High Third-order Optical Nonlinearity Research Articles

Rapid thermal annealing of chalcogenide thin films for mid-infrared sensing and nonlinear photonics

The influence of rapid thermal annealing (RTA) onto chalcogenide Ge-Sb-Se thin films is reported, focusing on changes in optical properties. These materials possess broad mid-infrared transparency covering the most critical absorption bands for (bio)chemical sensing and high third-order optical nonlinearities for potential applications in nonlinear photonics. The parameters of the RTA process within this study include the annealing temperature, heating rate, and the two sample processing methods – one by placing the sample inside the graphite susceptor and the other by simply laying the sample onto the silicon wafer. Selenide thin films were found to undergo a shift of the absorption edge upon the RTA, resulting in an optical bandgap energy increase (bleaching effect) and a notable refractive index decrease. As a result of structural relaxation, such changes show a great potential of RTA in fine-tuning of optical performance of chalcogenide thin films and planar chalcogenide waveguides. The authors acknowledge the IBAIA (101092723) Horizon Europe project, the ANR AQUAE (ANR-21-CE04-0011-04) project of the French National Research Agency (ANR), and project No. 22-05179S of the Czech Science Foundation (GAČR) for financial support.

Hydrazone organics with third-order nonlinear optical effect for femtosecond pulse generation and control in the L-band

Dissipative solitons are generalized solitons with much larger pulse energy and width than conventional solitons, and the pulse characteristics will be changed dramatically during transmission. Nonlinear photonic absorption device in nonlinear optical resonator can provide saturable absorption effect to generate ultrashort pulses. However, most of the nonlinear photonics devices are based on inorganic materials such as two-dimensional materials with complex production process, and there are relatively few such researches on organics. In this paper, nonlinear photonics absorption device based on hydrazone organics with high molecular polarizability and significant third-order optical nonlinearity generate ultrashort pulses. The dissipative soliton pulses are obtained by controlling the dispersion, and the pulses are compressed to the near-transformation limit of 408 fs with a compression ratio of 44.6. More importantly, the extra-cavity transport properties of dissipative solitons are discussed. Dissipative noise-like soliton pulses are discovered after additional anomalous dispersion fiber, further demonstrating that the physical laws and optical properties of dissipative solitons are completely different from those of traditional optical pulses. We expect that these experimental advances can provide some experimental basis and support for the propagation of dissipative solitons.

Giant and light modifiable third-order optical nonlinearity in a free-standing h-BN film

Recently, hexagonal boron nitride (h-BN) has become a promising nanophotonic platform for on-chip information devices due to the practicability in generating optically stable, ultra-bright quantum emitters. For an integrated information-processing chip, high optical nonlinearity is indispensable for various fundamental functionalities, such as all-optical modulation, high order harmonic generation, optical switching and so on. Here we study the third-order optical nonlinearity of free-standing h-BN thin films, which is an ideal platform for on-chip integration and device formation without the need of transfer. The films were synthesized by a solution-based method with abundant functional groups enabling high third-order optical nonlinearity. Unlike the highly inert pristine h-BN films synthesized by conventional methods, the free-standing h-BN films could be locally oxidized upon tailored femtosecond laser irradiation, which further enhances the third-order nonlinearity, especially the nonlinear refraction index, by more than 20 times. The combination of the free-standing h-BN films with laser activation and patterning capability establishes a new promising platform for high performance on-chip photonic devices with modifiable optical performance.

Ultrafast two-photon optical switch using single crystal hybrid halide perovskites

Photoelectronic performance of organic–inorganic hybrid perovskites has been investigated extensively. Advancement in optoelectronic characterization and application of this group of materials in solar cells, light-emitting diodes, and lasers has been pushed forward rapidly. However, nonlinear optical properties for applications in optical logic devices have not been exploited. Moreover, long-lived excitons and charges limit the response speed of such materials and further construction of ultrafast devices. In this work, we make use of the high third-order optical nonlinearity of ( C H 3 N H 3 ) P b B r 3 single crystals and develop a femtosecond optical switch (OS) based on strong two-photon processes through absorbing one pump and one probe photon. The fascinating band structure, with a steep edge at 2.22 eV between the transmission and strong absorption bands, endows single crystals of ( C H 3 N H 3 ) P b B r 3 with perfect characteristics for two-photon optical switching in the near infrared. An on–off ratio of the probe pulses of about 90% and a speed faster than 400 fs have been achieved for this OS device. To the best of our knowledge, such high efficiency has not been realized for an ultrafast OS with any other materials and devices. Moreover, this is a first exploration, to our best knowledge, of the third-order optical nonlinearity of organic–inorganic hybrid halide perovskites for application in ultrafast optical logic devices with high reliability and low threshold in the near infrared.

Spectral characterization of a supercontinuum source based on nonlinear broadening in an aqueous K2ZnCl4 salt solution.

We report on investigations concerning the shot-to-shot spectral stability properties of a supercontinuum source based on nonlinear processes such as self-phase modulation and optical wave-breaking in a highly concentrated K2ZnCl4 double salt solution. The use of a liquid medium offers both damage resistance and high third-order optical nonlinearity. Approximately 40μJ pulses spanning a spectral range between 390 and 960nm were produced with 3.8% RMS energy stability, using infrared input pulses of 500±50 fs FWHM durations and 2.42±0.04 mJ energies with an RMS stability of 2%. The spectral stability was quantified via acquiring single-shot spectra and studying shot-to-shot variation across a spectral range of 200-1100nm, as well as by considering spectral correlations. The regional spectral correlation variations were indicative of nonlinear processes leading to sideband generation. Spectral stability and efficiency of energy transfer into the supercontinuum were found to weakly improve with increasing driver pulse energy, suggesting that the nonlinear broadening processes are more stable when driven more strongly, or that self-guiding effects in a filament help to stabilize the supercontinuum generation.

High non-resonant third-order optical nonlinearity of Ag–glass nanocomposite fabricated by two-step ion exchange

We observe high third-order optical nonlinearity of Ag–glass nanocomposite fabricated by a two-step ion exchange method. The nonlinear refractive index and absorption are studied using the well-known Z-scan technique. In our experiments, we use a femtosecond Ti–Sapphire laser source operating at 800nm, which is far from the plasmon resonance of the Ag nanoparticles. Our nanoparticles possess a high positive nonlinear refractive index and also a very high saturable absorption. We estimate the nonlinear refractive index to be on the order of 10−10cm2/W and the saturable absorption to be on the order of 10−5cm/W.

High-quality crystallinity controlled ALD TiO_2 for waveguiding applications

We demonstrate a novel atomic layer deposition (ALD) process to make high-quality nanocrystalline titanium dioxide (TiO(2)) with intermediate Al(2)O(3) layers to limit the crystal size. The process is based on titanium chloride (TiCl(4))+water and trimethyl aluminum (TMA)+ozone processes at 250°C deposition temperature. The waveguide losses measured using a prism coupling method for 633 and 1551 nm wavelengths are as low as 0.2±0.1 dB/mm with the smallest crystal size, with losses increasing with crystal size. In comparison, plain TiO(2) deposited at 250°C without the intermediate Al(2)O(3) layers shows high scattering losses and is not viable as waveguide material. The third-order optical nonlinearity decreases with smaller crystal size as verified by third-harmonic generation microscopy but still remains high for all samples. Crystallinity controlled ALD-grown TiO(2) is an excellent candidate for various optical applications, where good thermal stability and high third-order optical nonlinearity are needed.

Donor–Acceptor (D–A)‐Substituted Polyyne Chromophores: Modulation of Their Optoelectronic Properties by Varying the Length of the Acetylene Spacer

A series of donor-acceptor-substituted alkynes, 2 a-f, was synthesized in which the length of the π-conjugated polyyne spacer between the N,N-diisopropylanilino donor and the 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) acceptor was systematically changed. The effect of this structural change on the optoelectronic properties of the molecules and, ultimately, their third-order optical nonlinearity was comprehensively investigated. The branched N,N-diisopropyl groups on the anilino donor moieties combined with the nonplanar geometry of 2 a-f imparted exceptionally high solubility to these chromophores. This important property allowed for performing INADEQUATE NMR measurements without (13) C labeling, which, in turn, resulted in a complete assignment of the carbon skeleton in chromophores 2 a-f and the determination of the (13) C-(13) C coupling constants. This body of data provided unprecedented insight into characteristic (13) C chemical shift patterns in push-pull-substituted polyynes. Electrochemical and UV/Vis spectroscopic studies showed that the HOMO-LUMO energy gap decreases with increasing length of the polyyne spacer, while this effect levels off for spacers with more than four acetylene units. The third-order optical nonlinearity of this series of molecules was determined by measuring the rotational averages of the third-order polarizabilities (γrot ) by degenerate four-wave mixing (DFWM). These latter studies revealed high third-order optical nonlinearities for the new chromophores; most importantly, they provided fundamental insight into the effect of the conjugated spacer length in D-A polyynes, that can be exploited in the future design of suitable charge-transfer chromophores for applications in optoelectronic devices.

Preparation and Characterization of Cd/CdS-SiO2 Composite Film with High Third-order Optical Nonlinearity by Electrochemically Induced Sol-Gel Method

Preparation and Characterization of Cd/CdS-SiO2 Composite Film with High Third-order Optical Nonlinearity by Electrochemically Induced Sol-Gel Method

Nonplanar Push–Pull Chromophores for Opto-Electronic Applications

Donor-substituted cyanoethynylethenes (CEEs) are planar push-pull chromophores featuring intense intramolecular charge-transfer (CT) interactions and high third-order optical nonlinearities. Their thermal stability allows for the formation of crystalline thin films by vapor-phase deposition. On the other hand, high-quality amorphous thin films are preferred for opto-electronic applications and such films can be prepared using nonplanar push-pull chromophores with a less pronounced propensity to crystallize. By taking advantage of a versatile, atom-economic 'click-chemistry'-type transformation, involving a formal [2 + 2] cycloaddition of tetracyanoethene (TCNE) to electron-rich alkynes, followed by cycloreversion, stable donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs) are obtained in high yield and large quantities. These nonplanar push-pull chromophores also feature intense intramolecular CT and, in many cases, high third-order optical nonlinearities. Some of these compounds form high-optical-quality amorphous thin films by vapor-phase deposition, and first applications in next-generation opto-electronic devices have already been demonstrated. Chiral derivatives display high helical twisting power and are efficient dopants to translate molecular into macroscopic chirality, by switching nematic into cholesteric liquid crystalline phases.

蒽二酮类化合物的合成及结构-非线性光学性能关系的研究

Six new anthracenedione compounds are synthesized and characterized by UV-vis,IR,1H NMR and elemental analysis.By using femtosecond laser,the off-resonant third-order optical nonlinearities of the compounds are measured with degenerate four-wave mixing technique.The third-order nonlinear optical susceptibilities χ(3) are 2.62～3.55×10-13.The nonlinear refractive indexes n2 are 4.82～6.52×10-12 esu.The second-order hyperpolarizabilities γ of the molecules are 2.57～3.25×10-31 esu.The response times τ are 91～116 fs.The influence of the molecular structure on the third-order optical nonlinearity is studied.The factors such as long conjugate chain,formation of donor-acceptor-donor srtucture,strong electron-donating abilities of substituents,and good coplanarity lead to the high third-order optical nonlinearity.

Non-planar push–pull chromophores

The development of a unique class of non-planar push-pull chromophores by means of [2 + 2] cycloaddition, followed by cycloreversion, of electron-deficient olefins, such as tetracyanoethene (TCNE), 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F(4)-TCNQ), as well as dicyanovinyl (DCV) and tricyanovinyl (TCV) derivatives, to donor-substituted alkynes is explored in this feature article. This high-yielding, "click-chemistry"-type transformation with acetylenic dendrimers affords dendritic electron sinks capable of multiple electron uptake within a narrow potential range. An [AB]-type oligomer with a dendralene backbone was synthesised by a one-pot, multi-component cascade reaction of polyyne oligomers with TCNE and tetrathiafulvalene (TTF). In most cases, the resulting chromophores feature intense intramolecular charge-transfer bands extending far into the near infrared region and some of them display high third-order optical nonlinearities. Despite substitution with strong donors, the electron-withdrawing moieties in the new chromophores remain potent acceptors and a number of them display positive first reduction potentials (vs. the ferrocenium/ferrocene (Fc(+)/Fc) couple in CH(2)Cl(2)), which rival those of parent TCNE, TCNQ and F(4)-TCNQ. The non-planarity of the chromophores strongly enhances their physical properties when compared to planar push-pull analogues. They feature high solubility, thermal stability and sublimability, which enables formation of amorphous, high-optical-quality thin films by vapour phase deposition and makes them interesting as advanced functional materials for novel opto-electronic devices.

Acetylene-Derived Strong Organic Acceptors for Planar and Nonplanar Push−Pull Chromophores

Though investigated for decades, interest in push-pull chromophores (D-pi-A), strong electron donors (D) connected by pi-conjugating spacers to strong electron acceptors (A), continues to grow. Such chromophores are of substantial interest for optoelectronic devices such as waveguides. Also, strong donors and acceptors form bimolecular charge-transfer (CT) complexes and salts, some of which exhibit electrical conductivity and magnetic behavior. Furthermore, strong organic acceptors are increasingly explored as dopants in the fabrication of organic light-emitting diodes (OLEDs) and solar cells. This Account describes systematic efforts pursued over the past decade in our laboratory to generate new families of organic electron acceptors (A) and conjugate them via pi-spacers to electron donors (D) under formation of push-pull systems with intense intramolecular CT interactions and high third-order optical nonlinearities. First, we describe donor-acceptor-substituted tetraethynylethenes (TEEs). In these chromophores, the peripherally attached p-nitrophenyl acceptors and N,N-dimethylanilino donors behave as nearly independent redox centers. Acetylenic scaffolding using TEE building blocks produces large all-carbon sheets, such as perethynylated dehydroannulenes, expanded radialenes, and radiaannulenes with potent electron-acceptor properties. Arylated TEEs act as molecular switches allowing two-way photochemical interconversion that is not perturbed by thermal isomerization pathways. Upon sequential substitution of the acetylene moieties in TEEs, we formed another family of potent acceptors, the cyanoethynylethenes (CEEs). Donor-substituted CEEs are planar CT chromophores with very high third-order optical nonlinearities. Their high environmental stability allows for the formation of thin films by vapor-phase deposition. Through careful analysis of the physicochemical properties of CEEs, we established useful guidelines for evaluating and tuning the optical gap in strong push-pull chromophores: increasing the length of the pi-spacer in D-pi-A systems reduces ground-state D-A conjugation and lowers the HOMO-LUMO gap. By taking advantage of "click-chemistry"-type [2 + 2] cycloadditions of tetracyanoethene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) with appropriately activated alkynes, followed by retro-electrocyclization, the formation of donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs), 1,1,2,4,4-pentacyanobuta-1,3-dienes (PCBDs), and novel TCNQ adducts is possible. Some of these stable, nonplanar CT chromophores form high optical quality amorphous thin films by vapor-phase deposition. Despite donor substitution, the new acceptors (TCBDs, PCBDs, and the TCNQ adducts) rival TCNE and TCNQ in their ease for reversible electron uptake. High-yielding cycloaddition/retro-electrocyclization cascades provide access to multivalent, dendritic chromophores acting as "molecular batteries" with a remarkable capacity for multiple electron uptake in a narrow potential range. Finally, we used a one-pot protocol for electronically controlled consecutive TCNE and tetrathiafulvalene (TTF) additions to end-capped polyynes to form [AB]-type oligomers with a dendralene-type backbone.

Synthesis and Characterization of Tetrathiafulvalene-Substituted Di- and Tetraethynylethenes with p-Nitrophenyl Acceptors

Novel di- and tetraethynylethene (DEE and TEE) compounds functionalized with tetrathiafulvalene (TTF) donor groups and p-nitrophenyl acceptor groups were synthesized by palladium-catalyzed cross-coupling reactions under various conditions. The molecules are strong chromophores and were investigated for their optical properties. Placement of two TTFs and two p-nitrophenyls about a central TEE core provides a molecule with a high third-order optical nonlinearity. The molecules experience reversible oxidations of the TTF units, and the optical properties of the oxidized species were elucidated by spectroelectrochemistry. The degree of quinoid character of the p-nitrophenyl in the molecules was determined by X-ray crystallography.

Property Tuning in Charge‐Transfer Chromophores by Systematic Modulation of the Spacer between Donor and Acceptor

A series of donor-acceptor chromophores was prepared in which the spacer separating 4-dimethylanilino (DMA) donor and C(CN)(2) acceptor moieties is systematically varied. All of the new push-pull systems, except 4 b, are thermally stable molecules. In series a, the DMA rings are directly attached to the central spacer, whereas in series b additional acetylene moieties are inserted. X-ray crystal structures were obtained for seven of the new, intensely colored target compounds. In series a, the DMA rings are sterically forced out of the mean plane of the residual pi system, whereas the entire conjugated pi system in series b is nearly planar. Support for strong donor-acceptor interactions was obtained through evaluation of the quinoid character of the DMA ring and by NMR and IR spectroscopy. The UV/Vis spectra feature bathochromically shifted, intense charge-transfer bands, with the lowest energy transitions and the smallest optical gap being measured for the two-dimensionally extended chromophores 6 a and 6 b. The redox behavior of the push-pull molecules was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). In the series 1 b, 2 b, 4 b, 5 b, in which the spacer between donor and acceptor moieties is systematically enlarged, the electrochemical gap decreases steadily from 1.94 V (1 b) to 1.53 V (5 b). This decrease is shown to be a consequence of a reduction in the D-A conjugation with increasing spacer length. Degenerate four-wave mixing experiments reveal high third-order optical nonlinearities, pointing to potentially interesting applications of some of the new chromophores in optoelectronic devices.

Donor‐Substituted 1,1,4,4‐Tetracyanobutadienes (TCBDs): New Chromophores with Efficient Intramolecular Charge‐Transfer Interactions by Atom‐Economic Synthesis.

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Donor‐Substituted 1,1,4,4‐Tetracyanobutadienes (TCBDs): New Chromophores with Efficient Intramolecular Charge‐Transfer Interactions by Atom‐Economic Synthesis

A wide variety of monomeric and oligomeric, donor-substituted 1,1,4,4-tetracyanobutadienes (TCBDs) have been synthesized by [2+2] cycloaddition between tetracyanoethylene (TNCE) and donor-substituted alkynes, followed by electrocyclic ring opening of the initially formed cyclobutenes. Reaction yields are often nearly quantitative but can be affected by the electron-donating power and steric demands of the alkyne substituents. The intramolecular charge-transfer (CT) interactions between the donor and TCBD acceptor moieties were comprehensively investigated by X-ray crystallography, electrochemistry, UV-visible spectroscopy, and theoretical calculations. Despite the nonplanarity of the new chromophores, which have a substantial twist between the two dicyanovinyl planes, efficient intramolecular CT interactions are observed, and the crystal structures demonstrate a high quinoid character in strong donor substituents, such as N,N-dimethylanilino (DMA) rings. The maxima of the CT bands shift bathochromically upon reduction of the amount of conjugative coupling between strong donor and acceptor moieties. Each TCBD moiety undergoes two reversible, one-electron reduction steps. Thus, a tri-TCBD derivative with a 1,3,5-trisubstituted benzene core shows six reversible reduction steps within an exceptionally narrow potential range of 1.0 V. The first reduction potential E(red,1) is strongly influenced by the donor substitution: introduction of more donor moieties causes an increasingly twisted TCBD structure, a fact that results in the elevation of the LUMO level and, consequently, a more difficult first reduction. The potentials are also strongly influenced by the nature of the donor residues and the extent of donor-acceptor coupling. A careful comparison of electrochemical data and the correlation with UV-visible spectra made it possible to estimate unknown physical parameters such as the E(red,1) of unsubstituted TCBD (-0.31 V vs Fc+/Fc) as well as the maxima of highly broadened CT bands. Donor-substituted TCBDs are stable molecules and can be sublimed without decomposition. With their high third-order optical nonlinearities, as revealed in preliminary measurements, they should become interesting chromophores for ultra-thin film formation by vapor deposition techniques and have applications in opto-electronic devices.

Single-mode Ag-As2Se3 strip-loaded waveguides for applications to all-optical devices

We report the fabrication and characterization of Ag-As2Se3 strip-loaded waveguides fabricated by the standard lift-off method and photodoping, where Ag-As2Se3 (i.e., Agx(As0.4Se0.6)100-x) is a very promising material for all-optical switching devices because of a high third-order optical nonlinearity. Absorption coefficient of the bulk glasses is also presented for different values of the Ag content x. The addition of Ag into an As2Se3 guiding layer increases its nonlinearity and eliminates or weakens a harmful photo-oxidation reaction of As2Se3 films. The measured propagation loss in the single-mode waveguides with the Ag content x=3 at.% was 0.5 dB/cm at a wavelength of 1.053 mum and was attributable to the absorption loss in the waveguide materials.

Synthesis and characterization of composition-spread (Sr,Ca)2CuO3 thin films with high third-order optical nonlinearity

The optical properties of composition-spread (SrxCa1−x)2CuO3(0⩽x⩽1) films showing high third-order optical nonlinearity are systematically investigated. Desired orientations and high crystallinity are obtained for the entire composition x by employing a composition-spread [(Sr2TiO4)x(LaSrAlO4)1−x] buffer layer and a temperature-gradient method in order to compensate the change in lattice mismatch and growth temperature depending on x. Linear absorption spectra and third-harmonic intensity with an exciting wavelength of 1.56 μm are measured as a function of x. As the Cu–O bond length increases with x, the charge-transfer gap decreases and the third-harmonic intensity increases, and both the charge-transfer gap and third-harmonic intensity change almost linearly with x between Ca2CuO3 and Sr2CuO3.

High third-order non-resonant optical non-linearities of surface modified CdS quantum dots embedded in BaTiO 3 thin film

Surface-modified nanocrystalline CdS has been integrated in amorphous BaTiO 3 films by a dip-coating process. The films were characterized by X-ray photoelectron spectra and transmission electron microscopy technique. The thin films exhibited unique absorption edge blue-shift with the increase of heat-treatment temperature due to the dielectric confinement in BaTiO 3 matrix and the thermal expansion mismatch between the semiconductor quantum dots and the host matrix. The strong fluorescence of CdS quantum dots at room temperature has been observed and is mainly attributed to the band-edge transition of CdS quantum dots. The composite films also exhibit ultra-fast non-linear response and high non-resonant, third-order optical non-linearities.