Journal of Nonlinear Optical Physics & Materials

The second-order nonlinear optical properties of two monomeric red fluorescent proteins, mStrawberry and mCherry, have been experimentally determined by frequency-resolved femtosecond hyper-Rayleigh scattering. These proteins were found to exhibit a stronger nonlinear response than the previously described eGFP, eYFP and DsRed,¹ confirming the trend that fluorophores with a more extended conjugated system, or a lower-energy band gap between ground and excited state, exhibit a larger static hyperpolarizability (β₀). Furthermore, these experimental data were complemented with quantum chemical calculations. A discrepancy was observed between experimental and theoretical results, but this could be explained by the chromophore model extracted from the available X-ray diffraction data. While eGFP showed a larger dynamic experimental response (β_HRS) due to the highest resonance enhancement, we measured an even higher signal for mCherry. Furthermore, mCherry also shows a better separation of the second harmonic signal and two-photon excited fluorescent signal, making this the preferred fluorescent protein for second harmonic imaging at 800 nm so far.

As the complex transmission coefficient for a Fabry–Perot dielectric plate is well-known, the density of states (DOS) approach by J. Bendickson et al. [Phys. Rev. E53, 4107 (1996)] was applied to the plate and a formula was found for the spectrum of its threshold gain for lasing from the dye-doped plate. Next, the dispersion relation and the spectrum of the threshold gain was discussed for the infinite helical cholesteric liquid crystal (CLC). Then the DOS approach was applied to the non-absorbing CLC layers having finite, different thickness. The threshold gain spectra were calculated and dependences were found of the minimum threshold gain on the layer thickness and the optical anisotropy of the material. Finally, the results of the DOS analytical technique were compared with numerical calculations based on the precise solutions of the Maxwell equations and, in such a way, the DOS technique has been proven. The contribution of the dye absorption was discussed separately. The experimental data presented in the paper are in good agreement with the threshold gain calculations.

We review our recent progress on power scaling of THz pulses generated from several nonlinear-optical crystals. By using a high-resistivity GaP crystal, we have significally increased the output peak power to as high as 722 W. By stacking three GaP wafers, we have further increased the highest output peak power to 2.36 kW. On the other hand, by using CO₂ laser pulses, we have obtained the average output power of 260 μW. We have also used these laser pulses to scale up the output power for the THz pulses to 29.8 μW by stacking GaAs wafers. Indeed, by stacking up to ten wafers, we have increased the output power by a factor of 160. Finally, by using ultrafast laser pulses, we have achieved record-high output powers for the THz pulses generated from multi-period periodically-poled LiNbO₃ crystals based on a backward configuration. The highest output power measured by us so far is 10.7 μW.

We give an overview of our recent work on improving the optical performance of polymeric materials via supramolecular concepts, i.e., by making use of spontaneous non-covalent interactions between the chromophores and the polymer backbone. More precisely, we show that the aggregation of dipolar chromophores in amorphous polymeric materials can be controlled by simply choosing a proper polymer matrix. This approach has a potential impact in a wide range of optical phenomena where chromophore aggregation or phase separation sets a limit to the system performance.

In this paper, we review ongoing progress in the development of novel on-chip, low loss planar molecular sensors that address the emerging need in the field of biochemical sensing. Chalcogenide glasses were identified as the material of choice for sensing due to their wide infrared transparency window. We report the details of manufacturing processes used to realize novel high-index-contrast, compact micro-disk resonators. Our findings demonstrate that our device can operate in dual modalities, for detection of the infrared optical absorption of a binding event using cavity enhanced spectroscopy, or sensing refractive index change due to surface molecular binding and extracting micro-structural evolution information via cavity enhanced refractometry.

We present a Raman generator based on a 35 cm long hollow core photonic crystal fiber entirely filled with ethanol emitting on a nearly-Gaussian beam. The power conversion efficiency from the pump at 532 nm to the first Stokes order of ethanol at 630 nm is 67%. Temporal profiles, mode patterns and power stability are also analyzed.

We observed an orientational photo-refractive effect in semiconductor CdSe/CdS nanorods-doped nematic liquid crystal cells homeotropically aligned. In order to study the photo-refractive and the orientational responses of this cell, we employed both asymmetric two-beam coupling (TBC) to measure two beam coupling gain under the influence of an applied electric field and forced light scattering technique. An optical index grating was written in the device in a holographic tilted configuration by overlapping two laser beams at 514 nm. Self diffraction of the writing beams and diffraction of a probe beam at 633 nm were measured. A high TBC gain coefficient of 270 cm^-1 was achieved at a DC field applied of 3.4 V/μm. First-order diffraction efficiencies were evaluated with a maximum efficiency of 7% in the dynamic grating with a nonlinear refractive index, n₂, 4 × 10^-2 cm²/W for E₀ = 3 V/μm.

A multi-wavelength Brillouin fiber laser (BFL) is demonstrated by using dual-wavelength fiber in a 7.7 km dispersion compensating fiber (DCF) that is incorporated in a 49 cm Bismuth erbium-doped fiber laser. The BFL operates in C-band, which is shifted by 0.08 nm and amplified by a Raman pump (RP). The threshold power for BP pumps and the Raman pump is achieved at 7 dBm and around 35 mW, respectively. The exclusive and practical benefits of using reduced high nonlinear Bismuth-erbium doped fiber (Bi-EDF) in implementing a four-wave mixing (FWM) method for multi-wavelengths Brillouin fiber laser (BFL) applications are experimentally demonstrated. Coupled interactions of Brillouin, Raman, and FWM scattering explain the unique feature of proposed configuration.

Intersubband optical absorption coefficient and refractive index changes of a weakly prolate ellipsoidal quantum dot, using the compact-density matrix formalism and iterative method, are investigated. In this regard, the linear and nonlinear intersubband optical absorption coefficient and refractive index changes of a GaAs/Al_xGa_1-x As ellipsoidal quantum dot, as functions of the dot radius, ellipticity constant, stoichiometric ratio and incident light intensity are calculated. The results indicate that absorption coefficient and refractive index changes strongly depend on the light intensity, size and geometry of the dot and structure parameters such as aluminium concentration in GaAs/Al_xGa_1-x As structures.

In this work, we study a system of coupled modified KdV (mKdV) equations. Multiple soliton solutions and multiple singular soliton solutions are derived by using the Hirota's bilinear method and the Hietarinta approach. The resonance phenomenon is examined.

We analyze the effect of pump configuration on the propagation velocity of an amplitude-modulated 1536 nm signal propagating through an ultra-highly doped erbium fiber amplifier. The nonlinear effects arising from the high value of the doping level lead to a strong variation of the pump power and signal gain along the fiber. We found that bidirectional pumping presents larger advancements than co-propagating and counter-propagating configurations. Numerical simulations allow us to explain the phenomenon in terms of the gain profile uniformity.

The incoherently coupled dark–dark, bright–bright, bright–dark screening-photovoltaic spatial soliton pairs are predicted in biased two-photon photovoltaic-photorefractive crystals under steady-state conditions. The result shows that these soliton pairs are established by the solitary beams which have the same polarization, wavelength, and are mutually incoherent. The screening-photovoltaic soliton pairs can be considered as the united form of screening soliton pairs and open or closed-circuit photovoltaic soliton pairs. They reduce to screening soliton pairs in the absence of photovoltaic effect, whereas they become photovoltaic soliton pairs under the open or closed-circuit conditions when the bias field is not applied. Moreover, the stability of these solitons has been investigated.

In this paper, we intend to determine periodic wave solutions for the modified nonlinear Schrödinger equation pertaining to negative index materials. We have treated the propagation equation possessing higher order linear and nonlinear dispersion terms with Jacobian elliptic function expansion method and arrived at the Jacobian elliptic periodic wave solutions. When the module of the Jacobian elliptic function m → 1, these solutions degenerate to the solitary wave solutions of the governing system.

The field in the entrance pupil of a high NA lens can be optimized such that, for given incident power, the electric field component in a given direction in the focal point is maximum. If the field component is chosen parallel to the optical axis, the longitudinal component is maximized and it is found that the optimum longitudinal component is narrower than the Airy spot. We discuss how this can be used to obtain higher resolution in photolithography when a resist is used that is sensitive to only the longitudinal component. We describe a proposition for realizing such resist.