Low Refractive Index Substrate Research Articles

High-Q 1D rod-based nanocavities.

We report an analysis of one-dimensional rod-based photonic crystal nanocavities. These cavities offer opportunities for dielectric materials which lack a matching low-refractive index substrate or are limited in under-etching possibilities to create slab-based PhC cavities. They offer high theoretical Q-values exceeding 106 for transverse magnetic polarized modes with modal volumes below 2.5(λ/n)3. For practical implementations, we propose embedding these structures in a low-refractive index polymer. An analysis of intentionally introduced variations in a rod diameter reveals which design directions should be followed in order to create cavities that are most robust for fabrication-induced variations.

Highly Efficient Dielectric Optical Incoupler for Quantum Well Infrared Photodetectors

In this letter, we propose a highly efficient all-dielectric optical incoupler for a quantum well infrared photodetector (QWIP) operating in the very long wavelength infrared regime from 14 to $16~\mu \text{m}$ . The microstructure is comprised of a thin top-contact/QWs/bottom-contact semiconductor layer sandwiched by a germanium layer drilled with periodic holes and a flat low-refractive index substrate. By exploiting the waveguide mode, the dielectric optical incoupler can convert infrared radiation efficiently into the QWs layer with strong infrared absorption for QWs at the specific wavelength of 14.9 $\mu \text{m}$ , of which the absorption increases 7.5-fold compared with an isotropic active region without any designed framework. More importantly, a large electrical field component ( $E_{z}$ ) normal to the plane of QWs can be obtained with a large value of coupling efficiency ( $\eta$ ) of $\vert \text{E}_{z} \vert ^{2}$ reaching up to 1.4. A slight dispersion (over 40°) and weak polarization dependence of the guided mode supported by the dielectric incoupler might be beneficial to high-performance infrared photodetectors.

Dual-surface flexible THz Fano metasensor

Sensing technologies based on terahertz waves have immense potential due to their non-destructive, transparent, and fingerprint spectral response of several materials that are opaque to other parts of the electromagnetic spectrum. Terahertz metasensors reported so far merely exploit the fringing electric field on the top of the subwavelength resonators. Here, we experimentally demonstrate an ultrathin flexible terahertz metamaterial sensor on a low refractive index substrate which enables sensing of analytes from the top and bottom surfaces of the metamaterial, opening up avenues for dual-surface sensing of analytes with fringing resonant fields on both front and rear sides of a metasurface. Since most of the real-world objects have 3D curvatures, the reported flexible metasensor with large mechanical strength and stability in free space would be an ideal platform for ultrasensitive sensing of dielectrics, chemicals, and biomolecules of extremely low concentrations with dual non-planar surfaces.

Optofluidic distributed feedback lasers with evanescent pumping: Reduced threshold and angular dispersion analysis

We demonstrate an evanescently pumped water-based optofluidic distributed feedback (DFB) laser with a record low pump threshold of ETH=520 nJ. The low threshold results from an optimized mode shape, which is achieved by a low refractive index substrate, and from the use of a mixed-order DFB grating. Investigating the photonic band structure via angular dispersion analysis both above and below lasing threshold allows us to measure the refractive index of the liquid gain layer and to determine the device parameters such as the waveguide core layer thickness. We show that it is possible to tailor the divergence of the lasing emission by varying the number of second order grating periods used for outcoupling.

Effects of the Substrate Refractive Index, the Exciting Light Propagation Direction, and the Relative Cube Orientation on the Plasmonic Coupling Behavior of Two Silver Nanocubes at Different Separations

In this paper, we presented a detailed theoretical investigation on the effects of changing the refractive index of the substrate and the interparticle separation distance on the electromagnetic field distribution around the face-to-face (FF) and the edge-to-edge (EE) oriented silver nanocube (Ag NC) dimers. We found that the contribution of the scattering to the absorption components is markedly decreased with increasing the substrate refractive index. This suggests that for surface-enhanced Raman spectroscopic (SERS) applications, the use of a substrate of very low refractive index is highly recommended. However, for photothermal applications, the use of substrate of high refractive index is recommended. Furthermore, we found that the wavelengths corresponding to the dipolar and multipolar modes are nearly unaffected by increasing the interparticle separation distances above 40 nm on the high refractive index substrate. However, on the low refractive index substrate, the dipolar peaks consistently red-s...

Alkanethiol-functionalized terahertz metamaterial as label-free, highly-sensitive and specificbiosensor

Specific biorecognition is essential for many biological processes, for which highly sensitive and label-free biosensors are strongly demanded. The recently developed metamaterials are a potential choice for biosensing due to their exotic properties. In the current work, a label-free and specific sensor for streptavidin–agarose (SA) was fabricated based on terahertz metamaterial functionlized by octadecanthiols and biotins. Both low and high frequency resonant modes from the metamaterials are found applicable for the detection of SA, and a redshift up to 6.76GHz for the high frequency mode was measured in the undiluted commercial solution. The low frequency mode is attributed to inductor–capacitor (LC) oscillation, while the high frequency mode originates from the plasmonic dipole oscillator, both of which are highly sensitive to the micro-environment change. Adsorption of SA of different concentrations causes different redshifts, and the replacement of high refractive-index substrate with low refractive-index substrate can efficiently promote the sensitivity, well agreeing with the numerical simulation. Moreover, for a particular biomolecule, the sensitivity can be further improved by optimizing the metamaterial design. This method might be very helpful for desirable biorecognition in biology, medicine, and drug industry.

Propagation and nanofocusing of infrared surface plasmons on tapered transmission lines: Influence of the substrate

We study the propagation of mid-infrared surface plasmons on non-tapered and tapered two-wire transmission lines on Si and CaF2 substrates, the two materials representing substrates with large and small refractive index, respectively. A comparative numerical study predicts a larger effective wavelength and an increased propagation length (i.e. weaker damping) for the CaF2 substrate. By near-field microscopy we image the near-field distribution along the transmission lines and experimentally verify surface plasmon propagation. Amplitude- and phase-resolved near-field images of a non-tapered transmission line on CaF2 reveal a standing wave pattern caused by back-reflection of the surface plasmons at the open-ended transmission line. Calculated and experimental near-field images of tapered transmission lines on Si and CaF2 demonstrate that for both substrates the mid-IR surface plasmons are compressed when propagating along the taper. Importantly, the nanofocus at the taper apex yields a stronger local field enhancement for the low-refractive index substrate CaF2. We assign the more efficient nanofocusing on CaF2 to the weaker damping of the surface plasmons.

Waveguides for nitride based quantum cascade lasers

AbstractWaveguide designs for AlGaN‐based near‐infrared quantum cascade lasers are investigated using a finite‐difference method mode solver. Because of the negligibly small refractive index difference between the AlN/GaN/AlGaN gain region and the surrounding AlGaN current injection and extraction layers, a low refractive index substrate (sapphire) and a low refractive index dielectric (SiO2) are used for vertical confinement of the optical field. A ridge waveguide with an off‐center contact metallization is used for lateral confinement. The off‐center contact allows for the propagation of a TM mode with low metal induced loss and sufficient optical confinement in the gain region. A viable waveguide design with a metal‐induced loss of 6.1 cm‐1 and a confinement factor of 0.52 is demonstrated. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dielectric waveguide with deep subwavelength mode confinement based on coupled nanowires

Dielectric waveguide with deep subwavelength mode confinement based on coupled semiconductor nanowires is proposed.Through the coupling between two adjacent nanowires with high refractive indexes, light can be efficiently confined in the nano-gap between the nanowires with a low refractive index. Numerical simulations indicate that the effective mode area of such a waveguide can be as small as Λ20/200, which is one order of magnitude smaller than that of a single nanowire, and such a mode confinement is comparable to that of hybrid plasmonic waveguide. It is also shown that from the view of real applications, possible existing low refractive index oxidization layers of nanowires, low refractive index substrate and small deviation of nanowire dimensions do not have significant influence on the property of the waveguide. As the propagation length is theoretically infinite for dielectric waveguides, such a coupled nanowire waveguide with deep subwavelength mode confinement may have important applications in future integrated photonic circuits.

Polymer photonic crystal slab waveguides

We present details of the fabrication, calculations, and transmission measurements for finite two-dimensional (2D) polymer photonic crystal (PC) slab waveguides, which were fabricated from a benzocyclobutene polymer on a low refractive index substrate from Teflon. A square air hole lattice (500 nm lattice constant, 300 nm hole diameter) was realized by electron beam lithography and reactive ion etching. Polarization and wavelength dependent transmission results show TE-like and TM-like stop gaps at 1.3 μm excitation wavelengths and are in good agreement with the calculated data obtained by 2D and three-dimensional finite difference time domain methods. Transmission was suppressed by 15 dB in the center of the TE-like stop gap for a PC length of ten lattice constants.