Rotated chirped Bragg gratings (r-CBGs) are an innovative optical component that spatially resolves spectra of optical beams redirecting spectral components in orthogonal planes along direction of beam propagation, and eliminating the need for free-space propagation. This enables ultra-compact spectrometers and pulse transformers. This concept was further extended by multiplexing r-CBGs and creating an X-CBG which enables compact multi-band spectroscopy. We will demonstrate different applications of r-CBGs including the synthesis of space-time wave packets with exceptional spectral resolution. This compact and versatile technology holds great promise for advancing optical systems in various fields, from materials identification to quantum optics.
We introduce a compact spectrometer based on a new optical component – rotated chirped volume Bragg grating (r-CBG) – with a compact footprint capable of spatially resolving the spectrum without the need for subsequent free-space propagation. Unlike conventional chirped Bragg gratings in which both the length and width of the device increase with operation bandwidth, the link between the length and width of r-CBG is severed, leading to a significantly reduced footprint for the same bandwidth. We fabricate and characterize such a device of total volume 25x6x6 mm3 in multiple spectral windows, we study their spectral resolution, and via FROG measurements we confirm that a pair of cascaded r-CBGs can resolve and combine the spectrum of a 100-fs pulse.
We introduce a new optical component – rotated chirped volume Bragg grating (r-CBG) – with a compact footprint capable of spatially resolving the spectrum without the need for subsequent free-space propagation. Unlike conventional chirped Bragg gratings in which both the length and width of the device increase with operation bandwidth, the link between the length and width of r-CBG is severed, leading to a significantly reduced footprint for the same bandwidth. We fabricate and characterize such a device of total volume 25x6x6 mm3 in multiple spectral windows, we study their spectral resolution, and via FROG measurements we confirm that a pair of cascaded r-CBGs can resolve and combine the spectrum of a 100-fs pulse.
Space-time wave packets (STWPs) are pulsed fields with non-separable spatio-temporal structure that yields exotic and useful behavior. To date, STWPs have been synthesized using bulky free-space optical systems that require precise alignment. Here, we describe a compact system based on an alternative strategy for spatially resolving the spectrum, which makes use of a volume chirped Bragg grating with Bragg structure rotated by 45degree with respect to device facets. Consequently, the field exits spectrally-resolved from a facet orthogonal to the entrance. We synthesize STWPs using a pair of such devices occupying a volume of 25×25×8mm3, which is orders-of-magnitude smaller than previously.
Space-time wave packets can travel in linear media invariantly, even in presence of chromatic dispersion. I review the current status of theoretical and experimental work on this ‘dispersion cancellation’ phenomenon, which is made possible by endowing the optical field with non-differentiable angular dispersion. Consequently, space-time wave packets can travel with no pulse broadening in dispersive media independently of the magnitude, sign, or order of the dispersion. These results may have useful consequences in phase-matching of nonlinear optical effects.
Space-time (ST) wave packets are pulsed beams endowed with tight spatio-temporal spectral correlations, whereby each spatial frequency is associated with a single wavelength resulting in propagation invariance at an arbitrary group velocity. Previously, our group introduced an experimental methodology to synthesize 2D ST wave packets, i.e. light sheets, based on 2D pulse shaper. Here, we demonstrate a new experimental method to synthesize ST wave packets localized in all dimensions, including 3D space and 1D time. We show that 3D ST wave packets propagate longer distances without spreading compared with traditional Bessel beams, and propagate at subluminal and superluminal
Space-time (ST) wave packets are pulsed beams endowed with tight spatio-temporal spectral correlations, whereby each spatial frequency is associated with a single wavelength resulting in propagation invariance at an arbitrary group velocity. Previously, our group introduced an experimental methodology to synthesize 2D ST wave packets, i.e. light sheets, based on 2D pulse shaper. Here, we demonstrate a new experimental method to synthesize ST wave packets localized in all dimensions, including 3D space and 1D time. We show that 3D ST wave packets propagate longer distances without spreading compared with traditional Bessel beams, and propagate at subluminal and superluminal group velocities.
We investigate space-time surface plasmon polariton (ST-SPP) wave packet, a conceptual correspondence of a surface-electromagnetic wave to the space-time wave (ST wave) that is excited on a metal surface through a light-SPP coupling at a nano-scaled ridge. In a framework of the finite-difference time-domain (FDTD) method, a pulsed excitation light was constructed by using a couple of hundred plane waves with different frequencies whose intensity and incident angle were determined to satisfy the conditions for generating ST wave according to the dispersion relation of SPP on the metal surface. The ST-SPP WP launched from the nano-ridge exhibited propagation invariance and tunability of the group velocity.
Refraction of an optical beam is a spatial phenomenon involving changes in the wave momentum while conserving energy. Although Snell’s law applies to monochromatic plane waves, its consequences are generally extended to a pulsed beam with an implicit assumption of separability of spatial and temporal degrees of freedom. Certain expectations are built into the law of refraction – that the speed of the transmitted light pulse is solely determined by the refractive index, hence independent of the angle of incidence to the interface. Here we show that introducing spatio-temporal correlation into a pulsed beam unveils a remarkable refractive phenomenon – tunable group velocity by varying angle of incidence. We present theoretical formulation as well as the experimental demonstration of this remarkable behavior by making use of 'space-time' wave-packets – propagation-invariant pulsed beam endowed with tight correlations between spatial and temporal frequencies.
‘Space-time’ (ST) wave packets are propagation-invariant pulsed optical beams whose group velocity can take on arbitrary values in free space. Such unique behavior is a consequence of tight spatio-temporal spectral correlations introduced into the field, which in turn results in a correlation between the temporal bandwidth and numerical aperture. Utilizing large temporal bandwidths or creating ultraslow group velocities both require excessively large numerical apertures. We introduce a methodology for spectral recycling that circumvents this obstacle, and confirm experimentally that the unique characteristics of the ST wave packet are retained. Furthermore, we synthesize a ST wave packet having a group velocity of c/14.3 at a low numerical aperture by exploiting spectral recycling.
Space-time (ST) wave packets are propagation-invariant pulsed beam endowed with a tight correlation between the underlying spatial and temporal frequencies. We present a theoretical formulation and experimental demonstration of sub-millimeter incoherent ST light sheets traversing 110 m without significant diffraction. We synthesize ST fields of beam width ~ 0.7 mm from a superluminescent diode with a bandwidth of ~20 nm centered at 840 nm and observe propagation-invariant behavior across a distance of over 100-m distance in free-space. Finally, we study the effect of the aperture on the propagation distance and far-field beam size of ST light sheets.
We introduce a new effect that we call the ‘space-time’ (ST) Talbot effect in which an optical field lattice that is periodic in both space and time undergoes periodic axial revivals after propagation in free space. Although the purely spatial and purely Talbot effects have been observed separately in optics, they have not been realized simultaneously due to the disparity between the spatial and temporal Talbot lengths. Indeed, the temporal Talbot effect has been observed to date in only single-mode fibers. By identifying a new pulsed beam configuration that we call a V-wave whose diffraction and dispersion lengths are intrinsically equal, we obtain a periodic spatio-temporal lattice by discretizing the spatio-temporal spectrum of a V-wave, and consequently observe the first example of self-imaging in space and time using an optical field.
Surface plasmon polaritons have been employed in a wide spectrum of sensing applications. However, as a surface plasmon polariton propagates along the surface of a metal-dielectric interface it rapidly diffracts and is absorbed reducing the sensitivity of a measurement. Attempts to prevent diffraction have included waveguides of many types but these require precise fabrication to mitigate dissipative losses. We propose using space-time surface plasmon polaritons for sensing. Our calculations show that proper selection of spatial frequency and temporal frequency correlations will produce a wave packet that propagates without diffraction or dispersion. This is possible without any waveguides or surface patterning.
Space-time wave packets -- a new class of pulsed optical beams in which the spatial and temporal degrees of freedom are inextricably intertwined -- provide a new platform for the study of structured light. Space-time wave packets can be designed to be propagation-invariant (diffraction-free and dispersion-free) and travel at arbitrary group velocities in free space and transparent dielectrics. I will discuss recent results obtained with space-time wave packets including the observation of anomalous refraction, tunable axial acceleration or deceleration, and novel Talbot effects observed in space-time rather than in space or time separately.
Following Fermat's principle, it is intuitively assumed that a pulse always slows down while traveling from low index to high index materials. Here we show that structuring a wave-packet both spatially and temporally challenges this well-established intuition, unveiling anomalous refractive phenomena – group velocity of a pulse increases while traversing denser media. We present a theoretical formulation as well as an experimental demonstration of this remarkable behavior by making use of 'space-time' wave-packets – propagation-invariant pulsed beam endowed with tight correlations between spatial and temporal degrees-of-freedom. We observe the boost of group velocity at the interface of various optical materials (BK7, MgF2, and Sapphire) with air.
The fundamental limits set by diffraction in optics have motivated the investigation of ‘diffraction-free’ beams. Examples of monochromatic diffraction-free beams include those whose profiles conform to Bessel, Mathieu, Weber, and Airy functions. For pulsed beams (wave packets or optical bullets), different functional forms of the field in space and time have been found to be propagation-invariant. Here we describe experiments on the synthesis and characterization of a unique class of pulsed optical beams called ‘space-time’ (ST) wave packets that are diffraction-free and dispersion-free in free space. The most salient features of such wave packets that determines their characteristics is the tight spatio-temporal spectral correlations underlying their construction. We identify 10 unique classes of ST wave packets according to the magnitude and sign of the group velocity, and whether the ST wave packet is ‘baseband’ or ‘sideband’; that is, whether low spatial frequencies are allowed in the wave-packet construction or are forbidden. Such wave packets can be propagation invariant even for extended distances. Furthermore, sculpting the spatio-temporal spectrum of the wave packet allows control over its propagation characteristics in optical materials, namely over the group velocity and group velocity dispersion, whether the material itself is dispersive or non-dispersive.
Beam confinement or “no free-space optics” via fiber transmission can achieve improved reliability, lower cost, and reduced component count for active sensing systems. For midinfrared delivery, mechanically robust chalcogenide (arsenic sulfide) single-mode fibers are of interest. A 12-μm core diameter fiber is shown to transport >10 W at 2053 nm, and a 25-μm core diameter fiber enables single-mode beam transport from a 4550-nm quantum cascade laser. As midinfrared sources continue to increase their output power capabilities, chalcogenide fibers will eventually be limited in their power-handling capacity due to optical nonlinearities or thermal failure. These limitations are discussed and analyzed in the context of single-mode chalcogenide fibers in order to provide a framework for power transmission limitations in various operating regimes.
Diffractive spreading is a fundamental property of light and inversely proportional to the beam waist of a propagating beam. For instance, a Gaussian beam at a wavelength of 800 nm focused into 7 micrometers full-width at half maximum (FWHM) at its beam waist would only have 137 micrometers Rayleigh range—the propagation distance from the waist for a beam to double its cross section. Here, we demonstrate a diffraction-free space-time light sheet (one-dimensional pulsed beam) with 7 micrometers FWHM propagating in free space for 25 mm while preserving its spatial features. By introducing a highly correlated spatio-temporal spectrum via a two-dimensional pulse shaper (consisting of a phase-only reflective spatial light modulator, a grating and a few cylindrical lenses), we generate various light sheets with arbitrary beam profiles at the pulse center and diffraction-free propagation distance of approximately 200 Rayleigh range that corresponds to its beam waist size at the pulse center. Arbitrary light-sheet profiles also include hollow sheets (bottle beams) and even Airy light sheets that are only transversely accelerating in the local time frame of the pulse and acceleration-free as a function of propagation distance. Moreover, we obtain the spatio-temporal beam profiles of the light sheets by experimentally measuring the complex spectra and performing computational two-dimensional Fourier transformations. Light sheets with arbitrary beam profiles and controllable spectrum properties may be instrumental in super-resolution light-sheet microscopy for 3D bio-imaging, nonlinear and multimodality spectroscopy, standoff detection of chemicals, and one-dimensional plasma and filamentation generation.
Defense sensing systems must be both productive and robust to accomplish their mission. Active infrared sensing devices consist of many components such as the active medium, mirrors, beam-splitters, modulators, gratings, detectors, etc. Each of these components is subject to damage by the laser beam itself or environmental factors. Misalignment of these components due to vibration and temperatures changes can also reduce performance. The result is a complex and expensive system subject to multiple points of degradation or complete failure. However, beam confinement or “no free-space optics” via fiber transmission and even component assembly within the fiber itself can achieve reliability and low cost for sensing systems with reduced component count and less susceptibility to misalignment.
We present measurements of high-power infrared laser beam transmission in chalcogenide fibers. The fiber compositions were As39S61 for the core and As38.5S61:5 for the cladding, resulting in a numerical aperture of 0.2. A polyetherimide jacket provided structural support. Multiwatt CW transmission was demonstrated in near single-mode 12 micron core fiber. Efficient coupling of quantum cascade lasing into anti-reflection coated chalcogenide fiber was also demonstrated. Efficient beam transport without damage to the fiber required careful coupling only into core modes. Beams with M2 ≥ 1.4 and powers higher than 1 W produced damage at the fiber entrance face. This was most likely due to heating of the highly absorptive polymer jacket by power not coupled into core modes. We will discuss current power limitations of chalcogenide fiber and schemes for significantly increasing power handling capabilities.
Coherent light traversing disordered media usually attains a random field both in amplitude and phase with independent Gaussian statistics and results in thermal light (associated with Bose-Einstein photon-number statistics) upon ensemble averaging. This is expected according to the central limit theorem, which dictates the addition of a large number of independent random variables leads to a normal (Gaussian) distribution. Here, we show that certain network topologies that light travels within preclude the central limit theorem and result in non-Gaussian statistics. We realize such networks in the form of evanescently-coupled waveguide arrays (photonic lattices) and obtain the photon statistics at the output by time gating and averaging over multiple realizations of disordered photonic lattices. The effect of lattice topology, however, only exists when the photonic lattice is endowed with chiral-symmetric eigenmode pairs a disorder-immune symmetry where the eigenmodes appear in pairs with oppositely signed eigenvalues and the coherent input field satisfies certain conditions. We specifically examine one-dimensional arrays of randomly coupled identical waveguides (off-diagonal disorder) arranged on linear and ring topologies. The emerging field exhibits super-thermal statistics (associated with modified Bose-Einstein photon-number statistics) only for ring lattices with even parity and linear lattices (independent of its parity), whereas input coherent fields traversing ring lattices with odd parity attain sub-thermal statistics. By controlling the relative phase of a coherent input field exciting two neighboring lattice sites, we also demonstrate a deterministic tuning of photon-number statistics, namely photon bunching, while maintaining the mean photon number fixed.
Due to the intrinsic absorption edge in silica near 2.4 μm, more exotic materials are required to transmit laser power in the IR such as fluoride or chalcogenide glasses (ChGs). In particular, ChG fibers offer broad IR transmission with low losses < 1 dB/m. Here, we report on the performance of in-house drawn multi-material chalcogenide fibers at four different infrared wavelengths: 2053 nm, 2520 nm and 4550 nm. Polymer clad ChG fibers were drawn with 12.3 μm and 25 μm core diameters. Testing at 2053 nm was accomplished using a > 15 W, CW Tm:fiber laser. Power handling up to 10.2 W with single mode beam quality has been demonstrated, limited only by the available Tm:fiber output power. Anti-reflective coatings were successfully deposited on the ChG fiber facets, allowing up to 90.6% transmission with 12.2 MW/cm2 intensity on the facet. Single mode guidance at 4550 nm was also demonstrated using a quantum cascade laser (QCL). A custom optical system was constructed to efficiently couple the 0.8 NA QCL radiation into the 0.2 NA ChG fiber, allowing for a maximum of 78% overlap between the QCL radiation and fundamental mode of the fiber. With an AR-coated, 25 μm core diameter fiber, >50 mW transmission was demonstrated with > 87% transmission. Finally, we present results on fiber coupling from a free space Cr:ZnSe resonator at 2520 nm.
Infrared (IR) fibers that transmit radiation at wavelengths from ~ 2 μm up to ~ 25 μm, a spectrum that extends across both the mid-IR (MIR) and far-IR (FIR), has gained extensive attention concomitant with the recent availability of MIR semiconductors sources and detectors. Chalcogenide glasses (ChGs) are a leading candidate for IR fibers by virtue of their wide optical transmission windows and high nonlinearity in the IR region. After extensive studies since the 1960s, the development and applications of ChG IR fibers are primarily hindered by their unfavorable mechanical properties. Here, we summarize our recent advances in low-cost, robust multimaterial ChG IR fibers with broad transmission windows and low optical losses, based on our multimaterial fiber preforms produced by several fabrication methodologies. Hundreds of meters of fibers are thermally drawn in an ambient atmosphere with desired step-index structure from a macroscopic multimaterial preform that contains few grams of ChG. These simple and efficient processes overcome many of the traditional obstacles, and therefore enable rapid production in an industrial setting.
Recent progress in combing multiple materials with distinct optical, electronic, and thermomechanical properties monolithically in a kilometer-long fiber drawn from a preform offers unique multifunctionality at a low cost. Here, we summarize the application of this nascent concept of multimaterial fibers to infrared fiber optics.
The utilization of infrared chalcogenide glass (ChG) fibers has long been hampered by the unfavorable mechanical characteristics typical of these glasses. Furthermore, the usual pathways to producing such fibers necessitate large-scale synthesis of high-purity glass, which represents a challenge in an academic environment, and thus presents an obstacle to the transfer of research results from academia to industry. Here we present our recent progress on multimaterial coextrusion technology that allows for high-efficiency disc-to-fiber manufacturing. A one-step extrusion from two glass discs (10-mm-diameter and 3-mm-thick) and a thermoplastic disc results in a robust step-index preform that is thermally drawn in an ambient atmosphere into continuous lengths of fiber with core diameters on the order of tens of micrometers. These results offer an alternative methodology that overcomes many of the traditional obstacles while potentially reducing the production cost.
The development of quantum cascade lasers that span mid-infrared wavelengths necessitate developing new infrared fibers capable of transmitting light in the 3 ‒ 12 micron range. The main material candidates for producing infrared fibers that cover this spectral region are polycrystalline silver halides and glassy tellurium-based chalcogenide glasses. The latter are more chemically stable, and thus represent a superior choice for infrared fibers. We adapt a fiber fabrication methodology that we recently developed for other chalcogenide glasses to tellurium-based chalcogenides. We introduce a novel infrared optical fiber with tellurium-based chalcogenide core and cladding, which is provided with a built-in polymer jacket. We prepare purified Ge-As-Se-Te glasses that are used in extruding a preform. This preform is then thermally drawn continuously in an ambient environment into extended robust infrared fibers that transmit light in the 3 ‒ 12 micron spectral range.
Luminescent Solar Concentrators (LSC’s) are a promising alternative for reducing the cost of solar power. Exploiting the advantages of optical fiber production, we present here a Fiber LSC (FLSC) in which the waveguide is a polymer optical fiber. We present modeling, fabrication and optical characterization of FLSC (conversion efficiency ~ 5.7%) with a hybrid fiber structure for two-stage concentration of incident light. Directional guiding in fiber allows for at least twofold geometrical gain improvement compared to conventional LSC. It also alleviates the size limitation of conventional LSC’s in one direction. Light-weight, flexible solar sheets assembled from such fibers can provide a means for mobile energy needs.
There exists a fundamental dimensional mismatch between the Hong-Ou-Mandel (HOM) interferometer and two-photon states: while the latter are represented using two temporal (or spectral) dimensions, the HOM interferometer allows access to only one temporal dimension owing to its single delay element. We introduce a linear two-photon interferometer containing two independent delays spanning the two-photon state. By unlocking the fixed phase relationship between the interfering two-photon probability amplitudes in a HOM interferometer, one of these probability amplitudes now serves as a delay-free two-photon reference against which the other beats, thereby resolving ambiguities in two-photon state identification typical of HOM interferometry. We discuss the operation of this phase-unlocked HOM on a variety of input states focusing on instances where this new interferometer outperforms a traditional HOM interferometer: frequency-correlated states and states produced by a pulse doublet pump. Additionally, this interferometer affords the opportunity to synchronize two-photon states in a manner analogous to an HOM interferometer; moreover, it extends that capability to the aforementioned class of states.
We discuss an approach to encoding qubits in the spatial distribution of single photons and entangled-photon pairs
based on utilizing the spatial parity of the photon wave fronts. Using simple optical components, that do not require
nonlinearities or moving parts, we discuss implementing rotations in the Hilbert space of spatial parity and
measurement of the state of parity. Using entangled photon pairs, we use spatial parity to demonstrate quantum
nonlocality by violating a Bell inequality using an EPR state. We also discuss generalizations of this scheme that
may allow for a larger number of qubits to be encoded per photon.
We describe an optical implementation of a CNOT gate in which the control qubit is the polarization of a single
photon and the target qubit is the spatial parity of the same photon. The gate is implemented with a polarizationsensitive
spatial light modulator. We characterize the operation of the gate using quantum process tomography and
the spatial parity is analyzed with a modified Mach-Zehnder interferometer. We also demonstrate the CNOT-gate
operation with arbitrary rotation of the target qubit and discuss the possibility of implementing multi-qubit CNOT
gates using the same approach.
We describe a novel fabrication method for producing polymer, glass, and metal micro- and nano-particles whose
diameters range from 200 microns to under 50 nanometers. This method relies on the Rayleigh capillary instability
in a multi-material fiber. The fiber core is made of the target material and has size close to the desired particle
diameter embedded in a sacrificial polymer matrix. The fiber temperature is elevated to reduce the core viscosity
and the Rayleigh instability results in the breakup of the core into a periodic string of spherical particles.
One dimensional nanostructure such as nanowires is typically fabricated by the wafer-based approach. Here we
report nanowires are fabricated by thermal drawing of fiber. A thin viscous semiconductor film internal to the
fiber undergoes filamentation driven by a fluid instability while retaining longitudinal structural integrity.
Arrays of centimeter-long crystalline nanowires by post-drawing crystallization process are electrically
contacted and yield a two-order-of-magnitude change in conductivity between dark and illuminated states.
These results hold promise for the nanowire-detector arrays that may be integrated with large-area electronics.
The process of optical imaging and the use of a glass lens have been hitherto inseparable since it is the lens that
is responsible for mapping incoming rays to form an image. While performing this critical role, the lens, by
virtue of its geometry and materials composition, presents constraints on the size, weight, angular field of view,
and environmental stability of an optical imaging system as a whole. Here, a new approach to optical imaging
is presented. Tough polymeric light-sensing fibers are suspended on a frame to form large-scale, low-density,
two- and three-dimensional photonic meshgrids. While a single grid can indeed locate a point of illumination, it
is the stacking of a multiplicity of such grids, afforded by their essential transparency, which allows for the
detection of the direction of illumination with a wide angular field of view. A surface-spanning-arrangement of
such fibers is used to extract an arbitrary optical intensity distribution in a plane using a tomographic algorithm.
Lensless imaging is achieved by a volumetric fiber assembly that extracts both the phase and intensity
distributions of an incoming electromagnetic field, enabling one to readily determine the object from which the
field originally emanated.
The process of optical imaging and the use of a glass lens have been hitherto inseparable since it is the lens that is
responsible for mapping incoming rays to form an image. While performing this critical role, the lens, by virtue of its
geometry and materials composition, presents constraints on the size, weight, angular field of view, and environmental
stability of an optical imaging system as a whole. Here, a new approach to optical imaging is presented. Tough
polymeric light-sensing fibers are suspended on a frame to form large-scale, low-density, two- and three-dimensional
photonic meshgrids. While a single grid can indeed locate a point of illumination, it is the stacking of a multiplicity of
such grids, afforded by their essential transparency, which allows for the detection of the direction of illumination with a
wide angular field of view. A surface-spanning-arrangement of such fibers is used to extract an arbitrary optical intensity
distribution in a plane using a tomographic algorithm. Lensless imaging is achieved by a volumetric fiber assembly that
extracts both the phase and intensity distributions of an incoming electromagnetic field, enabling one to readily
determine the object from which the field originally emanated.
We present a new quantum cryptography implementation that uses
frequency-correlated photon pairs to combine one-way operation
with an autocompensating feature that has hitherto only been
available in implementations that require the signal to make a
round trip between the users. Furthermore, we describe a new
scheme for creating frequency-correlated photon pairs
(auto-phase-matched spontaneous parametric down-conversion). The
new scheme offers several advantages over previous schemes,
including the ability to generate frequency-correlated photon
pairs regardless of the dispersion characteristics of the
system.
We present a novel quantum interferometric technique to perform ellipsometric measurements. Classical ellipsometric measurements are limited in their accuracy by virtue of the need for an absolutely calibrated source and detector. Mitigating this limitation requires the use of a well-characterized reference sample. Our technique relies on the use of a non-classical optical source, namely polarization-entangled twin photons generated by spontaneous parametric down-conversion from a nonlinear crystal, in conjunction with a coincidence-detection scheme. We have demonstrated that entangled-photon ellipsometry eliminates the necessity of constructing an interferometer altogether and is thereby self-referencing. The underlying physics that leads to this remarkable result is the presence of fourth-order (coincidence) quantum interference of the photon pairs in conjuction with polarization entanglement.
Entangled-photon beams, generated for example by spontaneous parametric down-conversion from a nonlinear crystal, may be used to implement holography based on quantum interferometry. One beam is split into a probe component that illuminates the object and another serving as a reference, and is detected with no spatial resolution. The other beam is measured with full spatial resolution. The rate of two-photon coincidence contains full holographic information on the object. Since the beams are not monochromatic, this is a form of higher-order white light holography.
The high accuracy required in traditional ellipsometric measurements necessitates the absolute calibration of both the source and the detector. We demonstrate that these requirements can be circumvented by using a non-classical source of light, namely, a twin-photon polarization-entangled source that produces type-II spontaneous parametric down-conversion, in conjunction with a novel polarization interferometer and coincidence-counting detection scheme. Our scheme exhibits two features that obviate the requirements of a calibrated source and detector. The first is the twin-photon nature of the source; we are guaranteed, on the detection of a photon in one of the arms of the setup, that its twin will be in the other, effectively serving as calibration of the source. The second is that the polarization entanglement of the source serves as an interferometer, thereby alleviating the need for calibrating the detector. The net result is that absolute ellipsometric data from a sample may be obtained. We present preliminary experimental results showing how the technique operates.
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