Coupling of light from microdisk lasers into plasmonic nano-antennas

An optical dipole nano-antenna can be constructed by placing a sub-wavelength dielectric (e.g., air) gap between two metallic regions. For typical applications using light in the infrared region, the gap width is generally in the range between 50 and 100 nm. Owing to the close proximity of the electrodes, these antennas can generate very intense electric fields that can be used to excite nonlinear effects. For example, it is possible to trigger surface Raman scattering on molecules placed in the vicinity of the nano-antenna, allowing the fabrication of biological sensors and imaging systems in the nanometric scale. However, since nano-antennas are passive devices, they need to receive light from external sources that are generally much larger than the antennas. In this article, we numerically study the coupling of light from microdisk lasers into plasmonic nanoantennas. We show that, by using micro-cavities, we can further enhance the electric fields inside the nano-antennas

Maximization of Gain in Slow-Light Silicon Raman Amplifiers

We theoretically study the problem of Raman gain maximization in uniform silicon photonic-crystal waveguides supporting slow optical modes. For the first time, an exact solution to this problem is obtained within the framework of the undepleted-pump approximation. Specifically, we derive analytical expressions for the maximum signal gain, optimal input pump power, and optimal length of a silicon Raman amplifier and demonstrate that the ultimate gain is achieved when the pump beam propagates at its maximum speed. If the signal’s group velocity can be reduced by a factor of 10 compared to its value in a bulk silicon, it may result in ultrahigh gains exceeding 100 dB. We also optimize the device parameters of a silicon Raman amplifier in the regime of strong pump depletion and come up with general design guidelines that can be used in practice

Optimized gold nanoshell ensembles for biomedical applications

We theoretically study the properties of the optimal size distribution in the ensemble of hollow gold nanoshells (HGNs) that exhibits the best performance at in vivo biomedical applications. For the first time, to the best of our knowledge, we analyze the dependence of the optimal geometric means of the nanoshells’ thicknesses and core radii on the excitation wavelength and the type of human tissue, while assuming lognormal fit to the size distribution in a real HGN ensemble. Regardless of the tissue type, short-wavelength, near-infrared lasers are found to be the most effective in both absorption- and scattering-based applications. We derive approximate analytical expressions enabling one to readily estimate the parameters of optimal distribution for which an HGN ensemble exhibits the maximum efficiency of absorption or scattering inside a human tissue irradiated by a near-infrared laser

Level Anticrossing of Impurity States in Semiconductor Nanocrystals

The size dependence of the quantized energies of elementary excitations is an essential feature of quantum nanostructures, underlying most of their applications in science and technology. Here we report on a fundamental property of impurity states in semiconductor nanocrystals that appears to have been overlooked—the anticrossing of energy levels exhibiting different size dependencies. We show that this property is inherent to the energy spectra of charge carriers whose spatial motion is simultaneously affected by the Coulomb potential of the impurity ion and the confining potential of the nanocrystal. The coupling of impurity states, which leads to the anticrossing, can be induced by interactions with elementary excitations residing inside the nanocrystal or an external electromagnetic field. We formulate physical conditions that allow a straightforward interpretation of level anticrossings in the nanocrystal energy spectrum and an accurate estimation of the states\u27 coupling strength

Visualization of electromagnetic-wave polarization evolution using the Poincaré sphere

For the first time to the best of our knowledge, we derive expressions for coordinates of the trajectory on the Poincaré sphere that represent polarization evolution in an arbitrary beam of completely polarized light. Our work substantially extends the mapping function of the Poincaré sphere, and opens up new possibilities for its use in optics. In particular, the obtained expressions allow one to visualize the results of the finite-difference timedomain modeling of light propagation through birefringent crystals, including simulations of polarization rotation experienced by ultrashort pulses in nonlinear media

ID 165644)

We revisit the problem of the optimization of a silicon-nanocrystal (Si-NC) waveguide, aiming to attain the maximum field confinement inside its nonlinear core and to ensure optimal waveguide performance for a given mode power. Using a Si-NC=SiO 2 slot waveguide as an example, we show that the common definition of the effective mode area may lead to significant errors in estimation of optical intensity governing the nonlinear optical response and, as a result, to poor strength evaluation of the associated nonlinear effects. A simple and physically meaningful definition of the effective mode area is given to relate the total mode power to the average field intensity inside the nonlinear region and is employed to study the optimal parameters of Si-NC slot waveguides. © 2012 Optical Society of America OCIS codes: 190.4400, 230.1150, 230.7370. Silica (SiO 2 ) embedded with silicon nanocrystals (Si-NCs) is considered a promising nonlinear material, as it exhibits a strong ultrafast Kerr effect and can also be used with the current complementary metal-oxidesemiconductor technologies where S z E × H ·ẑ is the time-averaged z component of the Poynting vector,ẑ is the unit vector along the waveguide axis, and the integration is over the entire x − y plane. In the weak-guidance approximation, implying that the refractive index varies slowly in the transverse direction, this definition leads to the well-known expression [4,6,8] jFx; yj 4 dxdy; Of primary importance for a nonlinear waveguide is the average field intensity I NL inside its nonlinear constituent, as it determines the efficiency of all nonlinear effects developing inside the waveguide. Without loss of generality, we focus on a quasi-TM mode with the dominant component of the electric field being in the x direction (so that F ≈ E x ). It turns out after some reflection that neither Eq. (1) nor Eq. (2) can be used to relate I NL to the total power P of this mode, as none of them explicitly contains the lateral dimensions of the waveguide. The equality I NL P=A eff would hold only if S z (or E x ) was uniform inside the nonlinear region and zero outside of it, in which case A eff is simply equal to the region's cross-section area a NL . It is not hard to construct a proper factor relating P and I NL , and we introduce a new EMA in the form where NL denotes integration over the nonlinear region. Since the surface integrals in this expression give the total mode power (numerator) and the power P NL transmitted through the nonlinear part of the waveguide (denominator), we can write I NL P NL =a NL P=a eff . Hence, while the effective area given in Eq. (1) or Eq. (2) determines the relative efficiency of the nonlinear effects within the framework of the NLSE, the quantity in Eq. (3) allows one to estimate the actual intensity of light inside the nonlinear waveguide. These equations are inapplicable to plasmonic waveguides, where S z has different signs inside metal and dielectric, and thus the total power flow may vanish. To better understand the difference between the preceding definitions, consider an optical fiber with a highly nonlinear core (e.g., a silicon-core fiber) and assume that June 15, 2012 / Vol. 37, No. 12 / OPTICS LETTERS 2295 0146-9592/12/122295-03$15.00/

Optimization of gain-assisted waveguiding in metal-dielectric nanowires

We theoretically demonstrate that, for a given diameter of the core-pumped metal-dielectric nanowire, there is an optimum thickness of the metallic cladding that provides the maximum propagation length of the lowest-order surface plasmon polariton (SPP) modes. If the nanowire is fabricated with the optimum cladding thickness, the lowest pumping power is required to fully compensate for the SPP propagation losses. We also show that a strong confinement of SPPs within the nanowire can be achieved, but at the expense of either high optical gains or large nanowire diameters. For example, a gain of 565 cm(-1) would suffice to make up for the decay of SPPs in a 250-nm-thick silver-GaAs nanowire; the confinement of optical power within such nanowires exceeds 90%, which makes them ideal interconnects for nanophotonic circuitry.The work of I. D. Rukhlenko and M. Premaratne was sponsored by the Australian Research Council through its Discovery Grant scheme under grants DP0877232 and DP110100713. C. Jagadish also gratefully acknowledges the Australian Research Council for financial support