Tunable Hyperbolic Phonon Polaritons in a Suspended van der Waals α‐MoO<sub>3</sub> with Gradient Gaps

Zheng, Zebo; Sun, Fengsheng; Xu, Ning; Huang, Wuchao; Chen, Xuexian; Ke, Yu; Zhan, Runze; Chen, Huanjun; Deng, Shaozhi

doi:10.1002/adom.202102057

Cited by 16 publications

(17 citation statements)

References 49 publications

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“…One of the practical means of controlling the dielectric environment is to transfer α-MoO 3 onto a prepatterned substrate consisting of areas of different dielectric constants. − In this regard, by using the photothermal-induced resonance technique, Schwartz et al investigated the effective tuning of substrate-mediated PhP propagation characteristics in RB2 by placing α-MoO 3 on a SiO 2 substrate with periodic trenches, where it experiences two dielectric environments underneath, air and SiO 2 . Zheng et al reported the opposite dependence of the PhP wavelength (λ PhP ) on the air gap distance for gradiently suspended α-MoO 3 in RB2 and RB3. However, a systematic investigation of the impact of dielectric environment on the PhP propagation in both RB2 and RB3 of α-MoO 3 is still lacking, and the underlying mechanism remains elusive.…”

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confidence: 99%

High-Q Phonon-polaritons in Spatially Confined Freestanding α-MoO₃

et al. 2022

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Highly confined and in-plane anisotropic phonon-polaritons (PhPs) in orthorhombic-phase molybdenum trioxide (α-MoO3) exist in three Reststrahlen bands (RB) in the mid-infrared regime where the crystal exhibits negative permittivity along three principal axes. However, PhP behaviors in geometrically confined α-MoO3 remain enigmatic. Here, we investigated PhPs confined in freestanding α-MoO3 covering submicron-width trenches. We remarkably observed opposite trends in terms of PhP wavelengths in two RBs for PhPs propagating on supported and spatially confined freestanding α-MoO3. Due to the geometric confinement in the submicron α-MoO3 freestanding channel, we resolved ultraconfined PhPs with the record-high quality (Q) factor of up to 40 at room temperature, 2 times higher than the previously reported highest value in α-MoO3. We further demonstrated PhPs guiding along a curved trajectory and, for the first time, proved PhPs could be guided to desired angles within spatially confined freestanding α-MoO3 channels. The outcomes of this work can be exploited for creating high-Q PhP waveguides toward directional nanophotonic and polaritonic devices.

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confidence: 99%

High-Q Phonon-polaritons in Spatially Confined Freestanding α-MoO₃

et al. 2022

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“…[33] Besides, the ambient dielectricity, such as the air gap (suspended) beneath the sample can also be detected through this polaritonic mode. [15,18,19,40] An inimitably latent functional application in α-MoO 3 falls in the energy range of 830-920 cm −1 , the lower band of RB2 and the upper band of RB1, where HPhPs launch in the form of an elliptic resonance-cone surface, and the "splitting mode" offers an operation to divide the HPhPs into a specific direction in real space. This property is promising for applications in light information transmission on the subwavelength scale.…”

Section: Resultsmentioning

confidence: 99%

In‐Plane Hyperbolic Phonon‐Polaritons in van der Waals Nanocrystals

Huang

Lei

Dong

et al. 2022

Advanced Optical Materials

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tunable energy range. [1][2][3][4] Among the emerging natural hyperbolic materials, a particular interest has been focused on orthorhombic-phase molybdenum trioxide (α-MoO 3 ) for sustaining extremely anisotropic HPhPs in the mid-IR range, which also provides multiple photonic operation routes such as the layer-twisting, [5][6][7][8] heterojunction, [9] artificial structure, [10][11][12][13][14][15] doping, [16,17] and ambient dielectric modulation. [11,18,19] These highly anisotropic HPhPs of α-MoO 3 stem from three socalled Reststrahlen bands (RBs) with opposite-signed refractive indices along the three principal crystal orientations in the mid-IR range, which results from the strong anisotropic structure. [2,3,20] The spectral energy in RB1 (545-850 cm −1 ) and RB2 (820-972 cm −1 ) supports in-plane hyperbolic polaritonic modes (along [001] and [100], respectively), while RB3 (958 to 1010 cm −1 ) sustains the in-plane elliptic polaritonic modes (Figure S1, Supporting Information). [3] To date, the investigations on the HPhP launching and propagating in α-MoO 3 have predominately been limited to photon wavenumbers in the range of 900-1010 cm −1 (9.9-11.1 µm) through scattering-type scanning near-field optical microscopy (s-SNOM), which covers RB3 and the upper part of RB2. Few spectral studies based on nano-FTIR have extended to the mid-IR transverse optical (TO) phonon frequency of RB2 (820 cm −1 ) using a broadband source, and the nano-imaging studies in this spectral range usually need to be reconstructed from the hyperspectral data. [2,10,21] Two possible reasons make it challenging to study HPhPs in this spectral band through s-SNOM. First, a polarized and high-power narrowband laser source in this mid-IR range (like the free-electron laser [22] ), or a broadband mid-IR radiation (like synchrotron source [21,23,24] ), is not readily accessible, which is essential to obtain a highquality near-field optical nano-image. [25][26][27][28] Second, the inherent damping rate of HPhPs in α-MoO 3 dramatically increases toward the TO phonon frequencies at 820 cm −1 (the lower band edge of RB2), which hinders the observation of HPhPs in this range. [1,3,12] Alternatively, Gao et al. employed an electron beam instead of a light source through electron energy loss spectroscopy (EELS) to investigate HPhPs in a large energy range that covers all three full RBs in mid-IR. [4] However, this nonoptical technique has a short penetration depth and a low spectral energy resolution limit. [4] The in-plane anisotropic HPhPs properties, especially in the spectral range that covers both RB1 Anisotropic hyperbolic phonon-polaritons (HPhPs) in a van der Waals material, orthorhombic-phase molybdenum trioxide (α-MoO 3 ), provide strategies in multiple dimensions to manipulate photons at the nanoscale. However, the nano-imaging studies of the in-plane HPhPs along orthogonal crystal orientations are still rare. In this work, the launching and propagating properties of HPhPs are studied in α-MoO 3 upon a holey silicon nitride microcavity ...

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“…(1) Among the four polaritons, experimental research on hyperbolic phonon polaritons (especially those appearing in hBN and α-MoO 3 ) 36,67,70,74,75,[77][78][79][140][141][142][143][144][145][146][147][148][149] is much more than on the other three polaritons. This could be due to the fact that hyperbolic phonon polaritons are the earliest discovered, and hBN and α-MoO 3 are relatively easier to achieve than the other 2D natural HMs.…”

Section: Discussionmentioning

confidence: 99%

“…The opposite alteration of polariton wavelengths in the lower and upper RBs has also been reported in gradually suspended α-MoO 3 . 79 As shown in Fig. 7(c), a nanoscale gradient air gap is formed between the SiO 2 substrate and the top α-MoO 3 flake.…”

Section: Hyperbolic Polaritons In 2d Natural Hmsmentioning

confidence: 91%