Third-harmonic Generation Spectroscopy Research Articles

Dynamical interplay between superconductivity and pseudogap in cuprates as revealed by terahertz third-harmonic generation spectroscopy

We report on nonlinear terahertz third-harmonic generation (THG) measurements on YBa2Cu3O6+xthin films. Different from conventional superconductors, the THG signal starts to appear in the normal state, which is consistent with the crossover temperatureT* of pseudogap over broad doping levels. Upon lowering the temperature, the THG signal shows an anomaly just belowTcin the optimally doped sample. Notably, we observe a beat pattern directly in the measured real-time waveform of the THG signal. We elaborate that the Higgs mode, which develops belowTc, couples to the mode already developed belowT*, resulting in an energy level splitting. However, this coupling effect is not evident in underdoped samples. We explore different potential explanations for the observed phenomena. Our research offers valuable insight into the interplay between superconductivity and pseudogap.

Elucidating low-frequency vibrational dynamics in calcite and water with time-resolved third-harmonic generation spectroscopy.

Low-frequency vibrations are foundational for material properties including thermal conductivity and chemical reactivity. To resolve the intrinsic molecular conformational dynamics in condensed phase, we implement time-resolved third-harmonic generation (TRTHG) spectroscopy to unravel collective skeletal motions in calcite, water, and aqueous salt solution in situ. The lifetime of three Raman-active modes in polycrystalline calcite at 155, 282 and 703 cm(-1) is found to be ca. 1.6 ps, 1.3 ps and 250 fs, respectively. The lifetime difference is due to crystallographic defects and anharmonic effects. By incorporating a home-built wire-guided liquid jet, we apply TRTHG to investigate pure water and ZnCl2 aqueous solution, revealing ultrafast dynamics of water intermolecular stretching and librational bands below 500 cm(-1) and a characteristic 280 cm(-1) vibrational mode in the ZnCl4(H2O)2(2-) complex. TRTHG proves to be a compact and versatile technique that directly uses the 800 nm fundamental laser pulse output to capture ultrafast low-frequency vibrational motion snapshots in condensed-phase materials including the omnipresent water, which provides the important time dimension to spectral characterization of molecular structure-function relationships.

In-situ characterization of femtosecond laser-induced crystallization in borosilicate glass using time-resolved surface third-harmonic generation

Coherent phonon dynamics in condensed-phase medium are responsible for important material properties including thermal and electrical conductivities. We report a structural dynamics technique, time-resolved surface third-harmonic generation (TRSTHG) spectroscopy, to capture transient phonon propagation near the surface of polycrystalline CaF2 and amorphous borosilicate (BK7) glass. Our approach time-resolves the background-free, high-sensitivity third harmonic generation (THG) signal in between the two crossing near-IR pulses. Pronounced intensity quantum beats reveal the impulsively excited low-frequency Raman mode evolution on the femtosecond to picosecond timescale. After amplified laser irradiation, danburite-crystal-like structure units form at the glass surface. This versatile TRSTHG setup paves the way to mechanistically study and design advanced thermoelectrics and photovoltaics.

Third-order nonlinear optical properties of a π-conjugated biradical molecule investigated by third-harmonic generation spectroscopy

Third-order optical nonlinearity of a biradical molecule with open-shell π-electron system, 1,4-bis- (4,5-diphenylimidazole-2-ylidene)-cyclohexa-2,5-diene (BDPI-2Y) was investigated by the third-harmonic generation method. The figure of merit for third-order optical nonlinearity in BDPI-2Y is comparable to those for conjugated polymers, which have elongated closed π-conjugated systems. This indicates that the biradical nature of BDPI-2Y enhances third-order optical nonlinearity.

Optical nonlinearities in hyperbranched polyyne studied by two-photon excited fluorescence and third-harmonic generation spectroscopy

The nonlinear optical properties of a hyperbranched polyyne (hb-Polyyne) have been measured at infrared wavelengths by using femtosecond and nanosecond pulsed excitation. This hyperbranched polyyne exhibited strong and intrinsic (simultaneous) two-photon absorption and upconverted blue fluorescent emission under femtosecond excitation around 800 nm. The hb-Polyyne in chloroform solution is characterized by a large two-photon absorption cross section of 9068 GM (1GM=10−50 cm4 s) and a fluorescence quantum yield of 0.57. On the other hand, by third-harmonic generation (THG) spectroscopy with nanosecond excitation, the measured third-order nonlinear susceptibility χ(3) for solid films of hb-Polyyne ranged from 2.4×10−11 to 6.1×10−11 esu in the spectral range of 1100–1600 nm, with results comparable to the values exhibited by the well-known conjugated polymer MEH:PPV, but with a much better transparency for visible wavelengths.

Slab waveguides of poly(p-phenylenevinylene)s for all-optical switching: impact of side-chain substitution

We have prepared thin film waveguides by spin-coating poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) and three related alkoxy-substituted PPVs, which have a different degree of long alkyl side-chains. We use UV-Vis-NIR and FTIR spectroscopy, prism coupling, and third-harmonic generation spectroscopy to determine the intrinsic absorption coefficient, refractive index and anisotropy, birefringence, infrared dichroism, mode-propagation loss, and third-order nonlinear optical susceptibility, respectively. Our results reveal structure–property relations of the impact of side-chain substitution on the morphology of the films. At increasing amounts of long alkyl substituents, the aggregation tendency of the PPV chains is reduced, which causes strongly reduced mode propagation losses from 20 dB/cm to less than 1 dB/cm. We also see a dilution effect of the conjugated π-electrons of the polymer backbones within a shell of alkyl chains, and varied orientations of PPV main chain segments with respect to the layer plane. These insights help to design suitable polymers for all-optical switching, as well as for other applications in optoelectronics.

Molecular Weight Effects in the Third-Harmonic Generation Spectroscopy of Thin Films of the Conjugated Polymer MEH-PPV

Thin spin-cast films of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) were prepared from samples whose weight-average molecular weight (Mw) was varied in the range of 10-1600 kg/mol. We have characterized the films by means of transmission and reflection ultraviolet-visible-near-infrared (UV-vis-NIR) spectroscopy to derive the linear optical constants, and third-harmonic generation spectroscopy with variable laser wavelengths to get the modulus and phase angles of the complex third-order nonlinear optical susceptibility chi(3). Increasing molecular weight yields films with significantly larger chi(3) values, absorption coefficients, and refractive indices. The chi(3) values of films from the largest and lowest Mw differ by a factor of 4, which is caused by chain orientation effects, local field effects, and changes of the effective conjugation length.

Sr2TMO3 (TM = Ni, Co) Compounds with 1D TM–O Chains

Sr2TMO3 (TM = Ni,Co) compounds with 1D TM-O chains have been successfully synthesized in the form of single-crystalline thin films using a combinatorial approach. The crystal structure is examined using X-ray diffraction and high-resolution TEM (see figure). Polarized absorption spectroscopy and third-harmonic generation spectroscopy reveal a 1D electronic structure and sizable nonlinear optical susceptibility, respectively, for Sr2NiO3.

Effect of Donor−Acceptor Substitution on the Nonlinear Optical Properties of Oligo(1,4-phenyleneethynylene)s Studied by Third Harmonic Generation Spectroscopy

Third-harmonic generation (THG) spectroscopy was performed for oligo(1,4-phenyleneethynylene)s (OPEs) with terminal donor-acceptor (DA) substitution and compared to the results of merely donor substituted OPEs and regular OPE chains with 2,5-dipropoxy benzene rings. Both, extension of the conjugation and push-pull effect enhance the molecular hyperpolarizability gamma, even for the DAOPEs, which exhibit a hypsochromic shift of the long-wavelength absorption for increasing length L of the conjugated chain.

Large Optical Nonlinearity in 1D and 2D Cuprates Investigated by Third Harmonic Generation Spectroscopy

Spectra of third-order nonlinear susceptibility |X (3) | have been investigated for 1D Mott insulators (Mls), Sr 2 CuO 3 and Ca 2 CuO 3 and 2D Mls, Nd 2 CuO 4 and La 2 CuO 4 , by using third-harmonic generation (THG) spectroscopy. The three-photon resonance to the odd-charge-transfer (CT) state enhances |X (3) |. The maximum values of |X (3) | in the 2D MIs (∼10 -10 esu) are one order smaller than those in the 1D Mls(∼10 -9 esu). The difference of the magnitudes and spectral shapes of |X (3) | between the 1D and 2D Mls can be explained by taking account of the concept of spin-charge separation.

Second- and third-harmonic generation spectroscopy of coupled microcavities formed from all-silicon photonic crystals

ABSTRACTThe enhancement of second- and third-harmonic generation (SHG and THG) in all-silicon coupled microcavities (CMC) formed from mesoporous silicon photonic crystals are observed at the resonance of the fundamental radiation with the CMC eigenmodes. Angular splitting of the peaks in intensity spectra of SHG and THG shows monotonous dependence on magnitude of coupling between two identical microcavity spacers controlled by the reflectivity of the intermediate Bragg reflector.

Large third-order optical nonlinearity of Cu-O chains investigated by third-harmonic generation spectroscopy.

Spectra of the third-order nonlinear susceptibility chi((3)) have been investigated for one-dimensional Mott insulators, Sr(2)CuO(3) and Ca(2)CuO(3), by applying the third-harmonic generation (THG) spectroscopy on their single-crystalline thin films. The three-photon resonance to the lowest charge-transfer (CT) state with odd parity strongly enhances chi((3)), which is of the order of 10(-9) esu. The two-photon resonant structure unravels the even-CT state, located close to the odd-CT state. Two types of chi((3)) spectra obtained from THG and the electroreflectance measurements are explained based on the concept of spin-charge separation.

Subpicosecond dynamics of excitons and photoexcited intrinsic polarons in the quasi-one-dimensional solid PtCl

Subpicosecond transient absorption (TA) spectroscopy was used to identify excited states, and to measure their relaxation kinetics as a function of excitation wavelength in the halogen-bridged transition-metal $(\mathrm{MX})$ compound PtCl. For pump photon energies ${E}_{\mathrm{pump}}$ between 1.3 and 1.8 eV, below the threshold for exciton formation, intrinsic polarons are photoexcited and decay within 1--3 ps. Excitation into the exciton absorption tail (2.0--2.3 eV) yields singlet excitons that rapidly decay into an even-parity state, which has a lifetime (\ensuremath{\sim}100 ps) varying with the intial photon energy. Excitation (3.1 eV) well above the exciton threshold appears to generate more slowly decaying excitons, as well as polarons that are formed upon exciton dissociation. The TA spectra also indicate that relaxation of excitons for ${E}_{\mathrm{pump}}>2\mathrm{}\mathrm{eV}$ is mediated by large-amplitude vibrations perpendicular to the chains. Using third-harmonic generation spectroscopy in addition to TA, we develop an energy-level diagram for the excited states, including polaron levels, and the lowest odd- and even-parity excitons.

Observation of electric-dipole-forbidden states in polydiacetylene by means of second-harmonic generation spectroscopy

We have observed second-harmonic generation (SHG) and third-harmonic generation (THG) spectra in polydiacetylene (PDA), poly[5,7-dodecadiyne-1,12-diyl-bis-ethylurethane], thin films. According to linear absorption and THG spectra in PDA, two dipole-allowed states, the lowest B u exciton and the band edge, exist around 2.0 eV and 2.6 eV, respectively. Therefore, a resonant peak appearing around 2.35 eV in SHG and THG spectra is due to the dipole-forbidden A g exciton state. Thus, the SHG spectroscopy serves as a powerful tool for investigating dipole-forbidden states as much as THG spectroscopy.

Subphthalocyanines: Novel Targets for Remarkable Second-Order Optical Nonlinearities

The design and synthesis of new molecular and supramolecular systems for nonlinear optics (NLO) is currently of great scientific and technological interest.1 Until now investigations on second-order nonlinear optical properties of organic molecules have concentrated mainly on one-dimensional dipolar systems. Recently, theoretical and experimental considerations have suggested the possibility of developing new synthetic approaches to molecules with nonlinearities of so-called “multipolar” origin.2-5 In this communication we show that subphthalocyanines (SubPcs), π-conjugated phthalocyanine-related compounds, present very large second-order molecular polarizabilities (â) and emerge as novel targets for second-order nonlinear optical applications. Thus, for example the trinitroSubPc 1, described here for the first time, has shown experimental values of 〈â2〉1/2(2ω) ) 2000 × 10-30 (static 〈â2(0)〉1/2 ) 407 × 10-30 esu) by hyper-Rayleigh scattering (HRS) experiments. These second-order polarizabilities are mostly associated with the octupolar contribution and are comparable to those found in the most efficient linear (or dipolar type) compounds, such as polyenes,6 and to the best octupolar molecules described until now. Phthalocyanines (Pcs) are two-dimensional macrocyclic 18π-electron conjugated systems with a great variety of technological applications.7 In the last years Pcs8 and related analogues9 have emerged also as a novel class of molecular materials for third-order NLO applications. SubPcs 1-3 (Figure 1) are Pc-related macrocyclic compounds formed by three coupled isoindole moieties having a delocalized 14-π-electron system.10 Despite their cone-shaped structure,11 these compounds have an aromatic nature.12 Few axially and peripherally substituted boron SubPcs10,13-15 and dimers of SubPcs12 have been described. Their NLO properties remain unexplored, except for a very recent study on the third-harmonic generation (THG) spectroscopy of an evaporated thin film of SubPc 2,16 where values of o(3), three times larger than those corresponding to Pcs in the same wavelength range, were measured. In that case, the possibility of “cascading mechanisms” related to important second-order NLO properties was suggested. Therefore, the main objective of this work has been to evaluate the potential of these new molecules for second-order NLO applications. In this work we follow a 2D strategy for the design of SubPcbased NLO systems. Thus, besides the nonsubstituted SubPc 2, we prepared SubPc 1, with three surrounding strong acceptor substituents (NO2) forming a trigonal arrangement (Figure 2), and SubPc 3, having three peripheral donor groups (tert-Bu). Compounds 210,16 and 312 were prepared as previously described. Trinitro-SubPc 1 was synthesized for the first time in a similar way, by condensation of 4-nitro-1,2-dicyanobenzene in the pres-