Readout Wavelength Research Articles

Plasmonic Fiber Grating Biosensors Demodulated Through Spectral Envelopes Intersection

Gold-coated tilted fiber Bragg gratings (TFBGs) have been highly studied over the last years, mainly for biosensing purposes. They present a comb-like spectrum of narrow-band cladding mode resonances that is often demodulated by tracking the change of a selected peak. In this paper, we report a twentyfold more sensitive demodulation method based on the intersection of the upper and lower envelopes of gold-coated TFBG spectra. This method has been successfully applied in biosensing experiments towards the detection of HER2 (Human Epidermal Growth Factor Receptor-2) proteins, a relevant biomarker for breast cancer. Practical improvements have also been implemented. First, a uniform FBG has been superimposed on the TFBG to reduce the read-out wavelength span to 10 nm instead of 70 nm while keeping the temperature-compensated measurements. Second, a micro-fluidic system has been implemented to smoothly deliver the samples to the sensor. These 3 originalities make this sensing platform even more attractive for use in practical applications.

Dosimetric calibration of GafChromic HD-V2, MD-V3, and EBT3 films for dose ranges up to 100 kGy.

A dosimetric calibration of three types of radiochromic films (GafChromicTM HD-V2, MD-V3, and EBT3) was carried out for absorbed doses (D) ranging up to 100 kGy using a 130 TBq Co60 γ-ray source. The optical densities (ODs) of the irradiated films were acquired with the transmission-mode flatbed film scanner EPSON GT-X980. The calibration data were cross-checked using the 20-MeV proton beam from the azimuthally varying field cyclotron at the Research Center for Nuclear Physics in Osaka University. These experimental results not only present the measurable dose ranges of the films depending on the readout wavelength, but also show consistency with our hypothesis that the OD response curve [log(OD)-log(D) curve] is determined by the volumetric average of the absorption dose and does not strongly depend on the type of radiation for the excitation.

Red-sensitive organic nanoparticle-polymer composite materials for volume holographic gratings with large refractive index modulation amplitudes

We demonstrate volume holographic recording at a wavelength of 640 nm in a photopolymerizable nanoparticle-polymer composite (NPC) film dispersed with ultrahigh refractive index hyperbranched-polymer (HBP) organic nanoparticles. We employ a new photosensitizer-initiator system consisting of cyanine dye, triazine compound and borate salt for efficient radical generation in the red. We investigate the electron transfer and radical generation processes of the system by measuring fluorescence quenching and photopolymerization dynamics to find the optimum composition of the system for volume holographic recording. We show that recorded volume gratings of 0.5-µm spacing possess the saturated peak-to-mean refractive index modulation amplitudes as large as 3×10−2 at a readout wavelength of 640 nm. Our results show the usefulness of photopolymerizable HBP-dispersed NPCs for volume holographic recording materials for various photonic applications including security and color holograms, and volume Bragg grating devices in head-mounted displays.

Dual-digital encoded suspension array based on Raman spectroscopy and laser induced breakdown spectroscopy for multiplexed biodetection

Aiming to satisfy the growing requirements of high-throughput multiplexed bioassays, a dual-digital encoded suspension array (SA) analysis based on Raman spectroscopy and laser induced breakdown spectroscopy (LIBS) is proposed. The SAs are prepared by the core-shell microbeads with Raman reporter molecules doped inside and elementary encoded nanoparticles (EENPs) assembled on the surface. Raman scattering spectra and LIBS spectra are obtained after excitation by laser, the specific peaks from the spectra are used for decoding. Highly sensitive and selective recognition of dual-digital encoded SA are realized by immunoassay experiments with a detection limit of 9.17 pM. Owing to the narrow bandwidths and unchangeable read-out wavelengths, the above two types of spectra provide high accuracy and stability for encoding. Meanwhile, Raman and LIBS spectra are independent from fluorescence, thus the cross talk between the encoding signal and fluorescent label on analytes can be avoided. Moreover, since the coding number grows exponentially along with the increase of types of encoding materials, the combination of two channels promote a considerable rise in detection throughput. All the above properties demonstrate the unique potential of dual-digital encoded SA in high-throughput biological detection.

Cr3+-doped Mg4Ga4Ge3O16 near-infrared phosphor membrane for optical information storage and recording

A new near-infrared phosphor Mg4Ga4Ge3O16: Cr3+ with multi-photo-functionalities has been developed. Such phosphors can act as luminescent down-shifting materials converting UV and visible photons into NIR light (600–850 nm). UV irradiation induced strong NIR long persistent luminescence (LPL) can persist for more than 10 h. With the prolonged decay time, the extinguished NIR LPL can be rejuvenated by external photo stimulus including not only 980 nm light but also visible light. Such NIR photostimulated persistent luminescence (PSPL) presents separated write-in and read-out wavelength ranges. Meanwhile, photochromism (PC), that the material surface undergoes reversible change between white/pale green and ronbrown when triggered alternatively by UV and visible irradiation/heating, was also observed. Several recycle measurements demonstrate the robust fatigue resistance of PC property. Owning to the multi-photo-functionalities of Mg4Ga4Ge3O16: Cr3+, the fabrication of flexible phosphor membranes endows their application for optical information storage and repeatable optical recording. The electron motion in traps was studied by thermoluminescence (TL) and the color center formation was verified with electron spin resonance (ESR) spectra, based on which an energy level diagram was proposed for the illustration of the multifunctional mechanism. The revealing of multi-photo-functional mechanism and fabrication of flexible phosphor membranes may promote the development of a universal design principle for further study.

A novel calibration device for quality assurance of therapeutic ultrasound

Photosensitive acrylate polymers can be spatially patterned to produce layers of alternating optical contrast using multi-beam interference patterns of laser light. This technology is widely used for the fabrication of photonic devices including optical sensors and holographic elements [1] , [2] . Here, the use of photonic Bragg structures fabricated in a pressure-sensitive photopolymer [3] for the calibration of therapeutic ultrasound pressure output is presented. On exposure to ultrasound pressure waves, the Bragg structure is compressed, inducing a change in the spacing of the periodic layers of varying optical contrast and, thus, changing the devices readout wavelength i.e. colour. The device colour change due to pressure can either be visually assessed or spectroscopically interrogated for more accurate pressure quantification. This technology offers a fast, straight-forward and low-cost method for regular mapping and calibration of ultrasound pressure output. This is crucial for quality assurance of therapeutic ultrasound scanners used in Extracorporeal Shock Wave Lithotripsy and High Intensity Focused Ultrasound. To date, the photopolymer composition and Bragg device structure has been optimised for MPa range pressures. Results from the optimisation and characterisation of the pressure-sensitive devices are presented.

Nonlinear Structured Illumination Using a Fluorescent Protein Activating at the Readout Wavelength.

Structured illumination microscopy (SIM) is a wide-field technique in fluorescence microscopy that provides fast data acquisition and two-fold resolution improvement beyond the Abbe limit. We observed a further resolution improvement using the nonlinear emission response of a fluorescent protein. We demonstrated a two-beam nonlinear structured illumination microscope by introducing only a minor change into the system used for linear SIM (LSIM). To achieve the required nonlinear dependence in nonlinear SIM (NL-SIM) we exploited the photoswitching of the recently introduced fluorophore Kohinoor. It is particularly suitable due to its positive contrast photoswitching characteristics. Contrary to other reversibly photoswitchable fluorescent proteins which only have high photostability in living cells, Kohinoor additionally showed little degradation in fixed cells over many switching cycles.

Red light holographic recording and readout on an azobenzene–LC polymer hybrid composite system

We present for the first time stable red light two-dimensional image holographic recording in azobenzene-containing polymer-liquid crystal material following a pre-illumination process. The red light hologram recording is based on cis-to-trans transition of the azobenzene and surrounding LC reorientation. The diffraction efficiency of about 2% has been achieved and clear holograms of a commercial 2D USAF resolution target have been recorded. Using the same readout wavelength as the recording wavelength, color distortion that exists in most hologram researches with azo-containing materials is naturally eliminated. Long-time stabilities such as one-day continuous non-destructive readout and up to 10 weeks hologram storage have been demonstrated. The holographic recording material remains erasable and rewritable.

Photothermal Conversion of Color Centers in $$\hbox {CaF}_{2}$$ CaF 2 Crystals: A Process Underlying the Use of Crystals as a Holographic Medium

Photothermal effects in a holographic medium of calcium fluoride crystals with color centers \((\hbox {CaF}_{2})\) are discussed. It is shown that photochromism of this crystal makes it possible to record volume holograms within its volume and to change their physical properties by photothermal treatments. The diffusion–drift mechanism of a hologram recording in \(\hbox {CaF}_{2}\) includes the conversion of centers and their redistribution over the crystal bulk. Depending on the readout wavelength, it is possible to read out amplitude, amplitude phase, or phase holograms. The opportunity to record thick holograms in \(\hbox {CaF}_{2}\) and their high resistance with respect to non-coherent illumination at an elevated temperature allows forming narrow-band angular and spectral holographic filters in \(\hbox {CaF}_{2}\).

Visible and near UV light-induced scattering of LiNbO3:Fe crystals and material characterization

We present an experimental study of reconstructing parasitic gratings that produce simultaneous wide-angle polarization-anisotropic and -isotropic light induced scattering. The gratings were recorded in lithium niobate crystals doped with iron (LiNbO3:Fe) using an ordinary-polarized pump beam in the visible and the near UV spectral ranges. The reconstruction was performed at different readout angles, wavelengths and linear polarization states. The main features of reconstructing the parasitic gratings at different readout conditions are qualitatively explained by a simple phenomenological model based on the Ewald sphere construction. The obtained results and the scattering pattern were applied to determine the birefringence, its sign and further to estimate the relative contribution of diffusion and bulk photovoltaic effect to the photorefractive effect, as well as the ratios of product of the Pockels and the photovoltaic tensors components that contribute to the readout of the gratings at different readout polarizations.

Colour hologram projection with an SLM by exploiting its full phase modulation range.

We demonstrate independent and simultaneous manipulation of light beams of different wavelengths by a single hologram, which is displayed on a phase-only liquid crystal spatial light modulator (SLM). The method uses the high dynamic phase modulation range of modern SLMs, which can shift the phase of each pixel in a range between 0 up to 10π, depending on the readout wavelength. The extended phase range offers additional degrees of freedom for hologram encoding. Knowing the phase modulation properties of the SLM (i.e. the so-called lookup table) in the entire exploited wavelength range, an exhaustive search algorithm allows to combine different independently calculated 2π-holograms into a multi-level hologram with a phase range extending over several multiples of 2π. The combined multi-level hologram then reconstructs the original diffractive patterns with only small phase errors at preselected wavelengths, thus projecting the desired image fields almost without any crosstalk. We demonstrate this feature by displaying a static hologram at an SLM which is read out with an incoherent red-green-blue (RGB) beam, projecting a color image at a camera chip. This is done for both, a Fourier setup which needs a lens for image focusing, and in a "lensless" Fresnel setup, which also avoids the appearance of a focused zero-order spot in the image center. The experimentally obtained efficiency of a two-colour combination is on the order of 83% for each wavelength, with a crosstalk level between the two colour channels below 2%, whereas a three-colour combination still reaches an efficiency of about 60% and a crosstalk level below 5%.

Broadband suppression of the zero diffraction order of an SLM using its extended phase modulation range.

The diffraction efficiency of a hologram displayed on a phase-only spatial light modulator (SLM) is maximal, if the SLM modulates the phase of the diffracted beam in a range between 0 and 2π. However, if the readout wavelength changes, or a broadband beam is used, due to dispersion this ideal modulation range cannot be maintained, which leads to lower diffraction efficiency and to the appearance of an undesired intense zero diffraction order. Here we show how an SLM with an extended phase modulation range of 4π can be used to display on-axis holograms with a strong suppression of the zero diffraction order in a wide spectral range, extending over 200 nm. The basic idea is to transform the original on-axis hologram into an off-axis hologram by adding a blazed grating and performing a modulo 2π operation, and then transforming it back by adding the conjugate grating, but without performing a subsequent modulo operation. The final hologram then spans over a phase range of 4π. The total diffracted field corresponds to that of the original on-axis hologram, but now the zero-order Fourier component is diffracted away from the optical axis. The same principle can be used to entangle the on-axis hologram with other phase structures, e.g. a random phase mask or a second hologram structure, followed by a subsequent addition of the conjugate mask, which may also suppress higher diffraction orders. The reconstructed holograms show a strong contrast enhancement in a broad wavelength range.

Holographic nanoparticle-polymer composites based on radical-mediated thiol-yne photopolymerizations: characterization and shift-multiplexed holographic digital data page storage

We investigate volume holographic recording in a photopolymerizable nanoparticle-polymer (NPC) composite film that employs radical-mediated thiol-yne step-growth photopolymerizations. Because each alkyne functional group can react consecutively with two thiol functional groups in thiol-yne photopolymerizatins, the thiol-yne based NPC system dispersed with inorganic nanoparticles has the potentiality to overcome the drawback of low crosslinking densities but to retain the advantage of low shrinkage that is possible by use of thiol-ene photopolymerizations. We show that a thiol-yne based NPC film dispersed with 25 vol.% SiO2 nanoparticles and 15 wt.% single functional co-monomer gives the saturated refractive index change as large as 0.008 and the material recording sensitivity as high as 2005 cm/J at a recording and readout wavelength of 532 nm. We find that while the shrinkage of a volume hologram recorded in a thiol-yne based NPC dispersed with organic nanopartices can be as low as 0.5%, it is approximately 1% with the dispersion of SiO2 nanoparticles due to the plasticizing effect of the doped co-monomer. On the other hand, the thermal stability is improved better with the dispersion of SiO2 nanoparticles. We also demonstrate shift-multiplexed holographic storage of 80 digital data pages in a thiol-yne based NPC film with high readout fidelity.

Study on measuring refractive index modulation of volume phase holographic gratings

In this paper, we present an approach to measure the refractive index modulation of volume phase holographic gratings (VPHG) by use of its angular selectivity, with the advantage of avoiding the diffraction efficiency errors induced by noise. Based on Kogelnik's coupled wave theory for volume holographic gratings, the wave vectors matching relationship for reflective VPHG at different readout wavelengths is deduced and the diffraction efficiency as a function of readout angle is also obtained. Then an all-fiber experimental system is designed to measure corresponding diffraction efficiency with different readout angles for VPHG recorded in dichromate gelatin. By seeking the Bragg angle where diffraction efficiency is maximum and the blank angle where diffraction efficiency drops to the minimum value, the refractive index modulation can be calculated. The experiment results proves the feasibility of the method.

Detuned surface plasmon resonance scattering of gold nanorods for continuous wave multilayered optical recording and readout

In a multilayered structure of absorptive optical recording media, continuous-wave laser operation is highly disadvantageous due to heavy beam extinction. For a gold nanorod based recording medium, the narrow surface plasmon resonance (SPR) profile of gold nanorods enables the variation of extinction through mulilayers by a simple detuning of the readout wavelength from the SPR peak. The level of signal extinction through the layers can then be greatly reduced, resulting more efficient readout at deeper layers. The scattering signal strength may be decreased at the detuned wavelength, but balancing these two factors results an optimal scattering peak wavelength that is specific to each layer. In this paper, we propose to use detuned SPR scattering from gold nanorods as a new mechanism for continuous-wave readout scheme on gold nanorod based multilayered optical storage. Using this detuned scattering method, readout using continuous-wave laser is demonstrated on a 16 layer optical recording medium doped with heavily distributed, randomly oriented gold nanorods. Compared to SPR on-resonant readout, this method reduced the required readout power more than one order of magnitude, with only 60 nm detuning from SPR peak. The proposed method will be highly beneficial to multilayered optical storage applications as well as applications using a continuous medium doped heavily with plasmonic nanoparticles.

Spectral dependence of photoinduced phase shift in optically addressed light modulator

Dependence of the photoinduced phase shift ΔΦλ in an optically addressed liquid crystal (LC) spatial light modulator on the record and readout wavelengths has been studied. An algorithm of ΔΦλ recalculation for two arbitrary wavelengths from the region of photoconductor sensitivity is proposed.

Analysis of diffracted image patterns from volume holographic imaging systems and applications to image processing

Diffracted image patterns from volume holograms that are used in volume holographic imaging systems (VHISs) are investigated. It is shown that, in VHISs, prior information about the shape and spectral properties of the diffracted patterns is important not only to determine the curvature and field of view of the image, but also for image registration and noise removal. A new methodology to study numerically and analytically the dependence of VHIS diffraction patterns with the hologram construction parameters and the readout wavelength is described. Modeling and experimental results demonstrate that, in most cases, VHIS diffracted shapes can be accurately represented by hyperbolas.

Green and red laser holographic recording in different glassy azocompounds

Holographic grating recording at 532 nm and 632.8 nm has been studied in five types of glassy molecular azocompound films. AFM and SEM measurements have been made. It was found that optimal azocompound structure depends on the recording wavelength, light polarization and readout mode. For the given sample recording efficiency depends not only on recording and readout wavelength but also on readout mode. Recording with p-polarized light is much more efficient than with s-polarized light. Introduction of Si atoms in organic glass molecules can improve the recording efficiency. Fast and slow recording mechanisms exist. Slow mechanism includes photoinduced mass transfer in both green and red light recording cases. Evidence of red light induced trans–cis photoisomerization is found. Other recording mechanisms also take place.

Analysis of photorefractive volume holographic lenses with different recording and read-out wavelengths

The photorefractive volume holographic lenses for converting a plane wave to a lateral-spread spherical wave with different recording and reconstructing wavelength has been studied. Using the coupled-wave theory, the half-analytical solutions for the amplitudes of the space harmonics of the field inside the transmission geometry are presented. The values of the off-Bragg parameter at the reconstructed process and the diffracted beam's amplitude distribution are analyzed. In addition, the dependences of diffraction properties on the focal length of recording spherical wave and on the geometrical dimensions of the grating are discussed. Furthermore, the focusing properties of this photorefractive holographic lens are analyzed.

GAFCHROMIC® EBT photospectral dose response dependence on temperature and implications for flat bed scanning

Various forms of GAFCHROMIC film have been used for several years as radiographic media for measuring dose distributions. GAFCHROMIC EBT (GC-EBT) film is particularly useful for clinical dose ranges. This thin film dosimeter develops a blue color (lambdamax approximately 635 nm) when irradiated with ionizing radiation. Temperature controlled photospectrometry was used to assess temperature related readout changes in GC-EBT type film dosimetry. For observing clinical impact of findings, multiple scan studies with rising flat bed scanner temperature were performed. The whole optical spectrum in the observed wavelength range of 450-700 nm shows a distinct spectrum shift linear with temperature toward lower wavelengths when readout temperature is increased. In addition, absorption decreases in maximum regions and increases in minimum regions of the absorbance spectrum. The most pronounced wavelength dependent readout differences occur in the slopes of the spectrum curves. Absorption readout differences of -1%/degree C for a 2.7 Gy irradiated film piece at a readout wavelength of approximately 650 nm can be found. The readout difference is strongly dependent on readout light spectral characteristic, irradiation dose, and temperature. Readout difference can be positive or negative. Characteristic temperature behavior patterns are present for each color channel of a flat bed scanner. All described effects are reversible within the measurement accuracy of this study. When using unsuitable readout light, careful control of the readout temperature is necessary in order to obtain consistent and accurate results. Adapted GC-EBT type film dosimetry guidelines are presented. Temperature dependent readout differences on a flat bed scanner can be avoided when using scanner bed temperature as a fixed dosimetry parameter.