nanowire lasers

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Polarization control in GaN nanowire lasers

Polarization control in GaN nanowire lasers

Semiconductor nanowires are quasi-one-dimensional structures with unique optical properties such as wave-guiding, high photon confinement, and high index contrast [1–4]. Nanowire- based monochromic-coherent light sources may enable a number of groundbreaking applications, such as compact high resolution biochemical imaging and spectroscopy [5]. For III-nitride (GaN-based) nanowires, advances in synthesis techniques [6–10] have given rise to high material quality and controlled nanowire geometries needed for lasing [11]. Moreover, increasing efforts have been dedicated recently to manipulating the fundamental lasing properties of these nanowire lasers to make them more suitable for practical applications. For example, Li et al [6] demonstrated single-mode lasing of individual GaN nanowires by precisely controlling the nanowire diameter and length. Xu et al. [12] and Gao et al. [13] both demonstrated single-mode lasing behavior from coupled GaN nanowires, which individually exhibited multimode behavior. Wright et al. [14] achieved single-mode lasing using distributed feedback by coupling GaN nanowire lasers to an external dielectric grating. Moreover, single-mode lasing was recently demonstrated from single GaN nanowire lasers when coupled to a lossy metal substrate due to the suppression of the emission from higher order transverse modes [15]. However, the polarization, another key feature essential for many practical applications, apart from a couple of recent studies [16, 17] has rarely been studied in nanowire lasers. In this letter we demonstrate the effective control of the polarization of the light emitted by GaN nanowire lasers by coupling these devices to an underlying gold substrate.

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Design and Characterisation of III-V Semiconductor Nanowire Lasers

Design and Characterisation of III-V Semiconductor Nanowire Lasers

β factor The β factor in nanowire lasers can be much larger compared to conventional, planar semiconductor lasers, because of their small dimensions and the high refractive index contrast. The β factor, as previously defined, depends on the number of resonant modes and the confinement of these modes in the nanowire. The β factor in nanowire lasers can be increased in two ways [32]. Firstly, the nanowire dimensions can be reduced, to limit the number of resonant modes. Secondly, the active region can be judiciously placed within the nanowire to maximise the coupling to a certain mode, thereby increasing the β factor for that mode. In fact, both these approaches can be used in conjunction to increase the β factor and thus obtain low threshold lasing. The β factors that have been reported to date for nanowire lasers is comparable to those realised in photonic crystal nanocavity and microcavity lasers. Nanowires have also been used to fabricate nanolasers with very high β factors [34, 35]. This has been achieved by coupling nanowires to metallic structures and achieving lasing from plasmonic modes, which are much more strongly confined and have much smaller mode volumes (below the diffraction limit) compared with photonic modes. While these lasers are promising for miniaturising lasers towards lengths scales that are compatible with current electronic devices, the threshold requirement for plasmonic lasers is very large. In this thesis, we will restrict our study to all-dielectric structures and thus only consider photonic mode lasing in semiconductor nanowires.

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Channel Plasmon Nanowire Lasers with V Groove Cavities

Channel Plasmon Nanowire Lasers with V Groove Cavities

With cylindrical geometry and strong two-dimensional confinement of electrons, holes, and photons, independent semiconductor nanowire is ideal for semiconductor laser with reduced threshold and compact size [1–6]. Up to date, room-temperature lasing emission has been realized in ZnO, GaN, CdS, and GaAs nanowires, covering optical spectrum from ultra-violet to near-infrared [7–12]. To continue shrinking dimensions of nanowires beyond the diffraction limit, plasmonic nanowire lasers has been pro- posed and experimentally demonstrated, including hybrid plasmonic nanowire lasers and high-order mode plasmon nanowire lasers [13–15]. Among them, hybrid plasmonic nanowire lasers achieved much smaller dimension limit. Recently, plasmonic nanowire laser showed its capability of integrating with plasmonic waveguides, using channel plasmon-polariton (CPP) modes in V-groove plasmonic waveguides [16]. The diameters adopted in the experiment are above 300 nm. CPPs are the plasmon polaritons guided by a V-shaped groove carved in metal, which was first theoretically suggested by Maradudin and co-workers [17]. CPPs showed strong confinement, low damping, and robustness against channel bending at near-infrared wavelengths [18–20].

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High-frequency dynamics of evanescently-coupled nanowire lasers

High-frequency dynamics of evanescently-coupled nanowire lasers

Finally, it is also important to note here that although this work focuses on laterally-coupled InP NW lasers with emission at 880 nm, the results of the present study, including the ultra-high values of oscillation frequen- cies predicted, are extendable to other types of NW lasers based on different material systems. The latter include existing GaN or ZnO NW lasers emitting at UV and visible wavelengths (see 2 and references therein). In addi-

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Integration of semiconductor nanowire lasers with polymeric waveguide devices on a mechanically flexible substrate

Integration of semiconductor nanowire lasers with polymeric waveguide devices on a mechanically flexible substrate

100’s of microns range. An alternative approach lies in the use of nanowire (NW) semiconductor laser devices. These sources have dimensions in the micron range and have been demonstrated in optically [8][9] and electrically [10] pumped variants, with average output powers in the nW regime [11]. There are two main methods for the integration of NWs with on-chip optical waveguides: direct growth of semiconductor NWs on the passive waveguide material [12, 13], or mechanical transfer of NWs from their growth substrate to the target devices [14, 15, 16]. Regrowth methods allow high accuracy alignment between physical structures on different material platforms through lithographic methods and have been reported with coupling efficiencies between NW array lasers and silicon waveguides of ~4%. Mechanical transfer techniques avoid subjecting the host devices to growth process conditions and allow the pre- screening of individual devices to be integrated. The crucial issue in the integration of these laser sources with PICs lies in the coupling of their optical output mode with the on-chip optics. A number of techniques for the mechanical positioning and organization of NWs have been reported in recent years, such as optical and optoelectronic tweezing [17], dry transfer techniques [18], mechanical pick-up using atomic force microscopy tips [19], fluidic [20] and electric field processes [21], Langmiur-Blodgett assembly protocols [22] and large scale transfer printing processes [23-26]. In all of these techniques there is a major tradeoff between the final NW placement accuracy and the potential scalability of the method. It has therefore been necessary to employ self-aligned coupling schemes to couple NW laser modes to guided wave structures, for example (with reported external coupling efficiencies given in parentheses), v-groove plasmonic guides (10%) [27], air-slot photonic crystals cavities [14] or lateral mechanical contacting of NW sections coupled to long nano-ribbon waveguides (50%) [28]. In addition to coupling efficiency of the NW output mode to external guiding structures, the form of and fabrication method of

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Transfer printing of semiconductor nanowire lasers

Transfer printing of semiconductor nanowire lasers

However, there are still key technological challenges for this to occur and for NW lasers to reach functional industrial products. One of these challenges, which directly arises from the ultra-small dimensions of NW lasers, make the manipulation and positioning of selected NWs at targeted locations on heterogeneous substrates with high precision and on industrial scales to produce functional systems a fundamental technological bottle-neck. This has limited greatly the transition of NW lasers from research laboratory environments into daily life products. Multiple techniques have been proposed for the manipulation of NWs. These include among others optical and optoelectronic tweezers [5], usage of microscope probes [6], large-scale printing techniques [7-11], and NWs’ assembly mechanisms relying either on liquid-assisted [12], electric field [13] or Langmuir- Blodgett processes [14]. Nevertheless, these techniques have diverse difficulties and drawbacks, which might require complex expensive procedures or would only allow operation in fluid media (with NWs in solution). Furthermore, they would not permit the transfer of NWs across different

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Vertically emitting indium phosphide nanowire lasers

Vertically emitting indium phosphide nanowire lasers

Semiconductor nanowires (NWs) have emerged as potential building blocks in advanced optical and nanophotonic devices and compact integrated circuits given their ultra-small dimensions and remarkable performance [1-6]. To date, intensive research has been conducted on the use of semiconducting NWs as gain media for smaller and faster lasers, critically important aspects for optical data transmission, sensing and imaging applications [7-10]. In order to demonstrate lasing from semiconductor NWs, these have usually been transferred from their growth substrate onto another secondary surface to ensure a large refractive index contrast between the NWs and their surrounding media [3,8]. Above lasing threshold, characteristic interference fringes can be observed from the NW lasers, similar to interference patterns generated by two coherent dipole emitters separated by the NW’s length [3], indicative of spatially coherent light emission from the NW’s end facets. However, in spite of the high promise of these nanoscale lasers for breakthroughs in nanophotonics, their light emission has a poor directionality with a diffracted beam resulting from the sub-wavelength size of NWs [11-14]. This leads to high coupling losses between NW lasers and optical fibers or waveguides. It currently limits their integration into three-dimensional on- chip devices, which indeed requires a small vertical divergence of the emission angle.

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III-V Compound Semiconductor Nanowire Terahertz Detectors

III-V Compound Semiconductor Nanowire Terahertz Detectors

core/shell, superlattice and branched structures with the desired composition, morphology and electrical properties through specific dopants incorporation and fine tuning of growth parameters. This largely extends the flexibility and functionality of nanowire-based electronic, optoelectronic and photovoltaic devices [32, 33]. In 2011, high-speed nanoscale electronic devices (both nanowire field-effect transistors and light-emitting diodes) were demonstrated for the first time using InP nanowires [34]. Subsequently, high-performance InAs/InP core-shell single nanowire field-effect transistors were reported with electron mobility reaching to 11500 cm 2 /Vs at room temperature [35]. In 2013, GaN-based nanowire light-emitting diodes with industry-standard quality had been achieved and were ready to reach market [36]. In the same year, a 13.8% efficient axial p-i-n InP nanowire solar cell was demonstrated [37], while GaAs-based nanowires were shown to lase at room-temperature [38]. Most recently, InP nanowire lasers [39], GaAs/AlGaAs multiple quantum well nanowire lasers [40] and a 15.3% efficiency p-n junction GaAs nanowire array solar cell [41] were reported. The significant development of III-V nanowire synthesis, property manipulation and device fabrication over the last ten years has proven the potential of III-V semiconductor nanowires as nano-building blocks for advanced integrated electronics and optoelectronics. Therefore, it is possible to design and grow III-V nanowires, and fabricate them into nano-THz-electronics to realise advanced THz systems. For example, a single nanowire THz detector can be used as a sub-wavelength detector element for near-field imaging [42], or integrated into an “on-chip” THz spectrometer [43, 44] avoiding the need for complex optical arrangements.

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Electronic and structural study of hexagonal TiC nanowires: ab initio study

Electronic and structural study of hexagonal TiC nanowires: ab initio study

The observed semiconducting nanowire of Titanium carbide have very narrow bandgap about (0.2- 0.35)ev which are also direct bandgap in nature. The narrow band gap structure with direct nature semiconducting material are supposed to be very important for the choice of many applications in the infrared region. Because in wide band gap semiconductors (band gap greater than Si) are not desirable for bipolar devices due to higher difference in mobility of electrons and hole for power application. The narrow band gap semiconducting materials are building block for novel tunnel devices and infrared super lattices [19]. Simple K-P theory suggest that the small gap materials basically means small effective mass which are obvious candidates to observe quantum confinement effects at larger dimensions[20]. The small masses with dual high conductivities and unique band offsets are the interesting part of narrow band gap materials for quantum application.

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Designing Sensors Using Nano Junctions

Designing Sensors Using Nano Junctions

Atomistic simulation methods such as first-principles quantum-mechanical methods [7] [8], molecular dynamics (MD) and Monte Carlo (MC) simulations [9] [10] are gen- erally accurate for the analysis of nanostructures. However, the extremely high compu- tational cost prohibits the application of the atomistic methods at the device level. On the other hand, classical continuum theories which are based on continuum assump- tions are efficient and accurate at macroscopic scale, but they may not be directly ap- plicable for devices with nanometer features. To achieve the goal of accurately captur- ing the atomistic physics and yet retaining the efficiency at various length scales, mul- ti-scale modeling and simulation techniques have recently gained significant interest. So we used NEGF theory for Si nanowire with different geometries in the different chemical environment to analyze the performance, the sensitivity of Si nanowire with different cross sections.

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Modified Neural Network Architecture based Expert System for Automated Disease Classification and Detection using PCA Algorithm

Modified Neural Network Architecture based Expert System for Automated Disease Classification and Detection using PCA Algorithm

and diagnose patients with diseases are explored. Challenges in disease diagnosis is identified and reported in this work. It is found that with the growth in population and with changes in environmental conditions human race is prone to various diseases. With shortage of medical practioners and doctors there is an immediate need for alternate solutions to cater to the needs of the common man. Thus with the technological advances in the areas of medical electronics, nanotechnology and VLSI technology, there is a need for a sophisticated, reliable and accurate system that automatically monitors the human behaviour and detects diseases in human beings. In this research work, an effort is taken up to develop an automated disease detection and drug delivery unit that can be used to detect diseases and monitor the diseases. Based on the mathematical models developed for the nanowire and nanowire based sensor array for cancer detection, an expert system based on neural network is proposed to detect and classify cancer. In order to validate the performances of the expert system, ovarian cancer is selected. Known set of data for ovarian cancer analysis is used for validation. The expert system is trained using known set of data samples and optimum weights for the classification problem are identified. Unknown data sets are used to identify the network performances. The network model developed using feed forward architecture classifies the test data correctly by 95%, and has an error of 4%. In order to further enhance the performances of the system, a modified technique based on Linear Discriminant Analysis (LDA) and Principal Component analysis (PCA) is modelled. In this method, from the data sets, features are extracted, and significant features are identified by performing PCA. Based on the PCA components identified, the network is trained. Optimum weights are identified during the training phase using the significant factors. The performances of the network are identified to increase from 95% to 98% times correctly classifying the input data sets. A PID control logic interfaced at the output of the expert system controls the drug diffusion unit based on the decision made at the expert system. The control logic considers the weight, height and age factor of the patient, identifies appropriate quantity of drug required to be diffused and monitor the process. References

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Ultrafast high repetition rate waveguide lasers

Ultrafast high repetition rate waveguide lasers

The widespread prominence of garnets and especially YAG as gain host crystals for bulk lasers has led to a wide interest in creating waveguides in such a material. Successful fabrication techniques have included ion implantation, LPE, PLD, laser inscription, and contact bonding [47-51]. In particular, contact bonding is especially interesting as it allows dissimilar materials to be bonded together to produce low-loss (~0.1 dB/cm) multi-layered structures with high numerical apertures, forming double-clad and large-mode-area structures and allowing high-power diode-pumped operation in a similar fashion to fiber lasers [52]. While emission spectra tend to be narrower than in glasses, double-tungstates, or sesquioxides, femtosecond pulses can be generated at high powers in bulk Yb:YAG lasers [53].

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Materialization of single multicomposite nanowire: entrapment of ZnO nanoparticles in polyaniline nanowire

Materialization of single multicomposite nanowire: entrapment of ZnO nanoparticles in polyaniline nanowire

the SMNWs were detached from the two electrodes by laterally scratching the surface and were then trans- ferred to a TEM grid using the nanomanipulator. HRTEM was carried out at an acceleration voltage of 200 kV and a camera constant of 25 cm. HRTEM was utilized to confirm entrapment and examine alignment and distribution of the ZnO NPs inside the SMNWs. Firmly, Raman Spectroscopy (inVia, Renishaw, Wotton- under-Edge, UK) confirmed that the fabricated nano- wires are materialized through electropolymerization as doped PANI in Figure 2b. Physical properties of the SMNWs were measured with I-V curves and deflection of the nanowire using a semiconductor device analyzer and FD measurements obtained from an AFM, respec- tively. Electrical conductivity was calculated from the measured I-V curve along with dimensions of the nano- wire. The applied force of 5 nN used in the AFM FD measurements was performed at the center of the nano- wire, with both ends supported.

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"Resistance of Single Ag Nanowire Junctions and Their Role in the Conductivity of Nanowire Networks"

"Resistance of Single Ag Nanowire Junctions and Their Role in the Conductivity of Nanowire Networks"

Prior to any junction resistance measurement it is essential to ensure that the four electrical contacts (two on either side) make good electrical contact with their respective wires (see Figure 1a, inset). Typically we find that there is a low resistance contact to each nanowire, which is likely the result of the large heat of condensation released during metal contact formation. If this is not the case these contacts need to be activated separately by driving current between electrodes 1-2 and 3-4. The next step involves activation of the junction itself. This must be performed using two contacts instead of four, as the resistance of the inner circuit in a 4-probe set-up is lower than that of the nanowire junction prior to activation. The insulating PVP layer prevents current flow up to a threshold, V FORM , beyond which the current rises sharply to a pre-defined current

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Holographic lasers

Holographic lasers

One of the main uses of phase conjugation is in the power scaling of lasers. It is comparatively easy to build a low power laser which operates on a single spatial and longitudinal mode. Developing higher power systems is a much more complicated endeavour. Heating of the laser medium due to the quantum defect and parasitic spectroscopic processes cause changes in refractive index, thermal expansion and stress induced birefringence. These effects are especially important now with the development of diode pumped solid state lasers. In the last 4 years diode power available from a single device has increased more than 10 fold from 40W diode bars up to the latest kilowatt stacks. With these powers comes the need to extract more heat than ever before.

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Lasers in Prosthetics

Lasers in Prosthetics

B. Treatment of undercut alveolar ridges: There are many causes of undercut alveolar ridges. Naturally occurring undercuts such as those found in the lower anterior alveolus or where a prominent pre-maxilla is present may be the cause of soft tissue trauma, ulceration, and pain when prosthesis is placed on such a ridge. Soft tissue surgery may be performed with any of the soft tissue lasers. Osseous surgery may be performed with the erbium family of lasers.

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Microchip lasers

Microchip lasers

The light used to probe the crystal was typically from a HeNe laser, which has a coherence length of approximately 30cm, much longer than the 0.2-1mm coherence length required to produce an interferogram. This long coherence length was problematic in that reflections from other surfaces, in particular the pumping lens close to the front surface of the microchip caused unwanted disturbance to the interferogram. To minimise this noise, various other light sources were investigated as alternatives to a HeNe laser, including filtered white light sources, diode lasers, and arc lamps. Overall, however, the uniform and high brightness illumination provided by the HeNe laser, or a green microchip laser were found to be the most suitable light sources for these experiments. To minimise the reflections from the pumping lens a suitable glass filter was employed which would transmit the pumping radiation at 810nm and absorb any incident 632.8nm HeNe light.

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Biofunctionalization of zinc oxide nanowires for DNA sensory applications

Biofunctionalization of zinc oxide nanowires for DNA sensory applications

Semiconductor nanowires have gained large interest as building blocks for low-cost, highly sensitive biosensors [1,2]. Due to the quasi-one-dimensional geometry of the nanowires and the large surface-to-volume ratio, sur- face-induced effects play a significant role on the electri- cal properties of nanowire-based devices. Even a single molecule attached to the nanowire surface is able to change the electrical properties of the nanowire consid- erably. Therefore, a functionalization of the nanowire surface gives way to highly sensitive sensors that can be arranged in a very dense assembly, owing to their nano- sized dimension. However, both the surface modification and the nanowire device arrangement have to be taken care of in order to prepare a complete nanowire-based DNA sensor.

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Frequency Noise Control of Heterogeneous Si/III V Lasers

Frequency Noise Control of Heterogeneous Si/III V Lasers

the imaginary part of the refractive index in the QWs. Therefore, for a given material, one cannot eliminate the presence of the linewidth enhancement factor. As discussed in Section 2.2.2, quantum-dots (QDs), for example, do not exhibit a linewidth enhancement factor, due to their delta function-like density of states. However, QDs still remain difficult to grow for certain materials and often exhibit lower material gain than their QW counterparts. The demonstration of the suppression of the linewidth enhancement factors at a relatively low frequency in our lasers means that using this approach, any laser with different material system can reduce to insignificance the effect of phase-amplitude coupling (i.e., the linewidth enhancement factor) at telecommunication relevant frequencies (i.e. a few GHz).

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Multicore rare earth doped fibres; application to amplifiers, filters and lasers

Multicore rare earth doped fibres; application to amplifiers, filters and lasers

Wavelength division multiplexing (WDM) is the preferred choice for expanding the capacity of optical communication systems. Using this technique bit rates exceeding 1 Terabits/s have been demonstrated. However to fully exploit the potential of WDM several key device developments are required. This report describes the results of an experimental program Multicore rare-earth doped fibres; application to amplifiers, filters and lasers funded under the ROPA scheme and targeted at new devices for WDM applications. The proposed program covered a three year program however funding was only awarded for two years. The key thrusts of the original submission were to introduce spatial hole burning into rare earth doped devices to controllably induce effective inhomogeneous broadening into the gain/loss medium. It was proposed to develop and optimise twincore erbium doped fibres to provide spatial hole burning and thus effective inhomogeneous broadening in the gain/loss medium. The development of channel equalising optical amplifiers, multi-wavelength and single frequency fibre lasers as well as passive tracking optical filters was targeted. These objectives were met. In addition multi-wavelength DFB fibre lasers have been developed as telecommunication sources and also applied to in-line pump a polarisation insensitive phase conjugator and as an active fibre temperature and strain sensor. Finally, a multi-wavelength (12 channel) Brillouin fibre laser has been demonstrated.

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