Photoinduced Refractive Index Change Research Articles

Transient Absorption Spectroscopy of Films: Impact of Refractive Index.

Transient absorption spectroscopy is a powerful technique to study the photoinduced phenomena in a wide range of states from solutions to solid film samples. It was designed and developed based on photoinduced absorption changes or that photoexcitation triggers a chain of reactions with intermediate states or reaction steps with presumably different absorption spectra. However, according to general electromagnetic theory, any change in the absorption properties of a medium is accompanied by a change in the refractive properties. Although this photoinduced change in refractive index has a negligible effect on solution measurements, it may significantly affect the measured response of thin films. In this Perspective paper, we examine why and how the measured responses of films differ from their expected "pure" absorption responses. The effect of photoinduced refractive index change can be concluded and studied by comparing the transmitted and reflected probe light responses. Another discussed aspect is the effect of light interference on thin films. Finally, new opportunities of monitoring the photocarrier migration in films and studying nontransparent samples using the reflected probe light response are discussed. Most of the examples provided in this article focus on studies involving perovskite, TiO2, and graphene-based films, but the general discussion and conclusions can be applicable to a wide range of semiconductor and thin metallic films.

Mid-infrared microfiber Bragg gratings.

We report mid-infrared (mid-IR) Bragg gratings fabricated on sub-wavelength-diameter chalcogenide glass (ChG) microfibers. ChG microfibers with diameters around 3 µm are tapered drawn from As2S3 glass fibers, and the mid-IR microfiber Bragg gratings (mFBGs) are inscribed on microfibers using interference patterns of near bandgap light at a 532 nm wavelength. At a wavelength of about 4.5 µm, the mFBG has an extinction ratio of 15 dB and a positive photo-induced refractive index change of 2×10-2. The dependence of the grating formation on accumulated influence of exposure power density and time is investigated. The mid-IR mFBGs demonstrated here may be used as building blocks for micro-photonic circuits or devices in the mid-IR spectral range.

Double line waveguide amplifiers written by femtosecond laser irradiation in rare-earth doped germanate glasses

We report the production of active double waveguides in Er/Yb doped GeO2-PbO glasses, by direct femtosecond laser writing. The glasses were produced using the melt-quenching technique and the active waveguides were written using 30 fs laser pulses, at 800 nm, with writing speed of 0.06 mm/s and pulse energy of 32 μJ. The photo-induced negative refractive index change was of −7.4 × 10−3. The Er/Yb doped sample showed a relative gain (signal enhancement of 7.5 dB/cm, for 105 mW of 980 nm pump power. The relative gain compensates both, the propagation losses and the absorption losses, and a positive maximum internal gain of 4.6 dB/cm can be obtained at the signal wavelength of 1550 nm. The results obtained in present work demonstrate that Er/Yb glasses are promising materials for the fabrication of integrated amplifiers, lossless components and lasers based on germanate glasses.

Optically induced birefringence in dye-doped blue phase liquid crystals

ABSTRACTThis paper investigates the photoinduced change of refractive index in dye (methyl red, MR)-doped blue phase (DDBP) cells by illumination of a pump beam. Through excitation of light irradiation with proper photon energy, MR can transform from trans-state to cis-state and successively diffuse and anisotropically adsorb on the inner glass substrate of the DDBP cell along the direction perpendicular to the polarisation of the pump beam. The adsorbed MR molecules can effectively rotate the orientation of the liquid crystal (LC) molecules and thereby modulate the effective refractive index of the DDBP cell. The SEM images of the adsorbed regions of the illuminated DDBP samples were also taken for discussing the relation between the pump intensity and the photoinduced birefringence.

Optical limiting performance of a GaAs/AlAs heterostructure microcavity in the near-infrared.

A multilayer GaAs/AlAs heterostructure forming a Fabry-Perot microcavity with a narrow resonance at 1.1 μm was produced by molecular-beam epitaxy. Under nanosecond pulsed laser radiation, a blueshift of the resonant line, associated with a photo-induced negative change in refractive index in GaAs, was experimentally registered by using an optical parametric oscillator. The spectral shift was accompanied by a reduction in peak transmittance, associated with nonlinear intracavity absorption. Such a cavity can be used as an optical limiter at the resonant wavelength when both the spectral shift and the transmittance reduction contribute to the limiting effect. An exceptionally low limiting threshold of about 1 mJ/cm2 was observed in the experiment.

The Sensing Characteristics of a Bragg Fiber Based Plasmonic Biosensor Using an As<sub>2</sub>S<sub>3</sub> Chalcogenide Layer

The sensing performances of a four layer Bragg fiber based plasmonic biosensor for aqueous solutions using an As2S3 chalcogenide layer are investigated with an analytical method. In comparison with a previously considered fiber structure with a GaP layer, the new geometry has the advantages of a smaller value of the full width at half maximum and a larger value of the signal-to-noise ratio; however, the maximum of amplitude sensitivity is smaller when an As2S3 layer is used in the place of the GaP layer. If the thickness of the As2S3 layer is increased to the value for which it is quarter wavelength, the power fraction carried at the resonant wavelength by the core guided mode in the analyte layer becomes comparable with the value obtained in an optical fiber with the GaP layer, evidencing that the higher value of the refractive index for the GaP layer is compensated by a larger value of the thickness for the As2S3 layer. Taking into account the 0.04 photoinduced refractive index change obtained by illumination the chalcogenide As2S3 layer with a laser beam, the resonant wavelength is decreased by 0.128 nm and the loss for the core mode is increased by 41dB/cm.

Depth profiling of the refractive index from probe beam deflections induced by a serrated pump illumination on GaAs

We demonstrate a method to experimentally quantify the distribution with depth of photo-induced refractive index generated by a pump illumination, which is spatially modulated by a serrated mask on the surface of bulk GaAs. This photo-induced refractive index change distributes inhomogeneously with depth thus deflecting the probe beam transversely passing through the depth direction. The refractive index distribution with depth is deduced from the set of deflections of probe beam with respect to the propagating distances, which is collected experimentally in a pump probe arrangement. The obtained set of deflections of probe beam is theoretically proved to be reliable.

All-optical controlling based on nonlinear graphene plasmonic waveguides.

We give the effective refractive index of graphene plasmonic waveguides with both linear and nonlinear effects based on the nonlinear cross-phase modulation, and address the effects of photo-induced refractive index change and absorption change. A non-resonant all-optical nonlinear graphene plasmonic switch with an ultra-compact size of 0.25 μm2 is proposed and numerically analyzed based on the dynamics of the photo-induced absorption change. The results show that the all-optical graphene plasmonic switch can realize a broad bandwidth over 5 THz, a potentially very high switching speed and an extinction ratio of 18.14 dB with the electric amplitude of the pump light of 1.5 × 107 V/m at the signal frequency of 28 THz. Our study could provide a possibility for future all-optical highly integrated optical components.

The photosensitivity of ytterbium-doped optical fibers with aluminophosphosilicate glass cores

The phenomenon of photoinduced refractive index change in active optical fibers with Al2O3/P2O5/Yb2O3/SiO2 core composition has been observed under the action of UV laser radiation with 193-nm wavelength. It is established that hydrogen treatment of active fibers plays a key role in ensuring a radical (about two orders of magnitude) increase in their photosensitivity. It is shown that intrafiber Bragg gratings with reflection coefficients above 99% can be formed, which are promising mirror reflectors for fiber laser cavities.

Hot-carrier cooling and photoinduced refractive index changes in organic-inorganic lead halide perovskites.

Metal-halide perovskites are at the frontier of optoelectronic research due to solution processability and excellent semiconductor properties. Here we use transient absorption spectroscopy to study hot-carrier distributions in CH3NH3PbI3 and quantify key semiconductor parameters. Above bandgap, non-resonant excitation creates quasi-thermalized carrier distributions within 100 fs. During carrier cooling, a sub-bandgap transient absorption signal arises at ∼1.6 eV, which is explained by the interplay of bandgap renormalization and hot-carrier distributions. At higher excitation densities, a ‘phonon bottleneck' substantially slows carrier cooling. This effect indicates a low contribution from inelastic carrier-impurity or phonon–impurity scattering in these polycrystalline materials, which supports high charge-carrier mobilities. Photoinduced reflectivity changes distort the shape of transient absorption spectra and must be included to extract physical constants. Using a simple band-filling model that accounts for these changes, we determine a small effective mass of mr=0.14 mo, which agrees with band structure calculations and high photovoltaic performance.

Nonlinear optical response of semiconductor-nanocrystals-embedded photonic band gap structure

Colloidal CdSe/ZnS core/shell nanocrystals (NCs), which were dispersed in SiO2 sol, were utilized to fabricate a SiO2:NCs/TiO2 all-dielectric photonic band gap (PBG) structure. The third-order nonlinear refractive index (n2) of the PBG structure was nearly triple of that of the SiO2:NCs film due to the local field enhancement in the PBG structure. The photoinduced change in refractive index (Δn) could shift the PBG band edge, so the PBG structure would show significant transmission modification, whose transmission change was ∼17 folds of that of the SiO2:NCs film. Under excitation of a 30 GW/cm2 femtosecond laser beam, a transmission decrease of 80% was realized.

Crystallization and absorption properties of novel photo-thermal refractive glasses with the addition of B2O3

A new boron-containing photo-thermal-refractive (boron-PTR) glass was prepared by two-step melt-quenching techniques at relative lower glass melting temperature. The influence of Ag+ doping concentration, UV exposure dosage and thermal development parameters on the boron-PTR glass' spectroscopic absorption and crystallization property were investigated and compared with that of the known boron-free PTR glass explored by L. B. Glebov et al. The introduction of B2O3, to some extent, can increase the strength of the silicate bonding thus preventing the liquid–liquid phase separation in this new PTR system when compared to the boron-free PTR system. The developed boron-PTR glass with the processing of 15s-UV exposure and appropriate thermal development (480°C/3h, 500°C/2h) exhibits good photo-induced crystallization performance, high VIS-near IR transmittance (670 to 2600nm) and moderate photo-induced refractive index change (1.1×10−3), which at present status enables this new PTR glass basic ability to record holographic elements.

A silica singmode fibre-chalcogenide multimode fibre-silica singlemode fibre structure

In this letter we theoretically and experimentally investigate a singlemode-multimode-singlemode (SMS) structure based on chalcogenide (As 2 S 3 ) multimode fibre and silica singlemodefibres. The experimental results show a general agreement with the numerical simulation results based on a wide angle-beam propagation method (WA-BPM). Multimode interference variation was observed as a result of photo-induced refractive index changes arising from either localized laser irradiation at a wavelength of 405 nm or the use of a UV lamp. Our result provides a platform for the development of compact, high-optical-quality, and robust photonic nonlinear devices. Full Text: PDF References Q. Wu, Y. Semenova, P. Wang, A. M. Hatta and G. Farrell, "Experimental demonstration of a simple displacement sensor based on a bent single-mode?multimode?single-mode fiber structure ", Meas. Sci. Technol., 22, 025203 (2011). CrossRef S. M. Tripathi, A. Kumar, R. K. Varshney, Y. B. P. Kumar, E. Marin, J.-P. Meunier, "Strain and Temperature Sensing Characteristics of Single-Mode?Multimode?Single-Mode Structures", J. Lightwave Tech., 27, 2348 (2009). CrossRef P. Wang, G. Brambilla, M. Ding, Y. Semenova, Q. Wu, G. Farrell, "High-sensitivity, evanescent field refractometric sensor based on a tapered, multimode fiber interference", Opt. Lett., 36, 2233 (2011). CrossRef Q. Wang and G. Farrell, "Multimode-fiber-based edge filter for optical wavelength measurement application and its design", Micro. Opt. Tech. Lett., 48, 900 (2006). CrossRef C. Grillet, S. N. Bian, E. C. Magi, and B. J. Eggleton, "Fiber taper coupling to chalcogenide microsphere modes ", Appl. Phys. Lett., 92, 171109 (2008). CrossRef Masaki Asobe, Hideki Kobayashi, Hiroki Itoh, and Terutoshi Kanamori, "Laser-diode-driven ultrafast all-optical switching by using highly nonlinear chalcogenide glass fiber", Opt. Lett., 18, 1056 (1993). CrossRef S. Shakeri and M. Hatami, "Self-tunable chalcogenide Raman laser", J. Opt. Soc. Am. B, 27, 679 (2010). CrossRef B. J. Eggleton, B. Luther-Davies, and K. Richardson, "Chalcogenide photonics", Nature Photonics, 5, 141 (2011). CrossRef P. Wang, G. Brambilla, M. Ding, Y. Semenova, Q. Wu, G. Farrell, "Investigation of single-mode?multimode?single-mode and single-mode?tapered-multimode?single-mode fiber structures and their application for refractive index sensing", J. Opt. Soc. Am. B, 28, 1180 (2011). CrossRef

Modulation of surface plasmon wave by photo-induced refractive index changes of CdSe quantum dots

We demonstrate an all-optical modulation of surface plasmon resonance by photo-irradiation induced dielectric constant change of CdSe quantum dots on the gold surface. The dipole-dipole interaction between the surface plasmon and the photo-induced excitons in the dots gives rise to a shift in the surface plasmon resonance peak. This shift is found to sensitively reflect a tiny change in the dielectric constant, which is useful for investigating the nonlinear effect in active plasmonic components.

Optical modulation of guided mode resonance in the waveguide grating structure incorporated with azo-doped-poly(methylmethacrylate) cladding layer

Optical modulation of guided mode resonance (GMR) is demonstrated in a waveguide grating structure (WGS) which contains a disperse-red1 (DR1)-doped poly(methylmethacrylate) (PMMA) cladding layer. The resonance wavelength of a GMR mode can be tuned by pumping the cladding layer with a 442 nm wavelength laser beam, because of photoinduced refractive index change in the layer. The resonance wavelength shifts to shorter wavelength side, and the shift increases with pumping power, up to a maximum shift of 5 nm. A detector was used to monitor the intensity of the light that was reflected from the WGS at the wavelengths of the GMR peak positions, and the WGS was found to exhibit optical modulation with a shortest switching time of less than 0.3s.

Photoinduced changes in refractive index of nanostructured shungite-containing polyimide systems

Photoinduced changes in the refractive index of a conjugate polyimide (PI) matrix sensitized by shungite carbon nanoparticles have been studied for the first time. The results are compared to the data of previous investigations of the photorefractive properties of PI matrices doped with fullerenes, carbon nanotubes, and quantum dots. The nonlinear refractive index of the proposed material has been determined using the dynamic holography techniques. The position of conjugate polymer materials of this type among the other nonlinear optical systems is considered.

Amplitude of the dynamic phase gratings in saturable Er-doped fibers

Dynamic population gratings recorded via local saturation of optical absorption/gain in rare-earth (Er, Yb) doped fibers demonstrate contributions of the amplitude and phase components. Quantitative comparison of the experimentally observed amplitude grating efficiency with that evaluated from spatially uniform saturation of the fiber optical absorption usually shows a significant discrepancy. The grating amplitude proves to be significantly weaker than its theoretical evaluation. The first results on quantitative comparison of the recorded phase grating amplitudes with spatially uniform photo-induced refractive index change in Er-doped fibers at three essentially different wavelengths (1485, 1526, and 1568 nm) are reported. As for the amplitude grating component, the experimentally observed phase grating amplitude is inferior to its theoretical evaluation; this discrepancy is spectrally dependent and is also significantly reduced in the short-wavelength region 1480–1490 nm on the Er3+ ion absorption.

Composition dependence and reversibility of photoinduced refractive index changes in chalcogenide glass

Photoinduced refractive index changes are investigated as a function of composition in Ge–As–Se chalcogenide glass by measuring the formation of Bragg reflectors photo-imprinted in polished glass discs. It is shown that the glass network connectivity has a strong effect on the photostructural changes which decrease dramatically when the rigidity percolates through the structure at coordination numbers ⟨r⟩ > 2.4. The Bragg reflector formation is also affected by the refractive index of the glass which varies sharply with ⟨r⟩ and correlates directly with the glass density. The photoinduced refractive index change is shown to reverse upon annealing near the glass transition following a non-exponential kinetic that closely matches the kinetics of structural enthalpy relaxation. The Bragg reflectors can also be reversibly erased and readjusted to a new wavelength through subsequent irradiation with a different photon energy. The composition dependence of photosensitivity is discussed in terms of structural constraints. It is suggested that over-constrained networks remain rigid despite photoexcitation of bond constraints therefore precluding photostructural rearrangements.

Role of point defects in the photosensitivity of hydrogen-loaded phosphosilicate glass

It is shown that point defect modifications in hydrogen-loaded phosphosilicate glass (PSG) do not play a central role in determining its photosensitivity. Photochemical reactions that involve a two-step point defect modification and pre-exposure effect are incapable of accounting for photoinduced refractive index changes. It seems likely that a key role in UV-induced refractive index modifications is played by structural changes in the PSG network. Experimental data are presented that demonstrate intricate network rearrangement dynamics during UV exposure of PSG.

Effects of aryl substituents in N-acetyl-α-dehydroarylalanine naphthyl ester additive on the photoinduced refractive index change of poly(methyl methacrylate) film

When poly(methyl methacrylate) (PMMA) film doped with (Z)-N-acetyl-α-dehydroarylalanine naphthyl esters ((Z)-1) was irradiated with unfiltered light from a high-pressure mercury lamp, (Z)-1 underwent heterolytic cleavage of the ester bond preferentially in PMMA film to give arylmethylene-substituted oxazolone and naphthol derivatives as major products. This photochemical transformation enhanced the refractive index of this polymer film in the range of 0.001–0.020, depending on the structure of the aryl substituents.