Free-space Photons Research Articles

A Path Loss Model for Non-Line-of-Sight Ultraviolet Multiple Scattering Channels

An ultraviolet (UV) signal transmission undergoes rich scattering and strong absorption by atmospheric particulates. We develop a path loss model for a Non-Line-of-Sight (NLOS) link. The model is built upon probability theory governing random migration of photons in free space, undergoing scattering, in terms of angular direction and distance. The model analytically captures the contributions of different scattering orders. Thus it relaxes the assumptions of single scattering theory and provides more realistic results. This allows us to assess the importance of high-order scattering, such as in a thick atmosphere environment, where short range NLOS UV communication is enhanced by hazy or foggy weather. By simulation, it is shown that the model coincides with a previously developed Monte Carlo model. Additional numerical examples are presented to demonstrate the effects of link geometry and atmospheric conditions. The results indicate the inherent tradeoffs in beamwidth, pointing angles, range, absorption, and scattering and so are valuable for NLOS communication system design.

Qualitative simulation of photon transport in free space based on monte carlo method and its parallel implementation.

During the past decade, Monte Carlo method has obtained wide applications in optical imaging to simulate photon transport process inside tissues. However, this method has not been effectively extended to the simulation of free-space photon transport at present. In this paper, a uniform framework for noncontact optical imaging is proposed based on Monte Carlo method, which consists of the simulation of photon transport both in tissues and in free space. Specifically, the simplification theory of lens system is utilized to model the camera lens equipped in the optical imaging system, and Monte Carlo method is employed to describe the energy transformation from the tissue surface to the CCD camera. Also, the focusing effect of camera lens is considered to establish the relationship of corresponding points between tissue surface and CCD camera. Furthermore, a parallel version of the framework is realized, making the simulation much more convenient and effective. The feasibility of the uniform framework and the effectiveness of the parallel version are demonstrated with a cylindrical phantom based on real experimental results.

Metal-Based Photonic Coatings from Electrochemical Deposition

Plasmonic materials offer the remarkable ability to manipulate light by metal–dielectric features significantly smaller than the wavelengths of free space photons. The fabrication of such minuscule features for the desired plasmonic properties, however, remains challenging. Here, we report an economical and versatile bottom-up approach that combines electrodeposition and de- alloying techniques for fabricating elaborate metal-based structures, achieving structural resolutions comparable to the present lithography techniques. We present our studies on the metal-based counterpart of the rugate filter. These metal-based photonic films are porous, magnetic, and feature strong optical responses in visible wavelengths, while their structural periodicities are an order of magnitude smaller than the light’s free space wavelength.

A Plasmonic Dimple Lens for Nanoscale Focusing of Light

Focusing electromagnetic energy to subwavelength dimensions has become an increasingly active field of research for a variety of applications such as heat-assisted magnetic recording, nanolithography, and nanoscale optical characterization of biological cells and single molecules using the near-field scanning optical microscopy technique. Double-sided surface plasmons in a metal-insulator-metal (MIM) geometry can have very small wavelengths for dielectric of thickness of less than 10 nm. A tapered dielectric structure sandwiched between metal can be used to efficiently couple electromagnetic energy from free space photons to the plasmonic wavelengths at the nanoscale. In this paper, we present the fabrication and characterization of a novel MIM plasmonic lens structure.

Strong interaction between light and a single trapped atom without the need for a cavity

An experiment that demonstrates efficient absorption of light by a single atom residing in free space should be helpful for designing interfaces for the transfer of quantum information from ‘flying’ qubits to stationary quantum systems, without the need for optical cavities. Many quantum information processing protocols require efficient transfer of quantum information from a flying photon to a stationary quantum system1,2,3. To transfer information, a photon must first be absorbed by the quantum system. This can be achieved, with a probability close to unity, by an atom residing in a high-finesse cavity1. However, it is unclear whether a photon can be absorbed effectively by an atom in a free space. Here, we report on an observation of substantial extinction of a light beam by a single 87Rb atom through focusing light to a small spot with a single lens. The measured extinction values can be directly compared to the predictions of existing free-space photon–atom coupling models4,5,6. Our result should open a new perspective on processing quantum information carried by light using atoms, in particular for experiments that require strong absorption of single photons by an atom in free space.

Theoretical analysis of light emission from a coupled system of a photonic nanocavity and a quantum dot

AbstractThis report quantum mechanically analyses a system composed of a two‐level system, a single mode cavity, and free space photon modes, which are coupled to each other. The analytical solution in the intermediate regime between the strong and weak coupling regimes is successfully obtained. It is found that the emission rate reaches its maximum when the cavity photon lifetime τcav satisfy 1/τcav = 4|g |, where 2|g | is the vacuum Rabi frequency. Also, Fano resonance is shown to occur under certain conditions. These results can be useful for high‐speed single photon sources and for fundamental understanding of the interaction between a quantum dot and a cavity mode. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A Nonvolatile Plasmonic Switch Employing Photochromic Molecules

We demonstrate a surface plasmon-polariton (SPP) waveguide all-optical switch that combines the unique physical properties of small molecules and metallic (plasmonic) nanostructures. The switch consists of a pair of gratings defined in an aluminum film coated with a 65 nm thick layer of photochromic (PC) molecules. The first grating couples a signal beam consisting of free space photons to SPPs that interact effectively with the PC molecules. These molecules can reversibly be switched between transparent and absorbing states using a free space optical pump. In the transparent (signal "on") state, the SPPs freely propagate through the molecular layer, and in the absorbing (signal "off") state, the SPPs are strongly attenuated. The second grating serves to decouple the SPPs back into a free space optical beam, enabling measurement of the modulated signal with a far-field detector. In a preliminary study, the switching behavior of the PC molecules themselves was confirmed and quantified by surface plasmon resonance spectroscopy. The excellent (16%) overlap of the SPP mode profile with the thin layer of switching molecules enabled efficient switching with power densities of approximately 6.0 mW/cm2 in 1.5 microm x 8 microm devices, resulting in plasmonic switching powers of 0.72 nW per device. Calculations further showed that modulation depths in access of 20 dB can easily be attained in optimized designs. The quantitative experimental and theoretical analysis of the nonvolatile switching behavior in this letter guides the design of future nanoscale optically or electrically pumped optical switches.

Observation of Plasmon Propagation, Redirection, and Fan-Out in Silver Nanowires

We report the coupling of free-space photons (vacuum wavelength of 830 nm) to surface plasmon modes of a silver nanowire. The launch of propagating plasmons, and the subsequent emission of photons, is selective and occurs only at ends and other discontinuities of the nanowire. In addition, we observe that the nanowires redirect the plasmons through turns of radii as small as 4 microm. We exploit the radiating nature of discontinuities to find a plasmon propagation length >3 +/- 1 microm. Finally, we observe that interwire plasmon coupling occurs for overlapping wires, demonstrating plasmon fan-out at subwavelength scales.

Entangled and non-line-of-sight (NLOS) free-space photon quantum communication [Invited

Feature Issue on Optical Wireless Communications (OWC) We present new quantum communication (QC) schemes suitable for free-space (wireless) QC implementation. In particular, we present several entangled QC schemes and propose non-line-of-sight (NLOS) free-space photon QC. It is shown that in the presence of atmospheric scattering media, UV photons can be used for NLOS QC. Non-Poisson quantum fluctuations in an invariance regime can propagate farther than coherent laser speckle. In such situations the non-Poissonian statistics survive over long distances and should be taggable with a polarization signature. Quantum noise observables scale markedly differently with scattering parameters compared to classical noise observables. Variation of the polarization should allow Yuen-Kumar QC using non-Poissonian statistics of the beam as an authentication of the signal. The NLOS communication in the UV would be expected to be possible to at least a range of 1 km and falls under the category of deliberately short-range QC. Applications of importance for this method include military stealth and optical communication in the presence of obstacles such as a forest or urban environment.

Switching devices with spatial and spectral resolution combining photonic crystal and MOEMS structures

One- and two-dimensional photonic crystal slabs have numerous applications in integrated optics. Unfortunately, due to their finite height, their in-plane guided modes can suffer outcoupling losses. Although considered as detrimental in the context of the development of photonic integrated circuits, it turns that the coupling of waveguided modes to radiative modes can be usefully exploited for the manipulation of photons in free space. It is shown in this paper that interaction of radiative and guided modes through a photonic crystal, especially under conditions where the latter correspond to extrema of the dispersion characteristics of the photonic crystal, results in resonance phenomena, which can be used practically for the development of new classes of devices, e.g., combining photonic crystal and microoptoelectromechanical systems structures.

Theoretical determination of the photon rest-mass

A mathematical expression for the photon rest-mass as a function of wavelength is derived by considering the fact that the refractive index of photons in free space depends upon wavelength and by considering the Sellmeier formula. In this context, free space does not coincide with vacuum.

Wigner functions for two-photon and three-photon entangled states

The space-time structure of two-photon and three-photon entangled states is analysed and the Wigner functions for these states are developed. The analysis of the Wigner functions is made in real position-momentum space where position goes with time in a retarded frame and the momentum of the photon in free space is proportional to frequency. The entanglement of the photons in frequencies and wave vectors is due to the phase matching conditions. By the use of narrow-band filters the dominant entanglement effect is due to phase matching conditions in frequencies, and under this condition the description of the three-photon entangled state becomes analogous to that of the two-photon. The motion of the entangled photons is seperated into the centre of mass motion and the relative motion between the photons, including two or three degrees of freedom for the two-photon and three-photon, respectively. Under the approximation of narrow-band filters we get for each degree of freedom a Gaussian wave packet in position momentum with the corresponding time frequency in retarded frames.

Space-time description of photon emission from an atom

Starting from the postulate that the electromagnetic field appearing in the transverse set of microscopic Maxwell-Lorentz equations governing field-matter interactions, properly normalized, can be looked upon as describing one-photon-emission and -absorption processes in space and time, a first-quantized initiation of photon emission by a single atom is presented. The wave function for the emerging photon is introduced as a six-vector object constructed from the complex analytical signals of the Riemann-Silberstein vectors belonging to opposite photon helicities. When the atom is no longer electrodynamically active, the emitted photon is described in first quantization by the so-called energy wave function well known for photons in free space. From the momentum representation of the emerging photon wave function a condition on the analytical part of the transverse atomic current density is established which ensures that precisely one photon is emitted. A propagator description of the emerged photon dynamics in the coordinate representation is established. The photon propagator is introduced as a two-component spinor, where upper and lower tensor components are constructed, respectively, from positive and negative helicity combinations of the propagators describing the time-space evolution of the transverse electric and magnetic fields. It is shown that the emission region for the photon coincides with the region in space where the transverse atomic current density is nonvanishing. For a photon emitted in an electric dipole transition the emission region essentially is the near-field zone of the atom, and this zone therefore determines the initial (and best) spatial confinement of the photon. The photon emerging from an atom active for a finite time necessarily is of the polychromatic sort and the associated wave packet essentially is confined between spherical shells moving outwards with the vacuum speed of light. To illustrate the main principles of the fundamental theory in a heuristic fashion we apply it to a study of the emission of a one-photon sinusoidal wavetrain from a pointlike atom. It is found that the atomic current density needed to create just one photon is independent of the oscillation period in the train and thus depends only on the number of periods in the wave train. An explicit expression for the one-photon energy is derived, and it is shown that only for extremely short pulse trains pronounced deviations from the textbook result, $E=\ensuremath{\Elzxh}{\ensuremath{\omega}}_{0},$ occur. The radial energy flow in the coupled atom-photon system in the near-field zone of the atom is investigated, and the cycle-averaged outwards energy transport carried by the emerging photon in a given distance from the atom is determined.

Quantum cryptography in free space

The range of quantum cryptography systems using optical fibers is limited to roughly 30 km because amplifiers cannot be used. A fully operational system for quantum cryptography based on the transmission of single photons in free space under daylight conditions has been demonstrated. The feasibility of a global system for quantum cryptography based on a network of ground stations and satellites is discussed.

Charge conjugation symmetry and the nonexistence of the photon's static magnetic field

It is shown that Evans' proposal of an axial magnetic field associated with a circularly polarized light wave or photon in free space cannot be correct because it would violate charge conjugation symmetry. However, an axial magnetic field induced by a circularly polarized light wave in an atomic or molecular medium conserves charge conjugation symmetry, being observed as the inverse Faraday effect.

Free-Space Photonics in Switching

Free-space digital optics is a new technology that exploits the ability of optics to handle thousands of light beams, or information channels, at once. This and other features of optics complement the strengths and weaknesses of purely electronic systems. Especially when combined with electronics, free-space optics allows the development of new architectures in digital systems. In particular, it offers large numbers of closely spaced interconnections inside digital processors that can be used to make large digital switching fabrics. In this paper, we outline some of the strengths and weaknesses of this emerging technology, and we briefly describe some of its experimental systems.