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The experimental observation of nonlinear two-photon pumped vortex lasing from perovskite metasurfaces is demonstrated. The vortex lasing beam is based on symmetry-protected quasi-bound states in the continuum (QBICs). The topological charge is estimated to be +1 according to the simulation result. The quality factor and lasing threshold are around 1100 and 4.28 mJ/cm 2 , respectively. Theoretical analysis reveals that the QBIC mode originates from the magnetic dipole mode. The lasing wavelength can be experimentally designed within a broad spectral range by changing the diameter and periodicity of the metasurface. The finite array size effect of QBIC can affect the quality factor of the lasing and be used to modulate the lasing. Results shown in this study can lead to more complex vortex beam lasing from a single chip and previously unidentified ways to obtain ultrafast modulation of the QBIC lasing via the finite array size effect.

Hot spots generated through the electrodynamic coupling of the local plasmons in metallic nanoparticles play an important role in enhancing surface-enhanced Raman spectroscopy (SERS) signal. Howeve...

In this article, the excitation of dipolar localized surface plasmon resonances (LSPRs) in both the far- and near-field regions is described in terms of the relevant static, dynamic, and radiative ...

The fundamental detection scheme of typical grating or photonic crystal (PhC) sensors relies on the correlation of the diffraction behaviors with the refractive index of target analyte. For biomaterial detection, a thin gold film is usually deposited on the sensor surface so that cross linkers can be anchored before functionalizing the device. The nanoscale metallic film suggests plasmonic resonances play a critical role in light-matter interaction. In this work, we demonstrated a compact two-dimensional (2D) hexagonal PhC biosensor with thin gold film coating. In such a device, surface plasmon polariton (SPP) on the metal structure and phase matching of optical wave with the PhC are considered when analyzing light-biomaterial interaction on the detector. Using Epstein-Barr nuclear antigen-1 (EBNA-1) antibody as an example, protein sensing along different lattice orientations of the PhC biosensor was investigated. Based on the dispersion relationship and phase matching conditions, we unravel the mechanisms involved in the wavelength shifts and sensitivities. The simulation results verify the phenomenon of tighter spatial confinement and higher local field intensity from the SPP, which helps detecting small variations of the refractive index on the sensor surface.

Selective multi-wavelength infrared light sources are important elements to achieve precise molecular detection by the usage of their intrinsic vibrational spectra. In this work, we proposed a double-stacked cross-shaped metal-dielectric-metal (MDM) resonator to achieve penta-wavelength mid-infrared thermal emission. Through the optimization of un-symmetric cross-shaped tri-layers incorporated with two sandwiched dielectric materials, four distinct emission bands associated with the magnetic resonances in stacked MDM resonators were realized, which shows nondispersive and polarization-dependent property due to the localized plasmon oscillations of the magnetic resonances. In addition, the phonon emission in the silicon dioxide layer also contributes one radiation peak at λ = 10 μm. Via a simple polarization rotator, the emission wavelengths can be tuned from 4.5 and 7.5 μm to 5.5 and 8.5 μm. This paves the way for simultaneous detection of multi-band molecular absorption fingerprint, and the polarization-tunable emission wavelengths also facilitate the possibility to achieve multi-compound sensing via one compact system.

The hexagonal silver-nanoparticles (Ag-NPs) array grown in self-assembled anodic aluminum oxide (AAO) nanochannels have been successfully demonstrated to show reliable enhanced Raman-scattering signal. In this work, we systematically explored the roles of multipolar resonances for Ag-NP/AAO substrates in affecting the signal amplification, the background noise, as well as the signal-to-noise ratio at three typical wavelengths for Raman spectroscopy (532 nm, 633nm, and 785 nm). In addition, a duel-resonant system consisting of Ag-NPs embedded in a sinusoidal-shaped AAO/Al substrate was also investigated. The grating structure enables the excitation of surface plasmon polaritons (SPPs) to further couple with the generated hots spots between adjacent Ag-NPs and meanwhile provides a diffraction dispersion which is beneficial for spectral detection.

An uncooled plasmonic infrared photodetector with selective narrow absorption band in long-wave-infrared was demonstrated. The hydrogenated amorphous silicon (a-Si:H) with high temperature coefficient of resistance characteristic was adopted as a detection layer in the plasmonic photodetector. A tri-layer gold/a-Si:H/gold arrangement with metallic gold strip was designed to sustain localized surface plasmon resonances (LSPRs). By altering the metallic linewidth and the dielectric thickness, the LSPRs can be adjusted freely. Therefore, the incoming infrared with specific wavelength corresponding to LSPRs will heat up the cavity directly, leading to the reduction of a-Si:H resistance along the current path and the signal would be detected. The optical and electrical characteristics of the photodetector were studied. The responsivity and perpendicular photoresponse ratio without suspended structure was enhanced by LSP resonances and directly heat up process.

Signal-to-background ratio of surface-enhanced Raman scattering (SERS) plays an important role in the analytic applications of SERS. Its dependences on excitation wavelength and gap between metal particles were studied theoretically and experimentally. We show that this ratio is higher at smaller gaps for the same excitation wavelength and is higher for the wavelengths having higher values of the absolute values of the dielectric function of the metal. This study thus results in design guidelines in the fabrication of SERS-active substrates with high signal-to-background ratio.

A wide-angle, wideband, and polarization-insensitive metamaterial (MM) absorber was studied based on the theoretical concept of uniaxial perfect matching layer (UPML) in the terahertz range. The MM absorber was designed as a multi-layered anisotropic array structure consisting of a conductive VIA, a bi-layered slot-FSS, and a split-ring resonator (SRR) separated by porous silica spacers. Each component is optimized to approach the required macroscopic uniaxial property for satisfying the reflection-less boundary condition. The SRRs and VIAs were found to play an important role in maintaining large absorption when the angle of oblique incidence increases under transverse electric (TE) and transverse magnetic (TM) polarization, respectively. The absorption achieves 90% in the frequency regime of 0.9 to 10.5 THz, corresponding to a bandwidth of 168% to the central frequency, and retains such high performance up to 60° oblique incidence for both TE and TM waves.

We recently introduced laminate metamaterials composed of a dielectric ABC layer sequence made by atomic-layer deposition. The ABC sequence breaks inversion symmetry, allowing for second-harmonic generation. Here, we discuss 3D polymeric woodpile photonic crystals conformally coated with such ABC laminate metamaterials (unpublished). In our experiments on such meta-crystals with 24 layers and 600 nm rod spacing at around 800-900 nm fundamental wavelength, we find up to 1000-fold enhancement of the second-harmonic conversion efficiency as compared to the same ABC laminate on a planar glass substrate (for 45 degrees angle of incidence with respect to the substrate and p-polarization). To clarify the underlying mechanism, we have performed extensive numerical calculations based on solving the full-wave problem for the fundamental wave, computing the second-harmonic 3D source-term distribution assuming tensor elements for the ABC laminate as found previously, and numerically computing the resulting emitted second-harmonic wave. This analysis indicates that the enhancement is consistent with guided-mode resonant excitations at the fundamental wavelength inside of the 3D meta-crystal slab, leading to a standing-wave behavior providing beneficial local-field enhancements.

The beauty of metasurfaces lies on their capability for controlling the amplitude, phase, and polarization response with subwavelength resolution via spatially-varying nano-scatterers. With the incorporation of multiple elements or multiple resonances of nano-resonators into metasurfaces, these integrated-resonant units (IRUs) provide additional degree of freedom to manipulate the multispectral optical property of scattering light over a continuous and broad bandwidth, which is especially essential for eliminating the chromatic effect that requires smooth and linear phase dispersion. Here, we demonstrated three achromatic meta-lens based on IRUs designs to show constant focal length in the range of 400 to 650 nm. In addition, a broadband versatile polarization generator operating in 400 nm to 1300 nm was demonstrated by using multi-nanorod IRUs as building blocks. The working efficiency of multi-nanorod design is enhanced above 10 times in the near infrared range compared to the optimization case of single nanorod design, demonstrating the capability of IRUs in achieving high conversion efficiency over a broad wavelength range.

We design a series of plasmonic bowtie antennas with broadly varying resonant wavelengths from 1.34 to 3.36 $\mu\hbox{m}$ under the same antenna footprint size. Based on the prototype of contour bowtie antennas (CBAs), we modify the geometry of CBAs with extensive resonant paths and, therefore, redshift resonances and maintain the gap enhancement factors, at least at the same level with that of the solid bowtie antenna.

An ultrathin planar cavity metasurface is proposed based on ultrathin film interference and its practicability for light manipulation in visible region is experimentally demonstrated. Phase of reflected light is modulated by finely adjusting the thickness of amorphous silicon (a-Si) by a few nanometers on an aluminum (Al) substrate via nontrivial phase shifts at the interfaces and interference of multireflections generated from the planar cavity. A phase shift of π, the basic requirement for two-level phase metasurface systems, can be accomplished with an 8 nm thick difference. For proof of concept, gradient metasurfaces for beam deflection, Fresnel zone plate metalens for light focusing, and metaholograms for image reconstruction are presented, demonstrating polarization-independent and broadband characteristics. This novel mechanism for phase modulation with ultrathin planar cavity provides diverse routes to construct advanced flat optical devices with versatile applications.

The functionalities of traditional optical component are mainly based on the phase accumulation through the propagation length, leading to a bulky optical component such as converging lens and waveplate. Metasurfaces composed of planar structures with artificial design have attracted a huge number of interests due to their ability on controlling the electromagnetic phase as well as amplitude at a subwavelength scale. The feasible applications based on metasurfaces include nonlinear dynamics, light beam shaping, quantum interference etc. Beside those promising characteristics, people now intend to discover the field of meta-devices, where we can attain optical properties and functionalities through changing the feature characteristics of metasurfaces in demand. They therefore pave a potential way for the development of flat optical devices and integrated optoelectronic systems and toward the far-reaching applications which are impossible previously. In this talk, four research topics for photonic applications with metasurfaces and meta-devices will be performed and discussed: high efficiency anomalous beam deflector, highly dimensional holographic imaging, versatile polarization control and metadevices with active property.

Metasurfaces, the two-dimensional (2D) sub-wavelength artificial structures, where light is not required to have a deep penetration, have shown the ability to tailor the amplitude, phase and polarization of light. The functionalities of various optical components can be realized by metasurface-based design, such as beam splitters, filters, waveplates, deflector, lens and holograms. Here, we propose a new type of metasurface based on the concept of ultra-thin film interference and experimentally demonstrate its feasibilities in beam deflector, light focusing and broadband meta-hologram in visible spectrum. Considering an ultra-thin thin film interference system, a sandwich structure, composed of air, a lossy material layer and a metallic mirror, the reflection of this system can be regarded as the linear superposition of the partial reflections from first interface and from the cavity after several roundtrips. First, we calculate the phases and reflections of various thicknesses of amorphous silicon (a-Si) on top of aluminum layer under normal illumination of an unpolarized light in the wavelength region from 400 to 850nm. 2 π phase coverage can be achieved by changing the film thickness of a-Si within 50 nanometers. We select two thicknesses (2-level phase modulation) for the demonstration of meta-devices. The ultra-flat grating metasurface for beam steering are designed. The reflection angles of grating metasurface can be modulated by changing its period, while the specular reflection is inhibited. We further demonstrate computer-generated holograms (CGH) based on ultra-thin interference metasurface. The holographic images are reconstructed by the combinations of phase- and amplitude- modulation. These devices show the great potential and CMOS-compatibility in the application of optics, display, security printing, and metasurface-based optical storage system.

Natural toroidal molecules, such as biomolecules and proteins, possess toroidal dipole moments that are hard to be detected, which leads to extensive studies of artificial toroidal materials. Recently, toroidal metamaterials have been widely investigated to enhance toroidal dipole moments while the other multipoles are eliminated due to the spacial symmetry. In this talk, we will show several cases on the plasmonic toroidal excitation by engineering the near-field coupling between metamaterials, including their promising applications. In addition, a novel design for a toroidal metamaterial with engineering anapole mode will also be discussed. Published version

The light-transmission resonant behavior of complex-shaped patterns can be difficult to estimate intuitively due to many possible resonant contours. In this paper, we propose a simple method to predict the effective resonant paths of onefold or twofold mirror-symmetry patterns, which exploits the antiphase property of certain field component along the resonant path and the symmetry requirement associated with the incident-wave polarization state. In addition, the resonant wavelengths for aperture-type patterns can further be estimated by a simple modified cutoff wavelength equation for a rectangular waveguide. Such prediction is validated by the simulated results of the finite-difference time domain method. In addition, we discuss how the separation distance between slit elements in the aperture affects the resonant wavelength, showing how the coupling between adjacent slits would play a role in the variation of the spectra. By studying the properties of such factors and how they interact in detail, we could manipulate the spectra with an additional degree of freedom, which could be important to structures with multielements in one unit cell.

The second harmonic generation (SHG) of vertical and planar split-ring resonators (SRRs) that are broken centro-symmetry configurations at the interface of metal surface and air is investigated. Strong interactions, better electromagnetic field confinements, and less leakage into the substrate for vertical SRRs are found. Experimental results show a 2.6-fold enhancement of SHG nonlinearity, which is in good agreement with simulations and calculations. Demonstrations of 3D metastructures and vertical SRRs with strong SHG nonlinearity majorly result from magnetic dipole and electric quadrupole clearly provides potential applications for photonics and sensing.

Rice (Oryza sativa L.) sucrose transporter 4 (OsSUT4) is a rice sucrose transporter family gene. During the early seed germination stage, OsSUT4 was substantially expressed in seed aleurone layer, scutellum, and embryonic vascular bundle. The OsSUT4 mRNA level in the embryos of seeds that were germinated at 30 °C was higher than that of seeds germinated at 20 °C. The temperature effect on the changes of OsSUT4 promoter activities was consistent with that on OsSUT4 mRNA levels. The OsSUT4 gene expression that occurs in response to temperature might be mediated by soluble sugars as signals that are transported from endosperms. The tissue-specific expression patterns of OsSUT4 found in spikelets were dependent on the developmental stages of rice panicles. The OsSUT4 transcript level in anthers during the post-heading stage was higher than that during the pre-heading stage, and the developmental expression pattern of the OsSUT4 gene in anthers was highly correlated with that of genes encoded sucrose metabolism and starch synthesis enzymes. The OsSUT4 promoter activity was also detected in mature and germinating pollen. Furthermore, the OsSUT4 expression in anthers was down-regulated at low temperature (i.e., 15/13 °C) compared with that at 30/25 and 35/30 °C. In conclusion, the levels of OsSUT4 expressions in germinating embryos and maturing pollen were regulated by temperature, and the expressions of OsSUT4 in embryos and pollen at higher temperature (i.e., 30 and 35 °C) were stronger than that at lower temperature condition (i.e., 15 and 20 °C).

Nanolaminate metamaterials recently attracted a lot of attention as a novel second-order nonlinear material that can be used in integrated photonic circuits. Here, we explore theoretically and numerically the opportunity to enhance the nonlinear response from such nanolaminates by exploiting Fano resonances supported in grating-coupled waveguides. The enhancement factor of the radiated second harmonic signal compared to a flat nanolaminate can reach values as large as 35 for gold gratings and even 7000 for MgFsub2/subgratings. For the MgFsub2/subgrating, extremely high-Q Fano resonances are excited in such all-dielectric system that result in strong local fields in the nonlinear waveguide layer to boost the nonlinear conversion. A significant portion of the nonlinear signal is also strongly coupled to a dark waveguide mode, which remains guided in the nanolaminate. The strong excitation of a dark mode at the second harmonic frequency provides a viable method for utilizing second-order nonlinearities for light generation and manipulation in integrated photonic circuits.

By exploiting the in-house finite-difference time-domain (FDTD) program, the optical properties of a designated 2-D array of U- to H-shaped holes perforated on a metallic film are analyzed. Some specific near-field features are found to be associated with the three mechanisms underlying the transmission spectral profiles, namely, excitation of surface electromagnetic (EM) modes, Wood's anomalies (WAs), and shape resonances (SRs), thus providing a way to reveal the physical nature of the enhanced transmission phenomenon.

We investigate the Dyakonov-like surface wave (DLSW) at the interface of a dielectric and a metal-dielectric multilayered (MDM) structure when this MDM structure serves as an elliptic medium according to the effective medium approximation (EMA). Different from the conventional Dyakonov surface waves, we find that this kind of DLSW possesses an unexpected leaky property due to an additional hyperbolic-like wave in the MDM structure, resulting in a significant increase of propagation loss compared to the results estimated by a simple effective model based on the EMA. Moreover, such leaky property is found to be sensitive to the period of the MDM structure. Thus, to diminish this non-negligible leaky loss, one can suppress the amplitude of the leaky component by designing the MDM structure with a smaller period. On the other hand, as the MDM structure sometimes can also support an additional elliptic-like dispersion when it is regarded as a hyperbolic medium, we discuss this condition for completeness. Based on this elliptic-like dispersion, an apparent leaky property is observed in the resultant DLSW. This DLSW propagates with a wider range of propagation direction but suffers from a poor field confinement to the interface it is propagating along.

Ultrafast active control of UV light with aluminum may become an efficient way for high-speed active UV devices. However, the nonlinear optical response of aluminum in the UV region is extremely small, which impedes the realization of the promising modulation depth on ultrafast control. Here, by using the surface plasmon resonance effect, we have achieved a 55-times enhancement in the modulation depth, as well as a short switching time of several picoseconds. Further investigation showed that such an enhancement mainly resulted from a two-order-of-magnitude boost in the response of the signal light to the lattice thermal variation at the plasmonic resonance condition. This improvement in the probing sensitivity could serve as an effective approach to resolve the dynamics of lattice vibrations in metals.

We report a Legendre pseudospectral modeling of silver sphere arrays illuminated by light waves. A special feature of the present computation is that the dispersive nature of silver is described by a Drude-Lorentz model with model parameters obtained by fitting the frequency-domain complex-valued dielectric constants measured experimentally. Numerical validations are conducted based on solving both the near and far fields of the Mie scattering problem, and we observe good convergence on the numerical fields. The far-field patterns of the Ag sphere arrays are simulated. The results are compared to those obtained by the dipole-coupling-model method, and we observe very good agreement between these results. The near-field patterns are also computed, and localized enhanced electric fields in the inter-particle gap regions due to plasmonic coupling between Ag particles are visualized. As the gap between Ag particles is reduced the strength of the localized enhanced electric field is increased.

The optical and electrical properties of a photonic-plasmonic nanostructure on the back contact of thin-film solar cells were investigated numerically through the three-dimensional (3D) finite-difference time-domain method and the 3D Poisson and drift-diffusion solver. The focusing effect and the Fabry-Perot resonances are identified as the main mechanisms for the enhancement of the optical generation rate as well as the short circuit current density. However, the surface topography of certain nanopattern structures is found to reduce the internal electrostatic field of the device, thus limiting charge collection. The optimized conditions for both optics and electronics have been analyzed in this paper.

A full-vectorial finite element method is developed to analyze the surface waves propagating at the interface between two media which could be dissipative particularly. The dissipative wave possessing a complex-valued propagation constant can be determined precisely for any given propagation direction and thus the property of losses could be thoroughly analyzed. Besides, by applying a special characteristic of the implicit circular block matrix, we reduce the computational consumptions in the analysis. By utilizing this method, the Dyakonov surface wave (DSW) at the interface between a dielectric and a metal-dielectric multilayered (MDM) structure which serves as a hyperbolic medium is discussed. Its propagation loss is smaller for larger period of the MDM structure but its field becomes less confined to the interface.

Ultrafast all-optical switching in metals can be an efficient way for high-speed active photonic devices. However, with the improvement in modulation speed, typically by reducing the optical switching pulse width from picoseconds to femtoseconds, the nonlinear optical response of the metal will decrease significantly, which hinders the realization of the sufficient modulation depth at femtosecond optical control. Here, by combining two optical nonlinear enhancement effects of surface plasmon polaritons, including their extreme sensitivity to refractive index change and their capability to induce strong localized optical fields, we have achieved an ∼50-times enhancement in the modulation depth simultaneously with a switching time of ∼75-fs. Such enhancement was found to be independent of the control intensity, which sets a basis for the future application of femtosecond switching at a minimum power.

A multi-band infrared thermal emitter with polarized waveguide resonances was investigated. The device is constructed by embedding the metallic grating strips within the resonant cavity of a metal/dielectric/metal (MDM) structure. The proposed arrangement makes it possible to generate waveguide resonances with mutually orthogonal polarization, thereby providing an additional degree of freedom to vary the resonant wavelengths and polarizations in the medium infrared region. The measured reflection spectra and the finite-difference time-domain (FDTD) simulation indicated that the electric fields of the waveguide modes with two orthogonal polarizations are distributed in different regions of the cavity. Resonant wavelengths in different polarizations can be adjusted by altering the period, the metallic line width, or the position of the embedded gold strips. The ratio of the full width at half maximum (FWHM) to the peak wavelength was achieved to be smaller than 0.035. This study demonstrated a multi-band infrared thermal emission featuring a narrow bandwidth and polarization characteristics, which is quite suitable to be applied to the non-dispersive infrared (NDIR) detection system.

Metasurfaces have become a rapidly growing field of research in recent years due to their exceptional abilities in light manipulation and versatility in ultrathin optical applications. They also significantly benefit from their simplified fabrication process compared to metamaterials and are promising for integration with on-chip nanophotonic devices owing to their planar profiles. The recent progress in metasurfaces is reviewed and they are classified into six categories according to their underlying physics for realizing full 2π phase manipulation. Starting from multi-resonance and gap-plasmon metasurfaces that rely on the geometric effect of plasmonic nanoantennas, Pancharatnam–Berry-phase metasurfaces, on the other hand, use identical nanoantennas with varying rotation angles. The recent development of Huygens' metasurfaces and all-dielectric metasurfaces especially benefit from highly efficient transmission applications. An overview of state-of-the-art fabrication technologies is introduced, ranging from the commonly used processes such as electron beam and focused-ion-beam lithography to some emerging techniques, such as self-assembly and nanoimprint lithography. A variety of functional materials incorporated to reconfigurable or tunable metasurfaces is also presented. Finally, a few of the current intriguing metasurface-based applications are discussed, and opinions on future prospects are provided.

The optical and electrical properties of a new type photonic-plasmonic nanostructure on the back contact of solar cells were investigated numerically through the three-dimensional (3D) finite-difference time-domain (FDTD) method and the Poisson and drift-diffusion (DDCC) solver. The focusing effect and the Fabry-Perot resonances are identified as the main mechanisms for the enhancement of the optical generation rate as well as the short circuit current density. In addition, the surface topography of the nanopattern has a strong effect on the device physics such as the potential and recombination profiles, and therefore influencing the electrode collecting efficiency of the photocurrents.

Large and periodic anti-ring arrays are fabricated by using a monolayer of polymer/nanosphere hybrid technique and applied as back reflectors in substrate-type hydrogenated amorphous silicon (a-Si:H) thin-film solar cells. The structure of each anti-ring comprises a nanodome centered inside a nanohole. The excitation of Bloch wave surface plasmon polaritons is observed in the Ag-coated anti-ring arrays. The nanodomes of the anti-ring arrays turn out to enhance large-angle light scattering and increase the effective optical path in the solar cell. The resulting efficiency of an ultrathin a-Si:H (thickness: 150 nm) solar cell is enhanced by 39% compared to that with a flat back reflector and by 13% compared to that with a nanohole back reflector.

The optical and electrical properties of a new type photonic-plasmonic nanostructure on the back contact of solar cells were investigated numerically through the three-dimensional (3D) finite-difference time-domain (FDTD) method and the poisson and drift-diffusion (DDCC) solver. The focusing effect and the Fabry-Perot resonances are identified as the main mechanisms for the enhancement of the optical generation rate as well as the short circuit current density. In addition, the surface topography of the nanopattern has a strong effect on the device physics such as the potential and recombination profiles, and therefore influencing the electrode collecting efficiency of the photocurrents.

Inspecting biological cells with bright-field light microscopy often engenders a challenge, owing to their optical transparency. We show that imaging contrast can be greatly enhanced as yeast cells are placed on a silver nanoparticle array. Its near- and far-field traits, revealed by electrodynamic simulations, illustrate that the enhancement is attributed to the sensitivity of its plasmonic characteristics to the attached cells. This study demonstrates that the silver nanoparticle array can serve as the agent for concurrently enhancing Raman scattering and imaging contrast of microorganisms for identification and examination.

This article studied the a-Si:H solar cell with a randomly rough surface for high-power conversion efficiency. A full 3D numerical modeling program developed by our group including 3D FD-TD for optics and 3D Poisson and drift-diffusion solver for electronic simulation are used to model the characteristics of a textured solar cell. The balance between the optical and electrical performance of the a-Si:H solar cell is studied in this work. For model verification, a solar cell with high 9.23 % power conversion efficiency is used to examine the model and parameters. This article figured out the electrical limit for the a-Si:H solar cell and studied the influence of different roughness scales.