이 발견은 무선 통신 및 광학 컴퓨팅 분야의 혁신적인 응용 분야의 토대를 마련합니다.
우리는 거울을 볼 때 우리를 바라보는 우리의 얼굴을 보는 데 익숙합니다. 반사된 이미지는 거울 표면에서 반사되는 전자기파에 의해 생성되어 공간 반사라고 하는 일반적인 현상을 생성합니다. 마찬가지로 음파의 공간적 반사는 우리의 말을 말한 순서대로 우리에게 되돌려주는 메아리를 형성합니다.
60년 이상 동안 과학자들은 파동 반사로 알려진 다른 형태의 파동 반사가 관찰될 수 있다고 가정했습니다. 내 시간또는 시간, 반사. 빛이나 음파가 공간의 특정 위치에서 거울이나 벽과 같은 경계에 부딪힐 때 발생하는 공간 반사와 달리 시간 반사는 파동이 갑자기 이동하여 그 특성을 변경할 때 나타납니다. 모든 공간. 그러한 경우 파동의 일부 시간이 반전되고 주파수가 새로운 주파수로 이동합니다.
현재까지 이러한 현상은 전자파에서 관찰되지 않았습니다. 이러한 증거 부족의 근본적인 이유는 재료의 광학적 특성이 시간 역전을 일으키는 속도와 크기로 쉽게 변경될 수 없기 때문입니다. 그러나 최근에 발표된 논문에서
“This has been really exciting to see, because of how long ago this counterintuitive phenomenon was predicted, and how different time-reflected waves behave compared to space-reflected ones,” said the paper’s corresponding author Andrea Alù, Distinguished Professor of Physics at The City University of New York Graduate Center and founding director of the CUNY ASRC Photonics Initiative. “Using a sophisticated metamaterial design, we were able to realize the conditions to change the material’s properties in time both abruptly and with a large contrast.”
This feat caused a significant portion of the broadband signals traveling in the metamaterial to be instantaneously time reversed and frequency converted. The effect forms a strange echo in which the last part of the signal is reflected first. As a result, if you were to look into a time mirror, your reflection would be flipped, and you would see your back instead of your face. In the acoustic version of this observation, you would hear sound similar to what is emitted during the rewinding of a tape.
The researchers also demonstrated that the duration of the time-reflected signals was stretched in time due to broadband frequency conversion. As a result, if the light signals were visible to our eyes, all their colors would be abruptly transformed, such that red would become green, orange would turn to blue, and yellow would appear violet.
To achieve their breakthrough, the researchers used engineered metamaterials. They injected broadband signals into a meandered strip of metal that was about 6 meters long, printed on a board and loaded with a dense array of electronic switches connected to reservoir capacitors. All the switches were then triggered at the same time, suddenly and uniformly doubling the impedance along the line. This quick and large change in electromagnetic properties produced a temporal interface, and the measured signals faithfully carried a time-reversed copy of the incoming signals.
The experiment demonstrated that it is possible to realize a time interface, producing efficient time reversal and frequency transformation of broadband electromagnetic waves. Both these operations offer new degrees of freedom for extreme wave control. The achievement can pave the way for exciting applications in wireless communications and for the development of small, low-energy, wave-based computers.
“The key roadblock that prevented time reflections in previous studies was the belief that it would require large amounts of energy to create a temporal interface,” said Gengyu Xu, the paper’s co-first author and a postdoctoral researcher at CUNY ASRC. “It is very difficult to change the properties of a medium quick enough, uniformly, and with enough contrast to time reflect electromagnetic signals because they oscillate very fast. Our idea was to avoid changing the properties of the host material, and instead create a metamaterial in which additional elements can be abruptly added or subtracted through fast switches.”
“The exotic electromagnetic properties of metamaterials have so far been engineered by combining in smart ways many spatial interfaces,” added co-first author Shixiong Yin, a graduate student at CUNY ASRC and at The City College of New York. “Our experiment shows that it is possible to add time interfaces into the mix, extending the degrees of freedom to manipulate waves. We also have been able to create a time version of a resonant cavity, which can be used to realize a new form of filtering technology for electromagnetic signals.”
The introduced metamaterial platform can powerfully combine multiple time interfaces, enabling electromagnetic time crystals and time metamaterials. Combined with tailored spatial interfaces, the discovery offers the potential to open new directions for photonic technologies, and new ways to enhance and manipulate wave-matter interactions.
Reference: “Observation of temporal reflection and broadband frequency translation at photonic time interfaces” by Hady Moussa, Gengyu Xu, Shixiong Yin, Emanuele Galiffi, Younes Ra’di and Andrea Alù, 13 March 2023, Nature Physics.
This research was partially supported by the Air Force Office of Scientific Research and the Simons Foundation.
“음악 팬. 매우 겸손한 탐험가. 분석가. 여행 괴짜. 익스트림 TV 전문가. 게이머.”