Giovanni Manfredi

Cellulose ternary photonic crystal created by solution processing

For the first time, a multiplanar photonic crystal structure has been obtained using cellulose as a structural material. This all-polymer system, made of cellulose, polyvinyl alcohol and poly(N-vinylcarbazole) is a ternary planar photonic crystal composed by 7 repeated trilayers produced by spin coating. Trimethylsilyl cellulose is used as a precursor to be converted to cellulose. Transverse Transmission Electron Microscopy analysis of our systems confirms the multilayered structure whose optical response can be theoretically accounted for. Preliminary results on the response of the photonic crystal to water vapors envisage the use of this system for humidity optical sensing.

Lasing from dot-in-rod nanocrystals in planar polymer microcavities

Colloidal nanocrystals attract considerable attention in the field of light emitting devices thanks to their high fluorescence quantum yield, low amplified spontaneous emission (ASE) threshold, and spectral tunability via electronic structure engineering and surface functionalization. Combining polymer microcavities with colloidal nanocrystals as gain material promises a solution-based fabrication route to plastic laser cavities as well as applications in the field of smart flexible large area light sources and sensors. Here we demonstrate lasing from polymer microcavities embedding solution processable dot-in-rod (DiR) CdSe/CdS nanocrystals. Two highly reflective polymer dielectric mirrors are prepared by spin-coating of alternated layers of polyacrylic acid and poly(N-vinyl carbazole), with their photonic band gap tailored to the emission of the DiRs. The DiRs are enclosed in the polymer microcavity by drop-cast deposition on one mirror, followed by pressing the mirrors onto each other. We obtain excellent overlap of the ASE band of the DiRs with the photonic band gap of the cavity and observe optically pumped lasing at 640 nm with a threshold of about 50 μJ cm−2.

Spin-Coated Polymer and Hybrid Multilayers and Microcavities

Polymer multilayer structures have attracted increasing attention in the recent years because of the straightforward and low-cost techniques that can be used for their fabrication. When the multilayers are composed of a periodical alternation of two materials with different refractive indexes and with layer thicknesses comparable with the wavelength of light, they take the name of distributed Bragg reflectors (DBR). They behave like planar one-dimensional photonic crystals (PhC) and exhibit a photonic band gap (PBG), a spectral region in which photons with suitable energy and wave vector are not allowed to propagate through the crystal. Moreover, within the PBG and at its edges, modifications of radiative photophysical processes occur. The spectral position, efficiency and linewidth of the PBG can be engineered by modifying the layer thicknesses and the refractive indexes of the two materials. While DBRs grown using inorganic materials are well known, polymer and colloidal particle DBRs are receiving a renewed interest due to the possibility to chemically engineer their structural properties and photonic functions; moreover, they can be free-standing and flexible thus being adaptable to any surface. Furthermore, polymers and porous structures can easily embed many other active materials, paving the way to a myriad of applications. In this chapter, we introduce polymer multilayers and planar microcavities fabricated using the spin coating technique, discussing the different materials employed and manufacturing challenges. We will also review different applications that exploit these kinds of photonic structures ranging from lasing to sensing.

Colorimetric Detection of Perfluorinated Compounds by All-Polymer Photonic Transducers

We report on the highly sensitive optical and colorimetric detection of perfluorinated compounds in the vapor phase achieved by all-polymer dielectric mirrors. High optical quality and uniformly distributed Bragg reflectors were fabricated by alternating thin films of poly(N-vinylcarbazole) and Hyflon AD polymers as high and low refractive index medium, respectively. A new processing procedure has been developed to compatibilize the deposition of poly(N-vinylcarbazole) with the highly solvophobic Hyflon AD polymer layers to achieve mutual processability between the two polymers and fabricate the devices. As a proof of principle, sensing measurements were performed using the Galden HT55 polymer as a prototype of the perfluorinated compound. The Bragg stacks show a strong chromatic response upon exposure to this compound, clearly detectable as both spectral and intensity variations. Conversely, Bragg mirrors fabricated without fluorinated polymers do not show any detectable response, demonstrating that the Hyflon AD polymer acts as the active and selective medium for sensing perfluorinated species. These results demonstrate that organic dielectric mirrors containing perfluorinated polymers can represent an innovative colorimetric monitoring system for fluorinated compounds, suitable to improve both environmental safety and quality of life.

Directional fluorescence shaping and lasing in all-polymer microcavities doped with CdSe/CdS dot-in-rod nanocrystals

Colloidal nanocrystals (NCs) have attracted considerable attention in the field of light emitting devices thanks to their high fluorescence quantum yield and spectral tunability via electronic structure engineering and surface functionalization.[1, 2] Their composition and shape can be adjusted to obtain materials with tailored and stable optoelectronic properties. Surface functionalization allows solubility in both organic solvents and water paving the way to a variety of solution-processing technologies[3]. Careful surface chemical engineering can drive NCs self-organization and preparation of nanocomposites, which is critical to create hybrid materials for photonic devices. Thanks to low costs, versatility, and compatibility with flexible thin film technologies, the combination of polymers and colloidal NCs are of great promise for lighting and photosensing. Combining polymer microcavities with NC as gain material promises a solution-based fabrication route to plastic laser cavities as well as applications in the field of smart flexible large area light sources and sensors[4-5]. The production of planar photonic structures such as Distributed Bragg Reflectors (DBRs) and microcavities with these simple techniques is very stimulating, because it allows the use of commercially available polymers and does not require expensive methods such as high vacuum deposition. Furthermore, the fabrication can be accomplished on many different substrates including glass, silicon, plastic, and elastic materials suitable for flexible devices.