Mid-infrared photothermal hyperspectral imaging of biomolecular systems

Thesis (Ph.D.)--Boston UniversityThe development of novel techniques in spectroscopy and microscopy that are label-free, contactless and accessible is useful among many scientific disciplines, ranging from Materials Science to Biomedical Engineering. Hyperspectral photothermal imaging using vibrational spectroscopy promises to be a new tool in the arsenal for analysis and characterization of materials. This technique can be used for understanding structural composition of a material that is advantageous to the materials scientist. A combination of microscopy and spectroscopy is also beneficial to the biologist or pathologist that analyzes a complex sample with rich morphology. Photothermal hyperspectral microscopy is a label-free nondestructive method that utilizes specific vibrational bands of a molecule giving spectral information to an image. The method is based on changes in the thermal state, and the associated change in the refractive index of the sample as it is irradiated with mid-infrared light. Photothermal microscopy has rapidly emerged as one of the most sensitive label-free optical spectroscopic methods, rivaling current well-established methods based on fluorescence. The method has been used to image single non-fluorescent molecules in room temperature and to directly characterize biological features such as mitochondria and red blood cells. Despite great breakthroughs in the visible regime, the method has not been explored in the mid-infrared regime where most of the biological molecules have characteristic vibrational modes that constitute an intrinsic molecular "fingerprint" . This thesis presents the development of a new technique to measure the linear and nonlinear mid-infrared photothermal response induced by tunable high power lasers such as Quantum Cascade Lasers (QCLs). Photothermal response can be measured in pump-probe heterodyne detection, using short wavelength visible lasers and compact fiber lasers as a probe. This allows for direct detection of the fingerprint mid-infrared vibrational modes through ultrasensitive photodetectors. Integrated into a mid-infrared microscope, the system facilitates the acquisition of spectra and images on condensed phase samples. Photothermal heterodyne mid-infrared hyperspectral vibrational technique is used to image biological samples such as bird brain and other biomolecules First photothermal images on specially designed plasmonic metamaterials, designed to either enhance or suppress a selected mid-infrared vibrational normal mode, are demonstrated. Plasmonic metamaterials can be engineered using electron beam lithography for functional studies on biomolecules enhancing selected vibrational infrared resonances. This study takes advantage of the strong interaction between light and matter and investigates properties of the material that are difficult to detect through conventional spectroscopic methods. The new technique has the ability to advance studies in many fields, as it is applicable to different types of materials, non-destructive, accessible and inexpensive

Wireless transfer of power by a 35-GHz metamaterial split-ring resonator rectenna

Wireless transfer of power via high frequency microwave radiation using a miniature split ring resonator rectenna is reported. RF power is converted into DC power by integrating a rectification circuit with the split ring resonator. The near-field behavior of the rectenna is investigated with microwave radiation in the frequency range between 20-40 GHz with a maximum power level of 17 dBm. The observed resonance peaks match those predicted by simulation. Polarization studies show the expected maximum in signal when the electric field is polarized along the edge of the split ring resonator with the gap and minimum for perpendicular orientation. The efficiency of the rectenna is on the order of 1% for a frequency of 37.2 GHz. By using a cascading array of 9 split ring resonators the output power was increased by a factor of 20

Dynamic Tuning of Surface Plasmon Polaritons via Thermally Controlled Liquid Crystals

We present a thermally controlled plasmonic substrate in contact with a liquid crystal offering spectral tunings similar to 18.7 nm within similar to 18 degrees C at LC nematic phase. Nematic/ isotropic transition also enables similar to 12 nm spectral change only within similar to 1 degrees C

Field-effect active plasmonics for ultracompact electro-optic switching

Merging of electronics and photonics at subwavelength dimensions could potentially allow development of ultracompact electro-optic modulators and active optical interconnects. Here, we introduce a field-effect active plasmonic modulator where the metallic ring serves as both a photonic resonator and a field electrode. By exploiting the simultaneous electronic and photonic functionalities of our plasmonic device, we show devices offering significantly improved modulation depths (as high as similar to 10.85 dB) compared to active dielectric micro-ring resonators. Device concepts introduced in this work are applicable in realization of various integrated components and could play an important role in development of active plasmonic circuits. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754139

Dual-band plasmonic resonator based on Jerusalem cross-shaped nanoapertures

In this paper, we both experimentally and numerically introduce a dual-resonant metamaterial based on subwavelength Jerusalem cross-shaped apertures. We numerically investigate the physical origin of the dual-resonant behavior, originating from the constituting aperture elements, through finite difference time domain calculations. Our numerical calculations show that at the dual-resonances, the aperture system supports large and easily accessible local electromagnetic fields. In order to experimentally realize the aperture system, we utilize a high-precision and lift-off free fabrication method based on electron-beam lithography. We also introduce a fine-tuning mechanism for controlling the dual-resonant spectral response through geometrical device parameters. Finally, we show the aperture system's highly advantageous far- and near-field characteristics through numerical calculations on refractive index sensitivity. The quantitative analyses on the availability of the local fields supported by the aperture system are employed to explain the grounds behind the sensitivity of each spectral feature within the dual-resonant behavior. Possessing dual-resonances with large and accessible electromagnetic fields, Jerusalem cross-shaped apertures can be highly advantageous for wide range of applications demanding multiple spectral features with strong nearfield characteristics. (C) 2015 Elsevier B.V. All rights reserved