Bio-inspired wrinkle microstructures for random lasing governed by surface roughness.

A method for fabricating bio-inspired scattering substrates based on polydimethylsiloxane (PDMS) for spatially incoherent random lasing is presented. The leaves of monstera and piper sarmentosum plants are used to mold PDMS polymer to form wrinkle-like scattering substrates, which are then used with a liquid gain medium for random lasing. Scattering is attributed to the surface roughness (Sa) of the samples. The rougher sample with 5.2 µm Sa shows a two-mode stable lasing with a 2 nm linewidth and a lower threshold fluence of 0.2mJ/cm2 compared to the sample with smaller Sa (3.6 µm) with a linewidth of 5 nm and a threshold fluence of 0.5mJ/cm2. The waveguide theory substantiates the results of incoherent random lasing through a relation between the microstructure feature size and the mean free path. Power Fourier transform analysis is used to deduce the resonant cavity length of 180 µm in the rougher sample, and the observed variations in cavity length with Sa validate the optical feedback. PDMS being hydrophobic, the scattering substrate can be reused by wiping off the gain medium. This Letter paves the way for facile fabrication methods of bio-inspired random lasers for sensing and imaging applications.

Electro-Ionic Control of Surface Plasmons in Graphene-Layered Heterostructures.

Precise control of light is indispensable to modern optical communication devices especially as the size of such devices approaches the subwavelength scale. Plasmonic devices are suitable for the development of these optical devices due to the extreme field confinement and its ability to be controlled by tuning the carrier density at the metal/dielectric interface. Here, an electro-ionic controlled plasmonic device consisting of Au/graphene/ion-gel is demonstrated as an optical switch, where an external electric field modulates the real part of the electrical conductivity. The graphene layer enhances charge penetration and charge separation at the Au/graphene interface resulting in an increased photoinduced voltage. The ion-gel immobilized on the Au/graphene further enables the electrical tunability of plasmons which modulates the intensity of the reflected laser light. This work paves the way for developing novel plasmonic electro-optic switches for potential applications such as integrated optical devices.

Particle free optical imaging of flow field by liquid crystal polarization.

This paper proposes and demonstrates a particle free method for flow field visualizations by analyzing liquid crystal polarizations. The proposed concept is implemented by imaging of liquid crystal flow under microfluidic environment using a crossed polarization microscopy configuration. Fringe patterns give good representation of flow characterizations for different nozzle/diffuser microchannel designs. The obtained results demonstrate that the flow field under various conditions can be evaluated. Visualizations of the flow fields are carried out by the liquid crystal polarization induced fringe patterns in nozzle/diffuser microchannels. We achieve good match between the flow field obtained by LC polarization and the simulated one. It is envisaged that the proposed methodology can make a potential impact in flow field visualization studies and related analysis.

Conoscopic analysis of electric field driven planar aligned nematic liquid crystal.

This paper illustrates the conoscopic observation of a molecular reconstruction occurring across a nematic liquid crystal (NLC) medium in the presence of an external electric field. Conoscopy is an optical interferometric method, employed to determine the orientation of an optic axis in uniaxial crystals. Here a planar aligned NLC medium is used, and the topological changes with respect to various applied voltages are monitored simultaneously. Homogenous planar alignment is obtained by providing suitable surface treatments to the ITO coated cell walls. The variation in the conoscopic interferometric patterns clearly demonstrates the transition from planar to homeotropic state through various intermediate states.

Imaging behind opaque obstacle: a potential method for guided in vitro needle placement.

We report a simple real time optical imaging concept using an axicon lens to image the object kept behind opaque obstacles in free space. The proposed concept underlines the importance and advantages of using an axicon lens compared to a conventional lens to image behind the obstacle. The potential of this imaging concept is demonstrated by imaging the insertion of surgical needle in biological specimen in real time, without blocking the field of view. It is envisaged that this proposed concepts and methodology can make a telling impact in a wide variety of areas especially for diagnostics, therapeutics and microscopy applications.

Quantitative optical coherence microscopy for the in situ investigation of the biofilm.

This paper explores the potential of optical coherence microscopy (OCM) for the

Nanoparticulate Contrast Agents for Multimodality Molecular Imaging.

Molecular imaging is rapidly developing as a powerful tool in research and medical diagnostic. By integrating complementary signal reporters into a single nanoparticulate contrast agent, multimodal molecular imaging can be performed as scalable images with high sensitivity, resolution and specificity. In this review, multifunctional nanoparticles (MFNPs) are classified into four types: conjugation, encapsulation, core/shell, and co-doping. Further, new constructs of MFNPs were reported recently which have used nanoparticulate contrast agent such as quantum dots (QDs), iron oxide nanoparticles (IONPs), Upconversion nanoparticles (UCNPs), carbon based nanoparticles, gold nanoparticles (Au-NPs), Metal-Organic Frameworks (MOFs), dendrimers and porphyrins based nanoparticles. Different surface modification strategies were also developed as well as ligands are attached to those NPs to render the biocompatibility and enable specific targeting. These new development in MFNPs are expected to introduce a paradigm shift in multi-modal molecular imaging and thereby opening up an era of personalized medicine and new diagnostic medical imaging tools.

Plasmon Resonant Silica-Coated Silver Nanoplates as Contrast agents for Optical Coherence Tomography.

Silica-coated silver nanostructures are identified as potential contrast agents for visible and near-infrared bio-imaging applications due to their high optical extinction caused by localized surface plasmon resonance (LSPR), improved chemical stability, and lower toxicity. We demonstrate the potential of plasmon resonant silica-coated silver nanoplates as a contrast agent for optical coherence tomography (OCT). It is shown that, triangular-shaped silica-coated silver nanoplates (SSNPs) with a side length of 170 ± 5 nm, base silver thickness of 10 ± 1 nm, and silica shell thickness of 40 ± 2 nm, exhibit higher optical extinction at a 1300 nm wavelength range, thus making them an excellent contrast agent for OCT imaging. Optical extinction characterization using OCT is found to be reasonably consistent with ultraviolet (UV)-Vis-near infrared (NIR) spectroscopy and finite difference time-domain (FDTD)-based analysis. Ex vivo studies on pig adipose tissue demonstrate that LSPR-induced enhanced scattering in SSNPs contributes to the OCT signal, leading to images with better contrast. Moreover, average A-scan profiles acquired at different time delays show the downward propagation of SSNPs and the extension of signal enhancement at the deeper regions. Speckle variance OCT images show that SSNPs are efficiently distributed over the targeted tissue region, demonstrating their applicability in a large lesion area.

Hybrid-modality ocular imaging using a clinical ultrasound system and nanosecond pulsed laser.

Hybrid optical modality imaging is a special type of multimodality imaging significantly used in the recent past in order to harness the strengths of different imaging methods as well as to furnish complementary information beyond that provided by any individual method. We present a hybrid-modality imaging system based on a commercial clinical ultrasound imaging (USI) system using a linear array ultrasound transducer (UST) and a tunable nanosecond pulsed laser as the source. The integrated system uses photoacoustic imaging (PAI) and USI for ocular imaging to provide the complementary absorption and structural information of the eye. In this system, B-mode images from PAI and USI are acquired at 10 Hz and about 40 Hz, respectively. A linear array UST makes the system much faster compared to other ocular imaging systems using a single-element UST to form B-mode images. The results show that the proposed instrumentation is able to incorporate PAI and USI in a single setup. The feasibility and efficiency of this developed probe system was illustrated by using enucleated pig eyes as test samples. It was demonstrated that PAI could successfully capture photoacoustic signals from the iris, anterior lens surface, and posterior pole, while USI could accomplish the mapping of the eye to reveal the structures like the cornea, anterior chamber, lens, iris, and posterior pole. This system and the proposed methodology are expected to enable ocular disease diagnostic applications and can be used as a preclinical imaging system.