Dr Adel Rahmani

Senior Lecturer, School of Mathematical Sciences
Associate Member, Research Strength Materials and Technology for Energy Efficiency Member
Core Member, Institute for Nanoscale Technology
Physics, Physics, PhD (Bourgogne)
 
Phone
+61 2 9514 4352
Fax
+61 2 9514 2260
Room
CB01.15.49

Book Chapters

Chaumet, P.C., Nieto-Vesperinas, M. & Rahmani, A. 2008, 'Near-field photonic forces' in Anatoly Zayats, David Richards (eds), Nano-Optics and Near-Field Optical Microscopy, Artech House Publishers, USA, pp. 21-46.

Conference Papers

Marks, B., Steel, M.J. & Rahmani, A. 2008, 'Modelling time reversal experiments in the optical domain', Australian Conference on Optical Fibre Technology, Sydney, Australia, July 2008 in 2008 Joint Conference Of The Opto-Electronics And Communications Conference And The Australian Conference On Optical Fibre Technology, ed IEEE, IEEE, New York, USA, pp. 1-2.
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We present a rigorous numerical study of the sub-wavelength focusing of electromagnetic waves using a time-reversal mirror, and show that the diffraction limit can be overcome in the optical domain.
Le Gac, G., Rahmani, A., Seassal, C., Picard, E., Hadji, E. & Callard, S. 2008, 'Active Near-Field Optical Microscopy', Conference On Lasers And Electro-Optics & Quantum Electronics And Laser Science Conference, San Jose, USA, May 2008 in 2008 Conference On Lasers And Electro-Optics & Quantum Electronics And Laser Science Conference, ed IEEE, IEEE, New York, USA, pp. 1-2.
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Near-field probe was used to tune the resonance wavelength of a linear cavity. Theoritical and experimental study are presented to show the effect of the probe material on the cavity resonance
Stewart, L., Marshall, G.D., Dawes, J., Withford, M. & Rahmani, A. 2008, 'Self-assembly around curved surfaces', Canberra, Australia, December 2007 in Photonics: Design, Technology, And Packaging Iii Book Series: Proceedings Of The Society Of Photo-Optical Instrumentation Engineers (SPIE), ed Krolikowski WZ; Soukoulis CM; Lam PK; Davis TJ; Fan S; Kivshar YS, SPIE-INT SOC OPTICAL ENGINEERING, Bellingham, pp. 1-9.
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We present a parametric study of self-assembled photonic crystal growth as a function of radius of curvature. To do this, we used a combination of microscope slides, glass capillaries and optical fibres as substrates to grow the self-assembled films on. Microscope and SEM images, as well as broadband transmission spectra were used to characterise the crystal, and the effect the changing surface curvature had on the crystal quality. Limitations for fabricating the crystals on highly curved surfaces willbe presented.
Bordas, F., Seassal, C., Dupuy, E., Regreny, P., Gendry, M., Steel, M.J. & Rahmani, A. 2007, 'Room-Temperature InAs/InP Quantum-Dot Photonic Crystal Microlasers Using Cavity-Confined Slow Light', CLEO 2007, Baltimore, USA, May 2007 in 2007 CONFERENCE ON LASERS & ELECTRO-OPTICS/QUANTUM ELECTRONICS AND LASER SCIENCE CONFERENCE (CLEO/QELS 2007), ed Optical Society of America, IEEE, New York, USA, pp. 1-2.
Photonic crystals (PCs) have attracted much attention for their ability to control light on a micrometer scale. Two major strategies are used, each of them exploiting a particular property of PCs. The first approach is based on the introduction of a defect in a 2D PC slab, which creates a localized level in the band gap. The careful design of the cavity border can yield very high quality factors and small modal volumes, which can lead to lasing provided a gain medium is present in the device [1]. The second approach builds upon the enhanced local density of electromagnetic states associated with the critical points of the band dispersion diagram. As the group velocity is zero at these points, the electromagnetic interaction between light and matter is greatly enhanced and allows efficient laser operation in the presence of a gain medium [2]. We propose to develop new active PC structures that combine the intrinsic high density of states associated with a slow light mode and the compactness of the microcavity, by using twodimensional cavity-confined slow light modes.
Bordas, F., Seassal, C., Steel, M.J. & Rahmani, A. 2007, 'Slow-Light Trapping in a Photonic Crystal Slab', CLEO2007, Baltimore, USA, May 2007 in 2007 CONFERENCE ON LASERS & ELECTRO-OPTICS/QUANTUM ELECTRONICS AND LASER SCIENCE CONFERENCE (CLEO/QELS 2007), ed Optical Society of America, IEEE, New York, USA, pp. 1-2.
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We present a general scheme to trap slow-light in a photonic crystal slab and achieve high Q/V ratio with a fabrication tolerant design, well suited for low-threshold microlasers and cavity quantum electrodynamics.
Marshall, G.D., Ams, M., little, D., Jovanovic, N., Fuerbach, A., Dekker, P., Rahmani, A., Dawes, J., Piper, J. & Withford, M. 2007, 'Direct writing of planar lightwave devices using ultrafast lasers', Australian conference on optical fibre technology, Melbourne, Australia, June 2007 in Joint international conference on the optical internet (COIN) and Australian conference on optical fibre technology (ACOFT) : proceedings, ed IEEE, IEEE, New York, USA, pp. 1-2.
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Ultrafast laser direct write micro-fabrication has emerged as a significant enabling technology creating new opportunities in microphotonics. The talk will present recent results of the performance of laser written microphotonic componentry in bulk glasses.
little, D., Dekker, P., Rahmani, A., Dawes, J., Marshall, G.D. & Withford, M. 2007, 'Characterisation of fs-laser written refractive index changes using near-field scanning optical microscopy', Canberra, Australia, December 2007 in PHOTONICS: DESIGN, TECHNOLOGY, AND PACKAGING III Book Series: Proceedings of the Socitey of Phot-optical Instrumentation Engineers (SPIE), ed Krolikowski WZ; Soukoulis CM; Lam PK; Davis TJ; Fan S; Kivshar YS, SPIE-INT SOC OPTICAL ENGINEERING, Bellingham, USA.
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We have performed measurements using a purpose-built Near-field Scanning Optical Microscope and shown that waveguides written with a fs laser in the kHz regime have an asymmetry associated with the unidirectional nature of the writing beam. Further, the asymmetry becomes more pronounced with increasing pulse energy. At very high pulse energies (5-10 J) the presence of multiple guided regions was also observed, indicating that the refractive index profile of the waveguide possesses several maxima, a result which is consistent with current studies on the filamentation process that high-powered laser pulses experience in a dielectric medium. In this paper we will present these observations, their subsequent analysis and implications for photonic device fabrication using this method

Journal Articles

Rahmani, A., Steel, M.J. & Chaumet, P.C. 2013, 'Invisibility and supervisibility: Radiation dynamics in a discrete electromagnetic cloak', Physical Review B, vol. 87, no. 4, pp. 045430-1-045430-1.
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We study the radiation dynamics of an electric dipole source placed near or inside a discrete invisibility cloak. We show that the main features of radiation dynamics can be understood in terms of the interaction of the source with a nonideal cloak in which local-field effects associated with the discrete geometry play a crucial role. As a result, radiation dynamics in a discrete cloak can differ drastically from what a source would experience in an ideal, continuous cloak. This can lead, for instance, to an enhancement of the emission by the cloak, thus making the source more visible to an outside observer than it would be without the cloak. The two main physical mechanisms for enhanced, or inhibited, radiation dynamics are the coupling of the source to leaky modes inside the cloak, and the coupling of the source with the lattice of the discrete cloak, via the local field. We also explore the robustness of the effect to material dispersion and losses.
Chaumet, P.C. & Rahmani, A. 2013, 'Optical binding of magnetodielectric Rayleigh particles', Physical Review B, vol. 87, pp. 1-8.
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We present a theoretical and numerical study of the optical binding and optical torque between two Rayleigh particles with arbitrary, complex, scalar dielectric permittivity and magnetic permeability. We use a computational approach based on the discrete dipole approximation to derive the optical force and torque experienced by the particles when illuminated by a linearly or circularly polarized plane wave. We show that optical binding between magnetodielectic particles is qualitatively different from the traditional case involving dielectric particles only. In particular, we show that for certain configurations, the system of two magnetodielectric particles will experience a long-range optical torque whose amplitude envelope does not decay with the separation between the particles.
Chaumet, P.C., Zhang, T., Rahmani, A., Gralak, B. & Belkebir, K. 2013, 'Discrete dipole approximation in time domain through the Laplace transform', Physical Review E, vol. 88, pp. 063303-1-063303-8.
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We present a form of the discrete dipole approximation for electromagnetic scattering computations in time domain. We show that the introduction of complex frequencies, through the Laplace transform, significantly improves the computation time. We also show that the Laplace transform and its inverse can be combined to extract the field inside a scatterer at a real resonance frequency
Chaumet, P.C., Rahmani, A., Zolla, F. & Nicolet, A. 2012, 'Electromagnetic Forces On A Discrete Spherical Invisibility Cloak Under Time-Harmonic Illumination', Physical Review E, vol. 85, no. 5, pp. 1-7.
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We study electromagnetic forces and torques on a discrete spherical invisibility cloak under time-harmonic illumination. We consider the influence of material absorption and losses, and we show that while the impact of absorption on the optical force rem
Vo, T., Mivelle, M., Callard, S., Rahmani, A., Baida, F., Charraut, D., Belarouci, A., Nedeljkovic, D., Seassal, C., Burr, G. & Grosjean, T. 2012, 'Near-Field Probing Of Slow Bloch Modes On Photonic Crystals With A Nanoantenna', Optics Express, vol. 20, no. 4, pp. 4124-4135.
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We study the near-field probing of the slow Bloch laser mode of a photonic crystal by a bowtie nano-aperture (BNA) positioned at the end of a metal-coated fiber probe. We show that the BNA acts as a polarizing nanoprobe allowing us to extract information
Grosjean, T., El Eter, A., Mivelle, M., Vo, T., Belkhir, A., Ecoffey, C., Le Gac, G., Nedeljkovic, D., Rahmani, A., Seassal, C., Callard, S. & Baida, F. 2012, 'Extraordinary blueshift of a photonic crystal nanocavity by reducing its mode volume with an opaque microtip', Applied Physics Letters, vol. 101, pp. 1-5.
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We demonstrate a method to reduce the mode volume of optical micro/nanocavities by positioning an opaque microtip in close proximity of the structures. This concept is used to blueshift the resonance of an active photonic crystal nanocavity by up to 16?nm. This tuning range is shown to be about 10 times larger than the redshift achieved with a bare dielectric microtip of the same size and shape. By imagining materials or multilayered devices with the ability to become transparent and opaque under external control, the blue and redshifts of the resonance would become possible with a single perturbing device.
Nedel, P., Letartre, X., Seassal, C., Auffeves, A., Ferrier, L., Drouard, E., Rahmani, A. & Viktorovitch, P. 2011, 'Design And Investigation Of Surface Addressable Photonic Crystal Cavity Confined Band Edge Modes For Quantum Photonic Devices', Optics Express, vol. 19, no. 6, pp. 5014-5025.
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We propose to use a localized +-point slow Bloch mode in a 2D-Photonic Crystal (PC) membrane to realize an efficient surface emitting source. This device can be used as a quantum photonic device, e.g. a single photon source. The physical mechanisms to increase the Q/V factor and to improve the directivity of the PC microcavity rely on a fine tuning of the geometry in the three directions of space. The PC lateral mirrors are first engineered in order to optimize photons confinement. Then, the effect of a Bragg mirror below the 2DPC membrane is investigated in terms of out-of-plane leakages and far field emission pattern. This photonic heterostructure allows for a strong lateral confinement of photons, with a modal volume of a few (/n)3 and a Purcell factor up to 80, as calculated by two different numerical methods. We finally discuss the efficiency of the single photon source for different collection set-up.
Chaumet, P.C., Belkebir, K. & Rahmani, A. 2011, 'Discrete Dipole Approximation For Time-Domain Computation Of Optical Forces On Magnetodielectric Scatterers', Optics Express, vol. 19, no. 3, pp. 2466-2475.
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We present a general approach, based on the discrete dipole approximation (DDA), for the computation of the exchange of momentum between light and a magnetodielectric, three-dimensional object with arbitrary geometry and linear permittivity and permeability tensors in time domain. The method can handle objects with an arbitrary shape, including objects with dispersive dielectric and/or magnetic material responses.
Chaumet, P.C., Rahmani, A., Zolla, F., Nicolet, A. & Belkebir, K. 2011, 'Optical force on a discrete invisibility cloak in time-dependent fields', Physical Review A, vol. 84, pp. 1-4.
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We study, in time domain, the exchange of momentum between an electromagnetic pulse and a threedimensional, discrete, spherical invisibility cloak.We find that a discrete cloak, initially at rest, would experience an electromagnetic force due to the pulse but would acquire zero net momentum and net displacement. On the other hand, we find that while the cloak may manage to conceal an object and shroud it from the electromagnetic forces associated with the pulse, the cloak itself can experience optomechanical stress on a scale much larger than the object would in the absence of the cloak. We also consider the effects of material dispersion and losses on the electromagnetic forces experienced by the cloak and show that they lead to a transfer of momentum from the pulse to the cloa
Steel, M.J., Marks, B. & Rahmani, A. 2010, 'Properties of sub-diffraction limited focusing by optical phase conjugation', Optics Express, vol. 18, no. 2, pp. 1487-1500.
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Recent work has demonstrated sub-diffraction limited focusing using time-reversal mirrors and sources in scattering media at--microwave frequencies. We numerically investigate the possibility of observing analogous effects in the optical domain using small cylindrical scatterers of realistic dielectric materials combined with an enclosing optical phase conjugate mirror in two-dimensional geometries. Such focusing is possible but appears not to significantly exceed the focusing available from an equivalent homogenized material, and is highly sensitive to precise scatterer configuration.
Chaumet, P.C., Belkebir, K. & Rahmani, A. 2010, 'Optical forces in time domain on arbitrary objects', Physical Review A, vol. 81, no. 2, pp. 1-4.
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We develop a general theoretical and computational framework to describe, in time domain, the exchange of momentum between light and arbitrary three-dimensional objects. Our fonnulation can be used to study the time evolution of optical forces on any object with linear material response, including inhomogeneous, dispersive, and absorbing dielectrics and metals. We illustrate our approach by studying the behavior of the Abraham force on an object illuminated by a sequence of electromagnetic pulses.
Jay, K., Chaumet, P.C., Langtry, T.N. & Rahmani, A. 2010, 'Optical binding of electrically small magnetodielectric particles', Journal of Nanophotonics, vol. 4, pp. 1-9.
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An ensemble of spherical particles with arbitrary dielectric permittivity and magnetic penneability was considered in the dipole approximation. Each particle was described by complex electric and magnetic polarizabilities. A computational approach based on the coupled dipole method, also called the discrete dipole approximation, was used to derive the optical force experienced by each particle due to an incident electromagnetiG..Ji.eld and the fields scattered by all other particles. This approach is general and can handle material dispersion and losses. In order to illustrate this approach, we studied the case of two spherical particles separated by a distance d, and illuminated by an incident plane wave whose wave vector is normal to the axis of the particles. We computed the optical force experienced by each particle in the direction of the beam (radiation pressure), and perpendicular to the beam (optical binding) for particles with positive and negative refractive indices. We also considered the effect of material losses.
Rahmani, A., Chaumet, P.C. & Bryant, G.W. 2010, 'Discrete Dipole Approximation for the Study of Radiation Dynamics in a Magnetodielectric Environment', Optics Express, vol. 18, no. 8, pp. 8499-8504.
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We develop a general computational approach, based on the discrete dipole approximation, for the study of radiation dynamics near or inside an object with arbitrary linear dielectric permittivity, and magnetic permeability tensors. Our method can account for dispersion and losses and provides insight on the role of local-field corrections in discrete magnetodielectric structures. We illustrate our method in the case of a source inside a magneto-dielectric, isotropic sphere for which the spontaneous emission rate of a source can be computed analytically. We show that our approach is in excellent agreement with the exact result, providing an approach capable of handling both the electric and magnetic response of advanced metamaterials.
Handmer, C.J., de Sterke, C.M., McPhedran, R.C., Botten, L.C., Steel, M.J. & Rahmani, A. 2010, 'Blazing evanescent grating orders: a spectral approach to beating the Rayleigh limit', Optics Letters, vol. 35, no. 17, pp. 2846-2848.
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We develop a way to enhance the amplitudes of the nonpropagating evanescent orders of resonant dielectric gratings. We use this blazing to design gratings with spectra tailored to generate steerable sub-Rayleigh field concentrations on a surface. We investigate the enhancement and customization of evanescent fields necessary to create a virtual and passive scanning probe with no moving parts. Spot size can be decreased 1 order of magnitude below the free-space Rayleigh limit.
Handmer, C.J., de Sterke, C.M., McPhedran, R.C., Botten, L.C., Steel, M.J. & Rahmani, A. 2010, 'Stacked dielectric gratings for sub-wavelength surface field synthesis', Journal of the Optical Society of America B: Optical Physics, vol. 27, no. 12, pp. 2580-2594.
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A method is developed to enhance the amplitudes of the non-propagating evanescent orders of resonant dielectric gratings. The origin of these resonances is analyzed in detail. The method relies on interactions between stacked gratings with different periods, and so a formalism is developed to model such stacks mathematically. In addition, a theoretical approach is developed to design gratings that enhance or blaze desired orders. These orders, controlled independently by incident fields from different angles, interfere and are optimized to produce steerable sub-Rayleigh field concentrations on a surface. These spots may function as a virtual scanning probe for non-invasive sub-Rayleigh microscopy. Optimization is conducted using a Monte Carlo Markov chain, and spots are generated which are both 1 order of magnitude narrower than the free space Rayleigh limit and robust to noise in the incident fields.
Vo, T., Rahmani, A., Belarouci, A., Seassal, C., Nedeljkovic, D. & Callard, S. 2010, 'Near-field and far-field analysis of an azimuthally polarized slow Bloch mode microlaser', Optics Express, vol. 18, no. 26, pp. 26879-26886.
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We report on the near- and far-field investigation of the slow Bloch modes associated with the G point of the Brillouin zone, for a honeycomb lattice photonic crystal, using near-field scanning optical microscopy (NSOM) and infra-red CCD camera. The array of doughnut-shaped monopolar mode (mode M) inside each unit cell, predicted previously by numerical simulation, is experimentally observed in the near-field by means of a metal-coated NSOM tip. In far-field, we detect the azimuthal polarization of the doughnut laser beam due to destructive and constructive interference of the mode radiating from the surface (mode TEM01*). A divergence of 2 for the laser beam and a mode size of (12.8 1) m for the slow Bloch mode at the surface of the crystal are also estimated.
Chaumet, P.C. & Rahmani, A. 2009, 'Coupled-dipole method for magnetic and negative refraction materials', Journal of Quantitative Spectroscopy and Radiative Transfer, vol. 110, no. 1-2, pp. 22-29.
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We present a derivation of the coupled-dipole method, also called discrete dipole approximation, for scatterers with arbitrary dielectric permittivity and magnetic permeability. We discuss the numerical implementation of the method and illustrate its application to magnetic and negative-refraction materials.
Chaumet, P.C. & Rahmani, A. 2009, 'Efficient iterative solution of the discrete dipole approximation for magneto-dielectric scatterers', Optics Letters, vol. 34, no. 7, pp. 917-919.
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The discrete dipole approximation (DDA) has been widely used to study light scattering by nonmagnetic objects. The electric field inside an arbitrary scatterer is found by solving a dense, symmetric, linear system using, in general, an iterative approach. However, when the scatterer has a nonzero magnetic susceptibility, the linear system becomes nonsymmetric, and some of the most commonly used iterative methods fail to work. We study the scattering of light by objects with both electric and magnetic linear responses and discuss the efficiency of several iterative solvers for the nonsymmetric DDA.
Bordas, F., Seassal, C., Dupuy, E., Regreny, P., Gendry, M., Viktorovitch, P., Steel, M.J. & Rahmani, A. 2009, 'Room temperature low-threshold InAs/InP quantum dot single mode photonic crystal microlasers at 1.5 ++m using cavity-confined slow light', Optics Express, vol. 17, no. 7, pp. 5439-5445.
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We have designed, fabricated, and characterized an InP photonic crystal slab structure that supports a cavity-confined slow-light mode, i.e. a bandgap-confined valence band-edge mode. Three dimensional finite difference in time domain calculations predict that this type of structure can support electromagnetic modes with large quality factors and small mode volumes. Moreover these modes are robust with respect to fabrication imperfections. In this paper, we demonstrate room-temperature laser operation at 1.5 m of a cavity-confined slow-light mode under pulsed excitation. The gain medium is a single layer of InAs/InP quantum dots. An effective peak pump power threshold of 80 W is reported.
Tomljenovic-Hanic, S., Rahmani, A., Steel, M.J. & de Sterke, M. 2009, 'Comparison of the sensitivity of air and dielectric modes in photonic crystal slab sensors', Optics Express, vol. 17, no. 17, pp. 14552-14557.
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Optical cavities provide a route to sensing through the shift of the optical resonant peak. However, effective sensing with optical cavities requires the optimization of the modal quality factor, Q, and the field overlap with the sample, f. For a photonic crystal slab (PCS) this figure of merit, M = fQ, involves two competing effects. The air modes usually have large f but small Q, whereas the dielectric modes have high-Q and small f. We compare the sensitivity of air and dielectric modes for different PCS cavity designs and account for loss associated with absorption by the sensed sample or its host liquid. We find that optimizing Q at the expense of f is the most beneficial strategy, and modes deriving from the dielectric bands are thus preferred.
Le Gac, G., Rahmani, A., Seassal, C., Picard, E., Hadji, E. & Callard, S. 2009, 'Tuning Of An Active Photonic Crystal Cavity By An Hybrid Silica/Silicon Near-Field Probe', Optics Express, vol. 17, no. 24, pp. 21672-21679.
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The influence of a near-field tip on the spectral characteristics of a resonant mode of an active photonic crystal micro-cavity was investigated. The wavelength shift of the mode was theoretically and experimentally demonstrated and evaluated as a function of the nature and the position of the tip above the cavity. Experiment showed that the shift induced is ten times higher with a Si-coated silica probe than with a bare silica tip: a shift until 2 nm was reached with Si-coated tip whereas the shift with bare silica tip is in the range of the tenth of nanometer, for wavelengths around 1,55 m.
Chaumet, P.C. & Rahmani, A. 2009, 'Electromagnetic Force And Torque On Magnetic And Negative-Index Scatterers', Optics Express, vol. 17, no. 4, pp. 2224-2234.
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We derive the analytic expressions of the electromagnetic force and torque on a dipolar particle, with arbitrary dielectric permittivity and magnetic permeability.We then develop a general framework, based on the coupled dipole method, for computing the electromagnetic force and torque experienced by an object with arbitrary shape, dielectric permittivity and magnetic permeability.
Jia, B., Norton, A.H., Li, J., Rahmani, A., Asatryan, A.A., Botten, L.C. & Gu, M. 2008, 'Local observation of modes from three-dimensional woodpile photonic crystals with near-field microspectroscopy under supercontinuum illumination', Optics Letters, vol. 33, no. 10, pp. 1093-1095.
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A near-field microscope coupled with a near-infrared (NIR) supercontinuum source is developed and applied to characterize optical modes in a three-dimensional (3D) woodpile photonic crystal (PC) possessing a NIR partial bandgap. Spatially resolved near-field intensity distributions under different illumination wavelengths demonstrate that the electric fields preferentially dwell in the polymer rods or in the gaps between rods, respectively, for frequencies below or above the stop gap, as predicted by the 3D finite-difference time-domain modeling. Near-field microspectroscopy further reveals that the position-dependent band-edge effect plays an important role in PC-based all-optical integrated devices.
Gardin, S., Bordas, F., Letartre, X., Seasall, C., Rahmani, A., Bozio, R. & Viktorovitch, P. 2008, 'Microlasers based on effective index confined slow light modes in photonic crystal waveguides', Optics Express, vol. 16, no. 9, pp. 6331-6339.
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We present the design, theory and experimental implementation of a low modal volume microlaser based on a line-defect 2D-photonic crystal waveguide. The lateral confinement of low-group velocity modes is controlled by the post-processing of 1 to 3?m wide PMMA strips on top of two dimensional photonic crystal waveguides. Modal volume around 1.3 (?/n)3 can be achieved using this scheme. We use this concept to fabricate microlaser devices from an InP-based heterostructure including InAs0.65P0.35 quantum wells emitting around 1550nm and bonded onto a fused silica wafer. We observe stable, room-temperature laser operation with an effective lasing threshold around 0.5mW.
Lee, A.J., Rahmani, A., Dawes, J., Marshall, G.D. & Withford, M. 2008, 'Point-by-point inscription of narrow-band gratings in polymer ridge waveguides', Applied Physics A, vol. 90, pp. 273-276.
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We report the point-by-point inscription of gratings in lithographically fabricated polymer ridge waveguides using direct-write laser processing. This device exhibited narrow-band-pass spectral features of ?0.8 nm full-width-half-maximum, suitable for wavelength division multiplexing applications.
Chaumet, P.C., Belkebir, K. & Rahmani, A. 2008, 'Coupled-dipole method in time domain', Optics Express, vol. 16, no. 25, pp. 20157-20165.
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We present a time-domain formulation of electrodynamics based on the self-consistent derivation of the electromagnetic field in a linear, dispersive, lossy object via the coupled dipole method.
Bordas, F., Steel, M.J., Seassal, C. & Rahmani, A. 2007, 'Confinement of Band-Edge Modes in a Photonic Crystal Slab', Optics Express, vol. 15, no. 17, pp. 10890-10902.
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We study the confinement of low group velocity band-edge modes in a photonic crystal slab. We use a rigorous, three dimensional, finite-difference time-domain method to compute the electromagnetic properties of the modes of the photonic structures. We show that by combining a defect mode approach with the high-density of states associated with bandedge modes, one can design compact, fabrication-tolerant, high-Q photonic microcavities. The electromagnetic confinement properties of these cavities can foster enhanced radiation dynamics and should be well suited for ultralow-threshold microlasers and cavity quantum electrodynamics.
Rahmani, A. & Chaumet, P.C. 2006, 'Optical Trapping near a Photonic Crystal', Optics Express, vol. 14, no. 13, pp. 6353-6358.
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We show that the photonic confinement induced by a photonic crystal can be exploited to trap nanoparticles. As demonstrated by the recent advances in the design and fabrication of photonic crystals slab structures, total internal reflection and multiple scattering can be combined to confine photons very efficiently. A consequence of this confinement is the existence of strong gradients of electromagnetic intensity in the near-field of the photonic structure. Hence, a nanoparticle placed in the vicinity of the crystal would experience an optical force which, with a proper design of the near-field optical landscape, can lead to trapping.
Bordas, F., Louvion, N., Callard, S., Chaumet, P.C. & Rahmani, A. 2006, 'Coupled dipole method for radiation dynamics in finite photonic crystal structures', Physical Review E, vol. 73, no. 5, pp. 056601-056607.
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We present a coupled-dipole treatment of radiation dynamics in the weak-coupling regime in a finite three-dimensional photonic crystal structure. The structure is discretized in real space and the self-consistent local field is computed. We illustrate the computation of radiation dynamics by calculating the spontaneous emission rate for a source located in a defect cavity inside a slab photonic crystal structure. We compute the cavity spectral response, the near-field modal structure, and the far-field radiation pattern of the microcavity. We also discuss our results in light of the recent experimental near-field observations of the optical modes of a photonic crystal microcavity.
Louvion, N., Rahmani, A., Seassal, C., Callard, S., Gerard, D. & de Fornel, F. 2006, 'Near-field observation of subwavelength confinement of photoluminescence by a photonic crystal microcavity', Optics Letters, vol. 31, no. 14, pp. 2160-2162.
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We present a direct, room-temperature near-field optical study of light confinement by a subwavelength defect microcavity in a photonic crystal slab containing quantum-well sources. The observations are compared with three-dimensional finite-difference time-domain calculations, and excellent agreement is found. Moreover, we use a subwavelength cavity to study the influence of a near-field probe on the imaging of localized optical modes.
Chaumet, P.C., Rahmani, A. & Nieto-Vesperinas, M. 2006, 'Local-field enhancement in an optical force metallic nanotrap: Application to single-molecule spectroscopy', Applied Optics, vol. 45, no. 21, pp. 5185-5190.
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We study the local-field enhancement in a nanocavity created by optical nanomanipulation. Recently we showed that a metallic probe can modify the optical force experienced by a metallic particle and generate a material selective trapping potential. We show that the same configuration used for optical forces can be used to control both in magnitude and tune the local-field enhancement around the particle at resonance. The spatial resolution and material selectivity of this technique, allied to its capability to manipulate particles at the nanometric level, may offer a new and versatile way to achieve surface-enhanced Raman scattering spectroscopy at the single-molecule level.
Rahmani, A., Chaumet, P.C., Sentenac, A. & Bryant, G.W. 2005, 'Efficient computation of optical forces with the coupled dipole method', Physical Review E, vol. 72, no. 4, pp. 046708-046712.
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We present computational techniques to compute in an efficient way optical forces on arbitrary nanoobjects using the coupled dipole method. We show how the time of computation can be reduced by several orders of magnitude with the help of fast-Fourier-transform techniques. We also discuss the influence of different formulations of the electric polarizability of a small scatterer on the accuracy and robustness of the computation of optical forces.
Louvion, N., Gerard, D., Mouette, J., de Fornel, F., Seassal, C., Letarte, X., Rahmani, A. & Callard, S. 2005, 'Local Observation and Spectroscopy of Optical Modes in Active Photonic Crystal Microcavity', Physical Review Letters, vol. 94, no. 11, pp. 1-4.
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We report the direct, room-temperature, near-field mapping and spectroscopy of the optical modes of a photonic-crystal microcavity containing quantum wells. We use a near-field optical probe to reveal the imprint of the cavity mode structure on the quantum-well emission. Furthermore, near-field spectroscopy allows us to demonstrate the strong spatial and spectral dependence of the coupling between the sources and the microcavity. This knowledge will be essential in devising future nanophotonic devices.
Chaumet, P.C., Rahmani, A. & Nieto-Vesperinas, M. 2005, 'Photonic force spectroscopy on metallic and absorbing nanoparticles', Physical Review B, vol. 71, no. 4, pp. 1-7.
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We present a detailed study of the optical trapping and manipulation of nanoparticles with complex permittivity using an apertureless near-field probe. We use a three-dimensional, self-consistent description of the electromagnetic scattering processes that accounts for retardation and the intricate many-body interaction between the substrate, the particle, and the probe. We analyze the influence of absorption on the optical force. For metals we describe how the optical force spectrum is influenced by the optical response of the metal, and in particular by plasmon resonances. We find that the optical force spectrum can provide an intrinsic signature of the particle composition which can be used to achieve a material-selective trapping and nanomanipulation.
Chaumet, P.C., Sentenac, A. & Rahmani, A. 2004, 'Coupled dipole method for scatterers with large permittivity', Physical Review E, vol. 70, no. 3, pp. 036606-036611.
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In the coupled dipole method, a three-dimensional scattering object is discretized over a lattice into a set of polarizable units that are coupled self-consistently. Starting from the volume integral equation for the field, we show that performing the integration of the free-space field susceptibility tensor over the lattice cell dramatically improves the accuracy of the method when the permittivity of the object is large. This integration, done without any approximation, allows us to define a prescription for the polarizability used in the coupled dipole method. Our derivation is not restricted to any particular shape of the scatterer or to a cubic discretization lattice.
Rahmani, A., Chaumet, P.C. & Bryant, G.W. 2004, 'On the Importance of Local-Field Corrections for Polarizable Particles on a Finite Lattice: Application to the Discrete Dipole Approximation', The Astrophysical Journal, vol. 607, no. 2, pp. 873-878.
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We investigate the influence of local-field effects on the electromagnetic response of a collection of dipoles. We derive the local-field corrected static polarizability for a collection of dipoles in the case of a scatterer with uniform depolarization. We then use this correction within the discrete dipole approximation to study the scattering of an electromagnetic wave by a spherical particle. The local-field correction leads to a new formulation of the discrete dipole approximation that is exact in the long-wavelength limit and more accurate at finite frequencies. We also discuss the feasibility of a generalization of the local-field correction to arbitrary scatterers.
Nieto-Vesperinas, M., Chaumet, P.C. & Rahmani, A. 2004, 'Near-field photonic forces', Invalid Code, vol. 362, no. 1817, pp. 719-737.
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A review of recent advancements in photonic forces is presented. We discuss in detail the interaction of light and sub-wavelength particles on a substrate illuminated by total internal reflection, and we study the opticalforces experienced by the particles. The effects of plasmon-mode excitations on the resulting photonic forces on metallic particles are also addressed. Moreover, we explore the possibility of using the metallic tip of a classical apertureless microscope to create optical tweezers, and thus to achieve a selective manipulation of nanoparticles.
Chaumet, P.C., Rahmani, A. & Bryant, G.W. 2003, 'Generalization of the coupled dipole method to periodic structures', Physical Review B, vol. 67, no. 16, pp. 1-5.
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We present a generalization of the coupled dipole method to the scattering of light by arbitrary periodic structures. This formulation of the coupled dipole method relies on the same direct-space discretization scheme that is widely used to study the scattering of light by finite objects. Therefore, all the knowledge acquired previously for finite systems can be transposed to the study of periodic structures.
Rahmani, A., Chaumet, P.C. & Bryant, G.W. 2002, 'Coupled dipole method with an exact long-wavelength limit and improved accuracy at finite frequencies', Optics Letters, vol. 27, no. 3, pp. 2118-2120.
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We present a new formulation of the coupled dipole method that accounts for local-field effects and is exact in the long-wavelength limit. This formulation also leads to improved accuracy of the description of light-scattering processes at finite frequencies.
Rahmani, A. & Bryant, G.W. 2002, 'Spontaneous emission in microcavity electrodynamics', Physical Review A, vol. 65, no. 3, pp. 033817-033828.
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We present a general, semimicroscopic, self-consistent treatment of spontaneous emission for a two-level atom in a dielectric microcavity with arbitrary shape. Both lossless and absorbing media are considered. Our approach is based on linear-response theory and the coupled-dipole method. We illustrate our method on spherical and cubic cavities. We discuss the Purcell effect, the frequency shift, the local-field effect, and the dipole-dipole coupling experienced by a source placed in a microcavity.
Chaumet, P.C., Rahmani, A. & Nieto-Vesperinas, M. 2002, 'Selective nanomanipulation using optical forces', Physical Review B, vol. 66, no. 19, pp. 1-11.
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We present a detailed theoretical study of the recent proposal for selective nanomanipulation of nanometric particles above a substrate using near-field optical forces [P.C. Chaumet et al., Phys. Rev. Lett. 88, 123601 (2002)]. Evanescent light scattering at the apex of an apertureless near-field probe is used to create an optical trap. The position of the trap is controlled on a nanometric scale via the probe, and small objects can be selectively trapped and manipulated. We discuss the influence of the geometry of the particles and the probe on the efficiency of the trap. We also consider the influence of multiple scattering among the particles on the substrate and its effect on the robustness of the trap.
Rahmani, A., Chaumet, P.C. & Bryant, G.W. 2002, 'Local-field correction for an interstitial impurity in a crystal', Optics Letters, vol. 27, no. 6, pp. 430-432.
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The local-field correction experienced by an interstitial impurity in a crystal with cubic symmetry is derived by use of a rigorous, self-consistent, semimicroscopic description of spontaneous emission in a microcavity. We compute the local-field factor for various positions of the impurity inside the crystal. Furthermore, we demonstrate that the local-field factor can be computed from a simple electrostatic model as a rapidly converging lattice sum. We show that the agreement between the predictions of this simple model and the rigorous calculations is remarkable, opening the way to a simple, general theory of a local-field effect for an impurity in a crystal with arbitrary symmetry.
Chaumet, P.C., Rahmani, A. & Nieto-Vesperinas, M. 2002, 'Optical trapping and manipulation of nano-objects with an apertureless probe', Physical Review Letters, vol. 88, no. 12, pp. 1-4.
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We propose a novel way to trap and manipulate nano-objects above a dielectric substrate using an apertureless near-field probe. A combination of evanescent illumination and light scattering at the probe apex is used to shape the optical field into a localized, three-dimensional optical trap. We use the coupled-dipole method and the Maxwell stress tensor to provide a self-consistent description of the optical force, including retardation and the influence of the substrate. We show that small objects can be selectively captured and manipulated under realistic conditions.
Rahmani, A., Chaumet, P.C. & de Fornel, F. 2001, 'Environment-induced modification of spontaneous emission: Single-molecule near- field probe', Physical Review A, vol. 63, pp. 023819-1-023819-11.
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The modification of lifetime experienced by a fluorescent molecule placed in an arbitrary environment is investigated theoretically within the framework of linear response theory. We present a complete description of the interaction of the particle with arbitrary structures on a plane substrate or inside a cavity. The theory is based on a self-consistent scattering procedure in which retardation effects and contributions from both homogeneous and evanescent modes of the electromagnetic field are included. The decay rate variations are computed and the concept of single-molecule near-field probe is discussed.