Emission spectroscopy in MIR using FTIR: an overview

Zhang Yong-gang*, Gu Yi, Wang Kai, Fang Xiang, Li Ai-zhen, Li Yao-yao, Liu Ke-hui

State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China

BIOGRAPHY Zhang Yong-gang: received B.S degree in semiconductor device from Nanjing Institute of Posts and Telecommunications, China in 1982, M.S and Ph.D. degrees in semiconductor physics from Shanghai Institute of Metallurgy, Chinese Academy of Sciences in 1987 and 1996 respectively. He had been a worker during 1975-1978, and a teacher during 1982-1984. He joined Shanghai Institute of Metallurgy (now named Shanghai Institute of Microsystem and Information Technology), Chinese Academy of Sciences since 1987, his research interests include III-V semiconductor optoelectronic materials, devices and applications. He had been a research professor at the State Key Laboratory of Functional Materials for Informatics since 1996, and supervised more than 20 Ph.D. and M. S. students there. He was a senior member of IEEE.

TECHNICAL ABSTRACT For most FTIR instruments an emission port has been included, through this port the spectral features from external

emission sources of materials and devices, such as lasing, photoluminescence (PL) and electroluminescence (EL), could be characterized. Based on this arrangement, accessional user configurations for different approaches have been developed [1-4]. Acquiring relatively strong PL signals apart from the background using conventional rapid scan mode of FTIR are valid. This mode is also effective for device characterization, such as the narrow band emission spectrum of quantum well lasers and quantum cascade lasers (QCLs) with enough lasing power in MIR, regardless of continuous wave or pulsed operation. However, because in certain cases the signal is often quite weak, and in many cases they are even much weaker than the room temperature thermal background radiation or stray light background, so extracting weak signal from the background and keeping the convenience of FTIR remains a challenge. In this work, with the comparative analysis of the system, an improved FTIR accessional configuration has been developed. In this approach three different modes of rapid scan, double modulation or step scan were software switchable without changing of the hardware or connections. The advantages and drawbacks of each mode were discussed in detail. Using this approach a group of emission samples in MIR with quite different intensities have been characterized to demonstrate the system validity.

As an example, Fig.1 shows the lasing spectrum of a pulsed DFB-QCL at about 7.7 μm using rapid scan or double modulation mode, in the measurements only one scan at the highest resolution (0.125 cm-1) of the instrument was used. Compared to the rapid scan mode, the using of double modulation mode restrained thermal background effectively, the signal enhanced remarkably. Another example was the PL measurements of two III-V samples in MIR at room temperature as shown in Fig.2, in which sample A was a MBE grown InSb with pn junction, sample B was a n-type InAs substrate. The PL was excited using a DPSS laser. For this extremely weak PL signal 32 scans at resolution of 16 cm-1 were used. The PL signals were submerged into the thermal background in rapid scan mode, whereas visualized in double modulation mode clearly. Results show that for emission samples with relatively strong signal out off the background, rapid scan mode is still preferable. For weaker emission samples overlapped with background, double modulation is the most effective mode. To get a better S/N when weaker PL or EL signals have been observed in double modulation mode, step scan mode should be an advisable option in despite of its long data acquiring time. Step scan mode is rarely used for lasing spectroscopy due to limited resolution and chirp of the laser during long acquiring time.

References: [1] K. A. Harris, S. Hwang, D. K. Blanks, J. W. Cook Jr., J. F. Schetzina, N. Otsuka, J. P. Baukus, and A. T. Hunter, Appl. Phys. Lett., 48, 396, (1986). [2] F. Fuchs, A. Lusson, J. Wagner and P. Koidl, Proc. SPIE 1145, 323 (1989). [3] Y. G. Zhang, K. J. Nan, and A. Z. Li, Spectrochimica Acta Part A 58, 2323, (2002). [4] J. Shao, W. Lu, X. Lü, F. Y. Yue, Z. F. Li, S. L. Guo, and J. H. Chu, Rev. Sci. Instrum. 77, 063104 (2006). Keywords: Emission spectroscopy, Mid-infrared, Fourier transform infrared, Lasing spectrum, Photoluminescence

*ygzhang@mail.sim.ac.cn; phone +8621-62511070x8312; www.sim.ac.cn

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Fig. 1. Lasing spectrum of a DFB-QCL at about 7.7 μm using rapid scan or double modulation mode.

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Fig. 2. PL of two III-V samples in MIR at room temperature using rapid scan or double modulation mode.