Cheng-Wei Qiu
Degree: PhD
Position: Associate Professor
Affiliation: NUS – Department of Electrical and Computer Engineering
Research Type: Experiment
Office: E4-05-17
Email: chengwei.qiu@nus.edu.sg
Contact: (65) 6516 2559
Website: https://www.ece.nus.edu.sg/staff/web.asp?id=eleqc
Research Interests:
Beam physics, tractor beams, and force between beam and particle
Silicon nanophotonics, optomechanics, Fano resonance in silicons
Graphene Metamaterials/Electronics, metamarerial antennas and waveguides chiral and birefringent materials
Waves interactions, plasmonic resonance and light scattering of materials with radial and Cartesian anisotropy
Macroscopic characterization techniques in electromagnetics: effective medium theory and dynamic homogenization method for periodic structures
CA2DM Publications:
2024 |
Guo, Qiangbing; Zhang, Qiuhong; Zhang, Tan; Zhou, Jun; Xiao, Shumin; Wang, Shijie; Feng, Yuan Ping; Qiu, Cheng-Wei Colossal in-plane optical anisotropy in a two-dimensional van der Waals crystal Journal Article NATURE PHOTONICS, 18 (11), 2024, ISSN: 1749-4885. @article{ISI:001288597300001, title = {Colossal in-plane optical anisotropy in a two-dimensional van der Waals crystal}, author = {Qiangbing Guo and Qiuhong Zhang and Tan Zhang and Jun Zhou and Shumin Xiao and Shijie Wang and Yuan Ping Feng and Cheng-Wei Qiu}, doi = {10.1038/s41566-024-01501-3}, times_cited = {1}, issn = {1749-4885}, year = {2024}, date = {2024-08-08}, journal = {NATURE PHOTONICS}, volume = {18}, number = {11}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Polarization, a fundamental property of light, has been widely exploited from quantum physics to high-dimensional optics. Materials with intrinsic optical anisotropy, such as dichroism and birefringence, are central to light polarization control, including the development of polarizers, waveplates, mirrors and phase-matching elements. Therefore, materials with strong optical anisotropy have been long-sought. Recently, two-dimensional van der Waals crystals show high optical anisotropy but are mostly restricted to the out-of-plane direction, which is challenging to access in optical engineering. Here we report a two-dimensional van der Waals material, NbOCl2, that exhibits sharp electronic and structural contrast between its in-plane orthogonal axes. Colossal in-plane optical anisotropy-linear dichroism (up to 99% in ultraviolet) and birefringence (0.26-0.46 within a wide visible-near-infrared transparency window)-is experimentally demonstrated. Our findings provide a powerful and easy-to-access recipe for ultracompact integrated polarization industries.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Polarization, a fundamental property of light, has been widely exploited from quantum physics to high-dimensional optics. Materials with intrinsic optical anisotropy, such as dichroism and birefringence, are central to light polarization control, including the development of polarizers, waveplates, mirrors and phase-matching elements. Therefore, materials with strong optical anisotropy have been long-sought. Recently, two-dimensional van der Waals crystals show high optical anisotropy but are mostly restricted to the out-of-plane direction, which is challenging to access in optical engineering. Here we report a two-dimensional van der Waals material, NbOCl2, that exhibits sharp electronic and structural contrast between its in-plane orthogonal axes. Colossal in-plane optical anisotropy-linear dichroism (up to 99% in ultraviolet) and birefringence (0.26-0.46 within a wide visible-near-infrared transparency window)-is experimentally demonstrated. Our findings provide a powerful and easy-to-access recipe for ultracompact integrated polarization industries. |
2023 |
Guo, Qiangbing; Qi, Xiao-Zhuo; Zhang, Lishu; Gao, Meng; Hu, Sanlue; Zhou, Wenju; Zang, Wenjie; Zhao, Xiaoxu; Wang, Junyong; Yan, Bingmin; Xu, Mingquan; Wu, Yun-Kun; Eda, Goki; Xiao, Zewen; Yang, Shengyuan A; Gou, Huiyang; Feng, Yuan Ping; Guo, Guang-Can; Zhou, Wu; Ren, Xi-Feng; Qiu, Cheng-Wei; Pennycook, Stephen J; Wee, Andrew T S Ultrathin quantum light source with van der Waals NbOCl2 crystal Journal Article 114 NATURE, 613 (7942), pp. 53-+, 2023, ISSN: 0028-0836. @article{ISI:000955711300008, title = {Ultrathin quantum light source with van der Waals NbOCl_{2} crystal}, author = {Qiangbing Guo and Xiao-Zhuo Qi and Lishu Zhang and Meng Gao and Sanlue Hu and Wenju Zhou and Wenjie Zang and Xiaoxu Zhao and Junyong Wang and Bingmin Yan and Mingquan Xu and Yun-Kun Wu and Goki Eda and Zewen Xiao and Shengyuan A Yang and Huiyang Gou and Yuan Ping Feng and Guang-Can Guo and Wu Zhou and Xi-Feng Ren and Cheng-Wei Qiu and Stephen J Pennycook and Andrew T S Wee}, doi = {10.1038/s41586-022-05393-7}, times_cited = {114}, issn = {0028-0836}, year = {2023}, date = {2023-01-05}, journal = {NATURE}, volume = {613}, number = {7942}, pages = {53-+}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Interlayer electronic coupling in two-dimensional materials enables tunable and emergent properties by stacking engineering. However, it also results in significant evolution of electronic structures and attenuation of excitonic effects in two-dimensional semiconductors as exemplified by quickly degrading excitonic photoluminescence and optical nonlinearities in transition metal dichalcogenides when monolayers are stacked into van der Waals structures. Here we report a van der Waals crystal, niobium oxide dichloride (NbOCl2), featuring vanishing interlayer electronic coupling and monolayer-like excitonic behaviour in the bulk form, along with a scalable second-harmonic generation intensity of up to three orders higher than that in monolayer WS2. Notably, the strong second-order nonlinearity enables correlated parametric photon pair generation, through a spontaneous parametric down-conversion (SPDC) process, in flakes as thin as about 46 nm. To our knowledge, this is the first SPDC source unambiguously demonstrated in two-dimensional layered materials, and the thinnest SPDC source ever reported. Our work opens an avenue towards developing van der Waals material-based ultracompact on-chip SPDC sources as well as high-performance photon modulators in both classical and quantum optical technologies(1-4).}, keywords = {}, pubstate = {published}, tppubtype = {article} } Interlayer electronic coupling in two-dimensional materials enables tunable and emergent properties by stacking engineering. However, it also results in significant evolution of electronic structures and attenuation of excitonic effects in two-dimensional semiconductors as exemplified by quickly degrading excitonic photoluminescence and optical nonlinearities in transition metal dichalcogenides when monolayers are stacked into van der Waals structures. Here we report a van der Waals crystal, niobium oxide dichloride (NbOCl2), featuring vanishing interlayer electronic coupling and monolayer-like excitonic behaviour in the bulk form, along with a scalable second-harmonic generation intensity of up to three orders higher than that in monolayer WS2. Notably, the strong second-order nonlinearity enables correlated parametric photon pair generation, through a spontaneous parametric down-conversion (SPDC) process, in flakes as thin as about 46 nm. To our knowledge, this is the first SPDC source unambiguously demonstrated in two-dimensional layered materials, and the thinnest SPDC source ever reported. Our work opens an avenue towards developing van der Waals material-based ultracompact on-chip SPDC sources as well as high-performance photon modulators in both classical and quantum optical technologies(1-4). |
2022 |
Jiang, Yucheng; He, Anpeng; Luo, Kai; Zhang, Jinlei; Liu, Guozhen; Zhao, Run; Zhang, Qing; Wang, Zhuo; Zhao, Chen; Wang, Lin; Qi, Yaping; Gao, Ju; Loh, Kian Ping; Wee, Andrew T S; Qiu, Cheng-Wei Giant bipolar unidirectional photomagnetoresistance Journal Article PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 119 (27), 2022, ISSN: 0027-8424. @article{ISI:000838706400010, title = {Giant bipolar unidirectional photomagnetoresistance}, author = {Yucheng Jiang and Anpeng He and Kai Luo and Jinlei Zhang and Guozhen Liu and Run Zhao and Qing Zhang and Zhuo Wang and Chen Zhao and Lin Wang and Yaping Qi and Ju Gao and Kian Ping Loh and Andrew T S Wee and Cheng-Wei Qiu}, doi = {10.1073/pnas.2115939119}, times_cited = {7}, issn = {0027-8424}, year = {2022}, date = {2022-07-05}, journal = {PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA}, volume = {119}, number = {27}, publisher = {NATL ACAD SCIENCES}, address = {2101 CONSTITUTION AVE NW, WASHINGTON, DC 20418 USA}, abstract = {Positive magnetoresistance (PMR) and negative magnetoresistance (NMR) describe two opposite responses of resistance induced by a magnetic field. Materials with giant PMR are usually distinct from those with giant NMR due to different physical natures. Here, we report the unusual photomagnetoresistance in the van der Waals heterojunctions of WSe2/quasi-two-dimensional electron gas, showing the coexistence of giant PMR and giant NMR. The PMR and NMR reach 1,007.5% at -9 T and -93.5% at 2.2 T in a single device, respectively. The magnetoresistance spans over two orders of magnitude on inversion of field direction, implying a giant unidirectional magnetoresistance (UMR). By adjusting the thickness of the WSe2 layer, we achieve the maxima of PMR and NMR, which are 4,900,000% and -99.8%, respectively. The unique magnetooptical transport shows the unity of giant UMR, PMR, and NMR, referred to as giant bipolar unidirectional photomagnetoresistance. These features originate from strong out-of-plane spin splitting, magnetic field-enhanced recombination of photocarriers, and the Zeeman effect through our experimental and theoretical investigations. This work offers directions for high-performance light-tunable spintronic devices.NMR)}, keywords = {}, pubstate = {published}, tppubtype = {article} } Positive magnetoresistance (PMR) and negative magnetoresistance (NMR) describe two opposite responses of resistance induced by a magnetic field. Materials with giant PMR are usually distinct from those with giant NMR due to different physical natures. Here, we report the unusual photomagnetoresistance in the van der Waals heterojunctions of WSe2/quasi-two-dimensional electron gas, showing the coexistence of giant PMR and giant NMR. The PMR and NMR reach 1,007.5% at -9 T and -93.5% at 2.2 T in a single device, respectively. The magnetoresistance spans over two orders of magnitude on inversion of field direction, implying a giant unidirectional magnetoresistance (UMR). By adjusting the thickness of the WSe2 layer, we achieve the maxima of PMR and NMR, which are 4,900,000% and -99.8%, respectively. The unique magnetooptical transport shows the unity of giant UMR, PMR, and NMR, referred to as giant bipolar unidirectional photomagnetoresistance. These features originate from strong out-of-plane spin splitting, magnetic field-enhanced recombination of photocarriers, and the Zeeman effect through our experimental and theoretical investigations. This work offers directions for high-performance light-tunable spintronic devices.NMR) |
2021 |
Zhu, Rui; Gao, Zhibin; Liang, Qijie; Hu, Junxiong; Wang, Jian-Sheng; Qiu, Cheng-Wei; Wee, Andrew Thye Shen Observation of Anisotropic Magnetoresistance in Layered Nonmagnetic Semiconducting PdSe2 Journal Article 13 ACS APPLIED MATERIALS & INTERFACES, 13 (31), pp. 37527-37534, 2021, ISSN: 1944-8244. @article{ISI:000685245800085, title = {Observation of Anisotropic Magnetoresistance in Layered Nonmagnetic Semiconducting PdSe_{2}}, author = {Rui Zhu and Zhibin Gao and Qijie Liang and Junxiong Hu and Jian-Sheng Wang and Cheng-Wei Qiu and Andrew Thye Shen Wee}, doi = {10.1021/acsami.1c10500}, times_cited = {13}, issn = {1944-8244}, year = {2021}, date = {2021-08-01}, journal = {ACS APPLIED MATERIALS & INTERFACES}, volume = {13}, number = {31}, pages = {37527-37534}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Anisotropy in crystals usually has remarkable consequences in two-dimensional (2D) materials, for example, black phosphorus, PdSe2, and SnS, arising from different lattice periodicities along different crystallographic directions. Electrical anisotropy has been successfully demonstrated in 2D materials, but anisotropic magnetoresistance in 2D materials is rarely studied. Herein, we report anisotropic magnetoresistance in layered non-magnetic semiconducting PdSe2 flakes. Anisotropic magnetoresistance along the two crystalline axes under a perpendicular magnetic field is demonstrated, and the magnetoresistance along the a-axis is apparently different from the magnetoresistance along the b-axis. The magnetoresistance can also be flexibly tuned by applying a gate voltage, leveraging the semiconductor properties of PdSe2. The computed anisotropic electronic density of states and electronic mobility with ab initio density functional calculations support the anisotropic and measured magnetoresistance. Our findings advance the understanding of magnetoresistance in anisotropic transition-metal dichalcogenides and pave the way for potential applications in anisotropic spintronic devices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Anisotropy in crystals usually has remarkable consequences in two-dimensional (2D) materials, for example, black phosphorus, PdSe2, and SnS, arising from different lattice periodicities along different crystallographic directions. Electrical anisotropy has been successfully demonstrated in 2D materials, but anisotropic magnetoresistance in 2D materials is rarely studied. Herein, we report anisotropic magnetoresistance in layered non-magnetic semiconducting PdSe2 flakes. Anisotropic magnetoresistance along the two crystalline axes under a perpendicular magnetic field is demonstrated, and the magnetoresistance along the a-axis is apparently different from the magnetoresistance along the b-axis. The magnetoresistance can also be flexibly tuned by applying a gate voltage, leveraging the semiconductor properties of PdSe2. The computed anisotropic electronic density of states and electronic mobility with ab initio density functional calculations support the anisotropic and measured magnetoresistance. Our findings advance the understanding of magnetoresistance in anisotropic transition-metal dichalcogenides and pave the way for potential applications in anisotropic spintronic devices. |
2020 |
Wu, Jing; Liu, Yanpeng; Liu, Yi; Cai, Yongqing; Zhao, Yunshan; Ng, Hong Kuan; Watanabe, Kenji; Taniguchi, Takashi; Zhang, Gang; Qiu, Cheng-Wei; Chi, Dongzhi; Neto, Castro A H; Thong, John T L; Loh, Kian Ping; Hippalgaonkar, Kedar Large enhancement of thermoelectric performance in MoS2/h-BN heterostructure due to vacancy-induced band hybridization (vol 117, pg 13929, 2020) Journal Article PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 117 (30), pp. 18127-18127, 2020, ISSN: 0027-8424. @article{ISI:000555849400010, title = {Large enhancement of thermoelectric performance in MoS_{2}/h-BN heterostructure due to vacancy-induced band hybridization (vol 117, pg 13929, 2020)}, author = {Jing Wu and Yanpeng Liu and Yi Liu and Yongqing Cai and Yunshan Zhao and Hong Kuan Ng and Kenji Watanabe and Takashi Taniguchi and Gang Zhang and Cheng-Wei Qiu and Dongzhi Chi and Castro A H Neto and John T L Thong and Kian Ping Loh and Kedar Hippalgaonkar}, doi = {10.1073/pnas.2012778117}, times_cited = {2}, issn = {0027-8424}, year = {2020}, date = {2020-07-28}, journal = {PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA}, volume = {117}, number = {30}, pages = {18127-18127}, publisher = {NATL ACAD SCIENCES}, address = {2101 CONSTITUTION AVE NW, WASHINGTON, DC 20418 USA}, keywords = {}, pubstate = {published}, tppubtype = {article} } |
Zhu, Rui; Zhang, Wen; Shen, Wei; Wong, Ping Kwan Johnny; Wang, Qixing; Liang, Qijie; Tian, Zhen; Zhai, Ya; Qiu, Cheng-wei; Wee, Andrew T S Exchange Bias in van der Waals CrCl3/Fe3GeTe2 Heterostructures Journal Article 98 NANO LETTERS, 20 (7), pp. 5030-5035, 2020, ISSN: 1530-6984. @article{ISI:000548893200044, title = {Exchange Bias in van der Waals CrCl_{3}/Fe_{3}GeTe_{2} Heterostructures}, author = {Rui Zhu and Wen Zhang and Wei Shen and Ping Kwan Johnny Wong and Qixing Wang and Qijie Liang and Zhen Tian and Ya Zhai and Cheng-wei Qiu and Andrew T S Wee}, doi = {10.1021/acs.nanolett.0c01149}, times_cited = {98}, issn = {1530-6984}, year = {2020}, date = {2020-07-08}, journal = {NANO LETTERS}, volume = {20}, number = {7}, pages = {5030-5035}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Exchange bias is a physical phenomenon whereby the spins of a ferromagnet are pinned by those of an antiferromagnet, and this phenomenon has played an undisputed role in magnetic data storage. Over the past few decades, this effect has been observed in a variety of antiferromagnet/ferromagnet systems. New aspects of this phenomenon are being discovered. With the increasing interest in van der Waals (vdW) magnets, we address the question whether the effect can exist in magnetic vdW heterostructures. Here, we report exchange-bias fields of over 50 mT in mechanically exfoliated CrCl3/Fe3GeTe2 heterostructures at 2.5 K, the value of which is highly tunable by the field-cooling process and the heterostructure thickness. We postulate an intuitive picture explaining how the effect arises in this vdW heterostructure, as well as explaining the practical difficulty associated with capturing the effect. This work opens up new routes toward designing spintronic devices made of atomically thin vdW magnets.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Exchange bias is a physical phenomenon whereby the spins of a ferromagnet are pinned by those of an antiferromagnet, and this phenomenon has played an undisputed role in magnetic data storage. Over the past few decades, this effect has been observed in a variety of antiferromagnet/ferromagnet systems. New aspects of this phenomenon are being discovered. With the increasing interest in van der Waals (vdW) magnets, we address the question whether the effect can exist in magnetic vdW heterostructures. Here, we report exchange-bias fields of over 50 mT in mechanically exfoliated CrCl3/Fe3GeTe2 heterostructures at 2.5 K, the value of which is highly tunable by the field-cooling process and the heterostructure thickness. We postulate an intuitive picture explaining how the effect arises in this vdW heterostructure, as well as explaining the practical difficulty associated with capturing the effect. This work opens up new routes toward designing spintronic devices made of atomically thin vdW magnets. |
Wu, Jing; Liu, Yanpeng; Liu, Yi; Cai, Yongqing; Zhao, Yunshan; Ng, Hong Kuan; Watanabe, Kenji; Taniguchi, Takashi; Zhang, Gang; Qiu, Cheng-Wei; Chi, Dongzhi; Neto, Castro A H; Thong, John T L; Loh, Kian Ping; Hippalgaonkar, Kedar Large enhancement of thermoelectric performance in MoS2/h-BN heterostructure due to vacancy -induced band hybridization Journal Article 44 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 117 (25), pp. 13929-13936, 2020, ISSN: 0027-8424. @article{ISI:000546763100018, title = {Large enhancement of thermoelectric performance in MoS_{2}/\textit{h}-BN heterostructure due to vacancy -induced band hybridization}, author = {Jing Wu and Yanpeng Liu and Yi Liu and Yongqing Cai and Yunshan Zhao and Hong Kuan Ng and Kenji Watanabe and Takashi Taniguchi and Gang Zhang and Cheng-Wei Qiu and Dongzhi Chi and Castro A H Neto and John T L Thong and Kian Ping Loh and Kedar Hippalgaonkar}, doi = {10.1073/pnas.2007495117}, times_cited = {44}, issn = {0027-8424}, year = {2020}, date = {2020-06-23}, journal = {PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA}, volume = {117}, number = {25}, pages = {13929-13936}, publisher = {NATL ACAD SCIENCES}, address = {2101 CONSTITUTION AVE NW, WASHINGTON, DC 20418 USA}, abstract = {Local impurity states arising from atomic vacancies in two-dimensional (2D) nanosheets are predicted to have a profound effect on charge transport due to resonant scattering and can be used to manipulate thermoelectric properties. However, the effects of these impurities are often masked by external fluctuations and turbostratic inter- faces; therefore, it is challenging to probe the correlation between vacancy impurities and thermoelectric parameters experimentally. In this work, we demonstrate that n-type molybdenum disulfide (MoS2 ) supported on hexagonal boron nitride (h-BN) substrate re- veals a large anomalous positive Seebeck coefficient with strong band hybridization. The presence of vacancies on MoS2 with a large conduction subband splitting of 50.0 +/- 5.0 meV may contribute to Kondo insulator-like properties. Furthermore, by tuning the chem- ical potential, the thermoelectric power factor can be enhanced by up to two orders of magnitude to 50 mW m(-1) K-2 . Our work shows that defect engineering in 2D materials provides an effective strat- egy for controlling band structure and tuning thermoelectric transport.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Local impurity states arising from atomic vacancies in two-dimensional (2D) nanosheets are predicted to have a profound effect on charge transport due to resonant scattering and can be used to manipulate thermoelectric properties. However, the effects of these impurities are often masked by external fluctuations and turbostratic inter- faces; therefore, it is challenging to probe the correlation between vacancy impurities and thermoelectric parameters experimentally. In this work, we demonstrate that n-type molybdenum disulfide (MoS2 ) supported on hexagonal boron nitride (h-BN) substrate re- veals a large anomalous positive Seebeck coefficient with strong band hybridization. The presence of vacancies on MoS2 with a large conduction subband splitting of 50.0 +/- 5.0 meV may contribute to Kondo insulator-like properties. Furthermore, by tuning the chem- ical potential, the thermoelectric power factor can be enhanced by up to two orders of magnitude to 50 mW m(-1) K-2 . Our work shows that defect engineering in 2D materials provides an effective strat- egy for controlling band structure and tuning thermoelectric transport. |
Li, Qi; Zhao, Xiaoxu; Deng, Longjiang; Shi, Zhongtai; Liu, Sheng; Wei, Qilin; Zhang, Linbo; Cheng, Yingchun; Zhang, Li; Lu, Haipeng; Gao, Weibo; Huang, Wei; Qiu, Cheng-Wei; Xiang, Gang; Pennycook, Stephen John; Xiong, Qihua; Loh, Kian Ping; Peng, Bo Enhanced Valley Zeeman Splitting in Fe-Doped Monolayer MoS2 Journal Article 85 ACS NANO, 14 (4), pp. 4636-4645, 2020, ISSN: 1936-0851. @article{ISI:000529895500083, title = {Enhanced Valley Zeeman Splitting in Fe-Doped Monolayer MoS_{2}}, author = {Qi Li and Xiaoxu Zhao and Longjiang Deng and Zhongtai Shi and Sheng Liu and Qilin Wei and Linbo Zhang and Yingchun Cheng and Li Zhang and Haipeng Lu and Weibo Gao and Wei Huang and Cheng-Wei Qiu and Gang Xiang and Stephen John Pennycook and Qihua Xiong and Kian Ping Loh and Bo Peng}, doi = {10.1021/acsnano.0c00291}, times_cited = {85}, issn = {1936-0851}, year = {2020}, date = {2020-04-28}, journal = {ACS NANO}, volume = {14}, number = {4}, pages = {4636-4645}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {The "Zeeman effect" offers unique opportunities for magnetic manipulation of the spin degree of freedom (DOF). Recently, valley Zeeman splitting, referring to the lifting of valley degeneracy, has been demonstrated in two-dimensional transition metal dichalcogenides (TMDs) at liquid helium temperature. However, to realize the practical applications of valley pseudospins, the valley DOF must be controllable by a magnetic field at room temperature, which remains a significant challenge. Magnetic doping in TMDs can enhance the Zeeman splitting; however, to achieve this experimentally is not easy. Here, we report unambiguous magnetic manipulation of valley Zeeman splitting at 300 K (g(eff) = -6.4) and 10 K (g(eff) = -11) in a CVD-grown Fe-doped MoS2 monolayer; the effective Lande g(eff )factor can be tuned to -20.7 by increasing the Fe dopant concentration, which represents an approximately 5-fold enhancement as compared to undoped MoS2. Our measurements and calculations reveal that the enhanced splitting and gaff factors are due to the Heisenberg exchange interaction of the localized magnetic moments (Fe 3d electrons) with MoS2 through the d-orbital hybridization.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The "Zeeman effect" offers unique opportunities for magnetic manipulation of the spin degree of freedom (DOF). Recently, valley Zeeman splitting, referring to the lifting of valley degeneracy, has been demonstrated in two-dimensional transition metal dichalcogenides (TMDs) at liquid helium temperature. However, to realize the practical applications of valley pseudospins, the valley DOF must be controllable by a magnetic field at room temperature, which remains a significant challenge. Magnetic doping in TMDs can enhance the Zeeman splitting; however, to achieve this experimentally is not easy. Here, we report unambiguous magnetic manipulation of valley Zeeman splitting at 300 K (g(eff) = -6.4) and 10 K (g(eff) = -11) in a CVD-grown Fe-doped MoS2 monolayer; the effective Lande g(eff )factor can be tuned to -20.7 by increasing the Fe dopant concentration, which represents an approximately 5-fold enhancement as compared to undoped MoS2. Our measurements and calculations reveal that the enhanced splitting and gaff factors are due to the Heisenberg exchange interaction of the localized magnetic moments (Fe 3d electrons) with MoS2 through the d-orbital hybridization. |
Hong, Xuanmiao; Hu, Guangwei; Zhao, Wenchao; Wang, Kai; Sun, Shang; Zhu, Rui; Wu, Jing; Liu, Weiwei; Loh, Kian Ping; Wee, Andrew Thye Shen; Wang, Bing; Alu, Andrea; Qiu, Cheng-Wei; Lu, Peixiang Structuring Nonlinear Wavefront Emitted from Monolayer Transition-Metal Dichalcogenides Journal Article 53 RESEARCH, 2020 , 2020. @article{ISI:000527258800001, title = {Structuring Nonlinear Wavefront Emitted from Monolayer Transition-Metal Dichalcogenides}, author = {Xuanmiao Hong and Guangwei Hu and Wenchao Zhao and Kai Wang and Shang Sun and Rui Zhu and Jing Wu and Weiwei Liu and Kian Ping Loh and Andrew Thye Shen Wee and Bing Wang and Andrea Alu and Cheng-Wei Qiu and Peixiang Lu}, doi = {10.34133/2020/9085782}, times_cited = {53}, year = {2020}, date = {2020-04-05}, journal = {RESEARCH}, volume = {2020}, publisher = {AMER ASSOC ADVANCEMENT SCIENCE}, address = {1200 NEW YORK AVE, NW, WASHINGTON, DC 20005 USA}, abstract = {The growing demand for tailored nonlinearity calls for a structure with unusual phase discontinuity that allows the realization of nonlinear optical chirality, holographic imaging, and nonlinear wavefront control. Transition-metal dichalcogenide (TMDC) monolayers offer giant optical nonlinearity within a few-angstrom thickness, but limitations in optical absorption and domain size impose restriction on wavefront control of nonlinear emissions using classical light sources. In contrast, noble metal-based plasmonic nanosieves support giant field enhancements and precise nonlinear phase control, with hundred-nanometer pixel-level resolution; however, they suffer from intrinsically weak nonlinear susceptibility. Here, we report a multifunctional nonlinear interface by integrating TMDC monolayers with plasmonic nanosieves, yielding drastically different nonlinear functionalities that cannot be accessed by either constituent. Such a hybrid nonlinear interface allows second-harmonic (SH) orbital angular momentum (OAM) generation, beam steering, versatile polarization control, and holograms, with an effective SH nonlinearity chi((2)) of similar to 25 nm/V. This designer platform synergizes the TMDC monolayer and plasmonic nanosieves to empower tunable geometric phases and large field enhancement, paving the way toward multifunctional and ultracompact nonlinear optical devices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The growing demand for tailored nonlinearity calls for a structure with unusual phase discontinuity that allows the realization of nonlinear optical chirality, holographic imaging, and nonlinear wavefront control. Transition-metal dichalcogenide (TMDC) monolayers offer giant optical nonlinearity within a few-angstrom thickness, but limitations in optical absorption and domain size impose restriction on wavefront control of nonlinear emissions using classical light sources. In contrast, noble metal-based plasmonic nanosieves support giant field enhancements and precise nonlinear phase control, with hundred-nanometer pixel-level resolution; however, they suffer from intrinsically weak nonlinear susceptibility. Here, we report a multifunctional nonlinear interface by integrating TMDC monolayers with plasmonic nanosieves, yielding drastically different nonlinear functionalities that cannot be accessed by either constituent. Such a hybrid nonlinear interface allows second-harmonic (SH) orbital angular momentum (OAM) generation, beam steering, versatile polarization control, and holograms, with an effective SH nonlinearity chi((2)) of similar to 25 nm/V. This designer platform synergizes the TMDC monolayer and plasmonic nanosieves to empower tunable geometric phases and large field enhancement, paving the way toward multifunctional and ultracompact nonlinear optical devices. |
2018 |
Wang, Zhuo; Dong, Zhaogang; Zhu, Hai; Jin, Lei; Chiu, Ming-Hui; Li, Lain-Jong; Xu, Qing-Hua; Eda, Goki; Maier, Stefan A; Wee, Andrew T S; Qiu, Cheng-Wei; Yang, Joel K W Selectively Plasmon-Enhanced Second-Harmonic Generation from Monolayer Tungsten Diselenide on Flexible Substrates Journal Article 116 ACS NANO, 12 (2), pp. 1859-+, 2018, ISSN: 1936-0851. @article{ISI:000426615600098, title = {Selectively Plasmon-Enhanced Second-Harmonic Generation from Monolayer Tungsten Diselenide on Flexible Substrates}, author = {Zhuo Wang and Zhaogang Dong and Hai Zhu and Lei Jin and Ming-Hui Chiu and Lain-Jong Li and Qing-Hua Xu and Goki Eda and Stefan A Maier and Andrew T S Wee and Cheng-Wei Qiu and Joel K W Yang}, doi = {10.1021/acsnano.7b08682}, times_cited = {116}, issn = {1936-0851}, year = {2018}, date = {2018-02-01}, journal = {ACS NANO}, volume = {12}, number = {2}, pages = {1859-+}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Monolayer two-dimensional transition-metal dichalcogenides (2D TMDCs) exhibit promising characteristics in miniaturized nonlinear optical frequency converters, due to their inversion asymmetry and large second-order nonlinear susceptibility. However, these materials usually have very short light interaction lengths with the pump laser because they are atomically thin, such that second-harmonic generation (SHG) is generally inefficient. In this paper, we fabricate a judiciously structured 150 nm-thick planar surface consisting of monolayer tungsten diselenide and sub-20 nm-wide gold trenches on flexible substrates, reporting similar to 7000-fold SHG enhancement without peak broadening or background in the spectra as compared to WSe2 on as-grown sapphire substrates. Our proof-of-concept experiment yields effective second-order nonlinear susceptibility of 2.1 X 10(4) pm/V. Three orders of magnitude enhancement is maintained with pump wavelength ranging from 800 to 900 nm, breaking the limitation of narrow pump wavelength range for cavity-enhanced SHG. In addition, SHG amplitude can be dynamically controlled via selective excitation of the lateral gap plasmon by rotating the laser polarization. Such a fully open, flat, and ultrathin profile enables a great variety of functional samples with high SHG from one patterned silicon substrate, favoring scalable production of nonlinear converters. The surface accessibility also enables integration with other optical components for information processing in an ultrathin and flexible form.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Monolayer two-dimensional transition-metal dichalcogenides (2D TMDCs) exhibit promising characteristics in miniaturized nonlinear optical frequency converters, due to their inversion asymmetry and large second-order nonlinear susceptibility. However, these materials usually have very short light interaction lengths with the pump laser because they are atomically thin, such that second-harmonic generation (SHG) is generally inefficient. In this paper, we fabricate a judiciously structured 150 nm-thick planar surface consisting of monolayer tungsten diselenide and sub-20 nm-wide gold trenches on flexible substrates, reporting similar to 7000-fold SHG enhancement without peak broadening or background in the spectra as compared to WSe2 on as-grown sapphire substrates. Our proof-of-concept experiment yields effective second-order nonlinear susceptibility of 2.1 X 10(4) pm/V. Three orders of magnitude enhancement is maintained with pump wavelength ranging from 800 to 900 nm, breaking the limitation of narrow pump wavelength range for cavity-enhanced SHG. In addition, SHG amplitude can be dynamically controlled via selective excitation of the lateral gap plasmon by rotating the laser polarization. Such a fully open, flat, and ultrathin profile enables a great variety of functional samples with high SHG from one patterned silicon substrate, favoring scalable production of nonlinear converters. The surface accessibility also enables integration with other optical components for information processing in an ultrathin and flexible form. |
2016 |
Lu, Yao; Song, Jingchao; Yuan, Jian; Zhang, Lei; Wu, Steve Qing Yang; Yu, Wenzhi; Zhao, Meng; Qiu, Cheng-Wei; Teng, Jinghua; Loh, Kian Ping; Zhang, Chao; Bao, Qiaoliang Highly efficient plasmon excitation in graphene-Bi2Te3 heterostructure Journal Article 15 JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS, 33 (9), pp. 1842-1846, 2016, ISSN: 0740-3224. @article{ISI:000385419900008, title = {Highly efficient plasmon excitation in graphene-Bi_{2}Te_{3} heterostructure}, author = {Yao Lu and Jingchao Song and Jian Yuan and Lei Zhang and Steve Qing Yang Wu and Wenzhi Yu and Meng Zhao and Cheng-Wei Qiu and Jinghua Teng and Kian Ping Loh and Chao Zhang and Qiaoliang Bao}, doi = {10.1364/JOSAB.33.001842}, times_cited = {15}, issn = {0740-3224}, year = {2016}, date = {2016-09-01}, journal = {JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS}, volume = {33}, number = {9}, pages = {1842-1846}, publisher = {OPTICAL SOC AMER}, address = {2010 MASSACHUSETTS AVE NW, WASHINGTON, DC 20036 USA}, abstract = {Graphene plasmons have attracted a lot of attention due to large confinement and small mode volume. However, the graphene-based plasmonic devices are still limited in the practical applications due to relatively small light absorption of graphene and limited light-matter coupling efficiency in general excitation strategy. Here, this work reported a strong plasmonic coupling effect observed in a novel graphene-Bi2Te3 heterostructure on the top of silicon gratings. It is interesting to find that the extinction spectra of the graphene-Bi2Te3 heterostructure has shown three times greater magnitude than that of graphene. This observation is mainly attributed to two factors: first, the coupling efficiency between the graphene and Bi2Te3; second, the higher light absorption in the graphene-Bi2Te3 heterostructure. Moreover, the plasmonic resonance peak of the graphene-Bi2Te3 heterostructure can be easily tuned by changing the grating period just like what happens in the graphene film. In all, this work utilizes the simple silicon grating to couple the light into the graphene-Bi2Te3 heterostructure, and further explores the hybridized Dirac plasmons in the graphene-Bi2Te3 heterostructure. We believe it will stimulate the interest to study the variant plasmonic heterostructure and trigger new terahertz device applications. (C) 2016 Optical Society of America}, keywords = {}, pubstate = {published}, tppubtype = {article} } Graphene plasmons have attracted a lot of attention due to large confinement and small mode volume. However, the graphene-based plasmonic devices are still limited in the practical applications due to relatively small light absorption of graphene and limited light-matter coupling efficiency in general excitation strategy. Here, this work reported a strong plasmonic coupling effect observed in a novel graphene-Bi2Te3 heterostructure on the top of silicon gratings. It is interesting to find that the extinction spectra of the graphene-Bi2Te3 heterostructure has shown three times greater magnitude than that of graphene. This observation is mainly attributed to two factors: first, the coupling efficiency between the graphene and Bi2Te3; second, the higher light absorption in the graphene-Bi2Te3 heterostructure. Moreover, the plasmonic resonance peak of the graphene-Bi2Te3 heterostructure can be easily tuned by changing the grating period just like what happens in the graphene film. In all, this work utilizes the simple silicon grating to couple the light into the graphene-Bi2Te3 heterostructure, and further explores the hybridized Dirac plasmons in the graphene-Bi2Te3 heterostructure. We believe it will stimulate the interest to study the variant plasmonic heterostructure and trigger new terahertz device applications. (C) 2016 Optical Society of America |
Wu, Yang; Niu, Jing; Danesh, Mohammad; Liu, Jingbo; Chen, Yuanfu; Ke, Lin; Qiu, Chengwei; Yang, Hyunsoo Localized surface plasmon resonance in graphene nanomesh with Au nanostructures Journal Article 11 APPLIED PHYSICS LETTERS, 109 (4), 2016, ISSN: 0003-6951. @article{ISI:000381688900006, title = {Localized surface plasmon resonance in graphene nanomesh with Au nanostructures}, author = {Yang Wu and Jing Niu and Mohammad Danesh and Jingbo Liu and Yuanfu Chen and Lin Ke and Chengwei Qiu and Hyunsoo Yang}, doi = {10.1063/1.4959833}, times_cited = {11}, issn = {0003-6951}, year = {2016}, date = {2016-07-25}, journal = {APPLIED PHYSICS LETTERS}, volume = {109}, number = {4}, publisher = {AMER INST PHYSICS}, address = {1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USA}, abstract = {A hybrid structure of a graphene nanomesh with the gold nanodisks is studied to enhance the light absorption by the localized surface plasmon resonance. From the reflection spectra of the visible range for graphene nanomesh samples without and with nanodisks, it is found that the absorption of graphene nanomesh structures is greatly enhanced in the presence of gold nanodisks around the resonance wavelength. Simulation results based on the finite-difference time-domain method support the experimental observations. This study demonstrates the potential of constructing graphene based photodetectors with a high light absorption efficiency and wavelength selectivity. Published by AIP Publishing.}, keywords = {}, pubstate = {published}, tppubtype = {article} } A hybrid structure of a graphene nanomesh with the gold nanodisks is studied to enhance the light absorption by the localized surface plasmon resonance. From the reflection spectra of the visible range for graphene nanomesh samples without and with nanodisks, it is found that the absorption of graphene nanomesh structures is greatly enhanced in the presence of gold nanodisks around the resonance wavelength. Simulation results based on the finite-difference time-domain method support the experimental observations. This study demonstrates the potential of constructing graphene based photodetectors with a high light absorption efficiency and wavelength selectivity. Published by AIP Publishing. |
Wang, Zhuo; Dong, Zhaogang; Gu, Yinghong; Chang, Yung-Huang; Zhang, Lei; Li, Lain-Jong; Zhao, Weijie; Eda, Goki; Zhang, Wenjing; Grinblat, Gustavo; Maier, Stefan A; Yang, Joel K W; Qiu, Cheng-Wei; Wee, Andrew T S Giant photoluminescence enhancement in tungsten-diselenide-gold plasmonic hybrid structures Journal Article 279 NATURE COMMUNICATIONS, 7 , 2016. @article{ISI:000375491100001, title = {Giant photoluminescence enhancement in tungsten-diselenide-gold plasmonic hybrid structures}, author = {Zhuo Wang and Zhaogang Dong and Yinghong Gu and Yung-Huang Chang and Lei Zhang and Lain-Jong Li and Weijie Zhao and Goki Eda and Wenjing Zhang and Gustavo Grinblat and Stefan A Maier and Joel K W Yang and Cheng-Wei Qiu and Andrew T S Wee}, doi = {10.1038/ncomms11283}, times_cited = {279}, year = {2016}, date = {2016-05-01}, journal = {NATURE COMMUNICATIONS}, volume = {7}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Impressive properties arise from the atomically thin nature of transition metal dichalcogenide two-dimensional materials. However, being atomically thin limits their optical absorption or emission. Hence, enhancing their photoluminescence by plasmonic nanostructures is critical for integrating these materials in optoelectronic and photonic devices. Typical photoluminescence enhancement from transition metal dichalcogenides is 100-fold, with recent enhancement of 1,000-fold achieved by simultaneously enhancing absorption, emission and directionality of the system. By suspending WSe2 flakes onto sub-20-nm-wide trenches in gold substrate, we report a giant photoluminescence enhancement of similar to 20,000-fold. It is attributed to an enhanced absorption of the pump laser due to the lateral gap plasmons confined in the trenches and the enhanced Purcell factor by the plasmonic nanostructure. This work demonstrates the feasibility of giant photoluminescence enhancement in WSe2 with judiciously designed plasmonic nanostructures and paves a way towards the implementation of plasmon-enhanced transition metal dichalcogenide photodetectors, sensors and emitters.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Impressive properties arise from the atomically thin nature of transition metal dichalcogenide two-dimensional materials. However, being atomically thin limits their optical absorption or emission. Hence, enhancing their photoluminescence by plasmonic nanostructures is critical for integrating these materials in optoelectronic and photonic devices. Typical photoluminescence enhancement from transition metal dichalcogenides is 100-fold, with recent enhancement of 1,000-fold achieved by simultaneously enhancing absorption, emission and directionality of the system. By suspending WSe2 flakes onto sub-20-nm-wide trenches in gold substrate, we report a giant photoluminescence enhancement of similar to 20,000-fold. It is attributed to an enhanced absorption of the pump laser due to the lateral gap plasmons confined in the trenches and the enhanced Purcell factor by the plasmonic nanostructure. This work demonstrates the feasibility of giant photoluminescence enhancement in WSe2 with judiciously designed plasmonic nanostructures and paves a way towards the implementation of plasmon-enhanced transition metal dichalcogenide photodetectors, sensors and emitters. |
Zhao, Meng; Zhang, Jie; Gao, Nengyue; Song, Peng; Bosman, Michel; Peng, Bo; Sun, Baoquan; Qiu, Cheng-Wei; Xu, Qing-Hua; Bao, Qiaoliang; Loh, Kian Ping Actively Tunable Visible Surface Plasmons in Bi2Te3 and their Energy-Harvesting Applications Journal Article 70 ADVANCED MATERIALS, 28 (16), pp. 3138-3144, 2016, ISSN: 0935-9648. @article{ISI:000374577400012, title = {Actively Tunable Visible Surface Plasmons in Bi_{2}Te_{3} and their Energy-Harvesting Applications}, author = {Meng Zhao and Jie Zhang and Nengyue Gao and Peng Song and Michel Bosman and Bo Peng and Baoquan Sun and Cheng-Wei Qiu and Qing-Hua Xu and Qiaoliang Bao and Kian Ping Loh}, doi = {10.1002/adma.201506367}, times_cited = {70}, issn = {0935-9648}, year = {2016}, date = {2016-04-27}, journal = {ADVANCED MATERIALS}, volume = {28}, number = {16}, pages = {3138-3144}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, abstract = {Hexagonal Bi2Te3 nanoplates support visible-range surface plasmons, of which the resonance energy is tuned as wide as 400 nm by Se doping and the resonance intensity is modulated by utilizing the phase change between the crystalline and amorphous states. The potential of Bi2Te3 for reconfigurable plasmonics, plasmon-enhanced solar cells, and photoluminescence is demonstrated.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Hexagonal Bi2Te3 nanoplates support visible-range surface plasmons, of which the resonance energy is tuned as wide as 400 nm by Se doping and the resonance intensity is modulated by utilizing the phase change between the crystalline and amorphous states. The potential of Bi2Te3 for reconfigurable plasmonics, plasmon-enhanced solar cells, and photoluminescence is demonstrated. |
Zhang, Kai; Zhang, Lei; Yap, Fung Ling; Song, Peng; Qiu, Cheng-Wei; Loh, Kian Ping Large-Area Graphene Nanodot Array for Plasmon-Enhanced Infrared Spectroscopy Journal Article 30 SMALL, 12 (10), pp. 1302-1308, 2016, ISSN: 1613-6810. @article{ISI:000372008200004, title = {Large-Area Graphene Nanodot Array for Plasmon-Enhanced Infrared Spectroscopy}, author = {Kai Zhang and Lei Zhang and Fung Ling Yap and Peng Song and Cheng-Wei Qiu and Kian Ping Loh}, doi = {10.1002/smll.201503016}, times_cited = {30}, issn = {1613-6810}, year = {2016}, date = {2016-03-09}, journal = {SMALL}, volume = {12}, number = {10}, pages = {1302-1308}, publisher = {WILEY-V C H VERLAG GMBH}, address = {POSTFACH 101161, 69451 WEINHEIM, GERMANY}, keywords = {}, pubstate = {published}, tppubtype = {article} } |