The threat of terrorism and the need for homeland security calls for advanced technologies to detect the concealed
explosives safely and efficiently. We demonstrated highly sensitive and selective detection of traces of nitroaromatic
explosive compounds by functionalizing gallium nitride (GaN) nanowires with titanium dioxide (TiO<sub>2</sub>)
nanoclusters to address this issue. The hybrid sensor devices were developed by fabricating two-terminal devices
using individual GaN nanowires (NWs) followed by the deposition of TiO<sub>2</sub> nanoclusters (NCs) using sputtering
technique. The photo-modulated GaN/TiO<sub>2</sub> NWNC hybrids showed remarkable selectivity to benzene and related
aromatic compounds, with no measureable response for other analytes at room temperature. This paper presents the
sensing characteristics of GaN/TiO<sub>2</sub> nanowire-nanocluster hybrids towards the different aromatic and nitroaromatic
compounds at room temperature. The GaN/TiO<sub>2</sub> hybrids were able to detect trinitrotoluene (TNT) concentrations as
low as 500 pmol/mol (ppt) in air and dinitrobenzene concentrations as low as 10 nmol/mol (ppb) in air in
approximately 30 s. The noted sensitivity range of the devices for TNT was from 8 ppm down to as low as 500 ppt.
The detection limit of Dinitrotoluene , nitrobenzene , nitrotoluene, toluene and benzene in air is 100 ppb with a
response time of ~ 75 s. The devices show very sensitive and selective response to TNT when compared to
interfering compounds like toluene. Integration of this nano-scale technology could lead to tiny, highly sensitive,
selective, low-power and smart explosive detectors that could be manufactured cheaply in large numbers.
Nanowire-nanocluster hybrid chemical sensors were realized by functionalizing gallium nitride (GaN) nanowires
with titanium dioxide (TiO<sub>2</sub>) nanoclusters for selectively sensing Benzene and other related aromatic compounds.
Hybrid sensor devices were developed by fabricating two-terminal devices using individual GaN nanowires followed by
the deposition of TiO<sub>2</sub> nanoclusters using RF magnetron sputtering. The sensor fabrication process employed all
standard micro-fabrication techniques. A change of current was observed for these hybrid sensors when exposed to
aromatic compounds such as Benzene, Toluene, Ethylbenzene, Xylene, and Chlorobenzene mixed with air. However,
these sensors did not show any sensitivity when exposed to Methanol, Ethanol, Isopropanol, Chloroform, Acetone, and
1, 3-Hexadiene. These sensors were capable of sensing the aromatic compounds only under ultraviolet excitation. The
sensitivity range for the above mentioned aromatic compounds varied from 1% down to 50 parts per billion (ppb) at
room-temperature. By combining the enhanced catalytic properties of the TiO<sub>2</sub> nanoclusters with the sensitive
transduction capability of the nanowires, an ultra-sensitive and highly selective chemical sensing architecture is
demonstrated. We have proposed a mechanism that could qualitatively explain the observed sensing behavior.
We discuss the present state-of-the-art concerning the growth mechanism, optical luminescence and electrical
properties for GaN nanowires grown with catalyst-free molecular beam epitaxy. These nanowires are essentially
defect-free and display long photoluminescence lifetimes and carrier mobilities relative to epitaxially grown GaN
films. The exclusion of crystalline defects comes from the ease with which strain-relieving dislocations can reach
the sidewalls and terminate. The growth mechanism is based on variations in Ga sticking coefficients and surface
energies of the sidewall planes and end facet planes. With control of the nucleation process through selective
epitaxy on patterned substrates, a high degree of diameter, length and position control can be achieved. Common
difficulties with interpretation of optical and electrical data with regard to internal quantum efficiency and mobility
are also addressed.
Hybrid glass parts composed of dissimilar glass sections are an attractive route to integrate multiple functions onto a single substrate and offer the potential to fabricate advanced laser sources, amplifiers, lossless splitters and other photonic devices such as Fabry-Perot etalons. We review the most promising bonding technologies, placing particular emphasis on techniques that do not require the use of high processing temperatures. In particular, we discuss in detail a recently developed low temperature bonding technology that relies on inorganic adhesives. Characterization of interfacial joints prepared with this inorganic technology indicate low insertion loss, high mechanical strength and chemical resistance to attack during the conventional lithographic and ion exchange steps employed to fabricate waveguide structures.
Phosphate glasses have become increasingly popular for planar waveguide devices owing in part to the development of a number of different commercial compositions with a wide range of optical, physical, chemical and laser properties. In addition, the recent development of low temperature bonding technology has made possible the fabrication of structures involving multiple glasses prepared as a single hybrid substrate. Combined, these new materials and technologies make possible the creation of devices with increasing integration and complexity. Here, we present passive characterization data collected on glass joints prepared with the low temperature bonding technology and active performance data of a hybrid DBR laser where the surface relief grating has been fabricated in the passive glass region of a hybrid substrate.
The ability to engineer glass properties through the selection and adjustment of chemical composition continues to make glass a leading material in both active and passive applications. The development of optimal glass compositions for integrated optical applications requires a number of considerations that are often at variance with one another. Of critical importance is that the glass offers compatibility with standard ion exchange technologies, allowing fabrication of guided wave structures. In addition, for application as an active material, the resultant structures must be characterized by absence of inclusions and low absorption at the lasing wavelength, putting demands on both the selection and identity of the raw materials used to prepare the glass. We report on the development of an optimized glass composition for integrated optic applications that combines good laser properties with good chemical durability allowing for a wide range of chemical processing steps to be employed without substrate deterioration. In addition, care was taken during the development of this glass to insure that the selected composition was consistent with manufacturing technology for producing high optical quality glass. We present the properties of the resultant glasses, including results of detailed chemical and laser properties, for use in the design and modeling of active waveguides prepared with these glasses.
Erbium and erbium/ytterbium co-doped silicate glass waveguide lasers have been fabricated by silver ion-exchange and their characteristics analyzed. We report on measurements and comparisons made in the lasing properties of these devices, including threshold, slope-efficiencies and pump tuning ranges. The results presented show that through proper choice of host glass, it is possible to make low-threshold lasers both in singly and co-doped devices.
A means of reproducibly fabricating stable cw channel waveguide lasers in rare-earth-doped Ti:LiNbO<SUB>3</SUB> is demonstrated, through careful choice of the light propagation direction. Z-propagating waveguides have been fabricated in Nd:Ti:LiNbO<SUB>3</SUB> and room-temperature cw laser operation has been obtained by pumping in the 800 nm-band, with greatly reduced photorefractive instability. The reduced photorefractive damage susceptibility in this waveguide configuration has been used to our advantage in the realization, for the first time, of a 980 nm-pumped laser in Er:Ti:LiNbO<SUB>3</SUB>. The device showed a lasing threshold of 10.5 mW of absorbed pump power and a slope efficiency of 8.5 percent.
Waveguide lasers formed by ion exchange in rare-earth-doped glasses have emerged as an attractive new technology on the threshold of commercial insertion. These devices can be used as both laser oscillators and optical amplifiers. In this article, we review ion exchange and glass composition. We then discuss the performance of ion-exchanged waveguide lasers made in silicate and phosphate glasses.
We examine measurement issues which arise in the testing of integrated optical devices subjected to ionizing radiation. Many of these issues are not addressed by measurement procedures developed for optical fibers. We outline the complexities involved in the measurement of integrated optics as they relate to size, function, and materials. Pertinent waveguide parameters include attenuation, changes in refractive index, photorefractive effects, and polarization effects. Optical measurement techniques are reviewed, with particular attention paid to spatial and temporal resolution, dynamic range, and the capacity for remote measurement. Suggestions are made to improve the reliability of testing and allow better comparison between laboratories.