Scientists build a nanolaser using a single atomic sheet
University of Washington scientists have built a new nanometer-sized laser — using the thinnest semiconductor available today — that is energy efficient, easy to build and compatible with existing electronics.
Lasers play essential roles in countless technologies, from medical therapies to metal cutters to electronic gadgets. But to meet modern needs in computation, communications, imaging and sensing, scientists are striving to create ever-smaller laser systems that also consume less energy.
The UW nanolaser, developed in collaboration with Stanford University, uses a tungsten-based semiconductor only three atoms thick as the “gain material” that emits light. The technology is described in a paper published in the March 16 online edition of Nature.
“This is a recently discovered, new type of semiconductor which is very thin and emits light efficiently,” said Sanfeng Wu, lead author and a UW doctoral candidate in physics. “Researchers are making transistors, light-emitting diodes, and solar cells based on this material because of its properties. And now, nanolasers.”
Nanolasers — which are so small they can’t be seen with the eye — have the potential to be used in a wide range of applications from next-generation computing to implantable microchips that monitor health problems. But nanolasers so far haven’t strayed far from the research lab.
Other nanolaser designs use gain materials that are either much thicker or that are embedded in the structure of the cavity that captures light. That makes them difficult to build and to integrate with modern electrical circuits and computing technologies.
The ultra-thin semiconductor, which is about 100,000 times thinner than a human hair, stretches across the top of the photonic cavity.Image credit: University of Washington.
The UW version, instead, uses a flat sheet that can be placed directly on top of a commonly used optical cavity, a tiny cave that confines and intensifies light. The ultrathin nature of the semiconductor — made from a single layer of a tungsten-based molecule — yields efficient coordination between the two key components of the laser.
The UW nanolaser requires only 27 nanowatts to kickstart its beam, which means it is very energy efficient.
This emission map of the nano-device shows the light is confined by and emitted from the photonic cavity. Image credit: University of Washington.
Other advantages of the UW team’s nanolaser are that it can be easily fabricated, and it can potentially work with silicon components common in modern electronics. Using a separate atomic sheet as the gain material offers versatility and the opportunity to more easily manipulate its properties.
“You can think of it as the difference between a cell phone where the SIM card is embedded into the phone versus one that’s removable,” said co-author Arka Majumdar, UW assistant professor of electrical engineering and of physics.
“When you’re working with other materials, your gain medium is embedded and you can’t change it. In our nanolasers, you can take the monolayer out or put it back, and it’s much easier to change around,” he said.
The researchers hope this and other recent innovations will enable them to produce an electrically-driven nanolaser that could open the door to using light, rather than electrons, to transfer information between computer chips and boards.
The current process can cause systems to overheat and wastes power, so companies such as Facebook, Oracle, HP, Google and Intel with massive data centers are keenly interested in more energy-efficient solutions.
Using photons rather than electrons to transfer that information would consume less energy and could enable next-generation computing that breaks current bandwidth and power limitations. The recently proven UW nanolaser technology is one step toward making optical computing and short distance optical communication a reality.
“We all want to make devices run faster with less energy consumption, so we need new technologies,” said co-author Xiaodong Xu, UW associate professor of materials science and engineering and of physics. “The real innovation in this new approach of ours, compared to the old nanolasers, is that we’re able to have scalability and more controls.”
Still, there’s more work to be done in the near future, Xu said. Next steps include investigating photon statistics to establish the coherent properties of the laser’s light.
Source: University of Washington; Posted in http://www.technology.org/2015/03/24/scientists-build-a-nanolaser-using-a-single-atomic-sheet/ on March 24, 2015
Plasmonic Nanoantennas: From Nanotweezers to Plasmonic Photography
Kimani C. Toussaint Jr., Brian J. Roxworthy, Hao Chen, Abdul M. Bhuiya and Qing Ding
Devices that convert optical radiation to spatially concentrated energy at the nanoscale are finding applications in data storage, spectroscopy, sensing and nanoscale optical manipulation. They can even serve as a plasmonic “film” for recording images or encoding sound.
For those of a certain generation, “antenna” conjures up early childhood memories of “rabbit ears”—telescopic, upward-pointing metal rods—perched on top of a television. Childhood observations allowed us to infer a few things about antenna functionality with no knowledge of antenna theory or electromagnetics. We observed that antennas were metal and typically long, and that the antenna’s relative orientation was crucial to signal reception. We might even have made an educated guess that antennas somehow converted the “television waves” of our favorite program, beamed through the air from the broadcasting station, into a signal that our television set could translate.
With the advances in personal electronics and communication devices over the last several decades, antennas have become ubiquitous. We generally are oblivious to their presence, however, because of their ubiquity and their increased miniaturization. Cellphone technology, for example, has moved from using carrier frequencies on the order of hundreds of MHz to a few GHz; this, in turn, has reduced the size of cellphone antennas from externally mounted projections to devices small enough to be concealed within the body of the cellphone itself. That’s because the characteristic dimensions of the antenna are proportional to the order of the radiation wavelength—and, thus, higher-frequency, shorter-wavelength devices will require smaller antennas.
By the same physical logic, an antenna for light—an electromagnetic wave oscillating at THz frequencies—would require nanoscale dimensions. And developments in nanotechnology in recent years have made the fabrication of optical antennas, or nanoantennas, quite viable. The ability of these devices to convert optical radiation to spatially concentrated energy at the nanoscale has found application in areas as diverse as data storage, spectroscopy, and sensing, as well as in nanoscale optical tweezers. And our group has recently shown that optical nanoantennas can even be modified to serve as photographic film for recording images or encoding sound in the near field—a finding that opens some fascinating potential new applications for the devices.
Artistic representation of pillar-supported bowtie nanoantennas. [Phil Saunders/spacechannel.org]
The deep subwavelength confinement and strong field enhancement associated with LSPRs combine to generate large near-field intensity gradients. Thus substantial amplification of optical gradient forces can be realized with nanoantennas, an effect that forms the basis of state-of-the art plasmonic nanotweezers for optical manipulation.
Plasmonic nanotweezers use metallic dimers as nano-focusing elements to concentrate incident electric fields into an engineered gap. The result is a pronounced increase of the trapping efficiency: Q = Ftot cm/P, where Ftot is the total trapping force, cm is the speed of light in the trapping medium, and P is the incident optical power. A value of Q = 1 implies that all of the incident optical momentum is converted to trapping force on an object. The augmentation ofQ enables low-input-power particle manipulation with remarkable trap stiffness and total force.
To that end, nanotweezers have featured a wide variety of geometries, including disks, rods, blocks and bowties. Concurrent with the resonant field enhancement, nanotweezers can also produce significant heating of the surrounding fluid medium containing the target particles. While this effect has generally been regarded as deleterious to trapping performance, our group has shown that by carefully tuning the resonance conditions—such as input wavelength and optical power—a rich set of trapping dynamics can be realized for micron-sized particles.
In these systems, the delicate balance between optical and thermally induced forces, generated by off-resonance illumination of dense arrays of bowtie nanonantennas, can produce different trapping states, or phases. Below a given power threshold, efficient single-particle trapping with Q> 1 is possible, which provides direct evidence that non-optical forces can contribute favorably to trapping performance. Above this empirically determined threshold, convective forces destabilize the single-particle trap, but multiple particles can be trapped simultaneously. By exploiting the particle-size dependence of these fluid forces, nanotweezers can be used for passive particle sorting.
A pillar-supported bowtie nanoantenna chip. [PROBE Lab/University of Illinois]
Plasmonic nanotweezers are particularly attractive in their potential application to manipulating live biological specimens. Interrogation of biological systems using conventional optical tweezers has already borne remarkable fruit, providing key insights into the operation of motor proteins and mechanical properties of nucleic acids. But application of the full power of optical tweezers to any biological specimen runs the risk of specimen damage due to intense optical radiation. Plasmonic antennas, by contrast, use much lower input power densities, and are also amenable to incorporation into optofluidic systems.
Moreover, conventional optical tweezers, being subject to the diffraction limit, have trapping volumes limited to hundreds of nm3—too large for confining nanoscale objects that are significantly perturbed by Brownian fluctuations. The extreme light confinement offered by plasmonic nanoantennas can reduce trapping volumes down to tens of nm3, and plasmonic nanotweezers have been employed for manipulation of individual proteins and dielectric objects with dimensions as small as 12 nm.
Steps in making pillar-supported bowtie nanoantennas (p-BNAs): 1. E-beam lithography and metal evaporation; 2. Deposit hard mask (Ni); 3. Reactive ion etching; 4. SEM image of p-BNAs.
An emerging area of interest for plasmonic nanotweezers is the use of a femtosecond pulsed excitation source. Pulsed light offers a potentially useful route for improving the nanotweezer trap stiffness, and could also open up new regimes of particle dynamics, in which objects can be selectively fused to plasmonic structures using mere microwatts of average power. This effect could be employed to modify the optical response of selected antennas, offering a route for novel sensing modalities or studies in nonlinear optics.
The lossy nature of noble metals at optical frequencies can be exploited for sensing applications. Specifically, such loss gives these metals a complex frequency-dependent dielectric function (or refractive index) ε(ω). In the case of a spherical plasmonic nanoparticle, for example, this means that its polarizability α(ω) is proportional to [(ε – εm)/( ε + 2εm)], where εm is the complex dielectric function of the surrounding medium.
Hence, the LSPR condition for this particle occurs when the real part of ε(ω) = –2εm. Different nanoparticle geometries will result in their own resonance condition. This strong divergence of the applied electric field, along with the field’s subwavelength confinement, leads to remarkable sensitivity to changes in the refractive index of the surrounding medium, and thereby makes nanoantennas ideal candidates for sensor applications.
Plasmonic nanoantennas have shown potential for sensitive detection of flammable gases such as hydrogen. The absorption of hydrogen by a single palladium (Pd) nanoparticle was detected as a shift in the LSPR of a gold nanoantenna placed in close proximity. A basic interpretation of the phenomenon is that hydrogen absorption by the Pd nanoparticle results in a change in the complex dielectric function of the Pd nanoparticle; consequently, the nearby plasmonic nanoantenna exhibits a corresponding plasmon resonance shift that can be optically detected.
Nanotweezers in action: In plasmonic nanotweezers, electromagnetic forces from the laser are amplified in the antenna gap, forming an optical trap; as the antennas are heated by electron motion, convection currents draw particles into the trapping volume surrounding the illuminated spot.
Pillar-supported or freestanding nanoantennas constitute even better candidates for sensing than their substrate-bound counterparts, due to their higher sensitivity to the local dielectric environment. Gold-coated mushroom arrays on photoresist pillars have been fabricated and used as biosensors to detect the proteins cytochrome c and alpha-fetoprotein. And freestanding nanoantennas have been shown to be especially advantageous for surface enhanced Raman spectroscopy (SERS) of adsorbed molecules on metal surfaces, as they expose a larger fraction of the enhanced-field region, or “hot spot,” to the environment containing the analytes of interest, and as raising the nanoantennas above the substrate increases the refractive-index sensitivity. In one case, 3-D star-shaped metallic Au nanostructures fabricated on silicon pillars were shown to be sensitive enough to detect single molecules by SERS, and to detect Raman signals from adenine molecules at concentrations as low as ~1 pM.
Enhancing data storage
Advances in magnetic recording have pushed data storage density to several hundred terabytes per m2, about half of the theoretical limit. Even so, to meet the burgeoning demand for more storage capacity, magnetic grain sizes must continue to decrease. But reduction in grain size for materials such as Fe, Co, and Ni results in thermal instability as the grains become superparamagnetic—and alternative materials that can achieve the desired nanometer grain sizes exhibit magnetic anisotropy beyond what can be handled by the magnetic switching fields of conventional recording heads.
A trapping phase diagram shows the different particle sorting regimes enabled by different parameters, for different particle sizes. [Adapted from Roxworthy et al., Nano Lett. 12, 796 (2012)]
Plasmonic nanoantennas offer an approach to mitigating this latter issue. Specifically, a uniquely designed nanoantenna, optically excited at its LSPR in either the visible or near-infrared, can exploit the lightning-rod effect to funnel electromagnetic energy to a nanoscale region. This, in turn, can locally heat a desired subwavelength spot of the magnetic material to above its Curie temperature (roughly 350 °C for FePt) and, in turn, radically reduce the coercivity of the heated region, so that the available switching field of the standard head is sufficient to record data.
While this approach to magnetic recording faces a number of challenges (among them, improving the coupling between the optical field and the nanoantenna, and increasing both the field confinement and local field intensity enhancement), it offers strong potential for extending the limits of next-generation magnetic storage devices.
Our group has recently shown that the p-BNA flavor of nanoantennas can be adapted to record the optical intensity in the near field. Owing to the subtle surface melting that occurs in the metal upon resonant excitation, morphological changes occur in the form of increased radius of curvature of each bowtie tip, increased gap spacing, and reduced in-plane height of each nano-triangle. (Interestingly, even though the bulk melting temperature of gold, the metal used for p-BNAs, is greater than 1,000 °C, surface melting causes this temperature to be significantly reduced at the nanoscale.)
The end result of these shape changes is a shift of the LSPR of up to 100 nm. Therefore, depending on input power, wavelength and polarization, the near-field intensity exposure history can be recorded by the p-BNAs in a manner that resembles classic photographic film. Compared with traditional film, the “development” in plasmonic photography happens in real time, thereby enabling instant feedback to nanoscale changes. Moreover, an optical microscope rather than a conventional point-and-shoot camera is used to transduce the optical information from a subject onto the plasmonic film—an effect that occurs at subwavelength distances.
To use such a nanocamera, one can take advantage of increasingly common auxiliary microscope components such as beam-steering galvo mirrors or a spatial light modulator to record a variety of patterns on the film—in principle, down to the level of a single pixel or bowtie (roughly 200 × 200 nm). Using this approach, our group was able to record the world’s first movie encoded on nanoantennas. And—in a manner reminiscent of the early history of talking pictures, when audio information was optically encoded and stored on film for later playback—we have also shown that audio recording using p-BNAs is possible. Once again, a microscope is the platform used for recording; the desired audio signal modulates a pair of galvo mirrors such that the audio signal can be stored on the p-BNA film either as a time-varying waveform or in the frequency domain as the corresponding amplitude and phase spectra.
Information is encoded on an array of gold nanoantennas (bottom) through subtle surface melting of the metal and a consequent change in the plasmonic response. [H. Chen et al., Sci. Rep. 5, 9125 (2015)]
Aside from the obvious “cool factor,” the emergence of plasmonic photography using p-BNAs has worthwhile scientific and engineering applications. The technology could serve as a useful component in developing plasmonic-based on-chip signal processing, where unwanted frequency components can simply be ablated away.
Another application comes from the realization that the nanoantennas do not have to be ablated to effect a significant spectral shift in their LSPR response—indeed, visible color changes in the reflectivity of white light from a p-BNA chip can be observed for an input power as low as 100 µW (around 50 times less power than from a standard laser pointer), over a focused spot diameter just less than 700 nm. Thus, for nanotweezer applications, the local gradient force can be biased by spatially exposing selected regions to a particular dosage of light intensity. This effect can be used to fabricate “optofluidic channels without walls” that could prove very attractive for lab-on-a-chip applications.
Nanoantennas in the technology toolbox
Which application emerges as the “killer app” that takes plasmonic nanoantennas to the technological mainstream will depend on a number of next steps. Applications such as plasmonic circuits and direct light harvesting appear to be hampered by the fact that metals such as gold and silver are lossy and, in view of their expense, are generally not attractive for the silicon manufacturing infrastructure of the traditional semiconductor industry. This has led to emerging efforts to fabricate nonmetallic nanoantennas using alternative materials such as titanium nitride and silicon.
But, as some of the nascent research discussed above has highlighted, losses in one application domain may be viewed as a resource in another domain. That suggests that a potpourri of nanoantennas for concentrating light, with different characteristics to choose from, could drive a range of applications as their properties continue to evolve. Optical nanoantennas, like their long-wavelength counterparts, may eventually become just another—very useful—component in the toolbox.
Kimani C. Toussaint Jr., Hao Chen, Abdul M. Bhuiya and Qing Ding are with the PROBE Lab at the University of Illinois at Urbana-Champaign, Urbana, Ill., USA. Brian J. Roxworthy is a National Research Council Postdoctoral Researcher in the Nanofabrication Research Group at the National Institute of Standards and Technology, Gaithersburg, Md., USA.
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Plasmonic nanostructures for enhanced LED efficiency
“Shih-Hao Chuang and Dong-Sing Wuu”
“A novel transparent electrode embedded with silver-based nanoparticles enhances the light extraction efficiency of LEDs, improving device performance by 88%.”
Due to the important role that they play in solid-state illumination, LEDs represent a promising candidate for next-generation lighting technology. However, confinement—which occurs due to the large difference in refractive index between the semiconductor and the ambient media—leads to severe total internal reflection at the interface, lowering the light extraction efficiency (LEE) of III-nitride LEDs. The light trapped inside of the LED device is eventually reabsorbed, thereby decreasing its efficiency. To achieve high light excitation and output performance, LEE enhancement is crucial.1
By modifying the chip shape or its surface morphology, the LEE can be improved. Several approaches have been proposed for the fabrication of different structures, either inside of the substrate or on its surface (e.g., photonic crystals, nanopyramids, a patterned substrate, surface roughness, and reflectors). The typical scale of these structures ranges from a few hundred nanometers to a few micrometers, depending on the resolution limit of the optical lithographic technology used. The texturing process still has several disadvantages, however, including non-uniformity, high cost, material degradation, and limited efficiency enhancement.
Due to its high optical transparency and low resistivity, a transparent conductive oxide layer (TCL) can be employed on the LED surface as an effective current-spreading layer and graded refractive index material.2, 3 We have developed a novel TCL embedded with a plasmonic nanostructure to enhance the effective light extraction in LEDs.4
Surface plasmons (collective charge oscillations, SPs), which are excited by the interaction between light and metal surfaces, can enhance light absorption in molecules and increase Raman scattering intensities. Fluctuations of the electromagnetic fields (surface plasmon polaritons, SPPs) accompany the charge fluctuations that arise due to SP oscillation. If the metal/semiconductor surface is flat, extracting light from the SPP mode is difficult, and because the SP is non-radiative, the SPP energy thermally dissipates. However, if roughness or nanostructures are introduced on the surface of the metal layer, the SPP energy can be extracted as light due to scattering of high-momentum SPPs. This scattering causes them to lose momentum and couple, forming radiative light.5
To fabricate the TCL, we embedded silver-based nanoparticles into indium tin oxide (ITO). We grew these nanoparticles on the positively-doped gallium arsenide (p-GaN) layer of blue LED wafers and analyzed them via transmission electron microscopy. Several nanoparticles, with an average size of 25±10nm, were distributed randomly in the ITO film (see Figure 1). These metallic nanoparticles are able to scatter the propagating light and couple the localized surface plasmons (SPs confined to areas on the material surface, LSPs) to the light that is internally trapped in the ITO/p-GaN interface and the ITO layer. This light subsequently radiates outside, enhancing the LEE of the LEDs.
Figure 1. Cross-sectional high-resolution transmission electron microscope images of an indium tin oxide (ITO) film embedded with silver-based nanoparticles. Ag2O: Silver oxide. d: The distance between adjacent planes.4
To embed the nanoparticles, we employed spin coating using a nanosilver solution (particle diameter ∼20nm). The silver layer was subsequently coated uniformly on the p-GaN top layer, leading to the formation of a nanoparticle morphology during the baking process. Because the optical response of the nanoparticles strongly depends on their size, shape, and density, we controlled the nanostructures by modulating the concentration of the nanosilver solution with the appropriate spin coating rate. Deposition of the ITO layer onto the nanoparticles via electron-beam evaporation resulted in the formation of a plasmonic nanostructure embedded within the TCL.
We measured device performance according to the optical output power as a function of the injection current (see Figure 2). An 88.10% enhancement in output power was achieved at an injection current of 350mA due to the excellent electrical characteristics and LSP resonance (LSPR) effects. As Figure 3 illustrates, the normal direction and large-angle far-field pattern of the LEDs were also subject to greater enhancement than those of conventional LEDs. These results indicate that the silver-based nanoparticles efficiently increase light extraction, even at large incident angles, and enhance light diffusion.
Figure 2. Light output power as a function of injection current for LEDs with a novel transparent conductive oxide layer (TCL), embedded with (blue line) and without (black line) a plasmonic nanostructure (silver nanoparticles, Ag). The inset shows the current-voltage characteristics of the devices.4
Figure 3. Far-field emission patterns of LEDs with (red) and without (black) silver nanoparticles at an injection current of 350mA.4
The transmission spectra of the silver nanoparticles on the glass substrate show an absorption peak at 437nm. This occurs due to the LSPR generated at the plane interface between the nanoparticles and glass under blue light (see Figure 4).6 The effect of LSP-light coupling on the high-energy side of the electroluminescence (EL) spectrum therefore leads to a blueshift of 2nm from its emission peak wavelength with respect to that of a conventional LED (see Figure 4). This spectral blueshift supports the existence of LSP-light coupling, resulting in an effective emission with a 70% increase in EL peak intensity.
Figure 4. Electroluminescence (EL) spectra of the LED samples measured at 20mA. The inset shows the transmission spectra of silver nanoparticles on the glass substrate.4 a.u.: Arbitrary units.
In summary, the excellent performance of our device indicates that the incorporation of a metallic solution via spin coating could enable fabrication of blue LEDs suitable for high-power solid-state lighting. If the resonance wavelength of the LSPs is closer to the LED emission wavelength, it should result in more efficient coupling to LSPs. We are now planning to precisely control the resonance wavelength by incorporating metallic nanoparticles with various sizes, shapes, and periodic distances. Our next challenge is to demonstrate fabrication of ultraviolet LEDs with an LEE lower than that achieved in visible-wavelength LEDs.
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S.-H. Chuang, C.-S. Tsung, C.-H. Chen, S.-L. Ou, R.-H. Horng, C.-Y. Lin, D.-S. Wuu, Transparent conductive oxide films embedded with plasmonic nanostructure for light-emitting diode applications, ACS Appl. Mater. Interfaces7(4), p. 2546-2553, 2015. doi:10.1021/am507481n
W. L. Barnes, Light-emitting devices: turning the tables on surface plasmons, Nat. Mater.3, p. 588-589, 2004.
K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, A. Scherer, Surface-plasmon-enhanced light emitters based on InGaN quantum wells, Nat. Mater.3(9), p. 601-605, 2004. doi:10.1038/nmat1198