根據(jù)德國馬克斯普朗克學會（Max Planck Institute）最近發(fā)布的一項報告顯示，通過石墨烯可望實現(xiàn)高功率的太赫茲（THz）激光光源。由于目前缺少太赫茲激光，而現(xiàn)有可用的THz級輻射光源又相當昂貴且體積龐大，如機場安全檢查站用來檢查衣物底下是否暗藏玄機的安全掃描儀。如今，馬克斯普朗克學會宣稱已證實石墨烯能用于制作出THz級激光器，并能進一步應用于探測高溫超導體。

通過外部光源激發(fā)單層石墨烯的示意圖

（來源：Max Planck Institute）

石墨烯是由純碳原子薄片組成，援引國際半導體技術(shù)藍圖（International Technology Roadmap for Semiconductors，ITRS）的預測表示，大約2023年石墨烯最終將取代硅晶微型晶片。然而，石墨烯由于缺乏能隙，因而被認為不適用于激光器。過去一向認為能隙是激光器中進行粒子數(shù)反轉(zhuǎn)的必要條件，大部份材料的電子由基態(tài)（價帶）穿越能隙而躍遷至激發(fā)狀態(tài)（導帶）然后再回到基態(tài)，并在這一過程中引發(fā)光子雪崩。

雖然石墨烯缺乏能隙，但馬克斯普朗克學會的研究人員們經(jīng)由實驗發(fā)現(xiàn)，石墨烯在THz頻率時經(jīng)由經(jīng)紅外線激光激發(fā)后仍能進行粒子數(shù)反轉(zhuǎn)，并產(chǎn)生短突波，使其在理論上應該也適用于脈沖THz激光的刺激發(fā)射。

“我們用第一個激光脈沖激發(fā)電子從價帶躍遷到導帶，并以光電子探測導帶中的電子停留多久才開始衰減回到價帶的基態(tài)，”Max Planck研究人員Isabella Gierz表示。

其結(jié)果是太赫茲波長發(fā)射長度約100飛秒左右，適于快速的脈沖激光。根據(jù)Gierz表示，許多應用都可從太赫茲激光器中受益，包括高溫超導體的研究等。

“高溫超導體以及其他材料在太赫茲范圍內(nèi)可展現(xiàn)多項應用啟發(fā)，”Gierz表示，“除了基礎(chǔ)研究以外，太赫茲光源可催生多項其他應用。”

截至目前為止，Gierz及其于Max Planck的研究團隊們已經(jīng)突破了幾項障礙，期望不久后可實現(xiàn)一款基于石墨烯的太赫茲激光器。

“激光器由三個部分組成：可放大光源的活性介質(zhì)、可供電的幫浦源，以及激光腔，”Gierz指出，“我們已經(jīng)成功完成第一和第二部份了，但還需要專門設計的激光腔。”

編譯：Susan Hong

原文參考：

Graphene Laser to Probe Superconductivity

PORTLAND, Ore. — Graphene could make the illusive terahertz laser possible, according to a recent report from the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg. Sources of terahertz radiation, such as that used by those security scanners that look underneath your clothing at airport security checkpoints, are expensive and bulky today, and no source exists for a terahertz laser. Now, Max Planck Institute claims to have demonstrated that graphene could be used to produce terahertz lasers, which could be useful in probing high-temperature superconductors.

Graphene -- comprising atomically thin sheets of pure carbon -- is already being developed as the successor to silicon, after the end of the International Technology Roadmap for Semiconductors （ITRS）circa 2023. However, graphene was thought to be unsuitable for lasers, since it does not have a bandgap. A bandgap was previously thought to be a necessary condition for population inversion in lasers, where most of a material's electrons are boosted from their ground state （valence band）across the bandgap to their exited state （conduction band）after which they decay back to the ground state, emitting an avalanche of photons in the process.

Despite its lack of a bandgap, Max Planck researchers were nevertheless able to demonstrate population inversion in graphene at terahertz frequencies after exciting it with an infrared laser, resulting in short bursts that theoretically make it suitable for the stimulated emission of a pulsed terahertz laser.

"We use a first laser pulse to excite electrons from the valence into the conduction band and use photoemission to probe how long the electrons stay in the conduction band before they decay back to their ground state in the valence band," Max Planck researcher Isabella Gierz told EE Times.

The result was terahertz wavelength emissions about 100 femtoseconds in length, suitable for a fast pulsed laser. Many applications could benefit from a terahertz laser, including research into high-temperature superconductors, according to Gierz who performed the work with colleagues at the Central Laser Facility in Harwell, England, and the Max Planck Institute for Solid State Research in Stuttgart, Germany.

"High-temperature superconductors -- but also other materials -- exhibit numerous excitations in the terahertz range," said Gierz. "Aside from fundamental research, terahertz light sources have numerous other applications."

So far, Gierz and her research group at Max Planck have surmounted two of the three hurdles to producing a graphene-based terahertz laser, with the last element on their to-do list.

"A laser consists of three parts: an active medium for light amplification, a pump source that supplies the power, and a laser cavity," Gierz told us. "We have demonstrated the first and second. The cavity, however, needs to be designed."

Gierz and colleagues also tried to show that graphene could be used in a manner opposite to a laser, that is, for light harvesting in solar cells. What they found was that one graphene electron can potentially release multiple photons for stimulated emission in a laser, but the opposite process -- one photon releasing multiple electrons as is necessary for solar cells -- was not observed, leading the group to believe that graphene will not work for photovoltaics.

By R. Colin Johnson