Summary form only given. Spontaneous degenerate parametric scattering [down-conversion] is a 3-photon process with one pump photon of frequency 2/spl omega/ and two output photons of frequency /spl omega/. An optical vortex is defined as a beam containing a phase singularity, i.e. the change of phase around the center of the beam equals 2 /spl pi/m; where the integer m is the topological charge. Our experimental results using a BaB/sub 2/O/sub 4/ crystal under type II phase matching conditions and numerical simulations suggest that the vortex in the pump beam is transformed into a speckle-like structure, effectively that structure is composed of randomly occurring vortex and Gaussian beams. The two down-converted output signals (signal and idler) appear as rings of orthogonal polarization and, therefore, can be easily distinguished.
Summary form only given. Spontaneous degenerate parametric scattering [down-conversion] is a 3-photon process with one pump photon of frequency 2/spl omega/ and two output photons of frequency /spl omega/. An optical vortex is defined as a beam containing a phase singularity, i.e. the change of phase around the center of the beam equals 2 /spl pi/m; where the integer m is the topological charge. Our experimental results using a BaB/sub 2/O/sub 4/ crystal under type II phase matching conditions and numerical simulations suggest that the vortex in the pump beam is transformed into a speckle-like structure, effectively that structure is composed of randomly occurring vortex and Gaussian beams. The two down-converted output signals (signal and idler) appear as rings of orthogonal polarization and, therefore, can be easily distinguished.
Summary form only given. Spontaneous degenerate parametric scattering [down-conversion] is a 3-photon process with one pump photon of frequency 2/spl omega/ and two output photons of frequency /spl omega/. We report the experimental observation of the down conversion with the optical vortex serving as a pump beam. An optical vortex is defined as a beam containing a phase singularity, i.e. The change of phase around the center of the beam equals 2 /spl pi/m; where the integer m is the topological charge. Each photon in the vortex beam possess orbital angular momentum L=m/spl planck/. The law of conservation of angular momentum is expected to be valid in a homogenous linear or nonlinear medium. Topological charge conservation was confirmed in second-harmonic generation experiments, and may be expected to hold under degenerate parametric scattering. However, in the quantum domain, the uncertainly relations, /spl Delta//spl phi//spl Delta/N>2/spl pi/, (where /spl phi/ is the phase and N is the number of photons) provides an opportunity to violate this conservation principle, since the phase of the two down-converted photons is undetermined to within /spl Delta//spl phi/=2/spl pi//2 for every converted fundamental photon. That is, the down converted signal may contain no information about the topological phase of the pump beam. Based on our experiments and numerical calculations of the down-converted signal we suggest that the phase information is indeed lost under down conversion, therefore violating the conservation of topological charge.
An optical vortex (OV) beam is a singular beam with a helical wavefront and a spatially varying phase profile, whereas spontaneous parametric down conversion (SPDC) is a well-established process of generating two-photon state of light. Here, we report photon generation via SPDC using a supercontinuum (SC) beam and a SC OV beam as pump. Specifically, we report generation of an integer charge SC OV beam with a spectral range of 420-480 nm by illuminating a narrow-band spiral phase plate (SPP) designed for 467 nm with a SC light beam of 390-500 nm spectral range. The broad bandwidth of the generated SC OV beam is confirmed using bandpass filters, and cylindrical lens based measurement is adapted to estimate the topological charge of the beam. The mode purity of the generated SC OV beam is determined by analysing its complex field, measured using a Mach–Zehnder interferometer. Besides, generation of fractional OV beam is observed around 400 nm, and the results are compared to the case of using a CW laser operating at 405 nm. In addition, we report generation of photon pairs via SPDC by pumping a β-BBO crystal with the SC beam and the integer charge SC OV beam. The down-conversion of the SC beam with a spectral range of 60 nm is confirmed using bandpass filters in the pump beam and recording the SPDC signal. A similar measurement is also conducted using the generated CW fractional OV beam. This investigation is expected to be useful in quantum imaging applications, where exploring new ways of generating photons and entanglement is of increasing interest.
An optical vortex (OV) beam is a singular beam with a helical wavefront and a spatially varying phase profile, whereas spontaneous parametric down conversion (SPDC) is a well-established process of generating two-photon state of light. Here, we report photon generation via SPDC using a supercontinuum (SC) beam and a SC OV beam as pump. Specifically, we report generation of an integer charge SC OV beam with a spectral range of 420-480 nm by illuminating a narrow-band spiral phase plate (SPP) designed for 467 nm with a SC light beam of 390-500 nm spectral range. The broad bandwidth of the generated SC OV beam is confirmed using bandpass filters, and cylindrical lens based measurement is adapted to estimate the topological charge of the beam. The mode purity of the generated SC OV beam is determined by analysing its complex field, measured using a Mach–Zehnder interferometer. Besides, generation of fractional OV beam is observed around 400 nm, and the results are compared to the case of using a CW laser operating at 405 nm. In addition, we report generation of photon pairs via SPDC by pumping a β-BBO crystal with the SC beam and the integer charge SC OV beam. The down-conversion of the SC beam with a spectral range of 60 nm is confirmed using bandpass filters in the pump beam and recording the SPDC signal. A similar measurement is also conducted using the generated CW fractional OV beam. This investigation is expected to be useful in quantum imaging applications, where exploring new ways of generating photons and entanglement is of increasing interest.
The "perfect" vortex is a new class of optical vortex beam having ring radius independent of its topological charge (order). One of the simplest techniques to generate such beams is the Fourier transformation of the Bessel-Gauss beams. The variation in ring radius of such vortices require Fourier lenses of different focal lengths and or complicated imaging setup. Here we report a novel experimental scheme to generate perfect vortex of any ring radius using a convex lens and an axicon. As a proof of principle, using a lens of focal length f = 200 mm, we have varied the radius of the vortex beam across 0.3-1.18 mm simply by adjusting the separation between the lens and axicon. This is also a simple scheme to measure the apex angle of an axicon with ease. Using such vortices we have studied non-collinear interaction of photons having orbital angular momentum (OAM) in spontaneous parametric down-conversion (SPDC) process and observed that the angular spectrum of the SPDC photons are independent of OAM of the pump photons rather depends on spatial profile of the pump beam. In the presence of spatial walk-off effect in nonlinear crystals, the SPDC photons have asymmetric angular spectrum with reducing asymmetry at increasing vortex radius.
We show that an optical vortex beam, implemented classically, can be transferred to the transverse amplitude of a heralded single photon. For this purpose we have relied on the process of spontaneous parametric downconversion (SPDC) for the generation of signal and idler photon pairs, using a pump in the form of a Bessel-Gauss (BG) beam with orbital angular momentum (specifically, with topological charge l = 1 and l = 2). We have designed our source so that it operates within the short SPDC crystal regime for which, the amplitude and phase of the pump may be transferred to a heralded single photon. In order to verify the vortex nature of our heralded single photon, we have shown that the conditional angular spectrum and the transverse intensity at the single-photon level match similar measurements carried out for the pump. In addition, we have shown that when our heralded single photon is diffracted through a triangular aperture, the far-field single-photon transverse intensity exhibits the expected triangular arrangement of intensity lobes associated with the presence of orbital angular momentum.
Summary form only given. The vast majority of experimental studies of entanglement is based on the use of a polarization entangled photon pair or multiples thereof. Typically, such pairs are produced by the process of spontaneous parametric down conversion in a crystal that exhibits a nonzero second-order nonlinear susceptibility and is cut for type II phase matching. These photons are orthogonally polarized along directions imposed by the symmetry, cut and orientation of the nonlinear crystal. Polarization-entangled pairs are then selected by spatial, and possibly spectral, filtering of the down-converted light. When the crystal is pumped with linearly polarized light a rather intuitive picture arises. If one selects the polarization of the signal photon to be linear, the idler photon will also be linearly polarized in such a way that /spl alpha//sub p/ = -(/spl alpha//sub s/ + /spl alpha//sub i/). Here /spl alpha//sub p,s,i/ refer to the polarization directions of pump, signal and idler, respectively. Their polarization state can be characterized by points on the equator of the Poincare sphere. If, for linear polarized pump, the signal is selected to be elliptically polarized, the idler will also be elliptically polarized. On the Poincare sphere signal and idler have equal latitude (on the same hemisphere), while their longitude is determined by the same relation as above. In all cases signal and idler are maximally entangled.