E-mail Alert
RSS
| Citation: |
Chen S, Wu XH, Fu CJ. Active tuning of anisotropic phonon polaritons in natural van der Waals crystals with negative permittivity substrates and its application in energy transport. Opto-Electron Sci 3, 240002 (2024). doi: 10.29026/oes.2024.240002
|
Active tuning of anisotropic phonon polaritons in natural van der Waals crystals with negative permittivity substrates and its application in energy transport
-
Abstract
Phonon polaritons (PhPs) exhibit directional in-plane propagation and ultralow losses in van der Waals (vdW) crystals, offering new possibilities for controlling the flow of light at the nanoscale. However, these PhPs, including their directional propagation, are inherently determined by the anisotropic crystal structure of the host materials. Although in-plane anisotropic PhPs can be manipulated by twisting engineering, such as twisting individual vdW slabs, dynamically adjusting their propagation presents a significant challenge. The limited application of the twisted bilayer structure in bare films further restricts its usage. In this study, we present a technique in which anisotropic PhPs supported by bare biaxial vdW slabs can be actively tuned by modifying their local dielectric environment. Excitingly, we predict that the iso-frequency contour of PhPs can be reoriented to enable propagation along forbidden directions when the crystal is placed on a substrate with a moderate negative permittivity. Besides, we systematically investigate the impact of polaritonic coupling on near-field radiative heat transfer (NFRHT) between heterostructures integrated with different substrates that have negative permittivity. Our main findings reveal that through the analysis of dispersion contour and photon transmission coefficient, the excitation and reorientation of the fundamental mode facilitate increased photon tunneling, thereby enhancing heat transfer between heterostructures. Conversely, the annihilation of the fundamental mode hinders heat transfer. Furthermore, we find the enhancement or suppression of radiative energy transport depends on the relative magnitude of the slab thickness and the vacuum gap width. Finally, the effect of negative permittivity substrates on NFRHT along the [001] crystalline direction of α-MoO3 is considered. The spectral band where the excited fundamental mode resulting from the negative permittivity substrates is shifted to the first Reststrahlen Band (RB 1) of α-MoO3 and is widened, resulting in more significant enhancement of heat flux from RB 1. We anticipate our results will motivate new direction for dynamical tunability of the PhPs in photonic devices. -
-
References
[1] Low T, Chaves A, Caldwell JD et al. Polaritons in layered two-dimensional materials. Nat Mater 16, 182–194 (2017). doi: 10.1038/nmat4792 [2] Li M, Hu GW, Chen X et al. Sci Adv 8, eadd6660 (2022). [3] Passler NC, Ni X, Hu GW et al. Hyperbolic shear polaritons in low-symmetry crystals. Nature 602, 595–600 (2022). doi: 10.1038/s41586-021-04328-y [4] Hu GW, Ma WL, Hu DB et al. Real-space nanoimaging of hyperbolic shear polaritons in a monoclinic crystal. Nat Nanotechnol 18, 64–70 (2023). doi: 10.1038/s41565-022-01264-4 [5] Ma WL, Hu GW, Hu DB et al. Ghost hyperbolic surface polaritons in bulk anisotropic crystals. Nature 596, 362–366 (2021). doi: 10.1038/s41586-021-03755-1 [6] Shahzadi M, Zheng CY, Ahmad S et al. Exciton-polariton based WS2 polarization modulator controlled by optical Stark beam. Opto-Electron Adv 5, 200066 (2022). doi: 10.29026/oea.2022.200066 [7] Dai S, Fei Z, Ma Q et al. Tunable phonon polaritons in atomically thin van der Waals crystals of boron nitride. Science 343, 1125–1129 (2014). doi: 10.1126/science.1246833 [8] Basov DN, Fogler MM, García de Abajo FJ. Polaritons in van der Waals materials. Science 354, aag1992 (2016). doi: 10.1126/science.aag1992 [9] Ma WL, Alonso-González P, Li SJ et al. In-plane anisotropic and ultra-low-loss polaritons in a natural van der Waals crystal. Nature 562, 557–562 (2018). doi: 10.1038/s41586-018-0618-9 [10] Caldwell JD, Aharonovich I, Cassabois G et al. Photonics with hexagonal boron nitride. Nat Rev Mater 4, 552–567 (2019). doi: 10.1038/s41578-019-0124-1 [11] Taboada-Gutiérrez J, Álvarez-Pérez G, Duan JH et al. Broad spectral tuning of ultra-low-loss polaritons in a van der Waals crystal by intercalation. Nat Mater 19, 964–968 (2020). doi: 10.1038/s41563-020-0665-0 [12] Wu YJ, Duan JH, Ma WL et al. Manipulating polaritons at the extreme scale in van der Waals materials. Nat Rev Phys 4, 578–594 (2022). doi: 10.1038/s42254-022-00472-0 [13] Poddubny A, Iorsh I, Belov P et al. Hyperbolic metamaterials. Nat Photon 7, 948–957 (2013). doi: 10.1038/nphoton.2013.243 [14] Zhang Q, Hu GW, Ma WL et al. Interface nano-optics with van der Waals polaritons. Nature 597, 187–195 (2021). doi: 10.1038/s41586-021-03581-5 [15] Li PN, Lewin M, Kretinin AV et al. Hyperbolic phonon-polaritons in boron nitride for near-field optical imaging and focusing. Nat Commun 6, 7507 (2015). doi: 10.1038/ncomms8507 [16] Dai S, Ma Q, Andersen T et al. Subdiffractional focusing and guiding of polaritonic rays in a natural hyperbolic material. Nat Commun 6, 6963 (2015). doi: 10.1038/ncomms7963 [17] Li Y, Li W, Han TC et al. Transforming heat transfer with thermal metamaterials and devices. Nat Rev Mater 6, 488–507 (2021). doi: 10.1038/s41578-021-00283-2 [18] Gomez-Diaz JS, Alù A. Flatland optics with hyperbolic metasurfaces. ACS Photonics 3, 2211–2224 (2016). doi: 10.1021/acsphotonics.6b00645 [19] Correas-Serrano D, Alù A, Gomez-Diaz JS. Plasmon canalization and tunneling over anisotropic metasurfaces. Phys Rev B 96, 075436 (2017). doi: 10.1103/PhysRevB.96.075436 [20] Li PN, Hu GW, Dolado I et al. Collective near-field coupling and nonlocal phenomena in infrared-phononic metasurfaces for nano-light canalization. Nat Commun 11, 3663 (2020). doi: 10.1038/s41467-020-17425-9 [21] Zhang Q, Zhen Z, Yang YF et al. Negative refraction inspired polariton lens in van der Waals lateral heterojunctions. Appl Phys Lett 114, 221101 (2019). doi: 10.1063/1.5098346 [22] Lin X, Yang Y, Rivera N et al. All-angle negative refraction of highly squeezed plasmon and phonon polaritons in graphene–boron nitride heterostructures. Proc Natl Acad Sci USA 114, 6717–6721 (2017). doi: 10.1073/pnas.1701830114 [23] Kumar A, Low T, Fung KH et al. Tunable light–matter interaction and the role of hyperbolicity in graphene-hBN system. Nano Lett 15, 3172–3180 (2015). doi: 10.1021/acs.nanolett.5b01191 [24] Bapat A, Dixit S, Gupta Y et al. Gate tunable light–matter interaction in natural biaxial hyperbolic van der Waals heterostructures. Nanophotonics 11, 2329–2340 (2022). doi: 10.1515/nanoph-2022-0034 [25] Dai S, Ma Q, Liu MK et al. Graphene on hexagonal boron nitride as a tunable hyperbolic metamaterial. Nat Nanotechnol 10, 682–686 (2015). doi: 10.1038/nnano.2015.131 [26] Brar VW, Jang MS, Sherrott M et al. Hybrid surface-phonon-plasmon polariton modes in graphene/monolayer h-BN heterostructures. Nano Lett 14, 3876–3880 (2014). doi: 10.1021/nl501096s [27] Zeng YL, Ou QD, Liu L et al. Tailoring topological transitions of anisotropic polaritons by interface engineering in biaxial crystals. Nano Lett 22, 4260–4268 (2022). doi: 10.1021/acs.nanolett.2c00399 [28] Hu GW, Krasnok A, Mazor Y et al. Moiré hyperbolic metasurfaces. Nano Lett 20, 3217–3224 (2020). doi: 10.1021/acs.nanolett.9b05319 [29] Hu GW, Ou QD, Si GY et al. Topological polaritons and photonic magic angles in twisted α-MoO3 bilayers. Nature 582, 209–213 (2020). doi: 10.1038/s41586-020-2359-9 [30] Chen MY, Lin X, Dinh TH et al. Configurable phonon polaritons in twisted α-MoO3. Nat Mater 19, 1307–1311 (2020). doi: 10.1038/s41563-020-0732-6 [31] Zhang Q, Ou QD, Hu GW et al. Hybridized hyperbolic surface phonon polaritons at α-MoO3 and polar dielectric interfaces. Nano Lett 21, 3112–3119 (2021). doi: 10.1021/acs.nanolett.1c00281 [32] Zheng ZB, Sun FS, Huang WC et al. Phonon polaritons in twisted double-layers of hyperbolic van der Waals crystals. Nano Lett 20, 5301–5308 (2020). doi: 10.1021/acs.nanolett.0c01627 [33] Wang C, Xie YG, Ma JW et al. Twist-angle and thickness-ratio tuning of plasmon polaritons in twisted bilayer van der Waals films. Nano Lett 23, 6907–6913 (2023). doi: 10.1021/acs.nanolett.3c01472 [34] Álvarez-Pérez G, González-Morán A, Capote-Robayna N et al. Active tuning of highly anisotropic phonon polaritons in van der Waals crystal slabs by gated graphene. ACS Photon 9, 383–390 (2022). [35] Hu H, Chen N, Teng HC et al. Doping-driven topological polaritons in graphene/α-MoO3 heterostructures. Nat Nanotechnol 17, 940–946 (2022). doi: 10.1038/s41565-022-01185-2 [36] Hu H, Chen N, Teng HC et al. Gate-tunable negative refraction of mid-infrared polaritons. Science 379, 558–561 (2023). doi: 10.1126/science.adf1251 [37] Yu SJ, Yao HL, Hu GW et al. Hyperbolic Polaritonic Rulers Based on van der Waals α-MoO3 Waveguides and Resonators. ACS Nano 17, 23057–23064 (2023). [38] Fali A, White ST, Folland TG et al. Refractive index-based control of hyperbolic phonon-polariton propagation. Nano Lett 19, 7725–7734 (2019). doi: 10.1021/acs.nanolett.9b02651 [39] Wehmeier L, Yu SJ, Chen XZ et al. Tunable Phonon Polariton Hybridization in a van der Waals Hetero‐Bicrystal. Adv Mater , 2401349 (2024). [40] Duan J, Álvarez-Pérez G, Voronin KV et al. Enabling propagation of anisotropic polaritons along forbidden directions via a topological transition. Sci Adv 7, eabf2690 (2021). doi: 10.1126/sciadv.abf2690 [41] Hong PL, Xu L, Rahmani M. Dual bound states in the continuum enhanced second harmonic generation with transition metal dichalcogenides monolayer. Opto-Electron Adv 5, 200097 (2022). doi: 10.29026/oea.2022.200097 [42] Yan SQ, Zuo Y, Xiao SS et al. Graphene photodetector employing double slot structure with enhanced responsivity and large bandwidth. Opto-Electron Adv 5, 210159 (2022). doi: 10.29026/oea.2022.210159 [43] Zhang WL, Çakıroğlu O, Al-Enizi A et al. Solvent-free fabrication of broadband WS2 photodetectors on paper. Opto-Electron Adv 6, 220101 (2023). doi: 10.29026/oea.2023.220101 [44] Lee IH, He MZ, Zhang X et al. Image polaritons in boron nitride for extreme polariton confinement with low losses. Nat Commun 11, 3649 (2020). doi: 10.1038/s41467-020-17424-w [45] Menabde SG, Heiden JT, Cox JD et al. Image polaritons in van der Waals crystals. Nanophotonics 11, 2433–2452 (2022). doi: 10.1515/nanoph-2021-0693 [46] Zheng ZB, Xu NS, Oscurato SL et al. A mid-infrared biaxial hyperbolic van der Waals crystal. Sci Adv 5, eaav8690 (2019). doi: 10.1126/sciadv.aav8690 [47] Álvarez-Pérez G, Voronin KV, Volkov VS et al. Analytical approximations for the dispersion of electromagnetic modes in slabs of biaxial crystals. Phys Rev B 100, 235408 (2019). doi: 10.1103/PhysRevB.100.235408 [48] Högström H, Valizadeh S, Ribbing CG. Optical excitation of surface phonon polaritons in silicon carbide by a hole array fabricated by a focused ion beam. Opt Mater 30, 328–333 (2007). doi: 10.1016/j.optmat.2006.11.064 [49] Gubbin CR, Martini F, Politi A et al. Strong and coherent coupling between localized and propagating phonon polaritons. Phys Rev Lett 116, 246402 (2016). doi: 10.1103/PhysRevLett.116.246402 [50] Sehmi HS, Langbein W, Muljarov EA. Optimizing the Drude-Lorentz model for material permittivity: Method, program, and examples for gold, silver, and copper. Phys Rev B 95, 115444 (2017). doi: 10.1103/PhysRevB.95.115444 [51] Ambrosio A, Tamagnone M, Chaudhary K et al. Selective excitation and imaging of ultraslow phonon polaritons in thin hexagonal boron nitride crystals. Light Sci Appl 7, 27 (2018). doi: 10.1038/s41377-018-0039-4 [52] Caldwell JD, Kretinin AV, Chen YG et al. Sub-diffractional volume-confined polaritons in the natural hyperbolic material hexagonal boron nitride. Nat Commun 5, 5221 (2014). doi: 10.1038/ncomms6221 [53] Dai SY, Tymchenko M, Yang YF et al. Manipulation and steering of hyperbolic surface polaritons in hexagonal boron nitride. Adv Mater 30, 1706358 (2018). doi: 10.1002/adma.201706358 [54] Narimanov EE. Dyakonov waves in biaxial anisotropic crystals. Phys Rev A 98 013818 (2018). [55] Zhang ZM. Nano/Microscale Heat Transfer (Springer, Cham, 2020). [56] Polder D, Van Hove M. Theory of radiative heat transfer between closely spaced bodies. Phys Rev B 4, 3303 (1971). doi: 10.1103/PhysRevB.4.3303 [57] St-Gelais R, Guha B, Zhu LX et al. Demonstration of strong near-field radiative heat transfer between integrated nanostructures. Nano Lett 14, 6971–6975 (2014). doi: 10.1021/nl503236k [58] Kim K, Song B, Fernández-Hurtado V et al. Radiative heat transfer in the extreme near field. Nature 528, 387–391 (2015). doi: 10.1038/nature16070 [59] Messina R, Antezza M, Ben-Abdallah P. Three-body amplification of photon heat tunneling. Phys Rev Lett 109, 244302 (2012). doi: 10.1103/PhysRevLett.109.244302 [60] Fernández-Hurtado V, García-Vidal FJ, Fan SH et al. Enhancing near-field radiative heat transfer with Si-based metasurfaces. Phys Rev Lett 118, 203901 (2017). doi: 10.1103/PhysRevLett.118.203901 [61] Rytov SM. Theory of Electric Fluctuations and Thermal Radiation (Air Force Cambridge Research Center, Bedford, 1953). [62] De Wilde Y, Formanek F, Carminati R et al. Thermal radiation scanning tunnelling microscopy. Nature 444, 740–743 (2006). doi: 10.1038/nature05265 [63] Challener WA, Peng CB, Itagi AV et al. Heat-assisted magnetic recording by a near-field transducer with efficient optical energy transfer. Nat Photon 3, 220–224 (2009). doi: 10.1038/nphoton.2009.26 [64] Elzouka M, Ndao S. Near-field NanoThermoMechanical memory. Appl Phys Lett 105, 243510 (2014). doi: 10.1063/1.4904828 [65] Zhao B, Zhang ZM. Enhanced photon tunneling by surface plasmon–phonon polaritons in Graphene/hBN heterostructures. J Heat Transfer 139 022701 (2017). [66] Wu XH, Fu CJ, Zhang ZM. Influence of hBN orientation on the near-field radiative heat transfer between graphene/hBN heterostructures. J Photon Energy 9, 032702 (2019). [67] Joulain K, Mulet JP, Marquier F et al. Surface electromagnetic waves thermally excited: radiative heat transfer, coherence properties and Casimir forces revisited in the near field. Surf Sci Rep 57, 59–112 (2005). doi: 10.1016/j.surfrep.2004.12.002 [68] Fu CJ, Zhang ZM. Nanoscale radiation heat transfer for silicon at different doping levels. Int J Heat Mass Tran 49, 1703–1718 (2006). doi: 10.1016/j.ijheatmasstransfer.2005.09.037 [69] Mittapally R, Lim JW, Meyhofer E et al. Quantifying the effect of nanofilms on near-field radiative heat transfer. ACS Photon 10, 2474–2480 (2023). doi: 10.1021/acsphotonics.3c00638 [70] Su CS, Fu CJ. Near-field radiative heat transfer between two α-quartz plates having hyperbolic and double-negative-permittivity bands. Int J Heat Mass Tran 196, 123235 (2022). doi: 10.1016/j.ijheatmasstransfer.2022.123235 [71] Liu XL, Zhang RZ, Zhang ZM. Near-field radiative heat transfer with doped-silicon nanostructured metamaterials. Int J Heat Mass Tran 73, 389–398 (2014). doi: 10.1016/j.ijheatmasstransfer.2014.02.021 [72] Basu S, Zhang ZM. Ultrasmall penetration depth in nanoscale thermal radiation. Appl Phys Lett 95 133104 (2009). -
Supplementary Information
Supplementary information for Active tuning of anisotropic phonon polaritons in natural van der Waals heterostructures by negative permittivity substrates and its application 
-
Access History
Figures(14)
Tables(1)
Article Metrics
Export File
Citation
Chen S, Wu XH, Fu CJ. Active tuning of anisotropic phonon polaritons in natural van der Waals crystals with negative permittivity substrates and its application in energy transport. Opto-Electron Sci 3, 240002 (2024). doi: 10.29026/oes.2024.240002
Format
Content
DownLoad:
-
Figure 1.
(a) Schematic of α-MoO3/substrate heterostructure, where the substrate is SiC or Au. (b) Real parts of the components of the dielectric tensor of α-MoO3 along the principal directions. The Reststrahlen bands are shaded in different colors.
-
Figure 2.
(a) The minimum of kx and ky calculated by Eq. (10). The blue shaded regions and black arrows denote the reorientation of the IFC of PhPs. (b) The local enlargement of the region is marked by the yellow panel in (a). The thickness of the α-MoO3 slab is 100 nm.
-
Figure 3.
Minimum k of the excited l = 0 mode along the [001] crystalline direction, i.e., y axis. (a) The permittivity of the substrate is purely real. Dashed lines indicate the LO phonon frequency along the x- (white), y- (green) and z- (purple) directions and solid lines represent the TO phonon frequency along the y- (green) and z- (purple) directions. The black arrows show the excitation region of reorientated polaritons, which are surrounded by black and blue dashed curves and are labeled as 1 and 2. (b) The effect of the dissipation of substrate on the k of reorientated polaritons. The region where reorientated polaritons can be excited is bounded by the blue dashed lines. The thickness of the α-MoO3 slab is fixed to 100 nm.
-
Figure 4.
Dispersion of PhPs in α-MoO3 slab with a thickness of 100 nm placed on the top of different substrates: (a) air; (b) SiC; (c) Au along the x (Left) and y (Right) directions. Dashed lines indicate the LO phonon frequency and solid lines represent the TO phonon frequency along the x- (red), y- (black) and z- (purple) directions. The false color plot displays the imaginary part of the Fresnel coefficient, rp (ω, k), obtained from the transfer-matrix method (see details in the Supplementary information), which illustrates the polaritonic dispersion.
-
Figure 5.
Dispersion and corresponding Im(rp) of PhPs in a 100-nm-thick α-MoO3 slab for air (a) and (c), SiC (b), and Au (d) substrates at different frequencies. At an incident frequency of 936 cm−1, the principal axis of the hyperbola-like curves lays along the [100] direction (a) while it is along [001] direction (b) when adopting SiC substrate. Dashed curves are calculated analytically from Eq. (2), and the blue and yellow curves present l = 0 and l = 1 modes, respectively. (e–h) The simulated near-field distribution, Re (Ez), is excited by an electric dipole on the α-MoO3 slab. The insets are the corresponding dispersions obtained by the Fourier transform of the electric distribution Re (Ez).
-
Figure 6.
Left: the normalized cross-sectional near-field distribution of the HPhPs propagating along the surface. Right: corresponding field profile at x = 0. Points A-D come from Fig. 5. Differently, (a) and (c) adopt air substrate while (b) and (d) correspond to SiC and Au substrates. Calculate the real part of x-component of the electric field distribution, Re (Ex) is shown for Points A, C, and D, while the real part of the y-component of the electric field distribution, Re (Ey) is presented for Point B. The Ex and Ey are normalized by the corresponding maximum value, and the x-axis and z-axis are normalized by the thickness of the film. The black solid lines denote the air / α-MoO3 interfaces and α-MoO3 / substrate interfaces.
-
Figure 7.
The calculated real and imaginary parts of qez at ω = 936 cm−1 in (a) and (b), and ω = 989 cm−1 in (c) and (d). An imaginary part-dominated qez represents the decaying along the direction perpendicular to the interface while the real part-dominated one corresponds to phase oscillation.
-
Figure 8.
Effect of substrate on NFRHT: (a) Schematic of NFRHT between two heterostructure systems composed of α-MoO3 slab with a thickness of t and practical substrates: SiC and Au. The vacuum gap width is d. The top structures is the emitter with a high temperature T1 (300 K) and the structure below the vacuum gap is receiver with a low temperature T2 (299 K). (b) The RHF (left) and relative enhancement coefficient η (right) for three kinds of structures as functions of vacuum gap width, d at thickness of α-MoO3 slab t =20 nm. The structures studied include (I) the individual α-MoO3 slab, (II) heterostructure consisting of α-MoO3 and SiC, and (III) heterostructure consisting of α-MoO3 and Au. The gay dotted line denotes the black body limit. (c) The enlargement of panel (b) for small gap size. According to the relative enhancement coefficient of SiC substrate, the whole region is divided into three parts: Region I (η ≈ 1); Region II annihilation (η < 1); Region III enhancement (η > 1).
-
Figure 9.
(a) Spectral heat flux for a vacuum gap size d =1000 nm at a thickness of α-MoO3 slab t =20 nm. The SR II represents the inside of RBs, and the SR I and SR III denote the outside of RBs. (b) The heat flux from three spectral regions for the three structures.
-
Figure 10.
In-plane PTC at frequencies corresponding to the peaks 1-6 labeled in Fig. 9(a) for the α-MoO3 /substrate heterostructure and suspended configurations.
-
Figure 11.
(a) The radiative heat fluxes (left) and relative enhancement coefficient η (right) for three kinds of structures as functions of the thickness of α-MoO3 at a vacuum gap d =20 nm. (b) Spectral heat fluxes for three kinds of structures, at t = 20 nm. The dark blue and gray shaded region indicate the reorientation and annihilation of the l = 0 mode in α-MoO3 / SiC heterostructure, respectively. The red shaded region represents the excitation of the l = 0 mode in α-MoO3 / Au heterostructure.
-
Figure 12.
In-plane PTC for α-MoO3 / SiC heterostructure (a–d); the individual α-MoO3 slab (e–h) at frequencies of the peaks 1–4 labeled in Fig. 11(b). (i–l) denote the attenuation length of the fundamental mode (l = 0). The arrows indicate the movement of dispersion curves when the α-MoO3 slab is placed on the top of SiC substrate. The white dashed curves indicate the symmetric and anti-symmetric branches of the coupled modes.
-
Figure 13.
In-plane PTC for α-MoO3 / Au heterostructure (a) and (b); the individual α-MoO3 slab (c) and (d) at a frequency of the points 5 and 6 labeled in Fig. 11(b). The blue and yellow dashed curves represent the dispersion relation of the l = 0 and l =1 modes, respectively. The white curves denote the coupled symmetry and anti-symmetry modes.
-
Figure 14.
NFRHT along the [001] crystalline direction for d = 20 nm. (a) Left: radiative heat flux varies with the thickness of the α-MoO3 slab for the cases with and without substrates. Right: the relative enhancement coefficient. (b) Spectral heat flux and total heat flux(inset) for the 10 nm-thickness α-MoO3 slab. (c) Heat flux from every RB. In-plane PTC at incident frequency of 760 cm−1 (RB 1) (d–f), 940 cm−1 (RB 2) (g–i) and 980cm−1 (RB 3) (j–l) for three structures: individual α-MoO3 slab, α-MoO3/ Au and α-MoO3/ SiC heterostructures.
