Resonant Sum-frequency Generation Research Articles

Vibronic coupling of Rhodamine 6G molecules studied by doubly resonant sum frequency generation spectroscopy with narrowband infrared and broadband visible.

Doubly resonant sum frequency generation (DR-SFG) serves as a potent characteristic technique for probing the electronic spectra and vibronic coupling of molecules on surfaces. In this study, we successfully developed a novel infrared (IR)-white light (WL) DR-SFG spectroscopy based on narrowband IR and tunable broadband WL. This novel method was employed to explore the excitation spectrum and vibronic couplings of sub-monolayer Rhodamine 6G molecules. Our findings elucidate that the xanthene skeleton vibrational modes exhibit strong coupling with the S0-S1 electronic transition. Notably, we observed not only the 0-0 transition of the S0-S1 electronic continuum but also the 0-1 transition, a first time observation in the realm of DR-SFG spectroscopy. This advanced DR-SFG spectroscopy methodology facilitates a more sensitive examination of electronic spectra and the coupling between electronic transitions and vibrational modes, heralding a significant advancement in the understanding of molecular interactions on surfaces.

Operando measurements of top-emission organic light-emitting diode using electric-field-induced doubly resonant sum-frequency generation vibrational spectroscopy

We performed sum-frequency generation (SFG) measurements on top-emission (TE) organic light-emitting diodes (OLEDs) during operation. These measurements require the detection of SFG light using probe laser beams transmitted through a 9 nm thick semitransparent Mg:Ag metal electrode. Only the SFG signals from the hole transport material (4,4′-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl, NPD) are observed for the device configuration of the OLEDs in this study. The SFG peak (at approximately 1603 cm−1) shows a linear dependence on the applied voltage. This indicates that the NPD-derived SFG is enhanced by an electric-field-induced (EFI) effect owing to injected charges. In addition, the SFG efficiency of the TE OLED is considerably higher than that of a bottom-emission (BE) OLED manufactured in the same batch using the same material and film thickness. The difference in the SFG responses of the BE and TE OLEDs is caused by structural differences rather than the layer composition. The results of this study indicate that EFI-doubly resonant-SFG operando measurements can be applied to analyze the carrier behavior and charge transport of not only BE OLEDs but also TE OLEDs, the latter of which are used more commonly as commercial products.

Development of Two-Dimensional Electronic-Vibrational Sum Frequency Generation (2D-EVSFG) for Vibronic and Solvent Couplings of Molecules at Interfaces and Surfaces.

Many photoinduced excited states' relaxation processes and chemical reactions occur at interfaces and surfaces, including charge transfer, energy transfer, proton transfer, proton-coupled electron transfer, configurational dynamics, conical intersections, etc. Of them, interactions of electronic and vibrational motions, namely, vibronic couplings, are the main determining factors for the relaxation processes or reaction pathways. However, time-resolved electronic-vibrational spectroscopy for interfaces and surfaces is lacking. Here we develop interface/surface-specific two-dimensional electronic-vibrational sum frequency generation spectroscopy (2D-EVSFG) for time-dependent vibronic coupling of excited states at interfaces and surfaces. We further demonstrate the fourth-order technique by investigating vibronic coupling, solvent correlation, and time evolution of the coupling for photoexcited interface-active molecules, crystal violet (CV), at the air/water interface as an example. The two vibronic absorption peaks for CV molecules at the interface from the 2D-EVSFG experiments were found to be more prominent than their counterparts in bulk from 2D-EV. Quantitative analysis of the vibronic peaks in 2D-EVSFG suggested that a non-Condon process participates in the photoexcitation of CV at the interface. We further reveal vibrational solvent coupling for the zeroth level on the electronic state with respect to that on the ground state, which is directly related to the magnitude of its change in solvent reorganization energy. The change in the solvent reorganization energy at the interface is much smaller than that in bulk methanol. Time-dependent center line slopes (CLSs) of 2D-EVSFG also showed that kinetic behaviors of CV at the air/water interface are significantly different from those in bulk methanol. Our ultrafast 2D-EVSFG experiments not only offer vibrational information on both excited states and the ground state as compared with the traditional doubly resonant sum frequency generation and electronic-vibrational coupling but also provide vibronic coupling, dynamical solvent effects, and time evolution of vibronic coupling at interfaces.

Probing the Gold/Water Interface with Surface-Specific Spectroscopy.

Water is an integral component in electrochemistry, in the generation of the electric double layer, and in the propagation of the interfacial electric fields into the solution; however, probing the molecular-level structure of interfacial water near functioning electrode surfaces remains challenging. Due to the surface-specificity, sum-frequency-generation (SFG) spectroscopy offers an opportunity to investigate the structure of water near working electrochemical interfaces but probing the hydrogen-bonded structure of water at this buried electrode-electrolyte interface was thought to be impossible. Propagating the laser beams through the solvent leads to a large attenuation of the infrared light due to the absorption of water, and interrogating the interface by sending the laser beams through the electrode normally obscures the SFG spectra due to the large nonlinear response of conduction band electrons. Here, we show that the latter limitation is removed when the gold layer is thin. To demonstrate this, we prepared Au gradient films on CaF2 with a thickness between 0 and 8 nm. SFG spectra of the Au gradient films in contact with H2O and D2O demonstrate that resonant water SFG spectra can be obtained using Au films with a thickness of ∼2 nm or less. The measured spectra are distinctively different from the frequency-dependent Fresnel factors of the interface, suggesting that the features we observe in the OH stretching region indeed do not arise from the nonresonant response of the Au films. With the newfound ability to probe interfacial solvent structure at electrode/aqueous interfaces, we hope to provide insights into more efficient electrolyte composition and electrode design.

All-experimental analysis of doubly resonant sum-frequency generation spectra for Franck–Condon and Herzberg–Teller vibronic modes

The transform technique applied to the analysis of doubly resonant sum-frequency generation (DR-SFG) spectra is extended to include Herzberg-Teller (HT) vibronic modes. The experimentally measured overlap spectral function generates all the energy resonant amplitudes of the DR-SFG excitation function for both Franck-Condon (FC) and HT modes. When FC modes dominate the DR-SFG spectra, a methodology is provided to perform efficient curve fitting and orientation analysis in order to extract FC activities of the various vibration modes from experimental spectra with the help of a molecular model. Determination of the FC or HT natures of the vibration modes from DR-SFG data is also shown to be possible through their visible line shapes with an appropriate choice of polarizations. As an example, experimental DR-SFG data suggest that a known HT-active mode in the vibronic structure of Rhodamine 6G monomers exhibits a FC behavior in molecular aggregates.

Phase-Sensitive Vibrationally Resonant Sum-Frequency Generation Microscopy in Multiplex Configuration at 80 MHz Repetition Rate.

Vibrationally resonant sum-frequency generation (VR SFG) microscopy is an advanced imaging technique that can map out the intensity contrast of infrared and Raman active vibrational modes with micron to submicron lateral resolution. To broaden its applications and to obtain a molecular level of understanding, further technical advancement is needed to enable high-speed measurements of VR SFG microspectra at every pixel. In this study, we demonstrate a new VR SFG hyperspectral imaging platform combined with an ultrafast laser system operated at a repetition rate of 80 MHz. The multiplex configuration with broadband mid-infrared pulses makes it possible to measure a single microspectrum of CH/CH2 stretching modes in biological samples, such as starch granules and type I collagen tissue, with an exposure time of hundreds of milliseconds. Switching from the homodyne- to heterodyne-detected VR SFG hyperspectral imaging can be achieved by inserting a pair of optics into the beam path for local oscillator generation and delay time adjustment, which enables self-phase-stabilized spectral interferometry. We investigate the relationship between phase images of several different C-H modes and the relative orientation of collagen triple-helix in fibril bundles. The results show that the new multiplex VR SFG microscope operated at a high repetition rate is a powerful approach to probe the structural features and spatial arrangements of biological systems in detail.

All-experimental analysis of doubly resonant sum-frequency generation spectra: Application to aggregated rhodamine films.

A new method is proposed to analyze Doubly Resonant infrared-visible Sum-Frequency Generation (DR-SFG) spectra. Based on the transform technique, this approach is free from assumptions about vibronic modes, energies, or line widths and accurately captures through the overlap spectral function all required aspects of the vibronic structure from simple experimental linear absorption spectra. Details and implementation of the method are provided along with three examples treating rhodamine thin films about one monolayer thick. The technique leads to a perfect agreement between experiment and simulations of the visible DR-SFG line shapes, even in the case of complex intermolecular interactions resulting from J-aggregated chromophores in heterogeneous films. For films with mixed H- and J-aggregates, separation of their responses shows that the J-aggregate DR-SFG response is dominant. Our analysis also accounts for the unexplained results published in the early times of DR-SFG experiments.

Probing Delocalized Current Densities in Selenophene by Resonant X-ray Sum-Frequency Generation

Time-resolved, resonant X-ray sum-frequency generation in aligned selenophene molecules is calculated. A wave packet of valence-excited states, prepared by an extreme-ultraviolet pump pulse, is probed by two 12-keV X-ray probe pulses resonant with the Se core-excited states for variable time delays. At these hard-X-ray frequencies, the angström wavelength of the X-ray probe is comparable to the molecular size. We thus employ a nonlocal description of the light-matter interaction based on the minimal-coupling Hamiltonian. The wavevector-resolved resonant stimulated sum-frequency-generation signal, obtained by varying the propagation direction of hard-X-ray pulses, can thus directly monitor the transition current densities between core and ground/valence states. This is in contrast to off-resonant diffraction, which detects the transition charge densities.

Doubly resonant SFG and DFG spectroscopies: An analytic model for data analysis including distorted and rotated vibronic levels. I. Theory.

We present an analytic description of doubly resonant infrared-visible sum (SFG) and difference frequency generation (DFG) spectroscopies. Within the Born-Oppenheimer and Condon approximations for harmonic oscillators, we extend the usual theory, limited to linear electron-vibration coupling, and introduce the quadratic coupling phenomena (mode distortion and mode mixing) in the excited state. The excitation spectra of vibrations in SFG and DFG experiments are calculated in integral form for arbitrary mode distortions and small amplitude mode mixing between pairs of modes. Mode distortion modifies all orders of vibronic coupling including the fundamental process, whereas mode mixing appears as a perturbation added to the distorted mode case. For small quadratic coupling amplitudes, the results may be recast in simple analytic forms after the introduction of the overlap spectral function and developed in sums and products of Lorentzian functions.

Doubly resonant SFG and DFG spectroscopies: An analytic model for data analysis including distorted and rotated vibronic levels. II. Applications.

The influence of vibration mode distortion and mode mixing in the vibronic structure of molecules on the doubly resonant sum (SFG) and difference frequency generation spectroscopies is systematically studied in several examples. These phenomena modify the spectral overlap function at the heart of the modeling of the doubly resonant processes. When the visible beam is tuned, under the Franck-Condon approximation, each vibration mode generates two main peaks corresponding to the resonance of the molecular electronic transition with the visible and SFG energies, together with higher order ones driven by the amplitudes of the vibronic displacements. For a single mode system, mode distortion modifies the positions of the main peaks and the excitations of the higher order ones. For a two-mode system, mode mixing induces in addition a deep change in the balance between the intensities of the mixed modes. It also changes the phases of the vibrations, which makes curve fitting difficult. For multimode systems, mode mixing may greatly enhance intensity of a poorly active mode. For all these reasons, it seems mandatory to take mode distortion and mode mixing into account for an accurate analysis of second-order nonlinear spectroscopic experimental data.

Dual Resonant Sum Frequency Generations from Two-Dimensional Materials.

We propose dual resonant optical sum frequency generation (SFG), where the two most singular resonances could be selected, and report for the monolayer (1L-) WSe2 when one (ω1) of two excitation pulses is resonant to A exciton and their sum frequency (ω1 + ω2) to D exciton. The dual resonant SFG confirms that, under an irradiation of ω1 and ω2 pulses with the same fluence of ∼1.4 × 1010 W/m2, its signal intensity could be enhanced about 20 times higher than the resonant SHG (i.e., 2ω1 to the D excitonic absorption). Further, the dual resonant SFG intensity of 1L-WSe2 is found to be 1 order of magnitude higher than the single resonant SFG intensity of 1L-WS2 under the same condition of two-pulse irradiation. Finally, observations of the dual resonant SFG are thoroughly examined using real-time time-dependent density functional theory (rt-TDDFT), and the relevant nonlinear optical characteristics are scrutinized using the Greenwood-Kubo formalism.

Resonant X-ray Sum-Frequency-Generation Spectroscopy of K-Edges in Acetyl Fluoride.

Resonant X-ray sum-frequency generation is calculated for excitations of the fluorine and the oxygen core K-edge in acetyl fluoride using real-time time-dependent density functional theory. The signal is generated by an extreme-ultraviolet pulse followed by an X-ray pulse with variable delay T. The X-ray pulse is tuned to different element-specific core excitations and used to probe the dynamics of a valence electronic wave packet. A two-dimensional signal is recorded depending on the dispersed X-ray pulse frequency and the frequency conjugated to T, revealing the couplings between core and valence excited states. Molecular orbital decomposition of the signal reveals which regions of the molecule contribute to the X-ray excitation.

Direct observations of the charge behavior of a high-efficiency blue organic light-emitting diode under operating conditions using electric-field-induced doubly resonant sum-frequency-generation vibrational spectroscopy

We investigated the carrier behavior in a high-efficiency multilayer blue organic light-emitting diode (OLED) that employs the triplet–triplet annihilation (TTA) phenomenon using electric-field-induced doubly resonant sum-frequency generation (EFI-DR-SFG) vibrational spectroscopy. We observed enhancements of the EFI-DR-SFG signals—particularly the peaks at 1404, 1464, and 1601 cm−1—when we applied forward- or reverse-bias voltages to the blue OLEDs. The results of our EFI-DR-SFG analyses of the carrier behavior in an operating blue OLED showed changes in the peak intensities at 1404 and 1464 cm−1, indicating that the electrons accumulate at the electron-blocking layer/emission layer interface. Conversely, the changes in the intensities of the peaks obtained from the hole-transport materials are caused by the cancelation of the interfacial polarization charges at the hole-injection layer/hole-transport layer interface. These results demonstrate that the EFI-DR-SFG technique is a powerful and effective tool for directly investigating the carrier behavior in practical multilayer organic devices with complicated operating mechanisms.

Doubly resonant sum frequency spectroscopy of mixed photochromic isomers on surfaces reveals conformation-specific vibronic effects.

Doubly resonant infrared-visible sum-frequency generation (DR-IVSFG) spectroscopy, encompassing coupled vibrational and electronic transitions, provides a powerful method to gain a deep understanding of nuclear motion in photoresponsive surface adsorbates and interfaces. Here, we use DR-IVSFG to elucidate the role of vibronic coupling in a surface-confined donor-acceptor substituted azobenzene. Our study reveals some unique features of DR-IVSFG that have not been previously reported. In particular, vibronic coupling resulted in prominent SFG signal enhancement of selective stretching modes that reveal electronic properties of coexisting photochromic isomers. Our analysis explores two concepts: (1) In partially isomerized azobenzene at the surface, coupling of the fundamental vibrations to the S0 → S1 transition is more prominent for the cis isomer due to symmetry breaking, whereas coupling to the S0 → S2 transition was dominant in the trans isomer. (2) A strong coupling between the fundamental vibrations and the valence π-electron density, promoted by the initial absorption of an infrared photon, may result in suppression of the intensity of the hot band vibronic transition. This may translate into a suppressed sum-frequency generation signal at sum frequency wavelengths resonant with the S0 → S2 transition of the trans isomer. The weaker coupling of the fundamental vibrations to the non-bonding electron density localized on the azo group can therefore produce detectable sum-frequency generation at the resonance wavelength of the weaker S0 → S1 transition in the cis form. These results are explained in the framework of a linear coupling model, involving both Franck-Condon and Herzberg-Teller coupling terms. Our theoretical analysis reveals the important role played by molecular conformation, orientation, and vibronic interference in DR-SFG spectroscopy.

Triple Modal Coherent Nonlinear Imaging with Vibrational Contrast

By combining spectroscopic contrast with imaging speed, coherent Raman scattering (CRS) microscopy techniques have reiterated the usefulness of vibrational microscopy. In the nonlinear optical (NLO) microscope, CRS is often combined with other nonlinear optical signals, such as two-photon excited fluorescence or second-harmonic generation, providing images with multimodal contrast. However, it is also possible to expand the vibrational capabilities of the NLO microscope, namely through incorporation of other vibrationally selective nonlinear optical signals beyond CRS. Recently, it has been shown that vibrationally resonant sum-frequency generation (SFG) microscopy can be integrated into the imaging platform as well, offering additional vibrational spectroscopic probing capabilities. In this contribution, we highlight the concept of triple-modal vibrational microscopy, which combines CRS, SFG, and third-order sum-frequency generation on a single laser-scanning microscope. These three vibrationally sensitive NLO imaging techniques provide complementary information about the sample, allowing for more detailed molecular spectroscopic studies of biological specimens. We review the principles of the three light-matter interactions that underlie the techniques and discuss some of the imaging properties of the triple-modal vibrational microscope. We provide several imaging examples and conclude with discussing some of the outstanding challenges.

Symmetric Raman Tensor Contributes to Chiral Vibrational Sum-Frequency Generation from Binaphthyl Amphiphile Monolayers on Water: Study of Electronic Resonance Amplitude and Phase Profiles

Heterodyne-detected vibrationally electronically doubly resonant chiral sum-frequency generation (HD-DR chiral SFG) spectra were observed for the first time on monolayers. Langmuir monolayers of R- and S-binaphthyl amphiphiles on water exhibited complex chiral vibrational sum frequency generation (VSFG) spectra whose amplitudes were approximately the same, but whose phases were different by π. By comparing the electronic resonance profiles of the chiral VSFG signal amplitude and phase with simulations based on a nonadiabatic resonance Raman theory, we concluded that the chiral VSFG signals from the monolayers originated not from the antisymmetric Raman tensor but from the symmetric one. This is the first experimental evidence that the symmetric Raman tensor indeed contributes to the chiral VSFG signals from oriented monolayer systems.

Aggregated States of Chalcogenorhodamine Dyes on Nanocrystalline Titania Revealed by Doubly Resonant Sum Frequency Spectroscopy

Broadband doubly resonant sum frequency generation (DR-SFG) spectroscopy was used to probe selectively the vibrational signatures of a series of chalcogenorhodamine dyes tethered to TiO2 either as monomers or as H-aggregates. This selectivity was achieved by virtue of the vibronic coupling associated with the unique electronic transitions for the corresponding states. It was found that clear spectral differences and polarization dependences were observed contingent on whether the outgoing SFG wavelength was in resonance with either the monomer or H-aggregate electronic excited states. Our results also indicate that the orientation and proximity of the xanthylium core to the semiconducting surface play an important role in this vibrational–electronic coupling and may affect the character of the charge-transfer complex. Our DR-SFG results also provide important clues into the anchoring motifs for carboxylic and phosphonic acid groups.

High-efficiency frequency upconversion of 1.5 μm laser based on a doubly resonant external ring cavity with a low finesse for signal field

A doubly resonant external ring cavity with a low finesse for the signal field is used to improve the frequency upconversion efficiency of a weak 1583 nm signal laser to 636 nm by mixing with a resonance power enhanced 1064 nm pump laser in a 25 mm periodically poled lithium niobate crystal. The process of frequency upconversion is described and optimized by the doubly resonant cavity-enhanced sum frequency generation theory under the condition of undepleted pump approximation. By selecting the suitable reflectivity of the signal input mirror and the incident pump power, a cavity-enhanced frequency conversion efficiency of 94.6% was obtained for signal powers up to 25 mW with an input pump power of 780 mW.

Double resonant sum-frequency generation in an external-cavity under high-efficiency frequency conversion

In recent years, more than 90% of the signal laser power can be up-converted based on the high-efficiency double resonant external cavity sum-frequency generation (SFG), especially when the whole system runs under the undepleted pump approximation scheme. Therefore it is difficult to directly achieve an error signal with a high signal-to-noise ratio through the signal laser to lock its frequency to the cavity mode. In this paper a novel method, based on the frequency modulation of signal laser and demodulation of the SFG laser, is used to obtain the error signal to realize the cascade frequency locking between the two fundamental lasers and the external cavity. In this experiment, 1064 nm laser is the pump laser and 1583 nm laser is the signal laser. They are coupled into a ring cavity inside which a 5% MgO-doped PPLN (25 mm1 mm0.5 mm) is used to produce the SFG laser of 636 nm. When the pump laser is resonant with the external cavity, a circulating power of 14.3 W is obtained with its input power of 1.3 W. The reflectivity of the input coupling mirror of signal laser is 10% to restrain the impendence mismatch. The temperature of PPLN is set at 68.5 ℃ to reach the optimum SFG temperature. In order to keep the signal laser resonance inside the external cavity, one needs to lock its frequency to the cavity mode. A 28.5 kHz sinusoidal voltage is used to modulate the frequency of the signal laser so that the frequency of 636 nm laser is modulated simultaneously. Then 5% of the output 636 nm laser power is sent into a Si photodiode detector the signal of which is demodulated at the modulation frequency by a lock-in amplifier. Finally the demodulated signal is feedback to the frequency control port of signal laser. Under these conditions, 73% of 1583 nm signal laser power can be converted into 636 nm laser power when the incident power varies from 10 W to demodulation of the transmitted cavity mode of 1583 nm when the incident signal laser power is below 12 mW. When the signal laser power increases from 50 mW to 295 mW, the conversion efficiency linearly drops to 60%, which is mainly caused by depleting the 1064 nm pump laser power. Finally a 440 mW of 636 nm laser is generated with an incident signal laser power of 295 mW. This scheme can realize a high-efficiency SFG with a low input signal laser power or poor single-pass SFG efficiency.

Theoretical investigation of impedance matching in the process of sum-frequency generation in an external resonator

The sum-frequency conversion efficiency is directly proportional to the product of two fundamental laser powers. Therefore, sum-frequency conversion efficiency is rather low when the fundamental beams pass through a nonlinear crystal only once. External resonant technique as an effective means of improving the powers of the fundamental light has been widely applied to the field of nonlinear frequency conversion. This technique can greatly improve the sum-frequency conversion efficiency and is particularly suitable for the situation in which the input power of the fundamental frequency lasers bas been limited. The implementation of high efficient sum-frequency generation in an external resonator requires that the fundamental frequency laser should be efficiently coupled to the external cavity. Therefore, the system needs to achieve impedance matching. In the part of theoretical analysis, first, we derive the enhancement factor when travelingwave cavity is resonant, and then, establish the theoretical models of doubly resonant and singly resonant sum-frequency generation in an external resonator respectively. The variation of enhancement factors as functions of reflectivity of the input couplers and power of the input fundamental light for doubly resonant and singly resonant sum-frequency systems is derived from Boyd-Kleinman theory in detail based on the theoretical models described in the text. The expressions of enhancement factors reflect the nonlinear correlation characteristics of two fundamental light beams in the process of sum-frequency generation. In the part of numerical simulation, firstly, we draw contour plots of output power as functions of reflectivity of the input couplers at two input frequencies in the doubly resonant sum-frequency system by theoretical simulation, and achieve an optimum reflectivity of the input couplers under the condition of different powers of input fundamental light. Secondly, we draw the contour plots of output power as functions of the reflectivity of the input coupler at the resonant frequency, and the input power of non-resonant frequency light in the singly resonant sum-frequency system by theoretical simulation, and achieve an optimum reflectivity of the input coupler at the resonant frequency. These optimum values enable the system to achieve impedance matching; consequently, the sum-frequency conversion efficiency is improved. Finally, this paper analyzes the influence of input power on the impedance matching, and shows that the optimal coupling mirror reflectivity of the resonant fundamental frequency will decrease with the increase of incident power of the other resonant or non-resonant fundamental frequency laser, otherwise, the resonant incident power of its own has less influence on the optimal coupling mirror reflectivity, whether the system undergoes doubly resonant or singly resonant sum-frequency. In addition, if the coupling mirror reflectivity exceeds the optimum value, the power of sum-frequency light will decrease rapidly, while if it is less than the optimum value, the power of sum-frequency light decreases relatively slowly. Therefore an input coupler that may yield over-coupling should be avoided. These results will have a certain guiding significance to related experiments.