Photoinduced Second Harmonic Generation Research Articles

Tunable photo-induced second-harmonic generation in a mode-engineered silicon nitride microresonator.

All-optical poling enables reconfigurable and efficient quasi-phase-matching for second-order parametric frequency conversion in silicon nitride integrated photonics. Here, we report broadly tunable milliwatt-level second-harmonic generation in a small free spectral range silicon nitride microresonator, where the pump and its second-harmonic are both always on the fundamental mode. By carefully engineering the light coupling region between the bus and microresonator, we simultaneously achieve critical coupling of the pump as well as efficient extraction of second-harmonic light from the cavity. Thermal tuning of second-harmonic generation is demonstrated with an integrated heater in a frequency grid of 47 GHz over a 10 nm band.

Considering photo-induced second harmonic generation as a DC-Kerr optical parametric oscillation or amplification process.

Photo-induced second harmonic generation (SHG) in centro-symmetric materials like silica and silicon nitride has been commonly explained as an effective second-order ( ) process mediated by a DC electric field and the medium's third-order ( ) nonlinearity. In this explanation, the coherent photogalvanic effect is the source of a DC electric field whose spatial periodicity naturally enables quasi-phase matching. While successful in explaining many observations from experiment, the behavior at low input powers, and in particular, the apparent existence of a threshold for efficient photo-induced SHG observed in some experiments has largely been overlooked theoretically. In this letter, we reconsider photo-induced SHG within the framework of four-wave mixing involving degenerate pump, second harmonic signal, and DC electric field. We propose a hypothesis that photo-induced SHG is a FWM-mediated DC-Kerr optical parametric oscillation/amplification process. This hypothesis can explain the threshold behavior, and moreover, predicts unconventional light amplification, both of which we verify by experiments in silicon nitride microresonators. Finally, we discuss the physical implications of our work in various platforms and future directions.

Second harmonic generation and spectroscopic characteristics of TiO2 doped Li2O–Al2O3–B2O3 glass matrix

In this present study, Li2O–Al2O3–B2O3 glass matrix doped with a small percentage of TiO2 (ranging from 0 to 1 mol%) was prepared and characterized by X-Ray Diffraction (XRD), FTIR and Differential Scanning Calorimetric (DSC) techniques. After that, Optical Absorption (OA), Photo Luminescence (PL), Electron Paramagnetic Resonance (EPR) and Second Harmonic Generation (SHG) have been performed as a function of TiO2 concentration. The outcome of these studies spelled out different oxidation states of Ti ions. However, sample containing up to about 0.8 mol% of the TiO2, mostly will be in the state of Ti4+ with substitutional octahedral arrangement in the glass chain and is responsible for PL. Beyond this percentage, it is noticed that some parts of Ti ions reduced Ti4+ to Ti3+ state. The results of Photo-Induced Second Harmonic Generation (PSHG) together with other spectroscopic studies were discussed for all chosen samples and indicated that the Photo-Induced Birefringence (PIB) of sample with 0.8 mol% TiO2 is the most suitable for NLO (non-linear optics) devices.

Two-component waveguide SiO2:TiO2 films fabricated by sol–gel technology for optoelectronic applications

The research results involving two-component waveguide films SiO2:TiO2 produced in sol–gel technology are presented. The waveguide films were deposited on soda-lime glass substrates using dip-coating method. The titled layers were fabricated as waveguide layers for application in planar evanescent wave sensors. The such prepared slab waveguides possess low optical losses and they demonstrate a possibility of formation of the photo induced optical second harmonic generation (SHG) which is observed due to photoinduced charge density non-centrosymmetry. In the present work we explore additionally the dependences of the photoinduced SHG versus the power density and the time of treatment.

BiB3O6:Pr3+ partially crystallized glasses for second harmonic generation

A series of praseodymium-doped BiB3O6 (with Pr3+ content 1 at.%) glass samples were prepared under conditions allowing their partial crystallization. It is well known that in pure glasses the second order optical effects like second harmonic generation (SHG) are forbidden by symmetry, however in the partially crystallized glasses during additional bicolor laser poling the SHG effects may be observed. The degree of crystallinity was measured with XRD ranges from 0 (purely amorphous glass) to 8.11 %. Photoinduced SHG was found to be dependent on the degree of crystallinity and demonstrates maximum for DC around 5 %. Similar results versus the degree of the crystallinity were obtained for linear electrooptics effect.

Specific features of photoinduced absorption and second harmonic generation of ferroic organic nanocomposites $$[\hbox {C}_{3}\hbox {H}_{7}\hbox {NH}_{3}]_{2}\hbox {MnCl}_{4}$$ [ C 3 H 7 NH 3 ] 2 MnCl 4

We have done the studies of the photoinduced absorption and of the related second harmonic generation (SHG) for the \([\hbox {C}_{3}\hbox {H}_{7}\hbox {NH}_{3}]_{2}\hbox {MeCl}_{4}\) (where Me=Cu, Mn). We have found substantial changes of the absorption under influence of the continuous wave lasers. The changes of the photoinduced optical SHG were stimulated by the two bicolour laser coherent beams. The results presented show that for the nanosecond 1,064 nm Nd:YAG laser beam principal changes were observed for the Cu-containing samples. The diethyl ammonium samples doped by Mn and Cu have shown substantially higher photo induced SHG up to the other samples. Simultaneously we have studied the pump-probe and size dependences. However for all the cases principal role was played by the octahedral \(\hbox {MeCl}_{6}\). Relation with the trapping levels actively interaction with phonon modes is principal here.

Structural, photoinduced optical effects and third-order nonlinear optical studies on Mn doped and Mn–Al codoped ZnO thin films under continuous wave laser irradiation

We report the photoinduced optical effects, third-order nonlinearity, and optical power limiting of Mn doped and Mn–Al codoped ZnO thin films. The thin films were prepared by the spray pyrolysis technique. The structural properties of the deposited films were examined by x-ray diffraction studies. Z-scan measurements were conducted to evaluate the nonlinear optical parameters using a He–Ne laser operating in continuous wave mode at 633 nm wavelength. The present study reveals that the introduction of Mn and Mn–Al into ZnO leads to significant changes in the third-order nonlinear susceptibility. Photoinduced second-harmonic generation studies also reveal the dependence of the nonlinear optical properties on the Mn content. Switching over from saturable absorption to reverse saturable absorption was observed. The optical limiting studies show that the films possess a lower limiting threshold and clamping level, which is essential for use in eye and sensor protection applications. Hence, the Mn doped and Mn–Al codoped ZnO thin films investigated here emerge as promising candidates for future optoelectronic and photonic device applications such as optical power limiters.

Label-Free Observation of Single Microtubules by Means of SHG Microscopy

The microtubule is a dynamic filamentous polymer involved in various important cell processes including cell division, cell migration and organelle transport. The dynamics of microtubules highly correlate with its structure. Despite of extensive studies, however, detecting detail structure of a single microtubule in living cell is still very challenging. Here we succeeded to visualize single microtubules by optical second-harmonic generation (SHG) microscopy which is one of powerful tools to characterize macroscopic structure of molecular assemblies in liquid. The technique allows us to detect nonlinear optical scattering signal arising from an electric polarization in a microtubule which is related to alignment of tubulins so that we can get information about structure of a microtubule without any labeling to sample. We demonstrate the results of SHG imaging of single microtubules in vitro using a home-built optical system. During the experiments, we found that photo-induced SHG (PISHG) signal from cover slips became dominant source of non-negligible background noise, and decreased signal-to-noise ratio. To suppress the PISHG, we used cover slips of highly-pure SiO2. As a result signal-to-noise ratio has been dramatically improved compared with the case we used borosilicate glasses, and we have successfully obtained clear images of microtubules. Furthermore, we have performed polarization-resolved measurements and obtained quantitative data which could be used to evaluate the structures of microtubules.

Optoelectronic operation in ferroic [NH2(C2H5)2]2CuxCo1−xCl4 nanocomposites

The complex studies of optically induced piezooptics and second harmonic generation (SHG) are performed for the [NH2(C2H5)2]2CuxCo1−xCl4 nanocomposites in a form of films and bulks. The corresponding nanoparticles (NP) were embedded into different polymer matrices with content of NP varying from 1 % up to 9 % in weight units. The changes of the photoinduced optical SHG were caused by simultaneous illumination of two bicolour coherent laser beams and simultaneously applied dc-electric field at different temperatures, above glass transition temperature. The fundamental 1,064 nm Nd: YAG laser duration was equal to 12 ns. A comparison of the same features with the analogous crystals doped with Fe2+ was carried out. The samples doped by Fe3+ ions have shown substantially less photo induced SHG up to one order. The time kinetics of the photoinduced changes after the switching off the external laser fields was studied. Temperature dependences of the piezooptics and the SHG near the incommensurate phase existence were explored.

β-BaTeMo2O9 microcrystals as promising optically operated materials

Studies of optical second harmonic generation (SHG) at fundamental wavelength of 1064 nm under photoinducing treatment of monoclinic piezoelectric β-BaTeMo2O9 (β-BTMO) were done. Continuous wave (CW) lasers generating at 808 and 1040 nm were used as photoinducing sources. The investigations were performed for the β-BTMO microcrystalline powder samples with grain sizes varying within the 25–300 μm range. We showed that depending on the microcrystallites size, the photoinduced changes of the SHG were substantially different depending on number of defects which were controlled by positron annihilation. The photoinduced SHG efficiency was substantially higher for more defective crystallites. The processes are completely reversible; however, their photoinduced time kinetics is very sensitive to the wavelength of the photoinducing CW laser beam. The possible reasons for the observed differences are discussed within a framework of intrinsic defect trapping levels and their interactions with phonon subsystem.

Electronic structure of non-centrosymmetric AgCd2GaS4 and AgCd2GaSe4 single crystals

X-ray photoelectron core-level and valence-band spectra for pristine and Ar+-ion irradiated (001) surfaces of AgCd2GaS4 and AgCd2GaSe4 single crystals grown, respectively, by the Bridgman method and the method of direct crystallization have been measured in the present work. The X-ray photoelectron spectroscopy (XPS) results reveal high chemical stability of (001) surfaces of AgCd2GaS4 and AgCd2GaSe4 single crystals. Electronic structure of AgCd2GaS4 has been calculated employing the full potential linearized augmented plane wave method. For the AgCd2GaS4 compound, the X-ray emission bands representing the energy distribution of the valence Ag d-, Cd d-, Ga p- and S p-like states were recorded and compared on a common energy scale with the XPS valence-band spectrum. The theoretical and experimental data regarding the occupation of the valence band of AgCd2GaS4 were found to be in excellent agreement to each other. Second harmonic generation (SHG) efficiency of AgCd2GaS4 by using the 320ns CO laser at 5.5μm has been recorded within the temperature range 80–300K. Substantial increase of the photoinduced SHG which in turn is substantially dependent on the temperature has been detected for the AgCd2GaS4 compound.

All-optical operation by optical susceptibilities of Ca4NdO(BO3)3:Yb single crystals

We have found substantial enhancement of the optical second harmonic generation (SHG) in Ca4NdO(BO3)3:Yb (1.9% of Yb in weight units) crystals after 10 min optical treatment by the continuous wave (cw) laser at 532 nm and after 15 min treatment by blue 405 nm cw laser. Treatment by a red laser (660 nm) leads to the suppression of this effect. Principally the effect is only slightly reversible and the successive illumination of the same sample leads to the enhanced SHG, which correlates with changes in the principal absorption bands around 300–330 nm and 400–430 nm corresponding to the optical transitions between the energy levels of Nd3+ ions. The effect does not depend on the photoinducing light polarization. The principal origin of the observed photoinduced optical SHG is caused by photopolarization of the Nd localized levels and formation of the stable polarized states.

A Large Enhancement of Photoinduced Second Harmonic Generation in CdI2−Cu Layered Nanocrystals

Photoinduced second harmonic generation (PISHG) in undoped as well as in various Cu-doped (0.05-1.2% Cu) CdI2 nanocrystals was measured at liquid nitrogen temperature (LNT). It was found that the PISHG increases with increasing Cu doping up to approximately 0.6% and then decreases almost to that for the undoped CdI2 for doping higher than approximately 1%. The values of the second-order susceptibility ranged from 0.50 to 0.67 pm V(-1) for the Cu-doped nanocrystals with a thickness of 0.5 nm. The Cu-doping dependence shown in a parabolic fashion suggests a crucial role of the Cu agglomerates in the observed effects. The PISHG in crystals with various nanosizes was also measured at LNT. The size dependence demonstrated the quantum-confined effect with a maximum PISHG for 0.5 nm and with a clear increase in the PISHG with decreasing thickness of the nanocrystal. The Raman scattering spectra at different pumping powers were taken for thin nanocrystals, and the phonon modes originating from interlayer phonons were observed in the spectra. The results were discussed within a model of photoinduced electron-phonon anharmonicity.

Size- and temperature-dependent second-order optical effects in copper-doped cadmium iodide nanocrystals

Size and temperature dependences of second-order optical effects in Cu-doped cadmium iodide nanocrystals are studied. A large enhancement of photoinduced second harmonic generation (PSHG) with decreasing thickness of the nanocrystals or temperature is observed. The PSHG is also found to be enhanced with increasing Cu doping. However, due to the formation of the metallic agglomerates, the PSHG for the crystals with high Cu contents decreases. The results are presented by discussing photoinduced kinetics of the electron-phonon anharmonic interaction and nanosized quantum confined effect, the crucial roles played in the observed effects.

Structural, Optical, and Theoretical Studies of a Thermochromic Organic Crystal with Reversibly Variable Second Harmonic Generation

N-(3,5-Ditert-butylsalicylidene)-2-aminopyridine (1) exhibits bulk second harmonic generation (SHG) activity (2 vs urea). X-ray diffraction measurements show a non-centrosymmetric structure (P32). Experimental data of polarization-dependent SHG match the theoretical calculations on the basis of the crystallographic data. 1 is also thermochromic in the crystalline state, and SHG at 120 K is 4 times as high as at room temperature. Comparison is made with the isostructural isomer, N-(3,5-ditert-butylsalicylidene)-4-aminopyridine (2), which exhibits reversible photoinduced SHG variations.

Optical and thermal tools for the phase transitions detection in the guanidine compounds

In the Reference 1 it was established that single crystals of guanidine compound [C(NH2)3]4Cl2SO4 possess two structural phase transitions of the first order at T = 352 K (to the intermediate orthorhombic) and T= 356 K (to the paraelectric tetragonal). In this report we present results of DSC measurements in heating-cooling regime with relatively low temperature rates- 0.3 K/min and 0.1 K/min. We compare the measured by us the DSC results with temperature dependent photo induced second harmonic generation (PISHG) data. The investigations show that at lowest heating temperature changes velocity (0.1 K/min) we obtain two wide peaks temperature separated. We conclude from this DSC measurement that [C(NH2)3]4Cl2SO4 single crystals undergo a two-step phase transitions. We have received a similar temperature splitting of the observed phase transformations and a 15 K shift between the temperature PISHG dependences and the principal DSC maxima.

Manifestation of the size effect in the optical and spectral properties of the (N(C2H5)4)2CoCl2Br2 nanoclusters incorporated into the polymer matrix

The spectral and non-linear properties of (N(C2H5)4)2CoCl2Br2 nanocrystallites incorporated into the polymer matrices were investigated. The temperature dependence of the photoinduced second harmonic generation (PISHG) demonstrates a remarkable dependence on temperature near the phase transitions. It has been found that the temperature of phase transition T2 in the nanostrustured ferroics could depend on host polymer matrix and is higher than those in the bulk material. It is suggested that this effect is connected with the surface tension coupled with polarization via electrostriction effect and the influence of the depolarization field arising around the surface of the nanocluster. The arising lateral pressure induces lowering of the symmetry of the metal-halogen complexes within the surface layer of the cluster. Their distortion is followed by more pronounced splitting of the degenerate energy levels due to the lower symmetry of the crystal field. The shift of the corresponding absorption bands as well as the suppression of the integral absorption in the region of the internal transition of Co2+ ion with diminishing of the nanocluster size is followed by considerable decreasing of PISHG intensity.

Photoinduced non-linear optics of Eu2O3 DOPED TeO2–GeO2–PbO glasses

During the coherent bicolour optical treatment of Eu2O3 doped telluride glasses at temperatures near the glassing points, an efficient second-order susceptibility equal to about 2.1 pm V−1 at fundamental wavelength 1907 nm was achieved. Bicolour optical treatment was performed by two coherent laser beams originating from 50 ps Nd-YAG laser (λ = 1.32 µm) exciting the high pressure hydrogen laser cell emitting at 1907 nm. The non-centrosymmetric grating of the medium was formed by coherent superposition of the fundamental laser at 1907 nm and the doubled frequency one at 953.5 nm. The maximal optical second harmonic generation (SHG) signal occurs for 0.5% of Eu2O3 (in wt%). It was discovered that the photoinduced SHG achieves saturation during the treatment at about 6–8 min. The ratio between the fundamental beam with power density of about 1.3 GW cm−2 and the writing beam of about 0.024 GW cm−2 corresponds to the optimal photoinduced SHG. The remaining value of the photoinduced SHG is larger for the samples with higher second-order susceptibility. It is necessary to emphasize that the efficient grating process is possible only within the narrow temperature range corresponding to the glassing phase transitions and the effect disappeared for nondoped glasses and for glasses doped with 5% of Eu2O3 (in wt%). Possible physical mechanisms of the phenomenon observed are discussed.

Photo-induced SHG as a sensitive tool for the detection of surface morphology for iron garnet derivative films deposited on GGG substrates

The influence of surface morphology on the photo-induced optical second harmonic generation (SHG) is established for specific iron garnet films deposited on (111) and (100) planes of the Gd 3Ga 5O 12 substrate, respectively. The anisotropy of the film surface is assigned to the substrate crystallographic plane used. It is shown that the decrease of the film's mean grain size enhances the output SHG signal. The increase of the mean grain size from 30 nm up to the 46 nm leads to a decrease of the photo-induced SHG (up to 30%). This could be used for the contactless diagnostic of garnet films' morphology.

Origin of the Photoinduced Optical Second Harmonic Generation Arising in N-Phenyl Microcrystalline Films

Photoinduced second-order nonlinear optical effects, particularly optical second harmonic generation (SHG) of N-phenyls with different numbers of aromatic rings deposited on glass substrates were studied. As a fundamental beam, a 5-ps pulsed Nd:YAG laser was used. Quantum chemical time-dependent density functional theory (TDDFT) simulations of the nonlinear optical properties were performed. The first-order hyperpolarizabilities of isolated molecules were calculated, under the influence of a polarized pumping beam, to evaluate the role played by the nanointerfaces separating the microcrystallites and the amorphous environment. Consideration was performed within a framework of steady-state Langevin order parameters for amorphous-like films. A strong dependence of the photoinduced SHG versus the number of aromatic rings determining the degree of film crystallinity was shown. A comparison of experimental data and theoretically evaluated results shows that for the photoinduced first-order nonlinear optical effect the dominant contribution is an amorphous-like structural component, unlike the transport properties, where the crucial role is played by the nanointerface region. This may reflect a specific feature of the multiphoton processes in such types of nanointerfaces because of nanoconfined effects.