Sub-micron-scale, micron-scale and greater than micron-scale, crack-free and regularly-shaped structures of high-contrast refractive index are provided in transparent storage media by controllably focusing ultrashort laser pulses in the bulk of virtually any transparent medium respectively during operation in a "low energy," a "high energy", and a "third" operating regime. In any operating regime, the crack-free and regularly-shaped structures of high-contrast refractive index may be controllably patterned in 2-D or 3-D so as to permanently store both digital and non-digital information in the bulk of the transparent storage medium. For digital-type information, greater than one (1) Terabit, and up to one hundred (100) Terabit, digital information storage capacity in a CD-ROM sized disc is provided. Virtually any non-digital information may be permanently stored therewithin, such as corporate logos, alphanumeric characters, security codes, and artistic images, or diffraction gratings, diffractive optical elements or other optical structures. Information permanently stored in 2-D or 3-D in the bulk of any transparent medium is read by the unaided eye, and by optical microscopy (scattered and transmitted light modes), phase contrast microscopy, laser DIC microscopy and confocal microscopy in dependance on the type of the information and on the operating regime. Information may be written or read in series or in parallel.
In one aspect, the present invention provides a silicon photodetector having a surface layer that is doped with sulfur inclusions with an average concentration in a range of about 0.5 atom percent to about 1.5 atom percent. The surface layer forms a diode junction with an underlying portion of the substrate. A plurality of electrical contacts allow application of a reverse bias voltage to the junction in order to facilitate generation of an electrical signal, e.g., a photocurrent, in response to irradiation of the surface layer. The photodetector exhibits a responsivity greater than about 1 A/W for incident wavelengths in a range of about 250 nm to about 1050 nm, and a responsivity greater than about 0.1 A/W for longer wavelengths, e.g., up to about 3.5 microns.
Complete absorption of electromagnetic waves is paramount in today’s applications, ranging from photovoltaics to cross-talk prevention into sensitive devices. In this context, we use a genetic algorithm (GA) strategy to optimize absorption properties of periodic arrays of truncated square-based pyramids made of alternating stacks of metal/dielectric layers. We target ultra-broadband quasi-perfect absorption of normally incident electromagnetic radiations in the visible and near-infrared ranges (wavelength comprised between 420 and 1600 nm). We compare the results one can obtain by considering one, two or three stacks of either Ni, Ti, Al, Cr, Ag, Cu, Au or W for the metal, and poly(methyl methacrylate) (PMMA) for the dielectric. More than 1017 configurations of geometrical parameters are explored and reduced to a few optimal ones. This extensive study shows that Ni/PMMA, Ti/PMMA, Cr/PMMA and W/PMMA provide high-quality solutions with an integrated absorptance higher than 99% over the considered wavelength range, when considering realistic implementation of these ultra-broadband perfect electromagnetic absorbers. Robustness of optimal solutions with respect to geometrical parameters is investigated and local absorption maps are provided. Moreover, we confirm that these optimal solutions maintain quasi-perfect broadband absorption properties over a broad angular range when changing the inclination of the incident radiation. The study also reveals that noble metals (Au, Ag, Cu) do not provide the highest performance for the present application.
Metamaterials with a Dirac-like cone dispersion at the center of the Brillouin zone behave like an isotropic and impedance-matched zero refractive index material at the Dirac-point frequency. Such metamaterials can be realized in the form of either bulk metamaterials with efficient coupling to free-space light or on-chip metamaterials that are efficiently coupled to integrated photonic circuits. These materials enable the interactions of a spatially uniform electromagnetic mode with matter over a large area in arbitrary shapes. This unique optical property paves the way for many applications, including arbitrarily shaped high-transmission waveguides, nonlinear enhancement, and phase mismatch-free nonlinear signal generation, and collective emission of many emitters. This review summarizes the Dirac-like cone-based zero-index metamaterials’ fundamental physics, design, experimental realizations, and potential applications.
Materials with a zero refractive index support electromagnetic modes that exhibit stationary phase profiles. While such materials have been realized across the visible and near-infrared spectral range, radiative and dissipative optical losses have hindered their development. We reduce losses in zero-index, on-chip photonic crystals by introducing high-Q resonances via resonance-trapped and symmetry-protected states. Using these approaches, we experimentally obtain quality factors of 2.6 × 103 and 7.8 × 103 at near-infrared wavelengths, corresponding to an order-of-magnitude reduction in propagation loss over previous designs. Our work presents a viable approach to fabricate zero-index on-chip nanophotonic devices with low-loss.
There is a great need in the biomedical field to efficiently, and cost-effectively, deliver membrane-impermeable molecules into the cellular cytoplasm. However, the cell membrane is a selectively permeable barrier, and large molecules often cannot pass through the phospholipid bilayer. We show that nanosecond laser-activated polymer surfaces of commercial polyvinyl tape and black polystyrene Petri dishes can transiently permeabilize cells for high-throughput, diverse cargo delivery of sizes of up to 150 kDa. The polymer surfaces are biocompatible and support normal cell growth of adherent cells. We determine the optimal irradiation conditions for poration, influx of fluorescent molecules into the cell, and post-treatment viability of the cells. The simple and low-cost substrates we use have no thin-metal structures, do not require cleanroom fabrication, and provide spatial selectivity and scalability for biomedical applications.
ABSTRACT: Spontaneous emission, stimulated emission and absorption are the three fundamental radiative processes describing light−matter interactions. Here, we theoretically study the behavior of these fundamental processes inside an unbounded medium exhibiting a vanishingly small refractive index, i.e., a near-zero-index (NZI) host medium. We present a generalized framework to study these processes and find that the spatial dimension of the NZI medium has profound effects on the nature of the fundamental radiative processes. Our formalism highlights the role of the number of available optical modes as well as the ability of an emitter to couple to these modes as a function of the dimension and the class of NZI media. We demonstrate that the fundamental radiative processes are inhibited in 3D homogeneous lossless zero-index materials but may be strongly enhanced in a zero-index medium of reduced dimensionality. Our findings have implications in thermal, nonlinear, and quantum optics as well as in designing quantum metamaterials at optical or microwave frequencies.
KEYWORDS: near-zero index materials, spontaneous emission, stimulated emission, absorption, fundamental radiative processes
The Fabry-Perot cavity in a miniature pulsed Fourier transform millimeter-wave spectrometer operating between 94 and 104 GHz is characterized in detail. The new device measures gas or volatile composition in situ and has a nominal volume of 12 cm3, which is 200 times smaller than cavities operating at comparable frequencies in laboratory gas spectrometers. Scans of mode amplitude are presented as a function of mirror spacing and transmitter frequency. Primary (TEM00) and secondary (TEM10) modes are both observed and are matched to an eigenmode calculation. The modes are well-behaved and have quality factors in the range of 1000–6000, which is a desirable compromise between field strength and mode width. Measurements of pulse bandwidth versus duration agree with time-bandwidth product predictions. Measurements of rotational transitions in N2O and CH3OH are plotted at various pressures and collisional broadening is resolved at mTorr pressures. Through these gas detections, we demonstrate that it is possible to significantly reduce the size of cavity spectrometers for in situ deployment. The new device opens new possibilities for molecular sensing in pollution monitoring, planetary science, and other fields.
The delivery of biomolecules into cells relies on porating the plasma membrane to allow exterior molecules to enter the cell via diffusion. Various established delivery methods, including electroporation and viral techniques, come with drawbacks such as low viability or immunotoxicity,
respectively. An optics-based delivery method that uses laser pulses to excite plasmonic titanium nitride (TiN) micropyramids presents an opportunity to overcome these shortcomings. This laser excitation generates localized nano-scale heating effects and bubbles, which produce transient pores in the cell membrane for payload entry. TiN is a promising plasmonic material due to its high hardness and thermal stability. In this study, two designs of TiN micropyramid arrays are constructed and tested. These designs include inverted and upright pyramid structures, each coated with a 50-nm layer of TiN. Simulation software shows that the inverted and upright designs reach temperatures of 875 °C and
307 °C, respectively, upon laser irradiation. Collectively, experimental results show that these reusable designs achieve maximum cell poration efficiency greater than 80% and viability greater than 90% when delivering calcein dye to target cells. Overall, we demonstrate that TiN microstructures are strong candidates for future use in biomedical devices for intracellular delivery and regenerative medicine.
This paper analyzes pre-post matched gains in the epistemological views of science students taking the introductory physics course at Beijing Normal University (BNU) in China. In this study we examined the attitudes and beliefs of science majors (n = 441) in four classes, one taught using traditional (lecture) teaching methods, and the other three taught with Peer Instruction (PI). In two of the PI classes, student peer groups were constantly changing throughout the semester, while in the other PI class student groups remained fixed for the duration of the semester. The results of the pre- and posttest using the Colorado Learning Attitudes about Science Survey showed that students in traditional lecture settings became significantly more novice-like in their beliefs about physics and learning physics over the course of a semester, a result consistent with what was reported in the literature. However, all three of the classes taught using the PI method improved student attitudes and beliefs about physics and learning physics. In the PI class with fixed peer groups, students exhibited a greater positive shift in attitudes and beliefs than in the other PI class with changing peer groups. The study also looked at gender differences in student learning attitudes. Gender results revealed that female science majors in the PI classes achieved a greater positive shift in attitudes and beliefs after instruction than did male students.
We discuss student participation in an online social annotation forum over two semesters of a flipped, introductory physics course at Harvard University. We find that students who engage in high-level discussion online, especially by providing answers to their peers’ questions, make more gains in conceptual understanding than students who do not. This is true regardless of students’ physics background. We find that we can steer online interaction towards more productive and engaging discussion by seeding the discussion and managing the size of the sections. Seeded sections produce higher quality annotations and a greater proportion of generative threads than unseeded sections. Larger sections produce longer threads; however, beyond a certain section size, the quality of the discussion decreases.
We present an on-chip Dirac-cone metamaterial with an impedance- matched zero refractive index at lambda = 1550nm. The design is a square array of air holes in 220-nm silicon- oninsulator (SOI) which offers compatibility with complementary metal-oxide-semiconductor (CMOS) technology.
Generation of entangled photons in nonlinear media constitutes a basic building block of modern photonic quantum technology. Current optical materials are severely limited in their ability to produce three or more entangled photons in a single event due to weak nonlinearities and challenges achieving phase-matching. We use integrated nanophotonics to enhance nonlinear interactions and develop protocols to design multimode waveguides that enable sustained phase-matching for third-order spontaneous parametric down-conversion (TOSPDC). We predict a generation efficiency of 0.13 triplets/s/mW of pump power in TiO2-based integrated waveguides, an order of magnitude higher than previous theoretical and experimental demonstrations. We experimentally verify our device design methods in TiO2 waveguides using third-harmonic generation (THG), the reverse process of TOSPDC that is subject to the same phase-matching constraints. We finally discuss the effect of finite detector bandwidth and photon losses on the energy- time coherence properties of the expected TOSPDC source.
Large-scale audiovisual data that measure group learning are time consuming to collect and analyze. As an initial step towards scaling qualitative classroom observation, we qualitatively coded classroom video using an established coding scheme with and without its audio cues. We find that interrater reliability is as high when using visual data only—without audio—as when using both visual and audio data to code. Also, interrater reliability is high when comparing use of visual and audio data to visual-only data. We see a small bias to code interactions as group discussion when visual and audio data are used compared with video-only data. This work establishes that meaningful educational observation can be made through visual information alone. Further, it suggests that after initial work to create a coding scheme and validate it in each environment, computer-automated visual coding could drastically increase the breadth of qualitative studies and allow for meaningful educational analysis on a far greater scale.
During development, a neuron transitions from a state of rapid growth to a stable morphology, and neurons within the adult mammalian CNS lose their ability to effectively regenerate in response to injury. Here, we identify a novel form of neuronal regeneration, which is remarkably independent of DLK-1/DLK, KGB-1/JNK, and other MAPK signaling factors known to mediate regeneration in Caenorhabditis elegans, Drosophila, and mammals. This DLK-independent regeneration in C. elegans has direct genetic and molecular links to a well-studied form of endogenous activity-dependent ectopic axon outgrowth in the same neuron type. Both neuron outgrowth types are triggered by physical lesion of the sensory dendrite or mutations disrupting sensory activity, calcium signaling, or genes that restrict outgrowth during neuronal maturation, such as SAX-1/NDR kinase or UNC-43/CaMKII. These connections suggest that ectopic outgrowth represents a powerful platform for gene discovery in neuronal regeneration. Moreover, we note numerous similarities between C. elegans DLK-independent regeneration and lesion conditioning, a phenomenon producing robust regeneration in the mammalian CNS. Both regeneration types are triggered by lesion of a sensory neurite via reduction of neuronal activity and enhanced by disrupting L-type calcium channels or elevating cAMP. Taken as a whole, our study unites disparate forms of neuronal outgrowth to uncover fresh molecular insights into activity-dependent control of the adult nervous system’s intrinsic regenerative capacity.
D’après le romancier français Marcel Proust, «Le véritable voyage de découverte ne consiste pas à chercher de nouveaux paysages, mais à avoir de nouveaux yeux » (Proust, 1923). Ainsi, l’un des principaux objectifs de l’enseignement des sciences est d’aider les étudiants à modifier leur vision du monde. Cela est particulièrement important en physique, car les étudiants ont souvent des idées préconçues qui vont à l’encontre de ce qu’on tente de leur enseigner (Bransford, Brown et Cocking, 2000 ; Knight et Burciaga, 2004 ; Redish, 2003), précisément en ce qui concerne les concepts newtoniens. Parmi ces des décennies (Clement, 1982 ; Halloun et Hestenes, 1985a ; Halloun et Hestenes, 1985b ; Minstrell, 1982 ; Viennot, 1979), on estime qu'un grand nombre sont profondément ancrées dans leur esprit et difficiles à modifier (Dunbar, Fugelsang et Stein, 2007 ; Posner et collab., 1982 ; Vosniadou, 1992 et 1994). Nous pré- sentons ici quelques découvertes qui ont transformé notre propre perception de la façon dont les étudiants apprennent la physique. Plusieurs des idées que nous soumettons pourraient aussi s'appliquer à d'autres disciplines que ce soit dans un programme préuniversitaire ou technique.
Using nonlinear scattering theory, we simulate nonlinear signal generation in 2-dimensional zero-index metamaterials based on a photonic Dirac cone at the Γ point. We observe unique phase- matching in multiple simultaneous directions as the index approaches zero.
Zero-refractive-index metamaterials have been proposed as potential candidates for super-coupling applications, where light is confined to sub- diffraction limited length scales on-chip. Such a device allows for efficient coupling between disparate modes and compact 90 degree bends, which are challenging to achieve using dielectric waveguides. We discuss the simulation and fabrication results of all-dielectric on-chip zero-index metamaterial-based couplers. We observe transmission normal to all faces, regardless of the structure's shape, highlighting an unexplored feature of zero index metamaterials for integrated photonics.
Femtosecond-laser hyperdoping of sulfur in silicon typically produces a concentration gradient that results in undesirable inhomogeneous material properties. Using a mathematical model of the doping process, we design a fabrication method consisting of a sequence of laser pulses with varying sulfur concentrations in the atmosphere, which produces hyperdoped silicon with a uniform concentration depth profile. Our measurements of the evolution of the concentration profiles with each laser pulse are consistent with our mathematical model of the doping mechanism, based on classical heat and solute diffusion coupled to the far-from-equilibrium dopant incorporation. The use of optimization methods opens an avenue for creating controllable hyperdoped materials on demand.
We fabricate 3D gratings and diffraction optics using direct laser writing. Diffraction patterns of gratings agree with Laue theory. We demonstrate zone plates for visible wavelengths. Direct laser writing is promising for integrated diffraction optics.