Emergence of coherent backscattering from sparse and finite disordered media.

Coherent backscattering (CBS) arises from complex interactions of a coherent beam with randomly positioned particles, which has been typically studied in media with large numbers of scatterers and high opacity. We develop a first-principles scattering model for scalar waves to study the CBS cone formation in finite-sized and sparse random media with specific geometries. The current study provides insights into the effects of density, volume size, and other relevant parameters on the angular characteristics of the CBS cone emerging from sparse and bounded random media for various types of illumination, with results consistent with well-known CBS studies which are typically based on samples with much larger number of scatterers and higher opacity. The enhancements are observed in scattering medium with dimensions between 10× and 40× wavelength and the number of particles as few as 370. This work also highlights some of the potentials and limitations of employing the CBS phenomenon to characterize disordered configurations. The method developed here provides a foundation for studies of complex electromagnetic fields beyond simple incident classical beams in randomized geometries, including structured wavefronts in illumination and quantized fields for investigating the effects of the quantum nature of light in multiple scattering, with no further numerical complications.

Focal stack based image forgery localization.

Image security is becoming an increasingly important issue due to advances in deep learning based image manipulations, such as deep image inpainting and deepfakes. There has been considerable work to date on detecting such image manipulations using improved algorithms, with little attention paid to the possible role that hardware advances may have for improving security. We propose to use a focal stack camera as a novel secure imaging device, to the best of our knowledge, that facilitates localizing modified regions in manipulated images. We show that applying convolutional neural network detection methods to focal stack images achieves significantly better detection accuracy compared to single image based forgery detection. This work demonstrates that focal stack images could be used as a novel secure image file format and opens up a new direction for secure imaging.

Strongly enhanced THz generation enabled by a graphene hot-carrier fast lane.

Semiconductor photoconductive switches are useful and versatile emitters of terahertz (THz) radiation with a broad range of applications in THz imaging and time-domain spectroscopy. One fundamental challenge for achieving efficient ultrafast switching, however, is the relatively long carrier lifetime in most common semiconductors. To obtain picosecond ultrafast pulses, especially when coupled with waveguides/transmission lines, semiconductors are typically engineered with high defect density to reduce the carrier lifetimes, which in turn lowers the overall power output of the photoconductive switches. To overcome this fundamental trade-off, here we present a new hybrid photoconductive switch design by engineering a hot-carrier fast lane using graphene on silicon. While photoexcited carriers are generated in the silicon layer, similar to a conventional switch, the hot carriers are transferred to the graphene layer for efficient collection at the contacts. As a result, the graphene-silicon hybrid photoconductive switch emits THz fields with up to 80 times amplitude enhancement compared to its graphene-free counterpart. These results both further the understanding of ultrafast hot carrier transport in such hybrid systems and lay the groundwork toward intrinsically more powerful THz devices based on 2D-3D hybrid heterostructures.

Tunable quantum two-photon interference with reconfigurable metasurfaces using phase-change materials.

The ability of phase-change materials to reversibly and rapidly switch between two stable phases has driven their use in a number of applications such as data storage and optical modulators. Incorporating such materials into metasurfaces enables new approaches to the control of optical fields. In this article we present the design of novel switchable metasurfaces that enable the control of the nonclassical two-photon quantum interference. These structures require no static power consumption, operate at room temperature, and have high switching speed. For the first adaptive metasurface presented in this article, tunable nonclassical two-photon interference from -97.7% (anti-coalescence) to 75.48% (coalescence) is predicted. For the second adaptive geometry, the quantum interference switches from -59.42% (anti-coalescence) to 86.09% (coalescence) upon a thermally driven crystallographic phase transition. The development of compact and rapidly controllable quantum devices is opening up promising paths to brand-new quantum applications as well as the possibility of improving free space quantum logic gates, linear-optics bell experiments, and quantum phase estimation systems.

Neural network based 3D tracking with a graphene transparent focal stack imaging system.

Recent years have seen the rapid growth of new approaches to optical imaging, with an emphasis on extracting three-dimensional (3D) information from what is normally a two-dimensional (2D) image capture. Perhaps most importantly, the rise of computational imaging enables both new physical layouts of optical components and new algorithms to be implemented. This paper concerns the convergence of two advances: the development of a transparent focal stack imaging system using graphene photodetector arrays, and the rapid expansion of the capabilities of machine learning including the development of powerful neural networks. This paper demonstrates 3D tracking of point-like objects with multilayer feedforward neural networks and the extension to tracking positions of multi-point objects. Computer simulations further demonstrate how this optical system can track extended objects in 3D, highlighting the promise of combining nanophotonic devices, new optical system designs, and machine learning for new frontiers in 3D imaging.

Terahertz circular dichroism spectroscopy of biomaterials enabled by kirigami polarization modulators.

Terahertz circular dichroism (TCD) offers multifaceted spectroscopic capabilities for understanding the mesoscale chiral architecture and low-energy vibrations of macromolecules in (bio)materials1-5. However, the lack of dynamic polarization modulators comparable to polarization optics for other parts of the electromagnetic spectrum is impeding the proliferation of TCD spectroscopy6-11. Here we show that tunable optical elements fabricated from patterned plasmonic sheets with periodic kirigami cuts make possible the polarization modulation of terahertz radiation under application of mechanical strain. A herringbone pattern of microscale metal stripes enables a dynamic range of polarization rotation modulation exceeding 80° over thousands of cycles. Following out-of-plane buckling, the plasmonic stripes function as reconfigurable semi-helices of variable pitch aligned along the terahertz propagation direction. Several biomaterials, exemplified by an elytron of the Chrysina gloriosa, revealed distinct TCD fingerprints associated with the helical substructure in the biocomposite. Analogous kirigami modulators will also enable other applications in terahertz optics, such as polarization-based terahertz imaging, line-of-sight telecommunication, information encryption and space exploration.

Dipole-like electrostatic asymmetry of gold nanorods.

The symmetry of metallic nanocolloids, typically envisaged as simple geometrical shapes, is rarely questioned. However, the symmetry considerations are so essential for understanding their electronic structure, optical properties, and biological effects that it is important to reexamine these foundational assumptions for nanocolloids. Gold nanorods (AuNRs) are generally presumed to have nearly perfect geometry of a cylinder and therefore are centrosymmetric. We show that AuNRs, in fact, have a built-in electrostatic potential gradient on their surface and behave as noncentrosymmetric particles. The electrostatic potential gradient of 0.11 to 0.07 V/nm along the long axes of nanorods is observed by off-axis electron holography. Kelvin probe microscopy, secondary electron imaging, energy-filtered transmission electron microscopy, and plasmon mapping reveal that the axial asymmetry is associated with a consistently unequal number of cetyltrimethylammonium bromide moieties capping the two ends of the AuNRs. Electrostatic field maps simulated for the AuNR surface reproduce the holography images. The dipole-like surface potential gradient explains previously puzzling discrepancies in nonlinear optical effects originating from the noncentrosymmetric nature of AuNRs. Similar considerations of symmetry breaking are applicable to other nanoscale structures for which the property-governing symmetry of the organic shell may differ from the apparent symmetry of inorganic core observed in standard electron microscopy images.

Mode control in a multimode fiber through acquiring its transmission matrix from a reference-less optical system.

A simple imaging system, together with complex semidefinite programming, is used to generate the transmission matrix (TM) of a multimode fiber. Once the TM is acquired, we can modulate the phase of the input signal to induce strong mode interference at the fiber output. The optical design does not contain a reference arm, no internal reference signal is used, and no interferometric measurements are required. We use a phase-only spatial light modulator to shape the profile of the propagating modes, and the output intensity patterns are collected. The semidefinite program uses a convex optimization algorithm to generate the TM of the optical system using intensity only measurements. This simple, yet powerful, method can be used to compensate for modal dispersion in multimode fiber communication systems. It also yields great promise for the next generation biomedical imaging, quantum communication, and cryptography.

Controlling Light Transmission Through Highly Scattering Media Using Semi-Definite Programming as a Phase Retrieval Computation Method.

Complex Semi-Definite Programming (SDP) is introduced as a novel approach to phase retrieval enabled control of monochromatic light transmission through highly scattering media. In a simple optical setup, a spatial light modulator is used to generate a random sequence of phase-modulated wavefronts, and the resulting intensity speckle patterns in the transmitted light are acquired on a camera. The SDP algorithm allows computation of the complex transmission matrix of the system from this sequence of intensity-only measurements, without need for a reference beam. Once the transmission matrix is determined, optimal wavefronts are computed that focus the incident beam to any position or sequence of positions on the far side of the scattering medium, without the need for any subsequent measurements or wavefront shaping iterations. The number of measurements required and the degree of enhancement of the intensity at focus is determined by the number of pixels controlled by the spatial light modulator.

Optical Asymmetry and Nonlinear Light Scattering from Colloidal Gold Nanorods.

A systematic study is presented of the intensity-dependent nonlinear light scattering spectra of gold nanorods under resonant excitation of the longitudinal surface plasmon resonance (SPR). The spectra exhibit features due to coherent second and third harmonic generation as well as a broadband feature that has been previously attributed to multiphoton photoluminescence arising primarily from interband optical transitions in the gold. A detailed study of the spectral dependence of the scaling of the scattered light with excitation intensity shows unexpected scaling behavior of the coherent signals, which is quantitatively accounted for by optically induced damping of the SPR mode through a Fermi liquid model of the electronic scattering. The broadband feature is shown to arise not from luminescence, but from scattering of the second-order longitudinal SPR mode with the electron gas, where efficient excitation of the second order mode arises from an optical asymmetry of the nanorod. The electronic-temperature-dependent plasmon damping and the Fermi-Dirac distribution together determine the intensity dependence of the broadband emission, and the structure-dependent absorption spectrum determines the spectral shape through the fluctuation-dissipation theorem. Hence a complete self-consistent picture of both coherent and incoherent light scattering is obtained with a single set of physical parameters.

Microscopic origins of the terahertz carrier relaxation and cooling dynamics in graphene.

The ultrafast dynamics of hot carriers in graphene are key to both understanding of fundamental carrier-carrier interactions and carrier-phonon relaxation processes in two-dimensional materials, and understanding of the physics underlying novel high-speed electronic and optoelectronic devices. Many recent experiments on hot carriers using terahertz spectroscopy and related techniques have interpreted the variety of observed signals within phenomenological frameworks, and sometimes invoke extrinsic effects such as disorder. Here, we present an integrated experimental and theoretical programme, using ultrafast time-resolved terahertz spectroscopy combined with microscopic modelling, to systematically investigate the hot-carrier dynamics in a wide array of graphene samples having varying amounts of disorder and with either high or low doping levels. The theory reproduces the observed dynamics quantitatively without the need to invoke any fitting parameters, phenomenological models or extrinsic effects such as disorder. We demonstrate that the dynamics are dominated by the combined effect of efficient carrier-carrier scattering, which maintains a thermalized carrier distribution, and carrier-optical-phonon scattering, which removes energy from the carrier liquid.

Scattering of entangled two-photon states.

We consider the scattering of entangled two-photon states from collections of small particles. We also study the related Mie problem of scattering from a sphere. In both cases, we calculate the entropy of entanglement and investigate the influence of the entanglement of the incident field on the entanglement of the scattered field.

Realization of mid-infrared graphene hyperbolic metamaterials.

While metal is the most common conducting constituent element in the fabrication of metamaterials, graphene provides another useful building block, that is, a truly two-dimensional conducting sheet whose conductivity can be controlled by doping. Here we report the experimental realization of a multilayer structure of alternating graphene and Al2O3 layers, a structure similar to the metal-dielectric multilayers commonly used in creating visible wavelength hyperbolic metamaterials. Chemical vapour deposited graphene rather than exfoliated or epitaxial graphene is used, because layer transfer methods are easily applied in fabrication. We employ a method of doping to increase the layer conductivity, and our analysis shows that the doped chemical vapour deposited graphene has good optical properties in the mid-infrared range. We therefore design the metamaterial for mid-infrared operation; our characterization with an infrared ellipsometer demonstrates that the metamaterial experiences an optical topological transition from elliptic to hyperbolic dispersion at a wavelength of 4.5 µm.

Electronic cooling via interlayer Coulomb coupling in multilayer epitaxial graphene.

In van der Waals bonded or rotationally disordered multilayer stacks of two-dimensional (2D) materials, the electronic states remain tightly confined within individual 2D layers. As a result, electron-phonon interactions occur primarily within layers and interlayer electrical conductivities are low. In addition, strong covalent in-plane intralayer bonding combined with weak van der Waals interlayer bonding results in weak phonon-mediated thermal coupling between the layers. We demonstrate here, however, that Coulomb interactions between electrons in different layers of multilayer epitaxial graphene provide an important mechanism for interlayer thermal transport, even though all electronic states are strongly confined within individual 2D layers. This effect is manifested in the relaxation dynamics of hot carriers in ultrafast time-resolved terahertz spectroscopy. We develop a theory of interlayer Coulomb coupling containing no free parameters that accounts for the experimentally observed trends in hot-carrier dynamics as temperature and the number of layers is varied.

Ultrafast Lateral Photo-Dember Effect in Graphene Induced by Nonequilibrium Hot Carrier Dynamics.

The photo-Dember effect arises from the asymmetric diffusivity of photoexcited electrons and holes, which creates a transient spatial charge distribution and hence the buildup of a voltage. Conventionally, a strong photo-Dember effect is only observed in semiconductors with a large asymmetry between the electron and hole mobilities, such as in GaAs or InAs, and is considered negligible in graphene due to its electron-hole symmetry. Here, we report the observation of a strong lateral photo-Dember effect induced by nonequilibrium hot carrier dynamics when exciting a graphene-metal interface with a femtosecond laser. Scanning photocurrent measurements reveal the extraction of photoexcited hot carriers is driven by the transient photo-Dember field, and the polarity of the photocurrent is determined by the device's mobility asymmetry. Furthermore, ultrafast pump-probe measurements indicate the magnitude of photocurrent is related to the hot carrier cooling rate. Our simulations also suggest that the lateral photo-Dember effect originates from graphene's 2D nature combined with its unique electrical and optical properties. Taken together, these results not only reveal a new ultrafast photocurrent generation mechanism in graphene but also suggest new types of terahertz sources based on 2D nanomaterials.

Graphene photodetectors with ultra-broadband and high responsivity at room temperature.

The ability to detect light over a broad spectral range is central to several technological applications in imaging, sensing, spectroscopy and communication. Graphene is a promising candidate material for ultra-broadband photodetectors, as its absorption spectrum covers the entire ultraviolet to far-infrared range. However, the responsivity of graphene-based photodetectors has so far been limited to tens of mA W(-1) (refs 5-10) due to the small optical absorption of a monolayer of carbon atoms. Integration of colloidal quantum dots in the light absorption layer can improve the responsivity of graphene photodetectors to ∼ 1 × 10(7) A W(-1) (ref. 11), but the spectral range of photodetection is reduced because light absorption occurs in the quantum dots. Here, we report an ultra-broadband photodetector design based on a graphene double-layer heterostructure. The detector is a phototransistor consisting of a pair of stacked graphene monolayers (top layer, gate; bottom layer, channel) separated by a thin tunnel barrier. Under optical illumination, photoexcited hot carriers generated in the top layer tunnel into the bottom layer, leading to a charge build-up on the gate and a strong photogating effect on the channel conductance. The devices demonstrated room-temperature photodetection from the visible to the mid-infrared range, with mid-infrared responsivity higher than 1 A W(-1), as required by most applications. These results address key challenges for broadband infrared detectors, and are promising for the development of graphene-based hot-carrier optoelectronic applications.

Unconventional terahertz carrier relaxation in graphene oxide: observation of enhanced auger recombination due to defect saturation.

Photoexcited carrier relaxation is a recurring topic in understanding the transient conductivity dynamics of graphene-based devices. For atomically thin graphene oxide (GO), a simple free-carrier Drude response is expected to govern the terahertz (THz) conductivity dynamics--same dynamics observed in conventional CVD-grown graphene. However, to date, no experimental testimony has been provided on the origin of photoinduced conductivity increase in GO. Here, using ultrafast THz spectroscopy, we show that the photoexcited carrier relaxation in GO exhibits a peculiar non-Drude behavior. Unlike graphene, the THz dynamics of GO show percolation behaviors: as the annealing temperature increases, transient THz conductivity rapidly increases and the associated carrier relaxation changes from mono- to biexponential decay. After saturating the recombination decay through defect trapping, a new ultrafast decay channel characterized by multiparticle Auger scattering is observed whose threshold pump fluence is found to be 50 µJ/cm2. The increased conductivity is rapidly suppressed within 1 ps due to the Auger recombination, and non-Drude THz absorptions are subsequently emerged as a result of the defect-trapped high-frequency oscillators.

Optical fiber-based in vivo quantification of growth factor receptors.

BACKGROUND: Growth factor receptors such as epidermal growth factor receptor 1 and human epidermal growth receptor 2 (HER2) are overexpressed in certain cancer cells. Antibodies against these receptors (eg. cetuximab and transtuzumab [Herceptin]) have shown therapeutic value in cancer treatment. The existing methods for the quantification of these receptors in tumors involve immunohistochemistry or DNA quantification, both in extracted tissue samples. The goal of the study was to evaluate whether an optical fiber-based technique can be used to quantify the expression of multiple growth factor receptors simultaneously. METHODS: The authors examined HER2 expression using the monoclonal antibody trastuzumab as a targeting ligand to test their system. They conjugated trastuzumab to 2 different Alexa Fluor dyes with different excitation and emission wavelengths. Two of the dye conjugates were subsequently injected intravenously into mice bearing HER2-expressing subcutaneous tumors. An optical fiber was then inserted into the tumor through a 30-gauge needle, and using a single laser beam as the excitation source, the fluorescence emitted by the 2 conjugates was identified and quantified by 2-photon optical fiber fluorescence. RESULTS: The 2 conjugates bound to the HER2-expressing tumor competitively in a receptor-specific fashion, but they failed to bind to a similar cell tumor that did not express HER2. The concentration of the conjugate present in the tumor as determined by 2-photon optical fiber fluorescence was shown to serve as an index of the HER2 expression levels. CONCLUSIONS: These studies offer a minimally invasive technique for the quantification of tumor receptors simultaneously.

Broadband optical ultrasound sensor with a unique open-cavity structure.

High-resolution ultrasound imaging requires quality sensors with wide bandwidth and high sensitivity, as shown in a wide range of applications, including intravascular imaging of cardiovascular diseases. However, piezoelectric technology, the current dominant approach for hydrophone fabrication, has encountered many technical limitations in the high-frequency range. Using optical techniques for the detection of high-frequency ultrasound signals has attracted much recent attention. One of the most studied approaches is based on a Fabry-Pérot interferometer, consisting of an optical cavity sandwiched between two mirrors. This technique offers promising sensitivity and bandwidth, and a potential alternative to piezoelectric polyvinylidene fluoride (PVDF) hydrophones. We propose an innovative optical ultrasound sensor using only a single mirror in a total-internal-reflection configuration. Besides retaining the advantages of Fabry-Pérot interferometer-based ultrasound sensors, this unique design provides a bandwidth of at least 160 MHz, a potential decrease in fabrication cost, and an increase in signal fidelity.

Microfluidic Droplet Consistency Monitoring and Cell Detection via Laser Excitation.

Microfluidic droplets formed in emulsions are used in a variety of analytical techniques and hold great potential for future scientific and commercial applications. Our experiments merge quantitative quality engineering methods into the microdroplet field. We present a unique microdroplet generation and consistency monitoring system with laser optics excitation and detection. Our setup analyzes each droplet with sub-millisecond signal resolution and single photon accuracy, and is compatible with process control methods. To demonstrate the consistency of microdroplet generation over time, we measure and examine the mean frequency of aqueous plug-shaped droplet (microplug) formation in oil phase, as well as the mean length of plugs, and the interval between consecutive droplets. We also demonstrate the detection of cancer cells encapsulated within aqueous microdroplets in continuous oil phase flow. Two-channel optical monitoring allows for the simultaneous and independent inspection of both microdroplet generation and identification of green fluorescent protein-labelled cancer cells within the droplets. Increased accuracy and consistency are central to many established and developing microfluidic technologies. A systematic, quantitative approach as demonstrated with our experiments may be essential in the development of advanced microfluidic concepts that require exacting reproducibility and would greatly benefit from incorporated automated measurement techniques for process control.