Large Macroscopic Optical Nonlinearity Research Articles

Broadband THz wave generation in organic benzothiazolium crystals at MHz repetition rates [Invited

We present broadband terahertz wave generation based on benzothiazolium crystals at a 100 MHz repetition rate. Among various benzothiazolium crystals, we chose PMB-4TFS (2-(4-(4-(hydroxymethyl)piperidin-1-yl)styryl)-3-methylbenzothiazol-3-ium 4-(trifluorome-thyl)benzenesulfonate) due to its large macroscopic optical nonlinearity, good crystal characteristics, and suppressed molecular phonon vibrations. When pumped at the telecommunication wavelength of 1560 nm, the generated THz spectrum covers up to ∼15 THz and the corresponding THz amplitude for a 0.29 mm thick crystal at normal incidence is by more than one order of magnitude higher compared to the inorganic standard, 1.0 mm thick ZnTe generator crystal.

A New Class of Organic Crystals with Extremely Large Hyperpolarizability: Efficient THz Wave Generation with Wide Flat‐Spectral‐Band

AbstractIn organic π‐conjugated crystals, enhancing molecular optical nonlinearity of chromophores (e.g., first hyperpolarizability β ≥ 300 × 10−30 esu) in most cases unfortunately results in zero macroscopic optical nonlinearity, which is a bottleneck in organic nonlinear optics. In this study, a new class of nonlinear optical organic crystals introducing a chromophore possessing an extremely large first hyperpolarizability is reported. With newly designed 4‐(4‐(4‐(hydroxymethyl)piperidin‐1‐yl)styryl)‐1‐(pyrimidin‐2‐yl)pyridin‐1‐ium (PMPR) chromophore, incorporating a head‐to‐tail cation‐anion OH⋯O hydrogen‐bonding synthon and an optimal selection of molecular anion into crystals results in extremely large macroscopic optical nonlinearity with effective first hyperpolarizability of 335 × 10−30 esu. This is in sharp contrast to zero value for previously reported analogous crystals. An ultrathin PMPR crystal with a thickness of ≈10 µm exhibits excellent terahertz (THz) wave generation performance. Both i) broadband THz wave generation with a wide flat‐spectral‐band in the range of 0.7–3.4 THz defined at −3 dB and high upper cut‐off generation frequency of > 7 THz as well as ii) high‐generation efficiency (5 times higher THz amplitude than ZnTe crystal with a mm‐scale thickness) are simultaneously achieved. Therefore, new PMPR crystals are highly promising materials for diverse applications in nonlinear optics and THz photonics.

Design Strategy of Highly Efficient Nonlinear Optical Orange‐Colored Crystals with Two Electron‐Withdrawing Groups

A new class of highly efficient nonlinear optical organic salt crystals is reported. In nonlinear optics based on organic materials, it is well known that using two electron‐withdrawing groups (EWGs) onto cationic electron acceptors instead of conventional one EWG remarkably enhances microscopic optical nonlinearity for chromophores. However, the corresponding organic crystals possessing enhanced large macroscopic optical nonlinearity have not been reported yet. Herein, a design strategy is proposed for obtaining highly efficient nonlinear optical crystals based on two EWGs in cationic electron acceptors. Introducing a phenolic electron donor, promoting a head‐to‐tail interionic assembly, along with a two‐EWG N‐pyrimidinyl pyridinium electron acceptor in cationic chromophores results in a preferred non‐centrosymmetric, perfectly parallel alignment of chromophores in crystal. Newly designed OPR (4‐(4‐hydroxystyryl)‐1‐(pyrimidin‐2‐yl)pyridinium) crystals exhibit approximately two times larger effective first hyperpolarizability than that of analogous N‐alkyl OHP (4‐(4‐hydroxystyryl)‐1‐methylpyridinium) crystals based on only one EWG. OPR crystals exhibit comparable second‐order optical nonlinearity to benchmark red‐colored DAST (4‐(4‐(dimethylamino)styryl)‐1‐methylpyridinium 4‐methylbenzenesulfonate) crystals, but a significant blue‐shifted absorption resulting in orange‐color crystals. Therefore, phenolic organic salt crystals using two EWGs are highly promising materials for various nonlinear optical applications.

Solid‐State Molecular Motions in Organic THz Generators

AbstractOrganic nonlinear optical salt crystals are widely used as efficient broadband THz generators. Although solid state molecular motions of organic crystals can greatly influence THz generation characteristics, their origin and effects on THz photonics are not clearly identified. In this work, the origin of solid‐state molecular motions of the state‐of‐the‐art nonlinear optical organic salt crystals and their effects on THz generation characteristics are theoretically investigated. A model crystal, HMQ‐TMS (2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium 2,4,6‐trimethylbenzenesulfonate) with large macroscopic optical nonlinearity, which is very attractive for intense broadband and narrowband THz wave generation, is chosen. The solid‐state molecular vibrations of HMQ‐TMS crystals can be classified in three frequency regions: phonon mode region, intramolecular motion region, and their mixing region. For the first time for ionic organic crystalline THz generators, the contributions of cationic chromophores and anionic matchmakers on each of vibrational modes are quantitatively separated. In addition, the influence of solid‐state molecular vibrations of HMQ‐TMS crystals on the generated THz spectra is investigated. These results provide an essential information for design of new organic nonlinear optical salt crystals for THz generators as well as detectors and for optimization of THz generation performance.

Efficient terahertz generation in highly nonlinear organic crystal HMB-TMS.

We report on generation of strong and broadband terahertz (THz) pulses via collinearly phase-matched optical rectification of near-infrared femtosecond pulses in the organic nonlinear optical HMB-TMS (2-(4-hydroxy-3-methoxystyryl)-3-methylbenzo[d]thiazol-3-ium 2,4,6-trimethylbenzenesulfonate) single crystals which exhibit optimal molecular orientation and large macroscopic optical nonlinearity for efficient THz wave generation. Single-cycle THz pulses with a peak electric field strength of 0.66 MV/cm and a bandwidth from 0.1 to 5.4 THz are achieved from an HMB-TMS crystal with only a 2-mm clear aperture pumped by 1350 nm pulses at moderate fluences. The generated THz energy is about 1 µJ and the corresponding pump-to-THz energy conversion efficiency reaches 0.23%.

Organic Three‐Component Single Crystals with Pseudo‐Isomorphic Cocrystallization for Nonlinear Optics and THz Photonics

AbstractA new organic three‐component single crystals are developed using the so‐called “pseudo‐isomorphic cocrystallization” for nonlinear optical and terahertz (THz) photonic applications. The pseudo‐isomorphic cocrystallization is based on two homocrystals exhibiting similar molecular ordering feature in the crystalline state, but different crystallographic space groups. Such new organic cocrystals consist of three components, highly nonlinear optical 2‐(4‐hydroxystyryl)‐1‐methylquinolinium (OHQ) cation, and two different counter anions. Compared to homocrystals having two components (OHQ cation and a single anion type), OHQ‐based cocrystals by isomorphic cocrystallization from isomorphic homocrystals exhibit an isomorphic crystal structure with very similar physical properties. In contrast, OHQ‐based cocrystals by pseudo‐isomorphic cocrystallization provide a different molecular ordering with a different crystallographic space group, resulting in remarkably distinguishable crystal characteristics and physical properties, while maintaining large macroscopic optical nonlinearity with excellent optical quality and morphology suitable for diverse optical experiments. To show a potential for nonlinear optical applications, THz wave generation is demonstrated by optical rectification pumped at fundamental wavelength of 1300 nm. A 0.92 mm thick OHQ‐based cocrystal by pseudo‐isomorphic cocrystallization delivers efficient optical‐to‐THz conversion with one order of magnitude higher peak‐to‐peak THz electric field than the 1.0 mm thick inorganic standard ZnTe crystal and presents a broad spectral bandwidth of up to 8 THz.

Benzothiazolium Single Crystals: A New Class of Nonlinear Optical Crystals with Efficient THz Wave Generation.

Highly efficient nonlinear optical organic crystals are very attractive for various photonic applications including terahertz (THz) wave generation. Up to now, only two classes of ionic crystals based on either pyridinium or quinolinium with extremely large macroscopic optical nonlinearity have been developed. This study reports on a new class of organic nonlinear optical crystals introducing electron-accepting benzothiazolium, which exhibit higher electron-withdrawing strength than pyridinium and quinolinium in benchmark crystals. The benzothiazolium crystals consisting of new acentric core HMB (2-(4-hydroxy-3-methoxystyryl)-3-methylbenzo[d]thiazol-3-ium) exhibit extremely large macroscopic optical nonlinearity with optimal molecular ordering for maximizing the diagonal second-order nonlinearity. HMB-based single crystals prepared by simple cleaving method satisfy all required crystal characteristics for intense THz wave generation such as large crystal size with parallel surfaces, moderate thickness and high optical quality with large optical transparency range (580-1620 nm). Optical rectification of 35 fs pulses at the technologically very important wavelength of 800 nm in 0.26 mm thick HMB crystal leads to one order of magnitude higher THz wave generation efficiency with remarkably broader bandwidth compared to standard inorganic 0.5 mm thick ZnTe crystal. Therefore, newly developed HMB crystals introducing benzothiazolium with extremely large macroscopic optical nonlinearity are very promising materials for intense broadband THz wave generation and other nonlinear optical applications.

Terahertz Phonon Mode Engineering of Highly Efficient Organic Terahertz Generators

For terahertz (THz) wave generators based on organic electrooptic crystals, their intrinsic phonon modes are playing an essential role in THz generation characteristics. Here, this study proposes an effective design strategy for THz phonon mode engineering of organic electrooptic salt crystals for efficient optical‐to‐THz frequency conversion. To reduce phonon‐mode intensity, strongly electronegative trifluoromethyl group acting as strong hydrogen‐bond acceptor is incorporated into molecular anions. New 2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium 4‐(trifluoromethyl)benzenesulfonate (HMQ‐4TFS) crystals exhibit a relatively small absorption coefficient in the THz spectral range between 0.5 and 4 THz, which is attributed to suppressed molecular vibrations due to strong hydrogen bonds involving the 4TFS anion. In addition, HMQ‐4TFS crystals possess a very large macroscopic optical nonlinearity, comparable (or even higher) to benchmark stilbazolium crystals. Based on the low‐intensity THz phonon modes and the large optical nonlinearity, a 0.37 mm thick HMQ‐4TFS crystal pumped with 150 fs infrared laser pulses facilitates very efficient THz wave generation by optical rectification, delivering 23 times higher peak‐to‐peak THz electric field than the widely used standard inorganic ZnTe crystal (1.0 mm thick) and a broader spectral bandwidth. Therefore, strongly electronegative groups introduced into molecular salt electrooptic crystals provide a very promising design strategy of THz phonon mode engineering for developing intense broadband THz sources.

Quinolinium single crystals with a high optical nonlinearity and unusual out-of-plane polar axis

New phenolic quinolinium crystals with very large macroscopic optical nonlinearity and good crystal-growth ability exhibit an out-of-plane polar axis, which is very unusual considering other state-of-the-art organic nonlinear optical crystals exhibiting an in-plane polar axis.

Crystal Engineering of Acentric Styryl Quinolinium Crystals with Strongly Hydrogen-Bonded Phenolic Anions

We report on new acentric styryl quinolinium crystals with phenolic sulfonate counteranions and investigate their supramolecular interactions that affect their quadratic nonlinear optical properties. The phenolic group acting as an electron-donor as well as hydrogen-bond donor site is located at one end of the anion, while the sulfonate group acting as an electron-acceptor as well as hydrogen-bond acceptor site is located at the opposite end of the anion. New styryl quinolinium crystals with 4-hydroxybenzenesulfonate and 6-hydroxynaphthalene-2-sulfonate counteranions exhibit a large macroscopic optical nonlinearity with very efficient second harmonic generation (SHG) efficiency. In styryl quinolinium 4-hydroxybenzenesulfonate crystals, the styryl quinolinium cation chromophores exhibit an acentric ordering with a high order parameter close to 1.0, which is optimal for electro-optic applications or THz-wave generation. The 4-hydroxybenzenesulfonate counteranions form strong head-to-tail hydrogen bonds, and...

Synthesis and characterization of thienyl-substituted pyridinium salts for second-order nonlinear optics

The synthesis of a series of thienyl-substituted pyridinium salts with different counter-anions and their second-order nonlinear optical (NLO) properties are described. Kurtz powder tests revealed that, out of the twelve new pyridinium salts, four possess noncentrosymmetric structures and the maximum powder second harmonic generation (SHG) efficiency is 0.6 times of DAST. Single crystals of three new salts were successfully obtained from methanol by slow evaporation method. X-Ray studies exhibited that they crystallized in the monoclinic space groupP21/c, P21, and P21/n, respectively. (E)-4-(2-(5-(dimethylamino)thiophen-2-yl)vinyl)-1-methylpyridinium 4-hydroxybenzenesulfonate (3c) forms a hydrated noncentrosymmetric phase with a large macroscopic optical nonlinearity 0.5 times of that of DAST at the non-resonant wavelength of λ = 2109 nm, while both (E)-4-(2-(5-(dimethylamino)thiophen-2-yl)vinyl)-1-methylpyridinium 2-amino-3,5-dimethylbenzenesulfonate (3i) and (E)-4-(2-(5-(dimethylamino)thiophen-2-yl)vinyl)-1-methylpyridinium iodide (3a) crystallize in centrosymmetric structures.

Highly Efficient Organic THz Generator Pumped at Near‐Infrared: Quinolinium Single Crystals

AbstractA novel highly efficient ionic electro‐optic quinolinium single crystals for THz wave applications is reported. Acentric quinolinium derivatives, HMQ‐T (2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium 4‐methylbenzenesulfonate) and HMQ‐MBS (2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium 4‐methoxybenzenesulfonate) exhibit high order parameters cos3θp = 0.92 and cos3θp = 1.0, respectively, as well as a large macroscopic optical nonlinearity, which is in the range of the benchmark stilbazolium DAST (N,N‐dimethylamino‐N’‐methylstilbazolium 4‐methylbenzenesulfonate) and phenolic polyene OH1 (2‐(3‐(4‐hydroxystyryl)‐5,5‐dimethylcyclohex‐2‐enylidene)malononitrile) crystals. As‐grown unpolished bulk HMQ‐T crystals with a side length of about 6 mm and thickness of 0.56 mm exhibit 3.1 times higher THz generation efficiency than 0.37 mm thick OH1 crystals and about 8.4 times higher than 1 mm thick inorganic standard ZnTe crystals at the near‐infrared fundamental wavelength of 836 nm. Therefore, HMQ crystals with high order parameter obviously have a very high potential for high power THz‐wave generation and its applications.

4-Nitrophenylhydrazone Crystals with Large Quadratic Nonlinear Optical Response by Optimal Molecular Packing

We investigate a novel nonlinear optical nitrophenylhydrazone (NPH) crystal, 3,4,5-trimethoxybenzaldehyde-4-nitrophenylhydrazone (TMO-NPH). Compared with many acentric NPH crystals having rod-shaped heads reported previously, the TMO-NPH molecule maintains the acentric intermolecular hydrogen bond synton through hydrazone and nitro groups but has distinct space filling characteristics due to a different shape of the headgroup, the disk-shaped trimethoxybenzaldehyde. We investigated the crystal structure and its characteristics, as well as details of the microscopic and the macroscopic nonlinear optical properties by quantum chemical calculations using density functional theory (DFT) and powder second-harmonic generation at nonresonant conditions. For comparison, DA-NPH (4-dimethylaminobenzaldehyde-4-nitrophenylhydrazone) having a rod-shaped head and known nonlinear optical properties is also investigated by the same analysis. TMO-NPH crystals have the monoclinic space group symmetry Pn and exhibit a large macroscopic optical nonlinearity due to a favorable molecular packing with a very high order parameter cos3(θp) = 0.90, while DA-NPH exhibits a lower order parameter cos3(θp) = 0.33. The diagonal effective hyperpolarizability tensor component of TMO-NPH crystals is 30% higher than the largest tensor component of DA-NPH, even though the microscopic molecular susceptibility of TMO-NPH molecule is only about half of the one of DANPH.

Design and synthesis of bichromophores for nonlinear optical applications in polymer films

Three bichromophores were designed and synthesized by linking a vinylthiophene-conjugated chromophore with different tether groups via esterification. Second harmonic generation measurements of nonlinear polymer films revealed that the second harmonic generation coefficients of the films containing the bichromophore and using 3-phenoxypropane-1,2-diyl succinic diester as the tether group were improved in comparison with the mono-chromophore-doped polymer films. However, the second harmonic generation coefficients of the polymer films containing bichromophores using isophthalic ester as the tether group were decreased. Overall, the results indicate that the 3-phenoxypropane-1,2-diyl succinic diester tether group has potential applications in the design and synthesis of multi-chromophoric dendrimers with large macroscopic optical nonlinearity.

Acentric nonlinear optical N-benzyl stilbazolium crystals with high environmental stability and enhanced molecular nonlinearity in solid state

We have developed a new cation core structure, N-benzyl stilbazolium nonlinear optical chromophore with a non-polar benzyl group to achieve acentric molecular ordering in the crystalline state. New N-benzyl stilbazolium crystal, BP3 (N,N-dimethylamino-N′-2,5-dimethylbenzyl-stilbazolium p-toluenesulfonate), exhibits an acentric crystal structure with the monoclinic P21 phase with a large macroscopic optical nonlinearity of 540 times the powder second harmonic generation efficiency of urea at the non-resonant wavelength of 1.9 µm. Compared to conventional rod-shaped N-alkyl stilbazolium crystals, an enhanced hyperpolarizability of the chromophoresβ0 in the solid state can be utilized in bent-shaped N-benzyl stilbazolium crystals. This is attributed to the decreased inter-chromophore interactions due to the larger chromophore–chromophore separation induced by the bulky and bent-shaped N-benzyl group, so-called site-isolation effect. Moreover, by introducing the non-polar dimethylbenzyl group, BP3 crystals show a high environmental stability: they exhibit almost one order of magnitude smaller solubility in water than conventional stilbazolium crystals and also do not form a hydrated centrosymmetric phase even if crystallized in water-containing solvents. We have grown good optical quality crystals large enough for optical characterization. With as-grown BP3 crystals without additional polishing and cutting procedures we have demonstrated THz generation by optical rectification using 180 fs pulses at the pump wavelength of 836 nm.

A 3-phenoxypropane-1, 2-diol based bichromophore for enhanced nonlinear optical properties

The structure of a novel bichromophore that comprised a readily hyperpolarisable chromophore with 3-phenoxypropane-1, 2-diol via esterification, was confirmed by elemental analysis, UV–visible absorption spectra and 1H NMR. Nonlinear optical polymer films were fabricated by doping the bichromophore and the corresponding monochromophore into poly(4-vinylphenol). In situ second harmonic generation measurements revealed that the bichromophore imparted a 2.2-fold enhancement in the second harmonic generation coefficients of the films, indicating that the novel 3-phenoxypropane-1, 2-diol derivative could be used in the design and synthesis of multi-chromophoric dendrimers with large macroscopic optical nonlinearity.