Friday, October 24, 2014

Flatland, we hardly knew ye: Unique 1-D metasurface acts as polarized beam splitter, allows novel form of holography

( —Traditional three-dimensional (3-D) plasmonic metamaterials with metallic structures – artificial materials that exploit coherent delocalized electron oscillations known as surface plasmons produced from the interaction of light with metal-dielectric materials – exhibit unique electromagnetic properties not found in natural materials, such as extraordinary transmission beyond the diffraction limit, efficient light-harvesting ability, plasmonic color filtering, and the ability to control the reflection or transmission direction of a light beam. However, they are difficult to fabricate, have a narrow usable bandwidth due to their resonant character, and exhibit low optical efficiency due to the inherent metal absorption. While two-dimensional metasurface structures have been proposed in an attempt to address these functional limitations, they still require complex designs and sophisticated fabrication procedures.

 by Stuart Mason Dambrot

Recently, scientists at Shanghai Jiao Tong University proposed a unique method for controlling photons with a 1-D metasurface that, by integrating diffraction, waveguide, and plasmonic effects, is fundamentally distinct from 2-D or 3-D methods. The 1-D metasurface they presented, based on bilayered metallic nanowire gratings, behaves as an ideal polarized beam splitter that produces highly anisotropic strong negative reflection for transverse-magnetic (TM) light and efficient reflection for transverse-electric (TE) light. (Anisotropy is the property of being directionally dependent, as opposed to isotropy, which implies identical properties in all directions.) Moreover, by combining this feature with the fringed structure of a hologram, the researchers demonstrated a higher-security anti-counterfeit metasurface holograph, that can be decoded only by TM light.
Dr. Zhicheng Ye discussed the paper that he, Dr. Jun Zheng and their co-authors published in Scientific Reports with A key factor is simplifying the metastructure to a one-dimensional metasurface using a bilayer metallic nanowire grating. "Designing three- or two-dimensional nanoantenna arrays can modify the optical wavefront with spatially-varying phase response," Ye tells "However, complex designs and sophisticated fabrication procedures are required. Simplifying the metastructure to one dimension is preferred for applications and large-scale production, but the structure that allows tunable parameters is limited. Therefore, the main challenges to achieve specific functions using bilayer metallic nanowire grating involve exploring and utilizing the intrinsic properties of surface plasmon waveguide, combining these with the diffraction characteristics of grating, and finding proper parameters."
Relatedly, Ye continues, using bilayer metallic nanowire gratings with nano-slits as 1-D metamaterials to realize both polarized negative reflection and reflection carries its own challenges. "Thus far, the dielectric or metal beam splitters based on gratings usually have narrow working spectrum and angle range. On the other hand, these studies have typically focused only on transmission and reflection properties – but polarized diffraction has not been considered," he explains. "We proposed the bilayer metallic nanowire gratings with nano-slits as 1-D metamaterials to realize polarized negative reflection for transverse-magnetic light and reflection for transverse-electric light, with a broad range of useable wavelengths and incident angles." The key point is that metasurface beam splitting does not rely on a resonant coupling mechanism, thereby enabling a broad range of useable wavelengths and incident angles, with flexible tunability in operation. "The main challenge was determining how to simultaneously realize broadband, angle-insensitivity and high polarization extinction ratios. Our research shows that, the grating pitch should be large enough to ensure a broad working spectrum; the slit should be wide enough to insure that transverse-magnetic light can be refracted strongly, but also thin enough to suppress TE refraction efficiently; and the proper slit height should significantly enhance the polarization extinction ratio."
Optical performance is not the only issue the scientists faced: The fabrication challenge is that in order to obtain high extinction ratio for both negative reflection and reflection, the ratio between the silt-width and pitch is always smaller than ½ – that is, the exposure condition for laser interference lithography (LIL) should be optimized. "The issue will be more challenging as the slit-refractive index increases, because the slit width should be further reduced to achieve the cut-off effect," Ye notes. "Smarter design of the grating structures is needed for the high extinction ratio of both reflection and negative reflection, especially for a very wide waveband device."
The third challenge was combining the intrinsically fringed structure of a hologram and the anisotropic character of metasurface beam splitter. "In a hologram, the interference fringes usually have random direction, pitch, duty cycle, and depth," Ye points out. "Therefore, to combine the hologram and beam splitter, photoresist processing and recording parameters should be well-designed in order to get deep and narrow slits for guaranteeing high-efficiency of TM light and inhibiting TE diffraction simultaneously. Otherwise, extinction ratios for both negative reflection and reflection will be lowered." Moreover, since a laser hologram is usually recorded on photoresist film in the form of fringes caused by two-beam interference, the patterns are typically fringed structure with pitch decided by the wavelength and angle between the interference beams. "To obtain maximum diffraction efficiency," Ye explains, "the fringes should be deep enough – but the contradiction is that deepening photoresist fringes causes slit width to increase, which in turn weakens anisotropy."
The researchers leveraged a range insights and techniques to address these challenges. "In our one-dimensional metasurface, the key point was fully combining the intrinsic properties of dielectric-metal-dielectric slit and the diffraction characteristics of grating, and finding the proper parameters to meet the required waveband," Ye recounts. "At the same time, the starting point of our research is physically as simple and fundamental as the description in classic optics textbooks that for diffraction to exist in a grating, the light should be able to enter into the slits. Therefore," he explains, "by considering the fact that in metal-insulator-metal waveguides TM mode is always supported while TE mode is dissipative in sub-wavelength size, we intuited that TM diffraction can be made to break the diffraction limit – corresponding to vector diffraction optics – while for TE light the diffraction should disappear without 'sensing' the slits, corresponding to geometrical optics." The demonstration of the hypothesis not only convincingly reveals a new way to control photons, but also gives a very clearly physical image for the transition between scalar optics (where the optical phenomena can be described by scalar diffraction) and nano-optics (where the wavelengths are much larger than the size and pitch of the resonant units so the diffraction is therefore absent.)
Spectra and photos of a 280-nm-pitch grating
Spectra and photos of a 280-nm-pitch grating. (a) Measured (solid lines) and simulated (dashed lines) NR (blue and cyan lines) and reflection (red and magenta lines) spectra for TM (blue and red lines) and TE (cyan and magenta lines) light with an incident angle of 60o. The inset is the simulated extinction ratio of NRTM/NRTE. (b) Photos of the device irradiated by un-polarized white light with an incident angle of approximately 60o and filtered by a polyvinyl alcohol (PVA) plastic polarizer (extinction ratio of 300). The yellow dotted line indicates the direction of the grating lines. The white dashed lines illustrate the electric field direction of the light passing through the polarizer. An SEM image of the device is also presented. The dielectric grating in the device was fabricated by laser interference, with an Al thickness of 50 nm. Credit: Zheng J et al. Highly anisotropic metasurface: a polarized beam splitter and hologram. Scientific Reports 4, 6491 (29 September 2014).
"Our technique is quite different from that generally employed in metamaterials research, where very fine nano-size resonators are often required to achieve a specific function, and where the diffraction effect – while more efficient and robust in either periodical or aperiodical structures – is rarely considered," Ye points out. Our study gave us the profound perspective that we can gain knowledge by utilizing long-established basic mechanisms to alleviate the difficulty of nano-optical device fabrication, rather than by pursuing very complex metastructures without considering cost and application practicality."
Summarizing their approach to creating a novel polarized beam splitters, Ye says that a dielectric grating was fabricated by nano-imprinting or laser interference on a silicon or glass substrate. "The grating was subsequently coated with ten nanometers of metal film by E-beam evaporation," he tells "In order to narrow the slits to restrain the TE diffraction, the deposition was optimized to make the metal film conform to the dielectric grating as far as possible. In doing so, the metal deposited on the side walls of the grating lines helped reduce the slit width."
The paper discusses how metasurface beam splitting enables a broad range of useable wavelengths and incident angles with flexible tunability in operation by not relying on a resonant coupling mechanism. "For the metasurface polarized beam splitter with TM negative reflection/TE reflection, the upper limit of the working wavelength is determined by the pitch, and the lower limit is decided by the slit width. Thus," he explains, "by choosing appropriate grating pitch and slit width, the polarized beam splitting can be achieved. For further improving the extinction ratio, it needs to optimize silt and metal thickness." Therefore, the device is flexible tunability in a wide range of spectra which is more robust than those beam splitters using resonant mechanism in which grating height, refractive index, pitch and duty circle must be precisely designed.
"Relatedly, our metasurface holograph – decodable only by transverse-magnetic polarized light –consists of a bilayer metal grating which ensures that only TM light can be diffracted. This means that the metasurface hologram can only be decodable by TM light, and the real image is reconstructed in the reflection side with the negative first order refraction." Periodically constructed metamaterials, in which permittivity (a measure of how an electric field affects, and is affected by, a dielectric medium) andpermeability (the degree of magnetization that a material obtains in response to an applied magnetic field) have simultaneous negative values, creating negative refraction (a dimensionless number that much light is bent when entering a material.
Metasurface hologram
Metasurface hologram. (a) Photoresist hologram fabrication scheme: The badge was placed perpendicularly to the photoresist film, and both were illuminated by a large cross-section collimated laser beam with an incident angle of 49o to the photoresist film. (b) Microscopy and SEM images of the top views of the metasurface hologram film at different scales. The film consists of randomly distributed B-MNGs with a pitch of 300 nm. The width of the dielectric PMMA is t1 = 150 nm, and the thicknesses of the PMMA and Al are h1 = 110 nm and h2 = 50 nm, respectively. (c) Real images reconstructed using TM and TE laser light, respectively. The metasurface hologram was illuminated by a TM or TE laser with a wavelength of 532 nm and an incident angle of 75o. The reconstructed real 3-D image was formed by the negative first-order diffraction mode with a diffractive angle of 55o and was imaged onto a black screen, as presented in the bottom left pictures. The image was also viewed with a camera, as presented in the top right pictures, or directly by eye. Credit: Zheng J et al. Highly anisotropic metasurface: a polarized beam splitter and hologram. Scientific Reports 4, 6491 (29 September 2014).
In addition to the 1-D metasurface applications discussed in the paper – displays, holograms, and laser optics – Ye mentions several others:
  • Compact reflective mirrors for laser cavities where the polarized output is required to meet the coherence for optical communication or laser interference
  • Diffractive gratings in optical spectrometers, where the TM diffraction efficiency is stably high in a wide waveband with high angle resolution since the pith is only several hundred nanometers, which has several advantages: as a reflection element it provides a range of benefits, such as allowing the controlling circuit to be placed below the reflective surface, and more advanced integrated circuit technology becomes available when the substrate materials are not limited by their opaqueness; and a folded light path wherein the input and output light beam share the same physical space, which offers a desirable compact arrangement
  • As a novel plasmonic , it can also be implemented in integrated optics devices for communication: TM light is diffracted into one waveguide and TE light is reflected into the other, allowing the two beams to be manipulated for further use
There are also long-term implications of a higher-security anti-counterfeit plasmonic hologram produced by adding the additional criterion of what Ye refers to as the "simple example" of polarization dependence demonstrated in the paper. "The greater enlightenment is that plasmonic effects can be used for higher-security anti-counterfeits," he confirms. "In fact, not only the polarized -change of the brightness but also the polarized reflection-change of the color can be utilized as new criterions to judge a hologram." Yet another advantage of the plasmonic anti-counter faking is that optical resonance can be well-engineered by the thickness of metal films, slit width and grating pitch, substantially increasing the cost and difficulty of fakery – especially for a complex pattern with several values of pitch, metal thickness and direction of the metallic nanowire grating. "In a word," Ye notes, "we open a new door for the application of metasurface devices in anti-counter faking."
Moving forward, in the near future the scientists are planning to create a theoretical 1-D metasurface model that presents a quantitative effective refractive index to analyze structural properties, thus facilitating further device design. "In the long-term," he adds, "for polarized beam splitting we plan to increase the waveband and extinction ratio for both reflection and negative reflection simultaneously by using a more delicate nanowire arrangement. Moreover, in addition to our current emphasis in anti-counter faking, we'll be looking at other applications, including integrated optics, spectrometers and laser cavities."
Another important innovation that the researchers might consider developing is combining actively tunable metasurfaces with other techniques, such as liquid crystals and electrowetting. "Tunability will enable metasurfaces to be applied to displays, optical storage and communications," Ye tells "Also, we're considering if the optical properties we've observed can be extended to other frequency spectra – for example, the terahertz range, since doing so may help us to build terahertz devices. In closing, Ye says that other areas of research, such as high-field laser physics, might also benefit from their study.
More information: Highly anisotropic metasurface: a polarized beam splitter and hologram, Scientific Reports 4, 6491 (29 September 2014), doi:10.1038/srep06491

Abstract-Temperature and Pressure Effects on Terahertz Time-Domain Spectroscopy of Crystalline Methedrine by Molecular Dynamics Simulation

Yu XinHassan Yousefi OderjiRan HaiCailong FuRaja AljarmouziHongbin Ding

In this study, the terahertz time-domain spectroscopy (THz-TDS) of crystalline methedrine, which is one of the illegal drugs, is performed using molecular dynamics simulation by the Fourier transform of time derivative auto-correlation functions of the dipole moment. In order to accurately detect the drugs from samples, it is necessary to build a complete database for terahertz spectra under different external conditions from theoretical calculation, which are hardly obtained from the experiments directly. Our results show remarkable consistency with the available experimental data in the frequency range of 10 - 100 cm-1indicating that the presented method has significant capability to simulate terahertz spectra at various conditions. We investigated the effects of temperature and pressure on THz-TDS by simulating the system at temperature range between 78.4 K and 400 K at pressures up to 100 atm. Results show the spectral features of THz-TDS both in intensity and profile are highly sensitive to the variation of temperature and with a lower magnitude to the variation of pressure. The vanishing, rebuilding and shifting of spectral peaks are due to the complex mechanisms such as the anharmonicity, shifting in the vibration energy levels, formation and destruction of hydrogen-binding and the deformation of the potential energy surface during the environment changing. This improved our understanding for complicated THz-TDS of crystalline methedrine and would be useful for assignment of the practical measurements.

Abstract-Tunable terahertz coherent perfect absorption in a monolayer graphene

Yuancheng Fan, Fuli Zhang, Qian Zhao, Zeyong Wei, and Hongqiang Li  »View Author Affiliations
Optics Letters, Vol. 39, Issue 21, pp. 6269-6272 (2014)

Coherent perfect absorber (CPA) was proposed as the time-reversed counterpart to laser: a resonator containing lossy medium instead of gain medium can absorb the coherent optical fields completely. Here, we exploit a monolayer graphene to realize the CPA in a nonresonant manner. It is found that quasi-CPA point exists in the terahertz regime for suspending monolayer graphene, and the CPA can be implemented with the assistance of proper phase modulation among two incident beams at the quasi-CPA frequencies. The graphene-based CPA is found of broadband angular selectivity: CPA point splits into two frequency bands for the orthogonal s and p polarizations at oblique incidence, and the two bands cover a wide frequency range starting from zero frequency. Furthermore, the coherent absorption can be tuned substantially by varying the gate-controlled Fermi energy. The findings of CPA with nonresonant graphene sheet can be generalized for potential applications in terahertz/infrared detections and signal processing with two-dimensional optoelectronic materials.
© 2014 Optical Society of America

OT-Team reveals molecular structure of water at gold electrodes

Schematic of the electrochemical cell – a silicon nitride (Si3N4) membrane separates the 
liquid from vacuum region of the x-ray source; a 20nm thin-film gold electrode is deposited on liquid side of the membrane. Detection of x-ray absorption is via fluorescence emission on the vacuum side or electron emission at the gold electrode. Credit: Berkeley Lab

When a solid material is immersed in a liquid, the liquid immediately next to its surface differs from that of the bulk liquid at the molecular level. This interfacial layer is critical to our understanding of a diverse set of phenomena from biology to materials science. When the solid surface is charged, just like an electrode in a working battery, it can drive further changes in the interfacial liquid. However, elucidating the molecular structure at the solid-liquid interface under these conditions has proven difficult.

Now, for the first time, researchers at the US Department of Energy's (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) have observed the  of liquid water at a  under different charging conditions.

Miquel Salmeron, a senior scientist in Berkeley Lab's Materials Sciences Division (MSD) and professor in UC Berkeley's Materials Science and Engineering Department, explains this in the context of a battery. "At an electrode surface, the build-up of electrical charge, driven by a potential difference (or voltage), produces a strong electric field that drives molecular rearrangements in the electrolyte next to the electrode."
Berkeley Lab researchers have developed a method not only to look at the molecules next to the electrode surface, but to determine their arrangement changes depending on the voltage.
With gold as a chemically inert electrode, and slightly-saline water as an electrolyte, Salmeron and colleagues used a new twist on x-ray absorption spectroscopy (XAS) to probe the interface and show how the interfacial molecules are arranged.
XAS itself is not new. In this process, a material absorbs x-ray photons at a specific rate as a function of photon energy. A plot of the absorption intensity as a function of energy is referred to as a spectrum which, like a fingerprint, is characteristic of a given material molecule and its chemical state. Our eyes recognize many materials by their characteristic colors, which are related to their visible light . The x-ray photons used in this study have energies that are about 250 times higher than those of visible light and are generated at Berkeley Lab's Advanced Light Source (ALS).
Typical XAS measurements are made under vacuum conditions, as x-rays are readily absorbed by matter, even the nitrogen molecules in air. But liquids will quickly evaporate in a vacuum. By using a very thin (100 nm, or a tenth of a micrometer) x-ray transparent window, with a thin coating of gold (20nm), on a sealed liquid sample holder, the Berkeley Lab team was able to expose  in the liquid to x-rays and collect their spectra.
Upon absorbing an x-ray photon, the excited water molecule can spew (emit) either charged particles (electrons) or light (photons). The amount of photon emission, or fluorescence, is one indicator of how many x-ray photons have been absorbed. However, fluorescing x-rays can be detected from molecules ranging from those at the gold surface to those deep (micrometers) inside the liquid far from the influence of the gold surface, and these dominate the measured spectrum.
"We are only really interested in a nanoscale interfacial region, and looking at the fluorescence photon signal we can't tell the difference between the interface and the interior electrolyte molecules," says Salmeron.
The challenge therefore was to collect a signal that would be dominated by the interfacial region. The team accomplished this by measuring electron emissions because electrons emitted from x-ray excited water molecules travel only nanometer distances through matter. The electrons arriving at the gold electrode surface can be detected as an electrical current traveling through a wire attached to it. This avoids confusion with signals from the interior electrolyte because electrons emitted from interior molecules don't travel far enough to be detected.
There's an additional problem that arises when studying liquids in contact with working electrodes because they carry a steady current as in batteries and other electrochemical systems. While the emitted electrons from nearby molecules are indeed detectable, this contribution to the current is dwarfed by the normal "Faradaic" current of the battery at finite voltages. When measuring current off the electrode, it is critical to determine which part is due to the x-rays and which is due to the regular battery current.
To overcome this problem, the researchers pulsed the incoming x-rays from the synchrotron at a known frequency. The current contribution resulting from electron emission by interfacial molecules is thus pulsed as well, and instruments can separate this nanoampere modulated current from the main Faradaic current.
These experiments result in absorption vs. x-ray energy curves (spectra) that reflect how water molecules within nanometers of the gold surface absorb the x-rays. To translate that information into molecular structure, a sophisticated theoretical analysis technique is needed.
David Prendergast, a staff scientist in the Molecular Foundry and researcher in the Joint Center for Energy Storage Research (JCESR), has developed computational techniques that allow his team to accomplish this translation.
Using supercomputer facilities at Berkeley Lab's National Energy Research Scientific Computing Center (NERSC), he conducted large molecular dynamics simulations of the gold-water interface and then predicted the x-ray absorption spectra of representative structures from those simulations.
"These are first-principles calculations," explains Prendergast. "We don't dictate the chemistry: we just choose what atomic elements are present and how many atoms. That's it. The chemistry is a result of the calculation."
It turns out that for a neutral gold surface, a significant number of water molecules (H2O) next to the gold surface orient with hydrogen (H) atoms pointing toward the gold. Water molecules are bound together by so-called hydrogen bonds, which orient the slightly positively-charged H atoms in each molecule towards the slightly negatively-charged oxygen (O) atoms of neighboring molecules. This network of hydrogen bonds is what holds water molecules together to make a liquid under conditions of temperature and pressure that we consider comfortable as humans. It is perhaps surprising that the inert gold surface can induce significant numbers of water molecules not to hydrogen-bond to each other but to bond to the gold instead. This number is enhanced when the gold is negatively charged and therefore attracting the more positive H atoms. Furthermore, positively-charged gold ions cause water molecules to orient their H atoms away from the gold, which strengthens the hydrogen bond network of the interfacial liquid.
"That's the main thing we know about the gold electrode surface from the x-ray absorption spectra: how many water molecules are tilted one way or another, and if their hydrogen bonds are broken or not," concludes Salmeron. "Water next to the electrode has a different molecular structure than it would in the absence of the electrode."
There are a couple of subtle things that are very important, notes Prendergast. First, the shape of the absorption spectra changes as a function of changing voltage. Since the measured spectra agree with the calculations one can draw conclusions about the molecular structure of the liquid interface as a function of voltage. The second is that in the calculations, the change in the structure of water is limited to the first two molecular layers above the surface and these two layers span only about 1 nanometer. To observe any difference in the experimental spectra with varying voltage means that measurements are sensitive to a shorter length scale than was thought possible.
"We had thought the sensitivity to be tens of nanometers, but it turns out to be subnanometer," says Prendergast. "That's spectacular!"
This study, which is reported in Science in a paper titled "The structure of interfacial water on gold electrodes studied by x-ray absorption spectroscopy," marks the first time that the scientific community has shown such high sensitivity in an in-situ environment under working electrode conditions.

More information: The structure of interfacial water on gold electrodes studied by x-ray absorption spectroscopy, Science, 2014.… 1126/science.1259437

Thursday, October 23, 2014

Abstract-Weak, strong, and coherent regimes of Fröhlich condensation and their applications to terahertz medicine and quantum consciousness

My Note: I included this older abstract, (from January, 2009), mainly because it discusses concepts I had never heard of before- this blog is intended to be a repository, I wanted to include it so I can easily find it, to read more about these concepts.
  1. Jeffrey R. Reimersa,1
  2. Laura K. McKemmisha
  3. Ross H. McKenzieb
  4. Alan E. Markc and 
  5. Noel S. Hushd
  1. Edited by Mark A. Ratner, Northwestern University, Evanston, IL, and approved January 22, 2009 (received for review June 30, 2008)

In 1968, Fröhlich showed that a driven set of oscillators can condense with nearly all of the supplied energy activating the vibrational mode of lowest frequency. This is a remarkable property usually compared with Bose–Einstein condensation, superconductivity, lasing, and other unique phenomena involving macroscopic quantum coherence. However, despite intense research, no unambiguous example has been documented. We determine the most likely experimental signatures of Fröhlich condensation and show that they are significant features remote from the extraordinary properties normally envisaged. Fröhlich condensates are classified into 3 types: weak condensates in which profound effects on chemical kinetics are possible, strong condensates in which an extremely large amount of energy is channeled into 1 vibrational mode, and coherent condensates in which this energy is placed in a single quantum state. Coherent condensates are shown to involve extremely large energies, to not be produced by the Wu–Austin dynamical Hamiltonian that provides the simplest depiction of Fröhlich condensates formed using mechanically supplied energy, and to be extremely fragile. They are inaccessible in a biological environment. Hence the Penrose–Hameroff orchestrated objective-reduction model and related theories for cognitive function that embody coherent Fröhlich condensation as an essential element are untenable. Weak condensates, however, may have profound effects on chemical and enzyme kinetics, and may be produced from biochemical energy or from radio frequency, microwave, or terahertz radiation. Pokorný's observed 8.085-MHz microtubulin resonance is identified as a possible candidate, with microwave reactors (green chemistry) and terahertz medicine appearing as other feasible sources.