We present an optical slot antenna and its application. By measuring transmission spectra and far-field radiation pattern of metallic slots with nanometer scale, we show that a metallic nanoslot has the properties of an antenna, which are resonance, polarization, and bidirectional far-field radiation pattern, and can be regarded as a magnetic dipole in optical region. Additionally, we also make the unidirectional radiation by adapting the geometry of RF Yagi-Uda antenna and applying slot antenna. By the aid of phase analysis based on 3-dimensional finite-difference time-domain simulation, we can increase the front-to-back ratio of an optical slot Yagi-Uda antenna up to about 5. As the application of a slot antenna, we integrate a metal-insulator-metal plasmonic waveguide with a slot antenna. A surface plasmon waveguide mode propagating in MIM structure is well-coupled to a slot antenna and radiates into free-space in form of dipole radiation. By adding an auxiliary structure that has the role of reflector as like a slot Yagi-Uda antenna, the direction of radiation from a slot antenna integrated with a plasmonic waveguide can be controlled efficiently. Besides the possibility of integration with a waveguide, we expect that a slot antenna can be applied to active devices such as light emitting diodes or lasers for the future.
Wearable devices often employ optical sensors, such as photoplethysmography sensors, for detecting heart rates or other biochemical factors. Pulse waveforms, rather than simply detecting heartbeats, can clarify arterial conditions. However, most optical sensor designs require close skin contact to reduce power consumption while obtaining good quality signals without distortion. We have designed a detection-gap-independent optical sensor array using divergence-beam-controlled slit lasers and distributed photodiodes in a pulse-detection device wearable over the wrist’s radial artery. It achieves high biosignal quality and low power consumption. The top surface of a vertical-cavity surface-emitting laser of 850 nm wavelength was covered by Au film with an open slit of width between 500 nm and 1500 nm, which generated laser emissions across a large divergence angle along an axis orthogonal to the slit direction. The sensing coverage of the slit laser diode (LD) marks a 50% improvement over nonslit LD sensor coverage. The slit LD sensor consumes 100% more input power than the nonslit LD sensor to obtain similar optical output power. The slit laser sensor showed intermediate performance between LD and light-emitting diode sensors. Thus, designing sensors with multiple-slit LD arrays can provide useful and convenient ways for incorporating optical sensors in wrist-wearable devices.
We present an optical slot antenna integrated with a metal-dielectric-metal (MIM) plasmonic waveguide. By integrating
optical slot antenna on top metal layer of MIM waveguide, we can couple the plasmon guide mode into the feed antenna
directly. The resonantly excited slot antenna works as a magnetic dipole and then radiates in dipole-like far-field pattern.
By adding an auxiliary groove structure along with the slot antenna, the radiation can be directed into the direction where
the structure determined. The demonstrated optical slot antenna integrated with a plasmonic waveguide can be used as a
“plasmonic via” in plasmonic nanocircuits.
Patterning of colloidal quantum dot (QD) of a nanometer resolution is important for potential applications in micro- or nanophotonics. Several patterning techniques such as polymer composites, molecular key-lock methods, inkjet printing, and the microcontact printing of QDs have been successfully developed and applied to various plasmonic applications. However, these methods are not easily adapted to conventional complementary metal-oxide semiconductor (CMOS)-compatible processes because of either limits in fabrication resolutions or difficulties in sub-100-nm alignment. Here, we present an adaptation of a conventional lift-off method for the patterning of colloidal QDs. This simple method can be later applied to CMOS processes by changing electron beam lithography to photolithography for building up photon-generation elements in various planar geometries. Various shapes formed by colloidal QD clusters such as straight lines, rings, and dot patterns with sub-100-nm size could be fabricated. The patterned structures show sub-10-nm positioning with good fluorescence properties and well-defined sidewall profiles. To demonstrate the applicability of our method, we present a surface plasmon generator from a QD cluster.
The patterning of colloidal quantum dots with nanometer resolution is essential for their application in photonics and plasmonics. Several patterning approaches, such as the use of polymer composites, molecular lock-and-key methods, inkjet printing, and microcontact printing of quantum dots, have limits in fabrication resolution, positioning and the variation of structural shapes. Herein, we present an adaptation of a conventional liftoff method for patterning colloidal quantum dots. This simple method is easy and requires no complicated processes. Using this method, we formed straight lines, rings, and dot patterns of colloidal quantum dots on metallic substrates. Notably, patterned lines approximately 10 nm wide were fabricated. The patterned structures display high resolution, accurate positioning, and well-defined sidewall profiles. To demonstrate the applicability of our method, we present a surface plasmon generator elaborated from quantum dots.
We emphasize the importance of optical confinement in the vertical direction for a successful two-dimensional photonic crystal waveguide (2D-PCW) performance by presenting optical properties of a 2D-PCW that has a special feature to provide a strong vertical confinement. The 2D-PCW, which was designed to operate in microwave regime, was composed of two parts: an ordinary 2D-PCW composed of alumina rods in air for lateral optical confinement and a pair of aluminum metal plates forming a metal waveguide in the vertical direction. Cylindrical alumina rods, each being 2 mm in diameter, were arranged in a square-lattice with its spatial period of 9 mm, which according to a simple photonic band calculation produces a photonic bandgap in the frequency range of 14.7~16.5 GHz for TM-polarized light (E-field parallel to the rods). This 2D-PCW was then embedded between two aluminum metal plates. Light propagation loss for a straight waveguide, which was estimated from transmission measurement as a function of waveguide length, is as low as 0.05 dB/cm whereas most of transmission loss could be attributed to the input and output couplings due to mode mismatch. When multiple 90°-bends were incorporated, on the other hand, estimated bending loss was only 0.1 dB/bend, which indicates that high performance 2D-PCWs are indeed possible if an appropriate strong confinement in the vertical direction is provided. Fabry-Perot oscillations seen in transmission spectra, whose oscillation period was observed dictated only by total waveguide length regardless of the number of bends, are another strong evidence for the low propagation and bending losses of our waveguide structure.
We introduce a compound semiconductor based omnidirectional reflector. A four layer pair stack of GaAs/AlAs was grown epitaxially using molecular beam epitaxy, which was then converted to a GaAs/Al<sub>2</sub>O<sub>3</sub> multilayer stack by selective oxidation of the AlAs layers. The resultant one-dimensional photonic crystal exhibited omnidirectional reflection properties in near infrared wavelength range below 1μm. Reflectance spectra measured at various incidence angles and polarizations were observed to be in good agreements with theoretically simulated results.