffraction

Quasi-phase-matching devices are usually fabricated by electric field poling over photolithographically defined electrode patterns on ferroelectric crystal substrates. For the optimal nonlinear optical performance of such devices, the micro-poled domain structure must ensure good fidelity to the designed grating structure. We present a nondestructive diffraction method to evaluate the quality of periodically poled lithium niobate crystals, by utilizing index modulation caused by the internal field effects. Our proposed method is much simpler than the conventional second-harmonic generation experiment, and provides a fast, low-cost but accurate means for micro-poling quality evaluation.

Terahertz-field-induced second-harmonic generation in a beta barium borate crystal and its application in terahertz detection

We report the observation of terahertz-field-induced second-harmonic (TFISH) generation in a beta barium borate (β-BBO) crystal based on a four-wave-mixing process. Using the conventional second-harmonic generation in the β-BBO crystal as a local oscillator, the crossed term between the TFISH and the local oscillator is proportional to the terahertz field. This phenomenon is used for coherent detection of intense terahertz waves, where a good linear response is observed for terahertz fields up to 160 kV/cm. In comparison, deviation from linear response and strong waveform distortion are observed when a 0.5-mm-thick ZnTe detector is used to characterize the same terahertz field.

Ex vivo and in vivo second-harmonic-generation imaging of dermal collagen fiber in skin: comparison of imaging characteristics between mode-locked Cr:forsterite and Ti:sapphire lasers

Second-harmonic-generation (SHG) microscopy is an interesting new tool for observing dermal collagen fiber in skin. However, conventional SHG microscopy using a mode-locked Ti:sapphire laser suffers from low penetration depth and a slow image acquisition rate caused by scattering and absorption in tissue, making it difficult to use for in vivo applications on human skin. We develop an SHG microscope equipped with a mode-locked Cr:forsterite laser with a long wavelength and compare its imaging characteristics with that of a Ti:sapphire-laser-based SHG microscope for the measurement of dermal collagen fiber in animal and human skins. The results indicate the suitability of the Cr:forsterite laser-based SHG microscope for in vivo imaging of human skin.

Giant enhancement of luminescence induced by second-harmonic surface plasmon resonance

Strong ultraviolet luminescence having intensity comparable with device-quality GaN epifilms has been observed in Au nanoparticles. It is identified that the luminescence involves radiative recombination of electrons in band 6 (sp conduction band) with holes in band 4 (secondary top d band), near the L symmetry point. We show that the strong emission is a consequence of the second-harmonic surface plasmon resonance (SHSPR), which is an inherent nature of metallic nanoparticles with high density of surface plasmons. The newly discovered SHSPR is very different from the conventional second-harmonic generation (SHG). In the conventional SHG, it requires an intense incident laser to generate two-photon absorption and radiation. However, for the SHSPR discussed here, we only need a weak pumping source to trigger the second-harmonic absorption of very dense surface plasmons in Au nanoclusters. In addition to Au nanoparticles, we demonstrate that SHSPR provides a very efficient way to enhance the luminescence of a material incorporated with metal nanoparticles. As an example, incorporation of Au nanoparticles into SiO2 nanoparticles can enhance the luminescence intensity by two orders of magnitude. We thus point out that SHSPR can serve as one of the underlying mechanisms responsible for surface-enhanced nonlinear optical phenomena.

Measurement of electric field by interferometric spectral trace observation

We present a new methodology that obtains, in an analytical way, the complex electric field of ultrashort pulses. This methodology is based only on Fourier analysis of the frequency components of spectrally resolved interferometric collinear autocorrelations. We present an experimental demonstration of this technique and the results are compared with the conventional second-harmonic generation frequency-resolved optical gating technique.

Comment on “Molecular orientation determined by second-harmonic generation: Self-assembled monolayers”

A recent paper by Eisert et al. [Phys. Rev. B 58, 10 860 (1998)] describes a second-harmonic generation (SHG) technique for probing molecular orientation in self-assembled monolayers (SAMs). Their results have raised some questions about the frequently used description of the optical properties of thin films. In their paper, the SAM film is considered anisotropic for SHG, but is assumed to be isotropic in terms of its linear optical response. It is likely that these different treatments of the same system have contributed to the unusual perspectives developed in the above paper. The purpose of this communication is to attempt a clarification of these observations. Here, a phenomenological model is outlined that treats linear and nonlinear optics of the interface in a single framework and can be used to analyze SHG from SAMs. It is also shown that, depending on the experimental system, conventional SHG measurements of phase and intensity may not be enough to determine the orientation of SAMs.

Second harmonic generation of dipolar dendrons in the assembled thin films

Dendritic macromolecules, called ‘dendrons’, having a branching structure modified with an electron donor/acceptor functionalized azobenzene chromophore have been synthesized. The structurally different dendrons possess a different number, 1, 3, 7, and 15, of chromophore units. The second harmonic generation (SHG) of dendrons was measured in the molecular assembled thin films prepared by the Langmuir–Blodgett (LB) transfer technique. The SHG results indicated that dendrons could be highly organized in these films, where film structure and molecular orientation were analogous to those in the conventional SHG active LB films. The SHG intensity for these molecular organized thin films revealed that the individual chromophores in dendrons were oriented non-centrosymmetrically and coherently contributed to the increase in their molecular hyperpolarizability. The SHG activity became larger as the number of branching units of dendrons increased. The highest molecular hyperpolarizability was found to be 4000×10−30 esu for a dendron containing 15 chromopholic units. The SHG result suggested that the structure of dendrons synthesized was uniaxally dipolar, and that there was a particular merit in this structure for the generation of SHG.

Enhancement of Second-Harmonic Generation in Helicoidal Distributed Feedback Structure of a Ferroelectric Liquid Crystal Using Two Counter-Propagating Fundamental Waves

Second-harmonic generation (SHG) was observed in an unwound ferroelectric smectic C* liquid crystal using two counter-propagating fundamental waves. A remarkable enhancement occurred when SH light wavelength approached to the optical pitch of the helicoid. This enhanced signal based on a helicoidal distributed feedback action is proportional to the fourth power of the sample thickness in thick cells, and becomes larger compared with the conventional phase-matched SHG signal in relatively thick samples. The crossover points of the SH intensities in the two phase matching types shift toward thinner cell thickness in materials with a larger dielectric anisotropy.

Surface order and symmetry determined by second-harmonic microscopy

Second-harmonic microscopy (SHM) is a fast, non-contact, surface-sensitive imaging technique for the quantitative in situ characterisation of a large variety of real interfaces. In conventional second-harmonic generation (SHG) experiments the surface symmetry and order averaged over the illuminated area are obtained. In contrast, SHM allows us to measure the susceptibility of any feature resolved in the images. The local susceptibility can be interpreted in terms of local molecular order and symmetry. The resolution limit of the stigmatic microscope is Abbe's limit for the frequency-doubled light.

Measurement of ultrashort optical pulses by third-harmonic generation

Interferometric and intensity autocorrelations by third-harmonic generation for the measurement of ultrashort optical pulses are demonstrated. The third-harmonic signal is generated by tight focusing of the ultrashort pulses upon the surface of an ordinary glass slide, replacing the high-optical-quality nonlinear crystals used in conventional second-harmonic generation measurements. The method is shown to be applicable to real-time measurements.