Archives December 2018

Resonant terahertz detection using graphene plasmons

Plasmons, collective oscillations of electron systems, can efficiently couple light and electric current, and thus can be used to create sub-wavelength photodetectors, radiation mixers, and on-chip spectrometers. Despite considerable effort, it has proven challenging to implement plasmonic devices operating at terahertz frequencies. The material capable to meet this challenge is graphene as it supports long-lived electrically tunable plasmons. Here we demonstrate plasmon-assisted resonant detection of terahertz radiation by antenna-coupled graphene transistors that act as both plasmonic Fabry-Perot cavities and rectifying elements. By varying the plasmon velocity using gate voltage, we tune our detectors between multiple resonant modes and exploit this functionality to measure plasmon wavelength and lifetime in bilayer graphene as well as to probe collective modes in its moiré minibands. Our devices offer a convenient tool for further plasmonic research that is often exceedingly difficult under non-ambient conditions (e.g. cryogenic temperatures) and promise a viable route for various photonic applications.

Graphene-based THz detectors. a Schematics of the encapsulated BLG FET used in this work. b 3D rendering of our resonant photodetector. THz radiation is focused to a broadband bow-tie antenna by a hemispherical silicon lens yielding modulation of the gate-to-source voltage, as indicated in a. c Optical photograph of one of our photodetectors. Scale bar is 200 μm. d Conductance of one of our BLG FETs as a function of the gate voltage V_g, measured at a few selected temperatures. Inset: zoomed-in photograph of c showing a two-terminal FET with gate and source terminals connected to the antenna. Scale bar is 10 μm.

Broadband operation

We intentionally start the photoresponse measurements at the low end of the sub-THz domain, where the plasma oscillations are overdamped (see below). This allows us to compare the performance of our detectors with previous reports.

Plasmon-assisted THz photodetection. a Responsivity measured at f = 130 GHz and three representative temperatures. Orange rectangle highlights an offset stemming from the rectification of incident radiation at the p-n junction between the p-doped graphene channel and the n-doped area near the contact. Upper inset: FET-factor F as a function of V_g at the same T. Lower inset: maximum R_a as a function of T. b Gate dependence of responsivity recorded under 2 THz radiation. The upper inset shows a zoomed-in region of the photovoltage for electron doping. Resonances are indicated by black arrows. Lower inset: resonant responsivity at liquid-nitrogen temperature.

Resonant responsivity is a universal phenomenon in ultra-clean graphene devices and is expected to be independent of the physical mechanisms behind the rectification of the ac field into a dc photovoltage. Nevertheless, it is important to establish possible nonlinearities responsible for the rectification, for example, in order to be able to increase the magnitude of responsivity. We first note that the aforementioned asymmetry in R_a (V_g) between electron and hole doping indicates rectification at the p–n junction formed in vicinity of the contacts. This rectification usually appears due to the thermoelectric effect arising as a result of non-uniform sample heating and the difference between the Seebeck coefficients in the graphene channel and contact regions.

For more information: https://www.nature.com/articles/s41467-018-07848-w

Generating Spin Current from Mid Infrared Plasmonic Metamaterial Absorbers

Wavelength selective spin current is generated in the mid infrared rage by combining plasmonic metamaterial absorbers with platinum/yttrium-iron-garnet spintronic devices. The wavelength selectivity is attributed to the plsmonic resonance of the metamaterial absorber.

For more information: https://www.osapublishing.org/abstract.cfm?uri=CLEO_QELS-2018-FF2F.8

A Plasmonic Sensor Designed with Graphene

Scientists at a Chinese university have designed an infrared sensor that exploits the plasmonic properties of graphene to detect multiple wavelengths.

Exploring plasmons and graphene

Many experiments with nanoscale sensor design have explored the possibilities of surface plasmon polaritons (SPPs)—electromagnetic surface waves stimulated by light. The resonance of the surface plasmons can shift dramatically due to small changes in the refractive index of the sensing medium. SPP experiments often use noble metals as plasmonic surfaces, but these materials respond only to visible wavelengths of light. Some previous sensors designed with patterned metamaterials detect only one particular frequency that is fixed at the time the sensor is built. The team at China Jiliang University in Hangzhou turned to graphene, which interacts well with infrared light, and proposed a design that employs it as the plasmonic material atop a dielectric substrate of calcium fluoride. Using computer simulations, the scientists explored what would happen if they shaped the graphene into 40-nm-radius disks, with each disk containing a small, off-center circular defect in its crystalline structure. An ion-gel layer on top of the graphene disks delivers a bias voltage to the setup.

Introducing defects

The presence of the defect stimulates a phenomenon called plasmon hybridization, which produces dual-band resonance peaks in the mid-infrared spectrum. Moving the location of the defects within the disks in the x-y plane and measuring the resulting changes in the transmission spectra shows that the array is polarization-sensitive. Scientists can change the sensitivity of the sensor to detect different substances by adjusting the applied voltage. Putting the sensor near a substance of interest changes the refractive index of the sensor, thus registering a detection.

The China Jiliang team reported that the sensitivity of their proposed design reached 550 cm^–1 per refractive index unit. Researchers from Zhejiang University of Technology in China and the Technical University of Denmark also contributed to the study.

For more information: doi: 10.1364/OME.9.000035

Our new paper in Journal of magnetism and magnetic materials

Congratulations to our new paper ” Unexpected large transverse magneto-optic Kerr effect at quasi-normal incidence in commercial disk-based magnetoplasmonic crystals” by Cichelero, M. A. Oskuei, M. V. Kataja, S.M. Hamidi, G. Herranz.

We investigate the transverse magneto-optic Kerr effect (TMOKE) of magnetoplasmonic crystals grown on top of commercial disks. From full angle-resolved scans we can identify Wood’s anomalies related to the excitation of plasmons of different orders. From these maps we also detect a wide range of wavelengths and angles of incidence for which the TMOKE signal is increased due to the interaction of light with plasmons. Remarkably, conditions are established for unexpectedly large responses at quasi-normal incidence, where, by fundamental symmetry reasons, the intrinsic TMOKE is insignificant. The key towards this unexpected outcome is to engineer the geometry of magnetoplasmonic crystals, so that first-order plasmon dispersion lines run up towards quasi-normal angles of incidence. These results provide general rules for magneto-optic enhancement and, in particular, show the potential of standard commercial disks as platforms for enhanced magneto-optic devices.

Our new paper in Scientific Reports

Congratulations to our new paper “Rectangular plasmonic interferometer for high sensitive glycerol sensor” by Zahra Khajemiri , Dukhyung Lee, Seyedeh Mehri Hamidi , and Dai-Sik Kim.

A novel plasmonic interferometric sensor intended for application to biochemical sensing has been investigated experimentally and theoretically. The sensor was included a slit surrounded by rectangular grooves using a thick gold film. A three-dimensional finite difference time-domain commercial software package was applied to simulate the structure. The Focused ion beam milling has been used as a mean to fabricate series of rectangular plasmonic interferometer with varying slit-groove distance L. Oscillation behavior is shown by transmission spectra in a broadband wavelength range between 400 nm and 800 nm in the distance between slit and grooves. Red-shifted interference spectrum is the result of increasing refractive indices. The proposed structure is functional from visible to near-infrared wavelength range and yields a sensitivity of 4923 nm/RIU and a figure of merit as high as 214 at 729 nm wavelength. In conclusion, this study indicates the possibility of fabricating a low cost, compact, and real-time high-throughput plasmonic interferometer.

Figure 2. (a) Microfluidic fabrication process. (b)  SEM image of the 7 × 7 fabricated plasmonic interferometer array. The center-to-center distance between each interferometer is 2 μm, and the sensor array footprint is 14 × 14 μm². Scale bar: 10 μm. (c) One of the interferometers .Scale bar: 2 μm. (d)The shematic of our measurement. (e) Normalized transmission as a function of number of grooves, ‘n’ at period of 200 nm for L= 850 nm.

Manipulation of the dephasing time by strong coupling between localized and propagating surface plasmon modes

Strong coupling between two resonance modes leads to the formation of new hybrid modes exhibiting disparate characteristics owing to the reversible exchange of information between different uncoupled modes. Here, we realize the strong coupling between the localized surface plasmon resonance and surface plasmon polariton Bloch wave using multilayer nanostructures. An anticrossing behavior with a splitting energy of 144 meV can be observed from the far-field spectra. More importantly, we investigate the near-field properties in both the frequency and time domains using photoemission electron microscopy. In the frequency domain, the near-field spectra visually demonstrate normal-mode splitting and display the extent of coupling. Importantly, the variation of the dephasing time of the hybrid modes against the detuning is observed directly in the time domain. These findings signify the evolution of the dissipation and the exchange of information in plasmonic strong coupling systems and pave the way to manipulate the dephasing time of plasmon modes, which can benefit many applications of plasmonics.

Structural characterization. The structure designed to realize the strong coupling is shown in Fig. 1. A 20-nm-thick gold film is deposited on an indium-tin-oxide (ITO)-coated glass substrate to support the SPP-Bloch wave. The ITO layer has a thickness of 150 nm, which makes the entire substrate surface suitably conductive for PEEM measurements. Then, a 25-nm-thick Al2O3 spacer is deposited using the atomic layer deposition technique.
The gold square nanoblock arrays are fabricated on the Al2O3 spacer via electron-beam lithography (EBL), followed by metal sputtering and lift-off, to support the LSPR modes. The sectional view (Fig. 1b) and top view (Fig. 1c) of the sample are acquired by a scanning transmission electron microscope (STEM) and a scanning electron microscope (SEM), respectively. In addition, energy-dispersive X-ray spectroscopy (EDS) is used to mark
different elements with a distinct color in the sectional view (Fig. 1d). The nanoblocks of different sizes (side lengths) are designed (100–160 nm) to tune the LSPR energy. Beyond that, the nanoblock array can provide the additional wave vector for the excitation light (K0) to excite the SPP supported on the thin metal by compensating the momentum mismatch between the excitation light and the SPP modes.

Experimental far-field spectral property. The measured extinction spectra of samples with different nanoblock sizes and fixed periods (400 or 500 nm) are presented in Fig. 2a, b, respectively. For the period of 400 nm, the left peak is almost entirely unshifted, and the right peak undergoes a redshift as the nanoblock size increases. The left and right peaks can be assigned to the SPP Bloch wave and LSPR mode, respectively. Moreover, in this case the two modes cannot couple well with each other, as is clearly shown by the dispersion curves of the two modes (Fig. 2c), where the SPP modes are kept unchanged while the nanoblock sizes change. Similarly, the dissipation of the LSPR mode (γLSPR = 98 meV) and the SPP-Bloch waves (γSPP = 38 meV) can be calculated from the experimental line widths with a period of 400 nm and a nanoblock size of 135 nm. For the period of 500 nm, the dispersion curves (Fig. 2d) extracted from the extinction spectra show an anticrossing behavior and can be fitted by the coupled oscillator model9,22,26 (details are shown in the Supplementary Note 1). The
splitting energy is calculated as 144 meV at ELSPR = ESPP. Then, we can determine that the interaction potential (V) is 78 meV.

Simulation results. To further understand these modes, we use the finite-difference time domain (FDTD) method to simulate the mode distribution. With the large nanoblock size (150 nm) and the small period (400 nm), two peaks appear on the extinction spectrum (blue line in Fig. 3a). Peak 1 has a narrow line width, and the electric field is confined mainly on the lower surface of the Au film. Peak 2 has a broad line width, and the electric field is located mainly at the interface between the nanoblocks and Al2O3, with much greater field enhancement, as shown in Fig. Therefore, we recognize that peaks 1 and 2 represent the SPPBloch wave and LSPR mode, respectively, and that the detuning between the LSPR mode and SPP-Bloch wave is large (~396 meV) in this case. The energy exchange gives rise to a higher near-field enhancement than the single SPP-Bloch wave and a longer oscillation time than the single LSPR mode, which demonstrates that the coupling between the LSPR mode and SPP-Bloch wave modifies the field distribution resulting from the normal-mode splitting with the small detuning. Notably, the near-field enhancement of  the two coupled modes (peaks 3 and 4) is both present and, in fact, slightly higher than that of the LSPR mode only (peak 2), which is shown by Fig.

For more information: DOI: 10.1038/s41467-018-07356-x

Nano “Solar Panels” Turn Yeast Cells into Biofactories

By decorating the outside of baker’s yeast cells with light-harvesting semiconductor nanoparticles, a research team from Harvard University and the University of Pennsylvania, USA, has turned the cells into tiny factories to churn out substances relevant to pharmaceutical and fine-chemical manufacture. The semiconductor fragments act as tiny “solar panels” that shunt photogenerated electrons into the yeast cell and thereby throw a natural process of biosynthesis into high gear. Beyond yeast, the research team believes that its technique enables a “mix and match approach” that could be extended to a range of cellular systems for chemical processing.

The need for speed

Chemical industries already use bacteria and fungi to provide a range of drugs and fine chemicals at scale. Baker’s yeast (Saccharomyces cerevisiae) is one such platform. In addition to puffing up loaves of bread, the organism’s complex metabolism allows it to produce commercially useful substances including shikimic acid, a precursor of the antiviral drug Tamiflu and a number of other medicines and fine chemicals. But there’s a catch—the shikimic acid reaction also depletes the cell of another important molecule, NADPH, involved in a redox reaction that’s key to providing the energy to drive production in the first place. That makes the whole process self-limiting; the faster the cell produces shikimic acid, the faster it “runs out of gas” to do so. Indeed, the researchers note that regeneration of NADPH is “a common bottleneck in the production of metabolites through microbial cell factories,” not just in yeast.

InP nanoparticles build a better biofactory

To get past that bottleneck, the research team hit on the idea of providing the cells with an external electron source to help them rebuild NADPH. Light-harvesting semiconductors seemed a natural choice. The team settled on indium-phosphide (InP) nanoparticles, which have a direct band gap that enables them to harvest energy from a broad swath of the solar spectrum. In principle, the InP particles, when attached to yeast cells and exposed to light, could serve as a source of photogenerated electrons that could boost NADPH levels in the cells, providing renewed energy to keep their production of shikimic acid going. The team then took those nanoparticles and assembled them in suspension onto S. cerevisiae cells, keeping the suspension well mixed to maximize the potential for collisions between the nanoparticles and the yeast. Finally, they tested the ability of the now-souped-up baker’s yeast cells to pump out shikimic acid under a variety of light and dark conditions.

An eleven-fold production increase

The researchers found that, when exposed to light, their hybrid yeast-InP system was able to churn out 11 times more shikimic acid than the hybrid cells without illumination. That showed, according to team leader Joshi, that “the energy transfer from light into the cell works very efficiently.” While the researchers worked specifically on yeast and shikimic acid, the team stresses that its system is “a modular bioinorganic hybrid platform,” and that the polyphenol-functionalized InP nanoparticles could be used with other microorganisms to boost production of other chemicals. The paper suggests that the technique should be “compatible with existing workhorse cellular chassis and a wide range of particle-cell combinations.” Indeed, lead author Guo, in a press release, argued that the approach “creates an entirely new design space for future biohybrid technologies.”

For more Information: doi: 10.1126/science.aat9777

Green Photonics

Lasers are not all that’s green in the photonics industry. Photonics technologies are helping to reduce energy consumption, they’re used in the manufacturing of renewable energy technologies, and many green, sustainable practices are adhered to throughout the photonics industry. And, yes, trend that it is – there is certainly green in going green.  New technologies and products are developed with energy efficiency in mind. This is not only because energy savings is a buzz word or marketing tool but also because customers are trying to cut their manufacturing costs – and saving electricity is a great way to start. As an example, Power Technology (PTI) of Little Rock, Ark., has replaced inefficient gas lasers with diode lasers. The IQ Micro (IQu) laser at 488 nm produces 60 mW of light and typically requires less than 10 W of energy to operate. The argon gas laser it replaces required 1500 W to generate the same power and wavelength.

Imaging in Green Apps 

Use of machine vision systems in the solar industry makes sense, as precision and high-quality output are of paramount importance. According to Gregory Hollows, director of machine vision solutions at Edmund Optics in Barrington, N.J., machine vision used in the solar industry is not just about looking for defects in wafers. “A subtle modification of one lens can be like reinventing the wheel,” Hollows said. The goal is to see with the camera what you see with the eye. It has to be done on the assembly line, and it has to be repeatable over and over. In some cases the material itself presents challenges – from highly reflective to curved to flat – “many properties come into play.”

Optics in Solar

The huge growth in solar energy installations has presented optics manufacturers with new opportunities for components that collect or focus the sun’s energy. Some of the components are similar to those made for other applications, such as homogenizing rods, light pipes or lightguides, according to Gregg Fales, product line manager at Edmund Optics. In some cases, solutions are customized for specific photovoltaic applications. For example, concentrated photovoltaics (CPV) requires a square aperture at one end of a concentrated parabolic concentrator (CPC) that’s bigger than the square aperture at the other end.

Solid-State Lighting

Optics engineers and other lighting experts are hard at work trying to improve solid-state lighting, driven in part by government mandates. According to a report by the US Department of Energy, 22 percent of the electricity generated in the US goes toward lighting applications. If light sources were converted to more energy-efficient LEDs, electrical use would be significantly reduced. To that end, governments are promoting research and development in the area of LEDs so that the technology can catch up with the demand for lighting applications. The US Department of Energy, for example, set a research goal of 160 lumens per watt by 2025.

Adhering to Green

Companies are also embracing sustainability efforts such as “lean” manufacturing, adoption of the Waste Electrical and Electronic Equipment (WEEE) initiative, compliance with RoHS standards and the REACH initiative. And many are taking a myriad of smaller measures that add up to a great amount of energy saved, such as recycling manufacturing byproducts, installing energy-efficient lighting and more.

For more information: https://www.photonics.com/Articles/Green_Photonics_/a41461