Single Light Beam Research Articles

Light beams with orbital angular momentum for free space optics

The light's orbital angular momentum (OAM) is a consequence of the spiral flow of the electromagnetic energy. In this paper, an analysis of light beams with OAM used for free space optics (FSO) is conducted. The basic description and conception of light's OAM are reviewed. Both encoding information into OAM states of single light beam and encoding information into spatial structure of the mixed optical vortex with OAM are discussed, and feasibility to improve the FSO's performance of security and obstruction of line of sight is examined.

Discussion on the improvement of opto-RF link properties by using a bipolar cascade laser source

This paper studies the possible improvement in RF link properties that can stem from electrically cascading several laser sources and combining the light from each source into a single information-carrying light beam. The effect of carrier recycling is first studied within a discrete architecture consisting of n individual laser diodes macroscopically connected in series. An RF link gain improvement proportional to n 2 , and a link noise figure improvement proportional to n is found. The model is validated by experimental data. The architecture nonetheless carries some drawbacks, including the need for a zero-loss optical combining device to benefit from the RF link gain improvement, and some bandwidth shortcomings. The effect of carrier recycling within an integrated laser device, a so-called bipolar cascade laser, is then studied. In order to push back the limitations of the discrete architecture, the device consists of n active regions integrated into a single laser cavity. A rate equation model is applied to this promising structure and it is found that, in good agreement with previously published results, the external efficiency is expected to increase by a factor of n, leading to a possible RF link gain improvement by a factor of n 2 . However, because the laser noise is dominated by the photon corpuscular noise, a weak influence of electrically cascading active junctions into a single laser cavity on the laser intensity noise, and thus on the link noise figure, is expected

Encoding information as orbital angular momentum states of light for wireless optical communications

The issues of security and obstruction of line of sight are concerns of wireless optical communications (WOC). In this paper, an analysis of light beams carrying orbital angular momentum (OAM) used for WOC is undertaken. The basic description and conception of OAM of light are given. Encoding information both as the OAM states of a single light beam and as the spatial structure of the mixed light beams with OAM is discussed and compared. Using OAM to encode the data offers an inherent security enhancement. Mixed vortex fields with OAM possess a self-reconstruction property that is able to regenerate and to reconstruct their initial spatial distributions of the complex amplitude in the condition of obstruction of line of sight.

Nanostructures produced by ultraviolet laser irradiation of silicon. II. Nanoprotrusions and nanoparticles

Two-dimensional ordered nanoprotrusion arrays were produced at silicon surface substrates by nanosecond ultraviolet laser irradiation using a Lloyd’s mirror set up. These protrusions are 40 to 70nm high and have a diameter of ∼60 to 100nm at their base, and in many cases display a regular rectangular lattice. Their origin and evolution were studied using scanning electron microscopy and atomic force microscopy. Nanoprotrusions originate from a subjacent laser-induced periodic surface structure upon continuing irradiation, under the same processing conditions that produced these ripples. Their evolution is discussed in terms of fingering instabilities of melted silicon consistent with a gradient of surface tension due to a temperature gradient. The temperature gradient is produced by the same mechanism responsible for the ripple formation. At slightly higher laser fluences, nanoparticles were observed to form using a single beam of nonpolarized laser light. The nanoparticles also span a linear ordered array, with line spacing that conforms to the grating equation. Their formation mechanism has been described previously as a result of ablation and redeposition, and thus is widely different from the formation of nanoprotrusions. The differences and similarities of nanoprotrusions and nanoparticles, and their connection with nanoripples, were studied in detail. In particular, when the ripple structure was still seen, nanoprotrusions were observed to form on ripple crests while nanoparticles were located in ripple valleys. Nanoprotrusions remained stable under a 2h thermal annealing at 1073K. By contrast, the aligned nanoparticle arrays became disordered as a result of a 2h anneal at a temperature as low as 423K, as nanoparticles moved on the surface.

Observation of superradiant Raman scattering in a Bose-Einstein condensate

We observed superradiant Raman scattering from a Bose-Einstein condensate irradiated by a single off-resonant light beam. Spontaneous mode selection of the scattered radiation turns probabilistic optical pumping into a stimulated Raman process, whereby the atoms are coherently transferred into a different hyperfine state. The population in the pumped state grew exponentially in time, demonstrating the emergence of bosonic stimulation in the optical pumping process.

Light deflection from ferroelectric domain boundaries

A new deflection phenomenon of light has been discovered in ferroelectric crystals that provides information about domain structures, their nucleation, and their dynamics: Illumination of a multi-domain crystal with a single light beam yields light patterns, if external electrical fields are applied. Deflection angles of up to ±8° are observed. We attribute this effect to light deflection from the boundaries of the domains. Besides providing a new method to study domains of any ferroelectric material, the method can be of practical importance for electro-optic beam scanning.

Sub-Doppler fluorescence on the atomic D 2 line of a submicron rubidium-vapor layer

An extremely thin cell (ETC) with the thickness of a Rb atomic vapor layer in the range of 100–300 nm was fabricated. It is demonstrated that a simple laser-diode technique with a single resonant light beam is sufficient to observe separately all of the atomic hyperfine transitions of the D2 line of Rb (780 nm) and also allows us to measure the relative transition probabilities of the hyperfine transitions. The onset of collisional self-broadening of the hyperfine transitions as the number density of atoms increases was studied. The detrimental role of the atoms with slow longitudinal velocity in the sub-Doppler response of the Rb ETC is demonstrated by studies in which the cell is tilted from normal incidence of the laser beam. It is also shown that using an ETC allows us to resolve in a moderate external magnetic field the Zeeman splitting of the hyperfine transitions of the 87Rb D1 transition Fg=1Fe=1,2.

Investigation of minority carrier diffusion length in shallow junctions by angle-resolved illumination technique

In this paper, we discuss a new approach to the determination of minority carrier diffusion length ( L d) on flat top surface shallow junction devices in which photocurrent is dominated by diffusion component as in solar cells. In our method, we propose the use of a single monochromatic light beam of appropriate wavelength, incident on the device surface at various angles. Under such experimental conditions, the short circuit current of the device is expressed as a function of the internal spectral response and device reflectance. Then diffusion length can be simply derived by the analysis of the experimental ratio between photocurrent measured at various incidence angles and when light impinges the device orthogonally, as function of the incident angle. An analytical model is proposed based on the modified absorption coefficient as a function of refractive angle and an experimental set-up for the evaluation of minority carrier diffusion length is proposed.

Mechanical manipulation of bone and cartilage cells with ‘optical tweezers’

The single beam optical gradient trap (optical tweezers) uses a single beam of laser light to non-invasively manipulate microscopic particles. Optical tweezers exerting a force of approximately 7 pN were applied to single bone and cartilage derived cells in culture and changes in intracellular calcium levels were observed using Fluo-3 labelling. Human derived osteoblasts responded to optical tweezers with an immediate increase in [Ca 2+] i that was inhibited by the addition of a calcium channel blocker nifedipine. Force applied to different regions of cells resulted in a variable response. [Ca 2+] i elevation in response to load was lower in rat femur derived osteoblasts, and not apparent in primary chondrocytes and the osteocytic cell line (MLO Y4).

Achieving a high sensitivity of measurements in optical systems for rewriting holograms by incoherent light

It is shown for the first time that holograms can be rewritten by a single beam of incoherent light without an accompanying growth of the frequency of the carrier fringes of the rewritten holograms. This removes the restriction on the number of rewrite cycles and thus permits the achievement of the maximum sensitivity of measurements.

Coherent Doppler narrowing in a thin vapor cell: Observation of the Dicke regime in the optical domain

Single light beam transmission spectroscopy in an ultrathin dilute vapor cell (10--100 \ensuremath{\mu}m) reveals sub-Doppler features. Indeed, atoms with fast (normal) velocity undergo a short interaction between the cell walls, and are shown to be less absorbing because they have not reached their steady state of interaction with light. The effect is observed in the linear-response regime with a simple two-level system, as ensured from the ultralow level of light irradiation ($l~1\ensuremath{\mu}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ for the Cs ${D}_{2}$ line). The effect, known for a long time in the microwave domain, is demonstrated here in the optical domain, and is extended to cell thicknesses large relative to the optical wavelength.

Fanning Laser Light Application to the Characterization of Bi<sub>12</sub>TiO<sub>20</sub> and Bi<sub>12</sub>SiO<sub>20</sub> Crystals

Bi 12 SiO 20 and Bi 12 TiO 20 crystals were grown using the melting technique. The crystals were characterized by DTA, X-ray diffraction, and chemical etching. The fanning effect is known as a self-scattering process in which a single laser light beam self-generates scattering in an asymmetrical way. The intensity distribution of fanning light in the far field measured under an applied electric field, normalized to the intensity measured without a field represents the two-wave mixing gain factory γ. For the experiments an argon laser with λ=515nm and the maximum power of 0.5 W was used and the scattered intensity was recorded. The factor γ(θ,φ) for a set of samples was calculated using the intensity distribution in the far field, recorded with a CCD camera. The values of γ(θ,φ) were correlated with the optical inhomogeneity of the crystals.

Coherence-multiplexed acousto-optic correlator for signal processing: theory and applications

A method of multichannel processing to compute correlation products simultaneously is introduced. The signals that process are encoded on a single light beam through the use of an electro-optic modulators that induce optical delays greater than the coherence length of light. The coherence-modulated light beam thus obtained is then spatially and temporally modulated through an acousto-optic Bragg cell. The potential number of channels is estimated to be approximately 5-10. The method can be combined with other existing systems, such as time-, space-, or frequency-multiplexed correlators, to increase the channel number. The method also applies to high-resolution spectral analysis.

Observation of quantum beating in a simple beam-splitting experiment: Two-particle entanglement in spin and space-time.

A single light beam, generated by type-II down-conversion, is split by a beam splitter. When a set of quartz plates is inserted into the single beam, the coincidence counting rate between the split beams exhibits a 100% frequency-beating modulation. This nonclassical phenomena is a manifestation of a two-photon entangled state in which the two-particle state is entangled simultaneously in spin and space-time.

Scatter rings in bismuth silicate illuminated by a single beam

We have observed cones of diffracted light by illuminating a crystal of bismuth silicate with a single beam of light while applying an electric field across the crystal. The scatter rings are a result of the preferential amplification of noise, which we explain here using the Ewald sphere construction.

Theory of radiation forces and momenta for mobile atoms in light fields.

A theory for a bound system of charges interacting with electromagnetic fields is developed, with special emphasis on the radiation-pressure effects imposed by the fields on the gross motion of the charges. The theory is particularly simple and transparent for the case of two opposite charges of finite mass M in the electric-dipole approximation. This enables a rigorous description to be given for the quantum electrodynamics of the problem with the various interaction terms affecting the gross motion being easily identified. Here, too, it becomes clear that, even in the dipole approximation, the conventional interaction term should be supplemented by the so-called R\ontgen term. Besides ensuring the consistency of the theory for the overall (charges plus fields) system regarding conservation laws, the R\ontgen term has dynamical consequences. This is established by explicit calculations of the expectation values of the mechanical momentum 〈MR\ifmmode \dot{}\else \.{}\fi{}〉 and the radiation pressure force 〈MR\ifmmode\ddot\else\textasciidieresis\fi{}〉 on a two-level atom in the dipole approximation. Results of calculations are displayed for the case of a single light beam and for counterpropagating beams. The feasibility of experimentally observing the transverse force is explored. Generalizations of the well-known friction force arising in one-dimensional optical molasses are given and discussed.

Photon noise reduction by controlled deletion techniques

We analyze different mechanisms for the generation of sub-Poissonian light beams within the framework of a simple photon model. In this model light is treated as a stream of discrete photon events that are localized in time. The course taken by each of these photons from its generation to its detection is evaluated according to the outcomes of several branching processes. We consider mechanisms in which the reduction of intensity noise below the shot-noise level is effected through control of photon deletion probabilities, both in a feedback configuration involving a single light beam and in a feedforward configuration involving twin light beams for which we explicitly include decorrelations. The resultant intensity noise spectra are evaluated analytically as well as numerically by Monte Carlo simulation. These results are compared with some corresponding results obtained from a semiclassical input–output analysis.

Optical Measurement of Unducted Fan Blade Deflections

A nonintrusive optical method for measuring unducted fan (or propeller) blade deflections is described and evaluated. The measurement does not depend on blade surface reflectivity. Deflection of a point at the leading edge and a point at the trailing edge in a plane nearly perpendicular to the pitch axis is obtained with a single light beam generated by a low-power, helium-neon laser. Quantitative analyses are performed from taped signals on a digital computer. Averaging techniques are employed to reduce random errors. Measured static deflections from a series of high-speed wind tunnel tests of a counterrotating unducted fan model are compared with available predicted deflections, which also are used to evaluate systematic errors.

Compliance of bacterial flagella measured with optical tweezers.

The development of the gradient force optical particle trap ('optical tweezers') has made it possible to manipulate biological materials using a single beam of laser light. Optical traps can produce forces in the microdyne range on intact cells without causing overt damage: such forces are sufficient to arrest actively swimming bacteria and can overcome torque generated by the flagellar motor of a bacterium tethered to a glass surface by a flagellar filament. By calibrating the trapping force against Stokes' drag and measuring the twist that is sustained by this force, we determined the torsional compliance of flagella in tethered Escherichia coli and a motile Streptococcus. Flagella behaved as linear torsion springs for roughly half a revolution, but became much more rigid when turned beyond this point in either direction.

Detection of flowing samples with a selective concentration gradient method

Refractive index gradients generated in the flowing medium are measured with the help of a single probing light beam (Schlieren optics). These gradients are associated with any solute present in the medium. Additionally, excitation energy is introduced to the detection volume. After the relaxation process, the temporal temperature gradient relates closely to the concentration gradient of the adsorbing solute only. Both universal (concentration gradient) and selective (temperature gradient) information associated with the solute can be acquired by monitoring the deflection of the probe light beam in nanoliter detection volume. These data can be utilized to differentiate between samples on the basis of their molar absorption coefficients. A one-dimensional mathematical model was developed to describe the heat transfer associated with the selective concentration gradient method in flowing streams. Theoretical results closely correlate with experimental data. This method has been applied to capillary liquid chromatography detection, characterization of freshness of fruit juices, and contamination of organic solvents (fuels) with water. The detection limit of the selective concentration gradient method is about 6 x 10/sup -6/ absorption unit for 1-mJ excitation pulses. This corresponds to a fraction of fentomole of congo red detected in a few nanoliter volume.