You are reading this article, probably either using your phone or using your computer. These electronic devices are all based on electronic chips. Simply put, it is to store and calculate information through whether the current is turned on or not. The competition in the chip industry is very fierce. To improve the computing power of electronic devices while reducing their volume, it is necessary to increase the number of transistors on the chip. For example, the latest Macbook Pro uses Macbook Pro with 20 billion transistors integrated on the M2 chip.
So far, the core technology of high-end chip manufacturing is still in the hands of a few foreign companies. For example, ASML's lithography machine can produce high-end chips with 5-nanometer process, which is also the reason why China ranked second among the 35 "bottleneck" technologies in 2019.
In addition to electronic chips, the concept of photonic chip has gradually risen in recent years. Photon can carry more information and transmit faster than electrons. Therefore, photon chips are considered to be the basis for building the next generation of low-energy, high-density and high-efficiency information devices.
has occupied a place in this field, and many countries have listed photonic chip technology as national strategies, such as EU 's Framework and Horizon 2020 plan. The development of photon chip mainly involves two key scientific issues: one is to break through the optical diffraction limit and realize the local area of photons on the subwavelength scale; the other is to accurately man-made regulation of the light field.
In order to achieve "overtaking on the curve" in the field of next-generation photonic chips, the National Natural Science Foundation of China has listed "light field regulation and its interaction with matter" as a priority development field.
Although the development prospects are tempting, the nano-local and artificial manipulation of photons is not easy. Unlike electrons, photons do not carry charges, and the interaction between light and matter is often weak, so it is difficult to regulate photons through traditional methods of regulating electrons such as electric field pressurization.
Fortunately, scientists discovered that photons can couple with other particles or quasi-particle such as electrons, phonons , excitons , etc., thereby producing a semi-light-semi-matter quasi-particle. The famous physicist Mr. Huang Kun called him a polarized zombie (as shown in Figure 1).
Because polarized anodes have the properties of multiple particles at the same time, it paves the way for precise photon manipulation on the nanoscale and provides ideas for solving the two major bottlenecks that plague the development of photon chips.
Recently, Dr. Duan Jiahua from the Quantum Nanoptics Group of the University of Oviedo, Spain, has the following new achievements: A series of studies on the near-field optical properties of polarized excitons in two-dimensional quantum materials help deepen the understanding of the basic optical phenomena of hyperbolic optical dispersion, and also pave the way for the potential application of hyperbolic polarized excitons.
Figure | Duan Jiahua (Source: Duan Jiahua)
For example, in building optical paths and even building nanophoton chips, a deep understanding of the propagation path of polarized elements is the basis for realizing photon integration and application. The most applicable potential in the research of
is the hyperbolic nano-optical cavity. Because of its unique optical properties such as enhanced optical density and near-field optical intensity, it can be used to achieve high sensitivity molecular detection, and it is even expected to achieve single biomolecular level detection.
Duan Jiahua is located in the Quantum Nanoptical Group of the University of Oviedo, has long been committed to achieving the precise manipulation of multi-frequency photons on the nanoscale.
In 2012 and 2014, its postdoctoral supervisor, Professor Pablo Alonso Gonzalez, realized real-space imaging of graphene plasmons and their refractive phenomena for the first time, verifying that the traditional Fresnel refractive law is still valid on the nanoscale.
graphene plasmons can draw mid-infrared light with a wavelength of about 11 microns locally to the nanoscale, that is, one-forty of the incident wavelength. However, electron-electron scattering in graphene will bring great optical propagation losses to the plasmon.
To solve this problem, Duan Jiahua's research group has been looking for other types of polarization elements. One typical representative is the phonon polarized exciton, which is a quasi-particle produced by coupling photons and optical phonons (as shown in Figure 1).
Figure 1 | (Source: Duan Jiahua)
In 2018, the team joined hands with other groups to report the phonon polarized anointed in molybdenum trioxide crystals, whose lifespan can reach 20 picoseconds, two orders of magnitude higher than graphene plasmon life.
What is particularly interesting is that as a van der Waals material, molybdenum trioxide has optical anisotropy in the mid-infrared band, and has completely different optical properties in the two directions in the plane: the dielectric constant along the [100] crystal direction is a negative number, that is, it is a metal; the dielectric constant along the [001] direction is a positive number, that is, it is a dielectric.
As shown in Figure 2, this optical anisotropy leads to: the inverted wavefront of the polarized anomaly, and the corresponding isofrequency curve is hyperbolic .
Figure 2 | (Source: Duan Jiahua)
hyperbolic optical dispersion has two very attractive characteristics: 1. The wave vector direction is limited but the size is not limited (traditional circular dispersion is opposite, that is, the wave vector direction is not limited but the size is limited). In theory, it can achieve infinite wave vector and infinite height local ability of light field; 2. There are many wave vectors close to the asymptomatic line of the hyperbolic line, which has a large optical state density. The two properties of
contain rich application potential, including optical imaging, biosensing, optical detection, photon integration, etc., and have also opened up the research direction of Hyperbolic nanophotonics.
When optical dispersion changes from traditional isotropic dispersion to abnormal hyperbolic dispersion, many basic optical phenomena such as refraction, reflection, focus, interference, etc. need to be re-studied and may exhibit completely counterintuitive behavior.
In 2021, Duan Jiahua's team published research on refraction and interference of hyperbolic polarization elements. As shown in Figure 3, at the interface between low refractive index and high refractive index environment, hyperbolic polarized excitons will refract, and the refractive angle is greater than the incident angle. The common refractive phenomenon of isotropic waves is that the refractive angle is smaller than the incident angle, such as light from air to water. Therefore, hyperbolic polarized refraction is opposite to common refraction phenomena.
Figure 3 | (Source: Duan Jiahua)
What is more interesting is that the wavelength of the hyperbolic polarized exciter after refraction becomes very small, and in theory it can even be infinitely small. Based on this property, the team designed and implemented the in-plane hyperbolic lens with the highest focal resolution to date, with a focus size of only one-sixth of the wavelength of the polarized excitation element, thus achieving nanofocusing of medium infrared light . Another study by the
research group showed that the interference of hyperbolic polarization elements is also a very complex optical phenomenon. This is because in hyperbolic optical dispersion, there are different types of wave vectors, including common wave vectors, high momentum wave vectors and infinite large wave vectors (as shown in Figure 4).
Figure 4 | (Source: Duan Jiahua)
. Through real-space imaging and theoretical calculations, the researchers found that due to the large optical state density of high momentum wave vectors playing a dominant role in interference, when multiple excitation sources form a disc shape, the interference of hyperbolic polarized excitons will lead to their focus on the deep subwavelength scale (as shown in Figure 4). Based on this, the team continued to study another basic optical phenomenon of hyperbolic polarization elements: reflection.
Reflection of light is a kind of physical common sense at the middle school level.This phenomenon can be seen everywhere in daily life, and reflection is indispensable in the mirror and the "moon in the water".
However, through research, the research team found that the reflection phenomenon of hyperbolic polarized excitons is counterintuitive: the reflection angle is not equal to the incident angle, and the incident wave and the reflected wave are located on the same side of the interface normal (i.e., negative reflection). These phenomena are contrary to the reflection law in textbooks - the incident angle is equal to the reflection angle and the separation between the normal.
To put it simply, if our world is hyperbolic optical dispersion, not isotropic dispersion anymore. Then, the best position we look at the mirror is no longer in front of the mirror, but may become a position almost parallel to the mirror (as shown in Figure 5).
Figure 5 | (Source: Duan Jiahua)
. In this study, near-field optical imaging technology helped the team achieve direct observation of negative reflection of hyperbolic polarized excitons (as shown in Figure 6). Through more in-depth research, it was found that when the reflection interface is designed as a hyperbolic profile, all hyperbolic polarized exciter wave vectors will return in the original path (i.e., back reflection).
Figure 6 | (Source: Duan Jiahua)
When the two hyperbolic profiles are close, a hyperbolic nano-cavity will be formed (as shown in Figure 7), which has very unique optical properties, such as enhanced near-field optical intensity, increased optical state density, and open optical cavity shape. These properties will have an important impact on many applications, especially optical detection or molecular detection.
Figure 7 | (Source: Duan Jiahua)
Recently, the relevant paper was published in Science Advances, with Duan Jiahua and Alvarez Perez as co-first authors [1] Duan Jiahua and Pablo Alonso Gonzalez serve as joint corresponding author .
Figure | Related papers (Source: Science Advanceds)
Reviewer A believes that the results displayed in the paper will attract widespread attention and interest in the field of nanophotonics. Reviewer B believes that relevant experiments have fully proved the negative reflection phenomenon of hyperbolic polarized elements, and the proposal of the concept of hyperbolic nanocavity is innovative and is very suitable for publishing on Science Advances. After the article was published, it was also selected as a highlight report by the editor of Science Advanceds journal.
33Duan Jiahua said that as early as 2018, he and his team decided to systematically study the basic optical phenomenon of hyperbolic optical dispersion. Specifically, this work can be divided into three steps:
First, it is necessary to establish a theoretical model suitable for hyperbolic optical dispersion. Molybdenum trioxide crystals are biaxial crystals, and their dielectric constants are also different when they follow different directions. Therefore, when calculating the hyperbolic polarized excitation dispersion, the research team needs to analyze complex equations.
Through this, the researchers obtained the equal frequency lines of the hyperbolic polarized exciton at different frequencies (as shown in Figure 6). Combined with the principle of conservation of momentum between the wave vector before and after reflection and the wave vector at the interface, they can accurately calculate the wave vector size, wave vector direction, and wave vector after reflection.
Next, the theoretical model must be verified with the help of experiments. In molybdenum trioxide crystals, the wavelength of the hyperbolic polarized element is about 1 micron. To obtain a clear image of its reflection phenomenon, the resolution of the imaging technology needs to reach the order of microns or nanometers.
For this reason, the team used a scattering-type Scanning Near-field Optical Microscopy (s-SNOM, scattering-type Scanning Near-field Optical Microscopy).Simply put, it is to focus incident light on a needle tip with a tip size of only 20 nanometers. This way, by collecting the near-field optical signal below the needle tip, the imaging spatial resolution can be increased to 20 nanometers.
When the polarized exciter is excited by the needle tip, it will propagate outward and reflect after touching the boundary. After returning to the needle tip, it will scatter to the detector. At this time, scan it point by point to obtain the near-field optical image of the sample (as shown in Figure 6). Experimental results show that the reflection phenomenon of hyperbolic polarized exciton is consistent with theoretical calculations.
To obtain a clearer hyperbolic polarized element reflection image, the research team carefully designed the size and direction of the reflection interface (as shown in Figure 8). When the size of the reflection interface is much larger than the wavelength of the polarized excitation element, the polarized excitation element excited by the needle tip will propagate and reflect in different directions, which will eventually cause very complex interference, which may mask the negative reflection phenomenon.
Figure 8 | (Source: Duan Jiahua)
To avoid this problem, the researchers designed a reflection interface at a subwavelength scale, which not only helps to obtain a clear image of the negative reflection phenomenon of hyperbolic polarized exciton, but also helps to accurately determine the Boyinting vector of reflective polarized exciton.
At the same time, you must also pay attention to the direction of the reflection interface. Because according to the imaging principle of s-SNOM, at the interface of certain angles, polarized elements that cannot be imaged will undergo reflection.
Finally, based on near-field optical experiments and theoretical calculations, the team designed a hyperbolic nanocavity and calculated its performance parameters.
Specifically, based on the principle of conservation of momentum before and after the reflection of polarized excitons, it found that the design of hyperbolic nanocavity is more like a mathematical problem: the wave vector of each point incident wave on the interface is perpendicular to the tangent direction of the interface.
Of course, physical optical loss must also be considered. From this point of view, the ideal reflection interface should be a hyperbolic section. At the end of the study, the research team also successfully obtained the theoretical model of hyperbolic nanocavity.
Two technical details have caused research on the micro-nano processing problem of molybdenum trioxide crystals in "resurrection". When preparing the polarized exciton reflective interface, it was found that in molybdenum oxide crystals, hyperbolic polarized excitons are very sensitive to micro-nano processing, and almost no near-field optical signals can be detected after processing.
Duan Jiahua said: "We were very frustrated at one time, and even thought that molybdenum oxide crystals might not be a good platform for observing hyperbolic polarization reflection phenomena, but we could not find other materials with similar properties." In order to solve this problem,
In order to solve this problem, it contacted scientific research teams from multiple countries. After joint research, it found that a specific metal mask can be used to protect molybdenum oxide crystals in micro-nano processing. Later, the research team found that after high temperature annealing, the polarized positive signal of molybdenum oxide can be restored. Ultimately, these two technical details make the research work "resurrect".
Duan Jiahua said: "What is more surprising is that after studying the physical principles behind the recovery of polarized exciton signals by annealing , we successfully discovered another optical phenomenon, which can also be said to be 'It is not a blessing to lose my horse'."
According to reports, Duan Jiahua is from Datong, Shanxi. Undergraduate degree in , Beijing Jiaotong University, majoring in Materials Chemistry, and graduated from the Institute of Physics, Chinese Academy of Sciences. In 2018, he went to the Quantum Nanoptics Group of the University of Oviedo in Spain to serve as a postdoctoral fellow and continue to engage in nanophotonics and near-field optics research.
He has been engaged in related research on polarization and published more than 30 SCI papers. He has long served as a reviewer of many international journals such as Nature, Nature Materials, Nature Communications, Physical Review Letters , and a project review expert of the Polish National Science Foundation of China.Speaking of subsequent career development, Duan Jiahua said: "I hope to return to the motherland as soon as possible. I am currently looking for a domestic position."
. The follow-up research plan for this research is also very clear:
On the one hand, the research team wants to push the hyperbolic polarization element to application, which first requires the hyperbolic nanocavity to be experimentally implemented. To this end, it is necessary to prepare nanocavity arrays, generalize from near-field optics to far-field optics, and combine with biomolecular detection.
On the other hand, the team will continue to study other optical phenomena of hyperbolic polarization elements and look for new polarization elements modes. The ultimate goal is to achieve precise manipulation of multiple frequency photons on the nanoscale.
Reference:
1.Álvarez-Pérez, G., Duan, J. et al. Negative Reflection of Nanoscale-Confined Polaritons in a Low-Loss Natural Medium. Science Advanceds, 8. 29, abp8486 (2022).https://doi.org/10.1126/sciadv.abp8486
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