Effect of Different Substitutions at the 1,7-Bay Positions of Perylenediimide Dyes on Their Optical and Laser Properties

Perylenediimide (PDI) compounds are widely used as the active units of thin-film organic lasers. Lately, PDIs bearing two sterically hindering diphenylphenoxy groups at the 1,7-bay positions have received attention because they provide a way to red-shift the emission with respect to bay-unsubstituted PDIs, while maintaining a good amplified spontaneous emission (ASE) performance at high doping rates. Here, we report the synthesis of a series of six PDI derivatives with different aryloxy groups (PDI 6 to PDI 10) or ethoxy groups (PDI 11) at the 1,7 positions of the PDI core, together with a complete characterization of their optical properties, including absorption, photoluminescence, and ASE. We aim to stablish structure-property relationships that help designing compounds with optimized ASE performance. Film experiments were accomplished at low PDI concentrations in the film, to resemble the isolated molecule behaviour, and at a range of increasing doping rates, to investigate concentration quenching effects. Compounds PDI 10 and PDI 7, bearing substituents in the 2' positions of the benzene ring (the one contiguous to the linking oxygen atom) attached to the 1,7 positions of the PDI core, have shown a better threshold performance, which is attributed to conformational (steric) effects. Films containing PDI 11 show dual ASE.

Holographic Lens Resolution Using the Convolution Theorem

The similarity between object and image of negative asymmetrical holographic lenses (HLs) stored in a low-toxicity photopolymer has been evaluated theoretically and experimentally. Asymmetrical experimental setups with negative focal lengths have been used to obtain HLs. For this purpose, the resolution of the HLs was calculated using the convolution theorem. A USAF 1951 test was used as an object and the impulse responses of the HLs, which in this case was the amplitude spread function (ASF), were obtained with two different methods: using a CCD sensor and a Hartmann Shack (HS) wavefront sensor. For a negative asymmetrically recorded HL a maximum resolution of 11.31 lp/mm was obtained. It was evaluated at 473 nm wavelength. A theoretical study of object-image similarity had carried out using the MSE (mean squared error) metric to evaluate the experimental results obtained quantitatively.

Green and wide acceptance angle solar concentrators

A low-toxicity photopolymer was employed to prepare holographic solar concentrators (HSCs). The main aim of this study is to obtain a versatile holographic element to concentrate the sunlight from different relative positions of the Sun during the day, avoiding the need of expensive tracking systems. Multiplexed holographic elements that combine symmetric and asymmetric holographic lenses of low frequency (545 l/mm) have been recorded in the same plate to concentrate the sunlight from sunrise to sunset. The holographic behavior of HSCs has been studied by measuring the angular diffraction efficiency at 633 nm (close to the maximum response of silicon cells). The efficiency of the complete system "HSC-solar cell" has been evaluated by measuring the short-circuit current (I_sc) under solar illumination at different incident angles. This study overcomes the trade-off between good efficiency and high incident acceptance angle, showing an important breakthrough to obtain wide acceptance angle systems.

Processing of Holographic Hydrogels in Liquid Media: A Study by High-Performance Liquid Chromatography and Diffraction Efficiency

The storage of time-stable holographic gratings in hydrogel matrices when the material is immersed in aqueous media is a real challenge at present. The optimization of the storage stages of the holograms must be properly investigated to identify the most suitable development processes. For this reason, this work is focused on the study of the optimization of the washing stages of the hydrogels based on acrylamide and N,N’-methylenebis(acrylamide) once unslanted transmission holograms have been stored. High-performance liquid chromatography and UV-visible measurements have been employed in our system to analyze the composition of the washing solutions. PBST and DMSO:H₂O are used as solvents in the washing stages. The diffraction efficiencies are measured during the washing stages and after the storing of the holograms during several days in PBST. Maximum diffraction efficiencies of 38 and 27.6% are reached when PBST and DMSO:H₂O are employed, respectively, for the washing process. Holograms show temporal stability after being stored immersed in PBST at 4 °C for 4 days.

Aberration-Based Quality Metrics in Holographic Lenses

Aberrations and the image quality of holographic lenses were evaluated by a Hartmann-Shack (HS) wavefront sensor. Two lenses, one recorded with a symmetrical configuration and the other with an asymmetrical one, were stored in a photopolymer called Biophotopol. Each was reconstructed with two different wavelengths, 473 nm and 633 nm. Different metrics were applied to determine and quantify the aberration of the lenses (Zernike coefficients, Seidel coefficients, Marechal tolerances, root-mean-square (RMS), peak to valley, critical fraction of the pupil), and the quality of the image they provided (Strehl ratio, entropy, cutoff frequency, modulation transfer function (MTF), and area under the MTF). Good agreement between the metrics related to optical quality was obtained. The negative asymmetric holographic lenses had less aberration than the positive symmetric ones.

LED-Cured Reflection Gratings Stored in an Acrylate-Based Photopolymer

The storage of volume holographic reflection gratings in low-toxicity photopolymers represents a challenge at present since they can be used in many important applications such as biosensors and holographic optical elements. In this context, an acrylate-based photopolymer developed in our research group was employed to study the recording of unslanted holographic reflection gratings at high spatial frequencies. The optimal preparation conditions of the photopolymer layers were determinated. The diffraction efficiencies are measured in both recording and curing stage and a comparative study of these values was realized. In addition, a theoretical study using Kogelnik's coupled wave theory was carried out with the aim of understanding the diffraction efficiency behaviour of both processes. In this work, a maximum diffraction efficiency of 14.1% was reached after a curing process in 150 µm layers at a recording wavelength of 488 nm. This value represents a good result compared to that reported in the literature and opens the way to reflection mode holography research using low-toxicity material.

Influence of Tert-Butylthiol and Tetrahydrofuran on the Holographic Characteristics of a Polymer Dispersed Liquid Crystal: A Research Line Toward a Specific Sensor for Natural Gas and Liquefied Petroleum Gas

Tert-Butylthiol (TBT) and tetrahydrothiophene (THT) are odorant substances added to natural gas and liquefied petroleum gas to help their detection by the human smell. In this research, TBT and THT are incorporated into a holographic polymer-dispersed liquid crystal and their influence in the main holographic characteristics of the photopolymer are studied in order to open the way towards the design of a holographic sensor to detect natural gas and liquefied petroleum gas.

Holographic Lenses in an Environment-Friendly Photopolymer

In this paper, we theoretically and experimentally evaluated the quality of volume phase transmission lenses stored in an environmentally friendly photopolymer. Holographic lenses (HLs) were obtained using symmetrical and asymmetrical experimental setups with the same positive and negative focal length and pupil diameter. The image quality was evaluated from the calculation of the modulation transfer function (MTF) by capturing the point spread function (PSF) with a charge-coupled device (CCD). A maximum frequency of 14 L/mm, reaching an MTF value of 0.1, was obtained for a negative asymmetrically recorded HL, evaluated at 473 nm wavelength. A theoretical study of aberrations was carried out to qualitatively evaluate the experimental results obtained.

Thickness dependence of amplified spontaneous emission in low-absorbing organic waveguides

The effect of varying film thickness (h) on the amplified spontaneous emission (ASE) properties of 0.5  wt.% perylenediimide-doped polystyrene waveguides is reported. The threshold dependence on h, not previously investigated in detail, is analyzed in terms of the film absorption and photoluminescence, the confinement of the fundamental waveguide mode (TE0), and the presence of high-order modes. For h<400  nm and down to 150 nm, the ASE wavelength blueshifts, while the linewidth and threshold increase. The detrimental ASE operation in very thin films is due to the low absorption as well as to the poor confinement of the TE0 mode.