Photoinduced Refractive Index 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.

Measurement of azo dye film with large photo-induced birefringence and complex refractive index

Azo dyes are valuable functional materials due to their sensitive response to and effective modulation of polarized light. It is essential to find out not only the optical anisotropy but also the complex refractive index of azo dye materials for photonics applications. We report here such measurements of highly ordered photoaligned azo dye films. The film has an apparent dichroic ratio of about 60. We model the photoaligned azo dye film systems using the Berreman transfer matrix. By means of regression fitting of the polarization dependent transmittance and reflectance data, we were able to determine the complex refractive indices of the aligned film at the wavelength range of 400–1000 nm. This material is found to have exceptionally large optical birefringence (Δn ∼ 0.75 at 632.5 nm and Δn ∼ 0.48 at 850.6 nm). The aligned azo dye film is valuable to applications such as coatable polarizers and optical modulation devices.

Direct observation of ultrafast singlet exciton fission in three dimensions

We present quantitative ultrafast interferometric pump-probe microscopy capable of tracking of photoexcitations with sub-10 nm spatial precision in three dimensions with 15 fs temporal resolution, through retrieval of the full transient photoinduced complex refractive index. We use this methodology to study the spatiotemporal dynamics of the quantum coherent photophysical process of ultrafast singlet exciton fission. Measurements on microcrystalline pentacene films grown on glass (SiO2) and boron nitride (hBN) reveal a 25 nm, 70 fs expansion of the joint-density-of-states along the crystal a,c-axes accompanied by a 6 nm, 115 fs change in the exciton density along the crystal b-axis. We propose that photogenerated singlet excitons expand along the direction of maximal orbital π-overlap in the crystal a,c-plane to form correlated triplet pairs, which subsequently electronically decouples into free triplets along the crystal b-axis due to molecular sliding motion of neighbouring pentacene molecules. Our methodology lays the foundation for the study of three dimensional transport on ultrafast timescales.

Three-Dimensional All-Optical Switching Using a Single Diffracting Bessel Beam

A single Bessel beam (BB) enables the scalable optical writing of waveguide devices with multiple inputs and outputs in a nonlinear medium. For this purpose, we experimentally propagate a diffracting zero-order BB in a strontium barium niobate photorefractive crystal. Such a configuration can be used for the self-writing of complex waveguiding structures. By tailoring different key parameters such as the diffraction phenomenon through the BB size, the nonlinearity strength through the applied electric field, and the photoinduced refractive-index modulation depth through input beam intensity, our optical platform enables not only the generation of optical splitters but also an all-optical control of the output intensity levels. We also analyze the stability in space and time of the different photoinduced channels. We show that adding a background illumination onto the medium permits a steady-state photoinduced refractive-index modulation with up to nine output channels. These results enrich the research on scalable optical writing techniques for the realization of complex interconnects and enlarge further possibilities for all-optical switching.

Externally Driven Nonlinear Time-Variant Metasurfaces

Resonant photonic nanostructures exhibiting enhanced nonlinear responses and efficient frequency conversion are an emergent platform in nonlinear optics. High-index semiconductor metasurfaces with rapidly tuned high-Q resonances enable a novel class of time-variant metasurfaces, which expands the toolbox of color management at the nanoscale. Here, we report on the dynamic control of the nonlinear optical response in time-variant semiconductor metasurfaces supporting high-quality factor resonances in the near-infrared spectral range. Pump-probe measurements of germanium metasurfaces at negative pump-probe time delays reveals frequency conversion in the fundamental beam and a blue-shift of 10~nm (3.05$\omega$) and 40% broadening in the third harmonic signal due to the photoinduced time-variant refractive index. A time-dependent coupled-mode theory, in excellent agreement with the experimental data, validated the time-variant nature of the system. Our findings expand the scope of time-variant metasurfaces and may serve as base for the next generation of nanoscale pulse shapers, optical switches and light sources.

Balanced-detection interferometric cavity-assisted photothermal spectroscopy employing an all-fiber-coupled probe laser configuration

The interferometric cavity-assisted photothermal spectroscopy (ICAPS) method has been proven highly suitable for sensitive and compact gas detection by application of an optical cavity as transducer for photothermal spectroscopy. This work reports on the implementation of an overall fiber-coupled probe laser configuration detecting the reflectance of the individual interferometers in a balanced-detection ICAPS system. The layout greatly improves the overall sensor system robustness. Two identical 1 mm path length cavities were used for balanced detection, enabling sensor operation close to the fundamental limit of shot noise by efficiently cancelling excess noise. A quantum cascade laser served as a mid-infrared excitation source to induce refractive index changes in the sample, and a near-infrared fiber laser served as probe source to monitor the photo-induced refractive index variations. The metrological figures of merit for the sensor were investigated by SO2 detection. For the targeted absorption band centered at 1380.93 cm-1, a 3 ppbv minimum detection limit was achieved with a 1 s integration time, corresponding to a normalized noise equivalent absorption of 4.5 × 10-9 cm-1 W Hz-1/2.

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.

Control and enhancement of photo-induced refractive index modifications in fused silica

A thorough study of the femtosecond photo-inscription of optical waveguides in fused silica is presented. Quantitative phase microscopy was used to study the variation of the index contrast of the waveguides as a function of the writing conditions. It is revealed that waveguides based exclusively on Type I refractive index modifications are difficult to form for pulses longer than 300 fs. We show that this limitation can be circumvented by scanning the laser beam multiple times at low pulse energy. We also demonstrate that by equally multiplying the scan speed and the number of passes, the index contrast can be increased, which was not expected for the low-repetition-rate regime. Based on the nonlinear ionization memory, we propose an explanation for this phenomenon. For shorter pulses, multiple passes of the beam allowed for the formation of waveguides with an enhanced index contrast while preserving the morphology and uniformity of Type I modifications. Index contrasts up to 9 × 10−3 in Heraeus F300 fused silica are reported. Using this method, waveguides that exhibits single mode operation at wavelengths of 405, 633, 980 and 1550 nm were successfully inscribed.

Photosensitive highly Er/Yb co-doped phosphosilicate optical fibers for continuous-wave single-frequency fiber laser applications

Highly Er/Yb co-doped phosphosilicate fibers with low optical losses were fabricated by the MCVD method, utilizing an all-gas-phase deposition technique developed “in house.” Important characteristics such as photodarkening and photosensitivity of these fibers were thoroughly investigated. An almost negligible impact of photodarkening-induced losses (no more than 1 dB/m) was discovered in the near-infrared spectral region. The photosensitivity of hydrogen-loaded fiber samples was studied using excimer laser irradiation at a wavelength of 193 nm. A photoinduced refractive index value of 1 × 10 − 3 at the total UV irradiation exposure dose of 550 J / c m 2 was achieved. A single-frequency laser with a Fabry–Perot short cavity was designed by inscribing highly reflective ( R > --> 99.9 % ) and partially reflective ( R = 94.3 % ) fiber Bragg gratings directly in the Er/Yb co-doped phosphosilicate fiber core. The laser showed stable CW output emitting with a narrow peak centered at 1551.5 nm in the absence of any signs of Y b 3 + parasite lasing in a wavelength range from 1000 to 1100 nm.

Comparison between the UV and X-ray Photosensitivities of Hybrid TiO2-SiO2 Thin Layers

The photo-induced effects on sol–gel-based organo TiO2-SiO2 thin layers deposited by the dip-coating technique have been investigated using two very different light sources: A light-emitting diode (LED) emitting in the UV (at 365 nm, 3.4 eV) and an X-ray tube producing 40 keV mean-energy photons. The impact of adding a photo-initiator (2,2-dimethoxy-2-phenylacetophenone-DMPA) on the sol–gel photosensitivity is characterized namely in terms of the photo-induced refractive index measured through M-line spectroscopy. Results show that both silica-titania sol–gel films with or without the photo-initiator are photosensitive to both photon sources. The induced refractive index values reveal several features where slightly higher refractive indexes are obtained for the sol–gel containing the photo-initiator. UV and X-ray-induced polymerization degrees are discussed using Fourier-transform infrared (FTIR) spectroscopy where the densification of hybrid TiO2-SiO2 layers is related to the consumption of the CH=C groups and to the decomposition of Si-OH and Si-O-CH3 bonds. X-rays are more efficient at densifying the TiO2-SiO2 inorganic and organic network with respect to the UV photons. Hard X-ray photolithography, where no cracks or damages are observed after intense exposition, can be a promising technique to design submicronic-structure patterns on TiO2-SiO2 thin layers for the building of optical sensors.

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.

Mechanisms of the Photodissociation of Aminoazobenzene Rydimers into Monomers, According to Data from Ultrafast Laser-Probing Spectroscopy

The results from ultrafast laser-probing spectroscopy of photoexcited states of trans-aminoazobenzene (t-AAB) by means of ultrafast transient lens (UTL) and transient absorption spectroscopy (TAS) are analyzed. The analysis relies on earlier elaborated views of the rydimeric structure of azo dyes. It is shown that earlier reported UTL and TAS signals of aminoazobenzene (solutions in ethanol and heptanol) obeying the same laws of kinetic attenuation (a drop in the photo-induced refractive index of solvents for UTL and transient photoexcited Vis states of the dye for TAS) are in reality the mechanism of rydimer dissociation into two monomeric pairs. One of these pairs (consisting of neutral monomers) relaxes into the initial t-AAB2 rydimer according to two exponential kinetic curves with constants (characteristic times) of 0.7 and 2 ps (UTL) or 0.6 and 1.9 ps (TAS). The other pair consists of a t-AAB monomer and a monomer with two phenylaminyl-type cations and Vis absorption at 600–750 nm; it undergoes trans–cis isomerization according to a mono-exponential kinetic law with constants of 0.3 ps (UTL) and 0.2 ps (TAS).

Graphene filled optical fiber Fabry‐Pérot cavity tunable filter

AbstractBefore saturated absorption, graphene possesses optical nonlinear refractive index, which is a photoinduced refractive index effect. Taking advantage of this effect, a wavelength tunable filter is realized based on a graphene filled all optical fiber Fabry‐Pérot microcavity. Experimental results show that the interference wavelength of the filter exhibits linear red‐shift with the increase of the tunning optical power and tunning efficiency increases with the increase of cavity length and filled graphene thickness. In this study, the maximum wavelength vs power tunning efficiency can get 66 pm/mW. This filter has the advantages of high tunning efficiency, simple structure, antienvironmental interference good mechanical performance, and repeatability.

Ultrafast Spectrally Resolved Photoinduced Complex Refractive Index Changes in CsPbBr3 Perovskites

The exceptional optoelectronic properties of metal halide perovskites have been illuminated by extensive spectroscopic studies. Recent measurements suggest strong photoinduced refractive index chan...

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.

Photo-induced refractive index and topographical surface gratings in functionalized nanocarbon solid film

We demonstrate that a single-walled carbon nanotube network noncovalently coupled with a pyrene-modified azo-benzene chromophore functions as a host matrix for a broad range of photo-orientation and photomechanical effects. The chromophore could be efficiently reoriented through repeated trans-cis-trans isomerization under linearly polarized 480 nm light, with Δn of 0.012 at 650 nm and fast characteristic rise-times of 0.12 s. Erasable phase diffraction gratings could also be written, with permanent surface relief gratings forming at sufficiently long irradiation times. In addition to demonstrating a mechanism for photo-manipulation of single-walled carbon nanotubes, these results show photo-orientation of chromophores in azo-functionalized single-walled carbon nanotube networks as a path towards the photosensitive tuning of the electrostatic environment of the nanotube.

Analysis and model of the photorefractive-like effect in non-conductive asymmetric liquid crystal hybrid cell

In this paper we describe a non-conductive liquid crystal hybrid cell with the photoconductive polymer (PP) layer. The orientation of the liquid crystal (LC) molecules in the described optical device is controlled by an external electric field and proper illumination. Method to calculate photoinduced refractive index distribution in liquid crystal layer has been presented. We propose model of the space charge formation in photoconductive polymer layer in the presence of light. We assumed that the carriers generated by the light in the PP diffuse into unlighted area and drift in external field and local field of the generated charge. In determining the distribution of the space charge field, we take into account the field-induced charges on the borders of the individual layers. After calculation of the final distribution of space charge and proper field distribution, we determine the director field distribution and refractive index distribution in LC layer. Full Text: PDF References A. Walczak, E. Nowinowski-Kruszelnicki, "Waveguide couplers induced optically over organic junction", Opt. Eng. 47(3), 035402 (2008). CrossRef A. Walczak, "Electromagnetic wave beam self - guidance in liquid crystal - semiconductor cell", Phot. Lett. Poland 2(2), 73 (2010). CrossRef A. Brignon, I. Bongrand, B. Loiseaux, J.P. Huignard, "Signal-beam amplification by two-wave mixing in a liquid-crystal light valve", Opt. Lett. 22(24), 1855 (1997). CrossRef G. Cook, C.A. Wyres, M.J. Deer, D.C. Jones, "Hybrid organic-inorganic photorefractives", Liquid Crystals VII, SPIE Annual Meeting (2003). CrossRef F. Kajzar, S. Bartkiewicz, A. Miniewicz, "Optical amplification with high gain in hybrid-polymer–liquid-crystal structures", Appl. Phys. Lett. 74(20), 2924 (1999) CrossRef S. Bartkiewicz, K. Matczyszyn, A. Miniewicz, F. Kajzar, "High gain of light in photoconducting polymer–nematic liquid crystal hybrid structures", Opt. Commun. 187, 257 (2001). CrossRef M. Kaczmarek, A. Dyadyusha, S. Slussarenko, I.C. Khoo, "The role of surface charge field in two-beam coupling in liquid crystal cells with photoconducting polymer layers", J. Appl. Phys. 96, 2616 (2004). CrossRef P. Moszczyński, A. Walczak, P. Marciniak, "Model for simulation of photo-induced charges inside the hybrid LC cells", Phot. Lett. Poland 5(1), 11 (2013). CrossRef N. Kukhtarev, V. Markov, S. Odulov, M. Soskin, V. Vinetskii, "Holographic storage in electrooptic crystals. i. steady state", Ferroelectrics 22, 949 (1979). CrossRef N. Kukhtarev, T. Kukhtareva, "A unified treatment of radiation-induced photorefractive, thermal, and neutron transmutation gratings", IEEE Proceedings 87(11), 1857 (1999). CrossRef J.S. Schildkraut, Y. Cui, "Zero-order and first-order theory of the formation of space?charge gratings in photoconductive polymers", J. Appl. Phys. 72(11), 5055 (1992). CrossRef J.S. Schildkraut, A.V. Buettner, "Theory and simulation of the formation and erasure of space-charge gratings in photoconductive polymers", J. Appl. Phys. 72(5), 1888 (1992). CrossRef A. Twarowski, "Geminate recombination in photorefractive crystals", J. Appl. Phys. 65(7), 2833 (1989). CrossRef W.D. Gill, "Drift mobilities in amorphous charge-transfer complexes of trinitrofluorenone and poly-n-vinylcarbazole", J. Appl. Phys. 43(12), 5033 (1972). CrossRef A. Walczak, P. Moszczyński, E. Nowinowski-Kruszelnicki, "Semiconductor–LC Layer Boundary and Photonic Structures", Mol. Cryst. Liq. Cryst. 559(1), 186 (2012). CrossRef