Backward Second-harmonic Generation Research Articles

Integrated Backward Second-Harmonic Generation through Optically Induced Quasi-Phase-Matching.

Quasi-phase-matching for efficient backward second-harmonic generation requires sub-μm poling periods, a nontrivial fabrication feat. For the first time, we report integrated first-order quasiphase-matched backward second-harmonic generation enabled by seeded all-optical poling. The self-organized grating inscription circumvents all fabrication challenges. We compare backward and forward processes and explain how grating period influences the conversion efficiency. These results showcase unique properties of the coherent photogalvanic effect and how it can bring new nonlinear functionalities to integrated photonics.

Broadband second-harmonic generation based on orientation-patterned II-VI crystals

Broadband second-harmonic generation (SHG) has important applications in many fields, such as femtosecond laser technology, optical communication, and biomedicine. The broadband SHG has been implemented in some crystals; however, it is not clear for forward and backward broadband quasi-phase-matching (QPM) SHGs in orientation-patterned Ⅱ-Ⅵ crystals. In this paper, the characters of orientation-patterned ZnS (OP-ZnS), orientation-patterned ZnSe (OP-ZnSe), and orientation-patterned CdTe (OP-CdTe) are, respectively, studied, including the forward broadband SHG and backward broadband SHG bandwidths at different temperatures. In addition, the influences of temperature on the forward broadband SHG bandwidth are investigated. The forward broadband QPM SHGs of OP-ZnS, OP-ZnSe, and OP-CdTe are obtained, and the backward broadband QPM SHGs cannot be obtained. The influence of temperature on the forward bandwidth is small.

Efficient first-order quasi-phase-matched backward second-harmonic generation.

We demonstrate first-order quasi-phase-matched backward second-harmonic generation (BSHG) with an efficiency of 18.7%. This represents an increase by two orders of magnitude from earlier experiments employing higher-order quasi-phase-matching. The efficient BSHG is demonstrated in bulk periodically poled Rb:KTiOPO4 with a poling period of 317 nm. Using these structures, the frequency doubling in the backward direction is achieved for the fundamental wavelength of 2309 nm. Here we report on the experimental investigation of the BSHG properties such as spectral bandwidth, temperature tuning, and temperature bandwidth by employing broadband and narrowband fundamental wavelength sources. The BSHG properties are compared with those of co-propagating second harmonic generation to reveal the BSHG potential for novel applications that were proposed theoretically but have not been realized in practice so far.

Label-free characterization of collagen fibers in cancerous esophagus tissues using ratiometric nonlinear optical microscopy.

The issue of classifying esophageal cancer at various developmental stages is crucial for determining the optimized treatment protocol for the patients, as well as the prognosis. Precision improvement in staging esophageal cancer keeps seeking quantitative and analytical imaging methods that could augment histopathological techniques. In this work, we used nonlinear optical microscopy for ratiometric analysis on the intrinsic signal of two-photon excited fluorescence (TPEF) and second harmonic generation (SHG) from single collagen fibers only in submucosa of esophageal squamous cell carcinoma (ESCC). The blind tests of TPEF/SHG and forward (F)/backward (B) SHG were demonstrated to compare with the histology conclusion. The discussion of sensitivity and specificity was provided via statistical comparison between the four stages of esophageal cancer. To the best of our knowledge, this is the first study of using these two ratios in combination for staging ESCC.

Quasi-Phase Matched Second Harmonic Generation in Plasmonic–Organic Hybrid Structures

In this article, we propose the design and analyze a plasmonic-organic-hybrid (POH) waveguide structure for efficient forward and backward second harmonic generation (SHG) to bridge the mid-infrared (IR) to near-IR. The required phase matching is satisfied by quasi-phase matching (QPM) technique through periodically altering the material of the metal-insulator-metal slot region between a nonlinear polymer (with second-order nonlinear coefficient of χ (2) ≠ 0) and Al2O3 (with χ (2) = 0) leading to modulation of χ (2) coefficient along the propagation direction. The strong field confinement offered by surface plasmon polaritons (SPPs) significantly enhances the normalized efficiency up to 1.75 × 106 W−1cm−2 in the case of backward SHG over a short device length of 17 μm with only 20 mW input power. Also, a slightly lower efficiency is achieved for forward configuration with broadband response, high fabrication tolerance, and small temperature-variation sensitivity. Furthermore, our proposed structure is based on standard nanofabrication technologies and can be used for all-optical signal processing, low-noise detection of a mid-IR signal in the near-IR windows, and (mirrorless) optical parametric oscillator.

Enhancement of efficiency of second-harmonic generation from MoS2 monolayers in 1D Fibonacci photonic crystals

In this article, the increase of the efficiency of second-harmonic generation (SHG) from MoS2 monolayers embedded in different one-dimensional Fibonacci photonic crystal structures is investigated. The systems contain ZnS, SiO2, and MoS2 layers with the same thickness of ZnS and SiO2 films, which are arranged on the basis of different Fibonacci sequences. The transfer matrix method is used to calculate the forward and backward SHG efficiencies in the undepleted pump approximation. The thicknesses of the ZnS and SiO2 films are changed to obtain the maximum frequency conversion in different structures. Tuning the thicknesses causes the second-harmonic waves generated in each MoS2 monolayer to interfere constructively. Also, the fundamental and second-harmonic wavelengths are both located at the photonic band gap edges, where the density of electromagnetic modes and the nonlinear interaction time are enhanced. Our results show that the SHG efficiencies were increased in quasiperiodic photonic crystals with respect to the periodic structure with the same number of MoS2 layers. In Fibonacci photonic crystal structures there are more geometrical parameters that can be tuned to obtain the highest efficiencies. Furthermore, we used a Bragg mirror at one side of each structure to control the propagation direction of the generated second-harmonic waves and to decrease the backward waves, which led to enhancement of the forward waves.

A numerical investigation of enhanced backward second-harmonic generation in one-dimensional PIM/NIM structure

In this paper, we have demonstrated a numerical investigation of an enhanced backward second-harmonic generation (BSHG) effect in a one-dimensional positive-index material/negative-index material (1D-PIM/NIM) structure with nonlinear deep grating. The 1D-PIM/NIM structure composed of common linear PIM layers and NIM layers, whose electric permittivity and magnetic permeability are described by Drude model for allowing negative refractive index behavior, embedded in nonlinear χ(2) material in periodically arrangement. To model BSHG phenomenon, we have developed a completed set of nonlinear coupled-mode equations (NCMEs) by perturbing nonlinear wave equation by a small factor with appropriate scale following a way of multiple-scale approach (MSA). Then, we have numerically solved the NCMEs to achieve the second-harmonic frequency output and conversion efficiency of backward field. We have also discussed a backward phase-matching (BWPM) condition, which has been satisfied by tuning a fundamental frequency (FF) at ω in a negative refractive index region and second-harmonic (SH) frequency at 2ω in a positive refractive index region, and band-edge local field enhancement condition, which has been created by rearranging PIM and NIM layers in optimal periodic fashion. By using both conditions, a conversion efficiency of BSHG can be dramatically enhanced.

Multimodal nonlinear optical microscopy reveals critical role of kinesin-1 in cartilage development.

We developed a multimodal nonlinear optical (NLO) microscope system by integrating stimulated Raman scattering (SRS), second harmonic generation (SHG) and two-photon excited fluorescence (TPEF) imaging. The system was used to study the morphological and biochemical characteristics of tibial cartilage in a kinesin-1 (Kif5b) knockout mouse model. The detailed structure of fibrillar collagen in the extracellular matrix of cartilage was visualized by the forward and backward SHG signals, while high resolution imaging of chondrocytes was achieved by capturing endogenous TPEF and SRS signals of the cells. The results demonstrate that collagen fibrils in the superficial surface of the articular cartilage decreased significantly in the absence of Kif5b. The distorted morphology along with accumulated intracellular collagen was observed in the Kif5b-deficient chondrocytes, indicating the critical roles of kinesin-1 in the chondrocyte morphogenesis and collagen secretion. The study shows that multimodal NLO imaging method is an effective approach to investigate early development of cartilage.

Use of Second Harmonic Imaging and Fourier Transformation to Analyze Cartilage Repair in Mice Injected with a Novel Peptide CK2.1

Second harmonic generation (SHG) imaging is a powerful imaging tool that allows to study the distribution of the collagen network in cartilage. This imaging modality has been successfully utilized for visualizing physiologic and pathological changes in tissues and organs. However only recently the quantification of SHG images by Fourier Transform analysis allowed for successful quantification of the collagen network in cartilage. Here, we demonstrate that systemic injection of CK2.1, a novel mimetic peptide based on the Bone Morphogenetic Protein (BMP) receptor type I sequence, induced cartilage formation in mice and reassembly of collagen fibers. Intra-articular injections of slow release HGPCK2.1 demonstrated a robust regeneration of the damaged cartilage tissue and the collagen network. Collection of the forward (aligned) and backward (non-aligned) SHG allowed us to quantify the fibrillar distribution in peptide induced cartilage formation and repair. Our data show that the Forward SHG obtained by the collagen structures were greater compared to Backward SHG in CK2.1 in the cartilage of CK2.1 injected mice. These data suggest the cartilage elasticity to withstand stresses placed on the tissue is increased. Our data provide us insights into the peptides capabilities as a future therapeutic agent that can regenerate the lost cartilage to its native form.

Phase matching of backward second harmonic generation assisted by lattice structure in collagen tissues.

Phase matching of backward second harmonic generation (SHG) in a periodic structure of collagen fibrils is investigated through theoretical modeling, simulation, and experiments. The lattice structure of collagen fibrils is considered to provide a virtual momentum for assisting the phase matching of backward SHG. Phase matching over a relatively wide excitation wavelength range is achieved by tilting the angle of the fundamental excitation and SHG wave vectors. The SHG intensity in the periodic structure is simulated to quantify the phase matching effect. The effect of the fundamental excitation and the SHG emission angles on the peak excitation wavelength of the SHG excitation spectrum is further validated in experiments, where the excitation and emission angles are controlled by spatial filtering. It is found that an optimized excitation wavelength exists for a certain collagen fibril structure, which shifts toward a shorter wavelength when the excitation and emission angles are increased. Our results show that the lattice structure of collagen fibrils can assist the phase matching, providing a mechanism for generating backward SHG in multiphoton microscopy.

Simultaneous negative refraction and focusing of fundamental frequency and second-harmonic fields by two-dimensional photonic crystals

Simultaneous negative refraction for both the fundamental frequency (FF) and second-harmonic (SH) fields in two-dimensional nonlinear photonic crystals have been found through both the physical analysis and exact numerical simulation. By combining such a property with the phase-matching condition and strong second-order susceptibility, we have designed a SH lens to realize focusing for both the FF and SH fields at the same time. Good-quality non-near field images for both FF and SH fields have been observed. The physical mechanism for such SH focusing phenomena has been disclosed, which is different from the backward SH generation as has been pointed out in the previous investigations. In addition, the effect of absorption losses on the phenomena has also been discussed. Thus, potential applications of these phenomena to biphotonic microscopy technique are anticipated.

Articular cartilage zonal differentiation via 3D Second-Harmonic Generation imaging microscopy

Purpose: The collagen structure throughout the patella has not been thoroughly investigated by 3D imaging, where the majority of the existing data come from histological cross sections. It is important to have a better understanding of the architecture in normal tissues, where this could then be applied to imaging of diseased states.Methods: To address this shortcoming, we investigated the combined use of collagen-specific Second-Harmonic Generation (SHG) imaging and measurement of bulk optical properties to characterize collagen fiber orientations of the histologically defined zones of bovine articular cartilage. Forward and backward SHG intensities of sections from superficial, middle and deep zones were collected as a function of depth and analyzed by Monte Carlo simulations to extract the SHG creation direction, which is related to the fibrillar assembly.Results: Our results revealed differences in SHG forward–backward response between the three zones, where these are consistent with a previously developed model of SHG emission. Some of the findings are consistent with that from other modalities; however, SHG analysis showed the middle zone had the most organized fibril assembly. While not distinct, we also report bulk optical property values for these different zones within the patella.Conclusions: Collectively, these results provide quantitative measurements of structural changes at both the fiber and fibril assembly of the different cartilage zones and reveals structural information not possible by other microscope modalities. This can provide quantitative insight to the collagen fiber network in normal cartilage, which may ultimately be developed as a biomarker for osteoarthritis.

Improving liver fibrosis diagnosis based on forward and backward second harmonic generation signals

The correlation of forward second harmonic generation (SHG) signal and backward SHG signal in different liver fibrosis stages was investigated. We found that three features, including the collagen percentage for forward SHG, the collagen percentage for backward SHG, and the average intensity ratio of two kinds of SHG signals, can quantitatively stage liver fibrosis in thioacetamide-induced rat model. We demonstrated that the combination of all three features by using a support vector machine classification algorithm can provide a more accurate prediction than each feature alone in fibrosis diagnosis.

Quasi-phase-matching of the dual-band nonlinear left-handed metamaterial

We demonstrate a type of nonlinear meta-atom creating a dual-band nonlinear left-handed metamaterial (DNLHM). The DNLHM operates at two distinct left-handed frequency bands where there is an interval of one octave between the two center frequencies. Under the illumination of a high-power signal at the first left-handed frequency band corresponding to fundamental frequency (FF), second-harmonic generation (SHG) is observed at the second left-handed band. This means that our DNLHM supports backward-propagating waves both at FF and second-harmonic (SH) frequency. We also experimentally demonstrate quasi-phase-matching configurations for the backward SHG. This fancy parametric process can significantly transmits the SH generated by an incident FF wave.

Backward frequency doubling of near infrared picosecond pulses

We report on backward second-harmonic generation using ps laser pulses in congruent lithium niobate with 3.2 µm periodic poling. Three resonant peaks were measured between 1530 and 1730 nm, corresponding to 16th, 17th and 18th quasi-phase-matching orders in the backward configuration, with a conversion efficiency of 4.75 x 10(-5%)/W for the 16th order. We could also discriminate the contributions from inverted domains randomized in duty-cycle.

Polarization-Resolved SHG towards Collagen Imaging

Collagen is a triple-helix protein that self-assembles into supramolecular fibrils, essential for the biomechanical integrity and physical properties of biological tissues and organs. Second Harmonic Generation(SHG) microscopy exploits a second order nonlinear coherent optical process to image macromolecules with non-centrosymmetric structure, as collagen fibers(1). Such a label-free approach provides three-dimensional(3D) imaging of the structural organization of the collagen fibers in tissues at a sub-cellular resolution with intrinsic optical sectioning, high deep penetration and reduction of out-of-focus photobleaching and phototoxicity(2). Here, we collected the endogenous SHG signal of the collagen to evaluate the hierarchical structure, organization and orientation of collagen fibers in different types of specimen, from ex-vivo biological tissues to bioengineered samples(3). To conduct this study, we coupled a femtosecond Ti:Sapphire Chameleon Ultra II laser(Coherent, Santa Clara, CA, USA) with a Leica TCS STED-CW confocal microscope(Leica-Microsystems, Mannheim, Germany). This allowed collecting the forward, the backward SHG signal and the TPE autofluorescence of the samples(2). Moreover, we also focused our attention on the SHG signal as function of polarization states of the incoming light to distinguish among different collagen fiber types and orientations(4). Analysis of the polarization of the outcoming light and the ratio FSHG/BSHG was carried out due to its potential in providing more detailed information about the sample, for instance a relationship with the diameter and the functional and structural organization of the collagen fibers variability in various tissues. For example, this allows realizing a set of features useful to evaluate the assembly of artificial tissues depending on the scaffold role in the collagen fibers assembly when mimicking a target tissue.1. Chen X. et al. Nat Protoc. 7(4),654(2012).2. Bianchini P. et al., J. Biophotonics, 1,6(2008).3. Scaglione S. et al. Nature Sci. Rep. 2,(274): 1-6(2012).4. Odin C. et al, Opt.Express, 16,20(2008).

Quantifying the backscattering of second harmonic generation in tissues with confocal multiphoton microscopy

The backward second harmonic generation (SHG) in mouse tissues is studied with a confocal multiphoton microscopy system. The total backward collected SHG (B-SHG) consists of the backward generated SHG and the backward-scattered forward-generated SHG (BS-SHG), which can be modeled by a Gaussian and a uniform distribution, respectively, at the confocal pinhole plane. By varying the pinhole size with a series of collection fibers, the proportion of the BS-SHG to the B-SHG and the proportion of BS-SHG to the forward generated SHG can be obtained. The approach is first validated by Monte Carlo simulation. It is then applied to two types of mouse tissues: mouse tail tendon and Achilles tendon. It is found that the BS-SHG contributes less to the B-SHG for the tail tendon than Achilles tendon with thicknesses of ~300 μm. With the thickness of the Achilles tendon tissue increased to 1000 μm but the focal plane kept at the same depth, as high as ~10% of the total forward SHG is backscattered and collected. The results indicate that BS-SHG may not be the major source of B-SHG in the tail tendon, but it may be the major source in the Achilles tendon. These methods and results provide a noninvasive method and supporting information for investigating the generation mechanism of SHG and help with optimizing backward SHG microscopy and spectroscopy measurements.

Quasi-phase-matched backward second-harmonic generation by complementary media in nonlinear metamaterials

High efficiency of the second-harmonic and sum-frequency generation can be obtained in optical superlattice by using the conventional quasi-phase-matched (QPM) method. Although this trick can be played on the acoustic wave, the media with negative nonlinear parameters are not common in acoustics. Furthermore, the QPM method used in acoustic metamaterials has been less studied. In this work, a protocol is provided to realize the QPM method by using nonlinear complementary media in acoustic metamaterials in order to obtain large backward second-harmonic generation. Compared with the conventional method, the method gains a broader bandwidth and can be used in both acoustic and electromagnetic waves.

From molecular structure to tissue architecture: collagen organization probed by SHG microscopy

Second-harmonic generation (SHG) microscopy is a fantastic tool for imaging collagen and probing its hierarchical organization from molecular scale up to tissue architectural level. In fact, SHG combines the advantages of a non-linear microscopy approach with a coherent modality able to probe molecular organization. In this manuscript we review the physical concepts describing SHG from collagen, highlighting how this optical process allows to probe structures ranging from molecular sizes to tissue architecture, through image pattern analysis and scoring methods. Starting from the description of the most relevant approaches employing SHG polarization anisotropy and forward - backward SHG detection, we then focus on the most relevant methods for imaging and characterizing collagen organization in tissues through image pattern analysis methods, highlighting advantages and limitations of the methods applied to tissue imaging and to potential clinical applications.

Fast Fourier Transform–Based Analysis of Second-Harmonic Generation Image in Keratoconic Cornea

To qualify and quantify structural alterations in keratoconic corneas ex vivo by the use of second-harmonic generation (SHG) microscopy and two-dimensional fast Fourier transform (2D-FFT) analysis. Two keratoconic and three normal human corneal specimens were imaged with a multiphoton microscope. Forward and backward second-harmonic generation images (FSHG and BSHG) of corneal stroma were obtained at high resolution and at different depths. The SHG images were further analyzed with a 2D-FFT algorithm to quantify the texture and orientation changes of collagen fibers. The results showed that the abnormality of collagen architecture was found through the whole layer of stroma. 2D-FFT analysis of SHG cornea images provided fiber orientation direction and an aspect ratio (AR) as a quantitative measure of fiber direction determination. It was found that for keratoconic cornea the average AR values are statistically greater than those of the normal cornea: 1.66 ± 0.42 (case I keratoconic cornea), 1.72 ± 0.44 (case II keratoconic cornea), and 1.34 ± 0.16 (average of three normal human corneas). Furthermore, the fiber directions determined by 2D-FFT analysis of BSHG and FSHG images were strongly correlated for large AR values (AR > 2). The high correlation between FSHG and BSHG fiber direction for large AR values shows that BSHG imaging can provide qualitative and quantitative information of the extent of structural changes in a keratoconic cornea. The ability to use BSHG for diagnosing and monitoring stroma abnormalities, even when cornea transparency is retained, demonstrates the clinical potential of this method: