6.2-GHz modulated terahertz light detection using fast terahertz quantum well photodetectors

Hua Li,1 Wen-Jian Wan,1 Zhi-Yong Tan,1 Zhang-Long Fu,1 Hai-Xia Wang,1 Tao Zhou,1 Zi-Ping Li,1 Chang Wang,1Xu-Guang Guo,2 and Jun-Cheng Cao1

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Introduction

太赫茲波的頻率定義在0.1和10太赫茲之間，由于其獨特的特性，如紙張和塑料材料的透明性，各種化學物質的豐富吸收“指紋”以及潛在的超寬通信帶寬，引起了人們極大的興趣。在過去幾十年中，太赫茲輻射源（如量子級聯激光器（QCL），單載波光電二極管，倍頻器等）和探測器（熱電，測輻射熱計，Golay-cell高萊等等）導致了太赫茲技術的顯著進步。例如太赫茲成像，數據傳輸和光譜學的實驗室應用已經被證實。太赫茲技術的進一步發展要求快速檢測千兆赫調頻太赫茲輻射。快速太赫茲檢測器是實現快速太赫茲應用的最后一個組件，因為使用量子級聯設計的太赫茲源已經被驗證可以快速調制到35 GHz 。對于太赫茲探測器，大多數工作在低調制頻率（幾十或幾百赫茲）的熱紅外探測器因此不能檢測到快速調制的太赫茲波。然而，由于在皮秒級別的快載流子弛豫時間，所以采用電子互帶躍遷的量子阱光探測器（QWP）應該在太赫茲區域中快速工作。實際上在中紅外波長范圍內，采用空氣橋技術和共面波導結構，已經在2006年已經展示了帶有小臺面量子阱紅外光電探測器的室溫110GHz高頻外差探測。然而，在太赫茲狀態下，由于衍射極限的原因，耦合到小型QWP器件的太赫茲光相對困難。因此，太赫茲QWP的高頻微波操作尚未見報道。

在這項工作中，利用精心設計的微波傳輸線有效提取射頻（RF）信號，我們首先展示了使用太赫茲QWP（器件面積為400×400μm2）的GHz調制太赫茲輻射的快速檢測。電整流測量顯示，QWP器件具有高達4.3GHz的調制響應帶寬。作為快速檢測的證明，使用快速太赫茲QWP，從長腔太赫茲QCL發射的6.2GHz調制太赫茲光被成功地用于模間光學拍音頻譜的檢測。

更多太赫茲QCL系統產品信息聯系我們，sales@gamtic.com，021-5197 0121

The terahertz wave1 with the frequency defined between 0.1 and 10?THz is of great interest due to its unique properties such as transparency for papers and plastic materials, abundant absorption “fingerprints” of various chemicals, and potential large communication bandwidth. In the previous dozens of years, the fast development of terahertz radiation sources (such as quantum cascade lasers (QCLs)2, 3, uni-travelling-carrier photodiodes4, frequency multipliers5, etc.) and detectors (pyroelectrics6, bolometers7, Golay-cell8, etc.) results in a significant progress of terahertz technology. Laboratory applications for example the terahertz imaging, data transmission, and spectroscopy have been already demonstrated9–13. Further advances in terahertz technology require fast detection of gigahertz frequency modulated terahertz radiation. The fast terahertz detector is the last component for implementing fast terahertz applications since the terahertz source employing the quantum cascade design has been already verified for being fast modulated upto 35?GHz14. Regarding the terahertz detectors, most thermal infrared detectors working at low modulation frequencies (dozens or hundreds of Hz) are therefore not able to detect the fast modulated terahertz wave. However, the quantum well photodectors (QWPs)15, 16 employing the electron intersubband transitions are supposed to work fast in the terahertz region due to the fast carrier relaxation time in picoseconds level. Actually in the mid-infrared wavelength range, room temperature high frequency heterodyne detection upto 110?GHz with small mesa quantum well infrared photodetectors has been already demonstrated in 2006 employing the air bridge technique and coplanar waveguide configuration17. However, in the terahertz regime, due to the diffraction limit the terahertz light coupling to a small QWP device is relatively difficult. And therefore, the high microwave frequency operation of terahertz QWP has been not yet reported.

Here in this work by utilizing a carefully designed microwave transmission line to efficiently extract the radio frequency (RF) signal, we first demonstrate the fast detection of GHz-modulated terahertz radiation using a terahertz QWP with a device area of 400?×?400?μm2. The electrical rectification measurements reveal that the QWP device has a modulation response bandwidth upto 4.3?GHz. As a proof for the fast detection, a 6.2-GHz modulated terahertz light emitted from a long cavity terahertz QCL was successfully detected in terms of inter-mode optical beat note spectrum using the fast terahertz QWP.

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Experimental setup

Figure 1 shows the experimental setup employed for the fast terahertz detection. The terahertz light is generated from an electrically-pump terahertz QCL. Due to the resonant cavity, the laser light is inherently modulated by the cavity round trip frequency of ~c/2ln with c being the speed of light, l the cavity length, and n the refractive index. Two parabolic mirrors are used to collect and focus the terahertz light onto the QWP. To couple the polarized terahertz light into the QWP mesa, a 45° edge facet geometry is used. To facilitate the extraction of the high frequency signal from the terahertz QWP mesa, a 50-Ω microwave transmission line is mounted as close as possible to the QWP mesa as shown in Fig. 1. The transmission line consists of an AlN dielectric substrate which is gold-coated on the backside and a thin metallic layer gold on top of the substrate. To efficiently collect the microwave signal via the transmission line, the design criteria is to make the dielectric layer the same thickness as the terahertz chip thickness around 200?μm and in the meantime set the transmission line impedance to 50-Ω by adapting the width of the metallic line. The top contact of the QWP mesa is wire bonded and connected to the central strip line while the back contact to the gold-plated holes which are connected to the ground. To reduce the microwave signal attenuation as much as possible and apply voltage onto the QWP mesa, high performance coaxial radio frequency cables (Huber?+?Suhner Sucoflex 100) and SMA connectors are used. A bias-T with a bandwidth of 40?GHz and DC current limit of 0.5A is used to inject DC voltage to the QWP and simultaneously extract the high frequency signal via the AC port. The weak microwave signal is firstly amplified by a low noise amplifier with a gain of 30?dB and then sent to a spectrum analyser (upto 26.5?GHz) for characterizations.

Figure 1

Experimental setup of the fast terahertz detection using a microwave transmission line equipped terahertz QWP. The modulated terahertz light is generated from a 6-mm long cavity terahertz QCL working in continuous wave mode. Two off-axis parabolic (OAP) ...

The lower left inset of Fig. 1 shows the focused terahertz beam pattern measured by replacing the QWP with a terahertz imager. It can be seen that with two parabolic mirrors, the terahertz light can be focused in a quasi-Gaussian area at the focus of the second parabolic mirror. The vertical and horizontal widths of the beam pattern are measured to be 290 and 210?μm, respectively, whilst the wavelength of the laser emission is around 71?μm. The upper right inset figure shows the schematic geometry of the QWP chip as well as the incident terahertz wave. If we ideally assume the incident terahertz beam has a gaussian pattern with a diameter of d?=?300?μm, the side length of the square mesa is supposed to be 2√d≈420? μm to fully absorb the incident terahertz emission. Because of this, we finally choose the mesa area of 400?×?400?μm2in this work to balance the trade-off between photoresponse and device capacitance.