Matthew Pietrzykowski

Strong broadband optical absorption in silicon nanowire films

The broadband optical absorption properties of silicon nanowire (SiNW) films fabricated on glass substrates by wet etching and chemical vapor deposition (CVD) have been measured and found to be higher than solid thin films of equivalent thickness. The observed behavior is adequately explained by light scattering and light trapping though some of the observed absorption is due to a high density of surface states in the nanowires films, as evidenced by the partial reduction in high residual sub-bandgap absorption after hydrogen passivation. Finite difference time domain simulations show strong resonance within and between the nanowires in a vertically oriented array and describe the experimental absorption data well. These structures may be of interest in optical films and optoelectronic device applications.

Tissue-like phantoms for near-infrared fluorescence imaging system assessment and the training of surgeons

We demonstrate how to construct calibrated, stable, and inexpensive tissue-like phantoms for near-IR (NIR) fluorescence imaging applications. The bulk phantom material is composed of gelatin, intralipid, hemoglobin, and indocyanine green (ICG). Absorbance, scatter, background fluorescence, and texture can be tuned as desired. NIR fluorescent inclusions are comprised of ICG-labeled polystyrene divinylbenzene beads and Pam78-labeled hydroxyapatite crystals. The former mimic tumor masses of controllable size and contrast agent concentration, and the latter mimic microcalcifications in breast cancer. NIR-fluorescent inclusions can be positioned precisely in phantoms, with one or more regions having different optical properties, and their position can be verified independently using microcomputed tomography. We demonstrate how these phantoms can be used to calibrate and compare imaging systems, and to train surgeons to operate under NIR fluorescence image guidance.

Direct probe of excitonic and continuum transitions in the photocurrent spectroscopy of individual carbon nanotube p-n diodes

The authors show that a carbon nanotube p-n diode is a very sensitive probe of optical transitions in individual single-walled carbon nanotubes. In the photocurrent spectra, an alternating sequence of resonant peaks from dissociation of excitons and exciton-phonon bound states, for the lowest and higher electronic subbands, is observed. At an intermediate energy, the onset of continuum is observed that allows measurement of exciton binding energies. Both the binding energy and the onset of continuum follow the inverse diameter relation as expected from general theory of optical transitions in nanotubes.

Data processing in multivariable RFID vapor sensors

Sensors for selective monitoring of gases and volatiles are needed for numerous applications including medical diagnostics, food safety, environmental, industrial, homeland protection, and many others. For these and other applications, we have developed passive radio frequency identification (RFID) sensors for vapor sensing where we apply a sensing film onto the resonant antenna of the RFID sensor, simultaneously measure several parameters of antenna impedance, and process these parameters using multivariate analysis tools. In this work, we critically analyze techniques of processing the impedance response of individual sensors coated with different sensing materials and the ability of these techniques to increase selectivity of developed sensors upon exposure to model vapors. Four types of investigated data processing techniques are based on unsupervised and supervised pattern recognition algorithms. Two evaluation criteria for these techniques involved their ability (1) to correctly identify types of vapors and (2) to provide the smallest error of prediction of concentrations of vapors.

Selective Vapor Monitoring Using Individual Multivariable RFID Sensors

We have developed multivariable radio frequency identification (RFID) sensors that quantify volatile organic compounds (VOCs) in their mixtures with water vapor. This selectivity is achieved by measuring several parameters of resonance impedance spectrum of the sensor followed by their multivariate analysis. Selectivity of developed individual RFID sensors promises to impact numerous sensing applications.