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A 2D material-based liquid crystal for deep-ultraviolet light modulation | Nature Nanotechnology
A 2D material-based liquid crystal for deep-ultraviolet light modulation
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Published: 16 August 2022 .
A 2D material-based liquid crystal for deep-ultraviolet light modulation .
Nature Nanotechnology ( 2022 ) Cite this article
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Subjects .
Optical properties and devices .
Photonic crystals .
Suspensions of 2D hexagonal boron nitride show an anomalously large specific Cotton–Mouton coefficient, enabling the fabrication of a magnetically tuneable and stable birefringent optical device. This device serves as a transmissive light modulator with wavelengths entering the ultraviolet (UV)-C region, representing a technological advance in deep-UV modulation.

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The problem .
Birefringence — that is, the dependence of the refractive index of certain materials on the light polarization — can be exploited for light manipulation. Indeed, widely-used electro-optical Kerr devices are based on birefringent materials, such as organic liquid crystals (LCs), in which their birefringence changes upon application of an electric field. LC displays are a well-known example of this technology, and have an annual global market of more than US$160 billion. Unfortunately, the application of organic LC Kerr devices in the deep-ultraviolet (DUV) region is hindered by the instability of organic LC molecules under UV exposure 1 and the high DUV absorption or scattering from device components, such as the electrodes. Thus, the modulation of DUV light has mainly been achieved through reflection. However, transmissive modulation of DUV light is highly desirable, because it permits light modulation without altering the optical path and, thus, offers applications in areas such as solar-blind optical communications, high-density data storage and high-precision photolithography. Nevertheless, DUV light modulation poses challenges in terms of material selection and the structural design of LC birefringent optical devices.
The solution .
To address this problem, we turned to 2D hexagonal boron nitride (h-BN), which has a six-membered ring structure and, owing to its wide bandgap of ~6 eV and unique electronic structure, has been applied in DUV optoelectronic devices, such as DUV light-emitting diodes and photodetectors 2 . Importantly, 2D h-BN has been reported to exhibit giant intrinsic birefringence 3 , which is a vital parameter in birefringent optics. Furthermore, 2D materials exhibit intriguing magneto-optical properties owing to the interplay between geometric, optical and magnetic aniositropies 4 , allowing for the observation of the giant magneto-birefringence effect and transmissive magneto-chromic effect 5 . These features endow 2D h-BN-based inorganic LCs with the potential for transmissive DUV modulation.
We thus performed magneto-optical measurements on an aqueous suspension of 2D h-BN flakes, produced by top-down exfoliation (Fig. 1a ). Upon application of a magnetic field, the otherwise randomly oriented 2D h-BN flakes align parallel to the field flux, as evidenced by the observation of polarization-dependent transmittance and magnetic anisotropy. The aligned h-BN flakes give rise to birefringence and thus phase retardation between the two components of DUV light, namely the out-of-plane polarization and in-plane polarization. The correspondence between the birefringence and magnetic field gives rise to a specific Cotton–Mouton coefficient of up to 8.0 × 10 6 T -2 m -1 , which is about five orders of magnitude higher than that of other potential DUV-transparent birefringent media. The physical origin of this giant Cotton–Mouton effect is ascribed to the large optical anisotropic factor and anisotropic magnetic susceptibility of the 2D h-BN flakes.
Fig. 1: Magneto-optical switching of a 2D h-BN-based inorganic LC. a , Images of a panda-patterned 2D h-BN suspension under different magnetic fields. According to Malus’s Law, the intensity of transmitted light for a birefringent medium between crossed polarizers increases with the birefringence. Without a magnetic field, the 2D h-BN flakes are randomly oriented in the solvent, with zero birefringence and therefore zero intensity of transmitted light. The alignment of the h-BN flakes can be controlled by application of a magnetic field, giving rise to ‘magneto-birefringence’. Brighter images can thus be obtained with a higher magnetic field. Scale bar, 5mm. b , Comparison of the stable spectral coverage of our 2D h-BN LC optical modulator with that of optical modulators based on commercial LCs and LCs currently in research and development (R&D). ? 2022, Xu, H. et al.
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To evaluate the DUV-modulation capability of the 2D h-BN LC, we performed a series of magneto-optical measurements in the UV-C range at a wavelength of 266?nm. Our 2D h-BN LC shows the desired performance, including hysteresis-free reversible behaviour, a fast magneto-response, low-temperature operation, and high material and operation stability. These features enabled us to fabricate a DUV LC modulator with high stability, and, notably, stability in the solar-blind UV-C spectral region (Fig. 1b ).
Future directions .
Using a 2D h-BN-based inorganic LC, we developed a stable transmissive modulator capable of tuning DUV light, including its phase retardation, intensity and polarization. Our research might extend the application of LC modulators to harsh environments, such as outer space and other environments with high DUV exposure or extreme temperatures.
Nevertheless, there are some open questions that remain to be addressed, such as the magnetic origin of the 2D h-BN flakes, how to further enlarge the magnetic and optical anisotropy, and how to quantify the effects of the geometric anisotropy of 2D materials on the results.
In addition to applications in extreme environments, DUV optical devices can be customized through gelation of the 2D material inside an optically isotropic matrix, with its birefringence finely controlled by an external magnetic field, the optical path distance and concentration. We also aim to explore the use of stimuli other than a magnetic field to tune the birefringence of 2D materials, as well as the use of other 2D materials.
Bilu Liu 1 and Hui-Ming Cheng 2
1 Tsinghua University, Shenzhen, China.
2 Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences, Shenzhen, China.
Expert opinion .
“This study demonstrates tuneable birefringence in h-BN-based liquid crystals that extends into the deep UV spectral range. There are few birefringent materials at UV wavelengths and hardly any for which the optical properties can be tuned. As the manipulation of UV light is crucial for many applications, these findings are a breakthrough for a wide range of UV photonic technologies”. Michael Kneissl, Technische Universit?t Berlin, Berlin, Germany.
Behind the paper .
When preparing the manuscript, we recalled that our initial attempts in making a DUV modulator with a 2D h-BN-based inorganic LC dated back to mid-2018, and that five years had passed in the blink of an eye. Through the study, we came to realize that it is important to use various approaches to elucidate the dominant factors that underpin the observed phenomena. Moreover, our ultimate research target is to push the boundaries beyond what can be achieved with existing technologies. Thus, an exciting moment for us was when we surprisingly found that a transmissive modulator based on a 2D h-BN LC can operate stably under exposure to DUV light. This discovery largely changes a long-held view in the field, namely that LC devices are inapplicable for DUV light. We eagerly expect that more efforts will be devoted to 2D materials in the field of tuneable optics. B. L. & H.-M. C.
From the editor .
“What’s striking about this work is the compelling implications for practical applications. Although the deep-UV birefringence of h-BN nanoplatelets has been demonstrated at the laboratory level, it is not hard to imagine how the vast knowledge of birefringent optics in the visible region can be extended to the deep-UV thanks to these results”. Alberto Moscatelli, Chief Editor, Nature Nanotechnology
References .
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This is a summary of: Xu, H. et al. Magnetically tunable and stable deep-ultraviolet birefringent optics using two-dimensional hexagonal boron nitride. Nat. Nanotechnol . https://doi.org/10.1038/s41565-022-01186-1 (2022).
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A 2D material-based liquid crystal for deep-ultraviolet light modulation. Nat. Nanotechnol. (2022). https://doi.org/10.1038/s41565-022-01192-3
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Published : 16 August 2022
DOI : https://doi.org/10.1038/s41565-022-01192-3
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Associated Content .
Magnetically tunable and stable deep-ultraviolet birefringent optics using two-dimensional hexagonal boron nitride .
Hao Xu .
Baofu Ding .
Bilu Liu .
Nature Nanotechnology Article 11 Aug 2022
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