We demonstrate a smartphone based spectrometer design that is standalone and supported on a wireless platform. The device is inherently low-cost and the power consumption is minimal making it portable to carry out a range of studies in the field. All essential components of the device like the light source, spectrometer, filters, microcontroller and wireless circuits have been assembled in a housing of dimensions 88 mm × 37 mm × 22 mm and the entire device weighs 48 g. The resolution of the spectrometer is 15 nm, delivering accurate and repeatable measurements. The device has a dedicated app interface on the smartphone to communicate, receive, plot and analyze spectral data. The performance of the smartphone spectrometer is comparable to existing bench-top spectrometers in terms of stability and wavelength resolution. Validations of the device were carried out by demonstrating non-destructive ripeness testing in fruit samples. Ultra-Violet (UV) fluorescence from Chlorophyll present in the skin was measured across various apple varieties during the ripening process and correlated with destructive firmness tests. A satisfactory agreement was observed between ripeness and fluorescence signals. This demonstration is a step towards possible consumer, bio-sensing and diagnostic applications that can be carried out in a rapid manner.

We report three-dimensional mirror-less lasing from non-cylindrical dye doped polystyrene fibers drawn using an electrospinning procedure where the fiber cross-sectional shape and dimension could be controlled. Signatures of three dimensional etalon like modes were observed corresponding to the transverse and axial quantization of the wave vector. Low lasing thresholds of the order of 200 nJ were achieved along with moderate Q factors.Optical microresonators and microlasers have shown to be promising for applications in optical communication, integrated optics, and sensors. 1 A large number of microresonator geometries such as disks, spirals, 2 rings, 3 cylinders, 4 squares, pentagons, stadiums, 5 or bubbles 6 have been investigated. In general, these lasers are based either on a onedimensional (1D) Fabry-Perot type or on a two-dimensional (2D) whispering gallery-like modes (WGM) feedback. Some of these 2D resonators serve as model platforms to study wave chaos as 2-D cavity contours can be tailored to prevent the stabilization of closed orbits. 5,7 It has been well predicted and demonstrated that there are sets of periodic orbits that dominate over other paths. 5 Unlike the extensive studies on 1-D and 2-D microcavities, 3-D microcavities are relatively less explored. "Bottle"-like microcavities have three-dimensional (3D) mode structures with coexistence of WGM and "bouncing ball" modes 9 where the latter are axial modes and azimuthal modes arise from WGM resonances. This combination, which is specific of a 3-D extension, allows such resonators to exhibit high Q-factors together with a larger dynamic tuning range than that of conventional resonators 10-13 leading to their application as add-drop filters. 14 In such cases, a common basic requirement is that of a fiber-like geometry in order to sustain axial modes.In this work, we explore the features of lasing in 3-D microcavities fabricated using relatively simple and controlled soft-methods. We present an approach based on electrospinning of dye-doped polymer fibers and utilize this versatile technique to spin fibers of various dimensions and geometries including coaxial structures. 15 It has been shown that fibers with a variety of cross-sections can be spun from certain polymer solutions like polystyrene/tetrahydrofuran (PS/THF). [16][17][18] The observation and the interpretation in the present case are different from previous observation of lasing in dyedoped electrospun nano-fibers with features of a 1D resonator. 19 The transverse dimensions in our case are large (k, where k is the lasing wavelength) with the ability to form 3D microresonators. It should be mentioned that the process of electrospinning is inherently large scale even with low input volume of the polymer solution which readily forms reproducible fibers of consistent size and shape. We observe signatures of 3D mode structure in the laser spectrum with feedback trajectories essentially dictated by the fiber geometry.Fibers were electrospun from a PS/THF (30 wt. %) solution with 4-dicyanometh...

A light harvesting device geometry is implemented for polymer solar cells that retain efficiency when the device area is scaled up. Patterning devices and incorporating suitable fluorescent dye doped polymers in the spaces between the patterns leads to 12% efficiency enhancement. Vacuum free deposition of electrodes is carried out using meltable alloys giving rise to device efficiency of 6%. The fluorescent layer along with the electrode serves as an active encapsulant leading to improved device stability.

Images obtained from photocurrent scanning of organic bulk heterojunction solar cell devices provide a direct measure of correlation of the morphology to the performance parameters. The peripheral photocurrent induced from light coupled to probe tips in the near-field regime of bulk heterojunction layers permits in situ scanning of active solar cells with asymmetric electrodes. We present a methodology involving a combination of atomic force microscopy, near-field optical microscopy, and near-field photocurrent microscopy to decipher the carrier generation and transport regions in the bulk heterojunction layer. The angular Fourier transformation technique is implemented on these images to rationalize the optimum blend concentration in crystalline and amorphous donor systems and provide insights into the role of the bulk heterojunction morphology.

We demonstrate a highly sensitive mobile phone based spectrometer that has potential to detect cancerous skin lesions in a rapid, non-invasive manner. Earlier reports of low cost spectrometers utilize the camera of the mobile phone to image the field after moving through a diffraction grating. These approaches are inherently limited by the closed nature of mobile phone image sensors and built in optical elements. The system presented uses a novel integrated grating and sensor that is compact, accurate and calibrated. Resolutions of about 10 nm can be achieved. Additionally, UV and visible LED excitation sources are built into the device. Data collection and analysis is simplified using the wireless interfaces and logical control on the smart phone. Furthermore, by utilizing an external sensor, the mobile phone camera can be used in conjunction with spectral measurements. We are exploring ways to use this device to measure endogenous fluorescence of skin in order to distinguish cancerous from non-cancerous lesions with a mobile phone based dermatoscope.