科學家發明室溫下的太赫茲激光器

據報道,這種世界上第一款室溫太赫激光器利用相當于光學外差法的技術來彌合太赫鴻溝。目前,太赫鴻溝存在于大多數半導體激光無法工作的地方,在微波波長(厘米波)和光波長(微米波)之間,其間是毫米波——太赫。

目前,在太赫茲頻率工作的唯一激光器是超冷卻量子級聯激光器(QCL)。最近,QCL的共同發明人(1944年在貝爾實驗室)、哈佛大學教授Federico Capasso已經證明,把在想要的太赫頻率上的、空間分隔的、兩個易于產生的光學頻率,在非線性材料中利用外差法注入進行混合,就可以得到室溫下的太赫激光器。

“這種非線性光學材料的有趣特征在于,當由兩個頻率激發時,它們的要素分子會連貫地振動,不僅僅在稱為‘泵頻’的驅動頻率上,而且在它們的差頻上,”哈佛大學教授Federico Capasso說道,“因此,在材料的輸出上,你不僅僅可以觀察到泵頻的光線,而且能夠觀察到差頻的光線,這個過程類似于在無線電中廣為采用的超外插原理。”

通過選擇易于在室溫產生的光學波長,但是,其頻差嚴格等于想要的太赫頻率,Capasso和哈佛研究協會的Mikhail Belkin回避了太赫鴻溝所存在的問題,獲得了工作在室溫的太赫激光器。Capasso的研究小組所采用的兩個光學激光器在室溫下被證明頻率分別為33.7-THz (8.9-微米波)和28.5-THz (10.5微米波),它們產生的差頻為5.2 THz。

“基本上,電子在這個頻率被驅動至完全同相振蕩,因此,產生相干太赫發射,”Capasso表示,“該器件的結構是紅外線雙中頻QCL以及非線性材料這兩者相結合,從而差生頻率差。因為兩個紅外線中頻是在室溫下產生的,它們的頻差顯然也是在室溫下產生的。以這種方式,我們成功地突破了THz QCL的極限,以前的器件迄今為止僅僅工作在低溫條件下。”

太赫掃描儀就像X射線一樣工作,但是,它的功率水平對于使用它的周圍的人來說是完全安全的。利用太赫掃描儀,機場可以檢測出隱藏在衣服中的武器,以及在行李中的有害和有毒物質。太赫激光器也可以被用于遠程監測在空氣中漂浮的有害氣體,從而為在一定距離上辨別臨時放置好的爆炸設備提供一種潛在的解決方案。

量子阱階梯

傳統的激光器給電子注入能量,然后,從半導體的導帶激發出一個中子,使之跳躍至價帶。相比之下,量子級聯激光器安排一個量子阱階梯,每一個處于漸變的較低能級,從而允許電子沿著能量階梯級聯下來,在每一個階梯上發射一顆中子。目前,量子級聯激光器如果不做過冷處理就無法在太赫頻段工作。然而,通過采用超外差架構,哈佛的研究人員證明,兩個量子級聯激光器的混合輸出可以覆蓋太赫茲頻段。

在非線性光學領域,超外差原理就是著名的差頻產生(DFG)技術。當光線撞擊非線性材料時,它們的行為就像線性諧振子一樣,只有當頻率匹配它們的自己的內部自然諧振頻率時才會振蕩。另一方面,像真空管和晶體管這樣的非線性器件可以被制成在兩個輸入的合頻以及差頻上諧振,從而允許無線電在各個頻帶之間搬移信號,或者,對信號進行編解碼。

其它研究人員已經證明了利用DFG實現太赫激光的可行性,但是,要采用碩大的外部“泵”激光來證明其原理。哈佛大學的研究小組利用半導體材料完成了這個任務,如果一切進展順利,最終將以廉價實現室溫下器件的大規模生產。

“我們的器件在一個微型半導體晶體上完成了一切,不需要利用碩大的外部激光做泵源,因此,優點在于緊湊、便攜、低功耗,”Capasso表示,“實際上,器件的材料被設計和生長為當偏置電流作用在它上面時,激光器不僅僅發射出所產生的兩個不同的紅外線中頻,而且以相應的差頻產生相干輻射,在我們的情形下,就是在5太赫茲。”

這種非線性器件像混頻(產生合頻與差頻)那樣工作的機制取決于所采用的材料。量子級聯激光器在制造過程中采用了分子束外延,從鎵和鋁構成的輪換層上一次制成原子層。每一層原子層均比它前面的一層稍薄。

下一步,哈佛的研究人員計劃最優化它們的設計,以努力把輸出功率從目前的納瓦級提高至幾個毫瓦。其中,一種解決方案就是把熱電冷卻裝置加入到激光器的襯底上,因為激光工作溫度越低,輸出功率越大。其次,該小組計劃把半導體材料的邊緣發射轉換為表面發射。

“我們的方法將極大地增加用于發射的表面積,”Capasso說道,“通過制成一種合適的光柵,由它垂直地把器件有源區產生的太赫茲輻射分散,就可以實現表面輻射。”

Belkin和Capasso的研究工作得到了Texas A&M大學的研究人員Feng Xie和Alexey Belyanin、以及瑞士蘇黎世的ETH大學的研究人員Milan Fischer、Andreas Wittmann和Jrme Faist等人的協作。研究資金由美國Air Force Office of Scientific Research、國家科學基金以及兩個基于哈佛的研究中心、美國納米級科學和工程中心以及美國國家納米科技基礎設施網絡下屬的納米級系統中心等單位提供。

翻頁查看英文原文:

Room-temperature terahertz laser invented

What's claimed to be the world's first room-temperature terahertz laser harnesses the optical equivalent of heterodyning to bridge the terahertz gap. Today, a terahertz-gap exists where most semiconductor lasers fail to operate--between microwave wavelengths (centimeters) and optical wavelengths (microns). In between are the millimeter wavelengths--terahertz frequencies (1-10 THz).

The only semiconductor lasers that run at terahertz frequencies today are supercooled quantum cascade lasers (QCL). Now, the co-inventor of the QCL (while at Bell Labs in 1994), professor Federico Capasso at Harvard University, has demonstrated a heterodyning method cast in nonlinear materials that mixes two easy-to-generate optical frequencies spaced apart at the desired terahertz frequency, resulting in a room-temperature terahertz laser.

"This class of nonlinear optical materials has the interesting property that, when illuminated by two frequencies, their constituent molecules vibrate coherently, not only at the driving frequencies, known as 'pump' frequencies, but also at their difference frequency," said Harvard professor, Federico Capasso. "As a result, at the output of the material one not only observes light at the pump frequencies, but also at the difference frequency--a process similar to the heterodyne principle widely used in radio."

By choosing optical wavelengths that are easy to generate at room temperature--but whose difference is exactly the desired terahertz frequency--Capasso and Harvard research associate Mikhail Belkin sidestepped the terahertz-gap problem, resulting in a terahertz laser that operates at room temperature. The two optical lasers used by Capasso's group in its room-temperature demonstration were at 33.7-THz (8.9-micron wavelength) and 28.5-THz (10.5-micron wavelength), which produced a difference frequency of 5.2 THz.

"Basically, electrons are driven to oscillate all in phase at this frequency, thus producing coherent terahertz emission," said Capasso. "The device structure is both a two frequency mid-infrared QCL and a nonlinear material, which generates the frequency difference. Since the two mid-infrared frequencies are generated at room temperature, their difference obviously is, as well. In this way we have circumvented the limitation of THz QCLs, which operate so far only at cryogenic temperatures."

Terahertz scanners act like x-rays, but at power levels that are completely safe to use around people. Using a terahertz scanner, airports could detect hidden weapons under clothing, as well as hazardous and toxic materials inside luggage. Terahertz lasers could also remotely detect hazardous gases floating in the air, offering a potential solution to identifying improvised explosive devices from a distance.

A stair-step of quantum wells

Conventional lasers energize electrons, which then emit a single photon by jumping from the semiconductor's conduction band to its valence band. Quantum cascade lasers, on the other hand, arrange a stair-step of quantum wells--each at a progressively lower energy level--that allow electrons to cascade down an energy staircase, emitting a photon at each step. Today, quantum cascade lasers lose their ability to work in the terahertz gap without supercooling. But by using a heterodyning architecture, the Harvard researchers demonstrated twin quantum cascade lasers, whose mixed output is in the terahertz gap.

The heterodyning principle is well known in nonlinear optics as difference frequency generation (DFG). Most materials act like linear harmonic oscillators when light impinges on them, oscillating only when the frequency matches their own internal natural resonant frequency. Nonlinear materials like vacuum tubes and transistors, on the other hand, can be made to resonate at the sum and difference frequencies of two inputs, enabling radios to move signals between bands, or to encode and decode them.

Others have demonstrated the feasibility of terahertz lasers using DFG, but bulky external "pump" lasers were used just to prove the principle. The Harvard group accomplished the task with semiconductor materials that, if all goes well, eventually could be mass produced for inexpensive room-temperature devices.

"Our device does everything in one small semiconductor crystal with no need for bulky external lasers for pumping; hence, the advantages of compactness, portability and low power consumption," said Capasso. "In essence, the material of the device is designed and grown so that when a bias current is applied to it, not only are laser beams emitting at two different mid-infrared frequencies generated, but also coherent radiation at the difference frequency corresponding, in our case, to 5 Terahertz".

The mechanism by which nonlinear devices perform operations like mixing--generating sum and difference frequencies--depends on the materials used. The quantum cascade laser is fabricated using molecular-beam epitaxy, a layer of atoms at a time, from alternating layers of gallium and aluminum. Each layer is slightly thinner than the one before it.

Next, the Harvard researchers plan to optimize their design in an attempt to increase the output power to milliwatts, from its nanowatt levels today. One way is to add low-cost thermoelectric coolers to the laser's substrate--since the cooler the laser runs, the higher its output power. Secondly, the group plans to switch from edge emission to surface emission for their semiconductor material.

"Our approach will be to greatly increase the surface area used for emission," said Capasso. "Surface emission will be achieved by fabricating a suitable grating to scatter vertically the terahertz radiation generated in the device's active region."

Belkin and Capasso performed the work in cooperation with researchers Feng Xie and Alexey Belyanin, at Texas A&M University (College Station), and researchers Milan Fischer, Andreas Wittmann, and Jrme Faist, at ETH (Zurich, Switzerland). Funding was provided by the Air Force Office of Scientific Research, the National Science Foundation and two Harvard-based centers, the Nanoscale Science and Engineering Center and the Center for Nanoscale Systems, a member of the National Nanotechnology Infrastructure Network.