Free-carrier density dependent relaxation lifetime in Si quantum dot optical absorption modulator Chung-Lun Wu, Sheng-Pin Su and Gong-Ru Lin* Graduate Institute of Photonics and Optoelectronics, and Department of Electrical Engineering, National Taiwan University No.1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan, Republic of China *E-mail: grlin@ntu.edu.tw Abstract: A 4-nm large Si-QD doped SiO x rib waveguide based optical absorption modulator with free-carrier density dependent relaxation lifetime is demonstrated. The switching response is shortened to 9 μs by reducing pumping duty-cycle to 0.5 μs. I. INTRODUCTION Due to the quantum confinement effect, the quantum efficiency and radiative recombination rate of silicon quantum dots (Si-QDs) can be significantly enhanced when the grain size of Si shrinking from bulk to few nanometers. Recently, to demonstrate the stimulated emission from Si-QDs, the micro-disk, waveguide amplifier and distributed Bragg reflector structure have been introduced to the Si-QD based light emitting device. However, only the spontaneous emission and amplified spontaneous emission have been discovered. When increasing the pumping power to achieve the higher optical net gain, the larger free carrier absorption (FCA) loss is also enhanced. Although the FCA limited the optical gain in the Si-QD based devices, the FCA effect can be utilized to demonstrate the all-optical modulator or switch at optical communication region. In 2009, Kekatpure et. al have proven that the FCA absorption cross-section in Si-QD is one-order higher than that of in the bulk Si [1]. Recently, the Si-QD based FCA modulator has been reported. Typically, the pump-probe system with the chopper is utilized to demonstrate the FCA, and the modulation frequency for the chopper is ~1 kHz [2, 3]. In our work, the electrical pattern generator is utilized to replace the chopper, and directly modulates the pumping laser. The probe signal power degradation caused by thermal effect is significantly decreased and the pattern generator enables the faster modulation on the pumping laser than the chopper. In addition, the relation between the pumping duty-cycle and the free carrier absorption loss as well as modulation depth of the Si-QD based FCA modulator has been discussed. Furthermore, the pumping power-dependent relaxation time is discovered in our experiment. The variation of the relaxation lifetime can be correlated to the free carrier occupation rates in Si-QDs. II. EXPERIMENTAL SETUP The geometric structure of the Si-QD based rib waveguide and the cross-section view recorded by scanning electron microscope (SEM) are shown in the inset of Fig. 1. First, the 1-μm thick of Si-rich SiO x film is deposited on the 3-μm thick buffered SiO 2 by using the plasma-enhanced chemical vapor deposition (PECVD). Then, the Si-QDs are self-assembly formed during the thermal annealing process at 1100 o C for 90 mins in N 2 environment. The averaged Si-QD size is ~4.3 nm with the size distribution of 0.9 nm, which are analyzed by the High-resolution transmission electron microscopy (HRTEM). Finally, the Si-QD based rib waveguide is fabricated by using the e-beam lithography and reactive ion etching. The system diagram of the pump-probe measurement is shown in the Fig. 1. The GaN laser diode is directly modulated by the electrical pattern generator (HP81101A). The pumping wavelength is 405-nm, and the pumping beam is focused by using the cylindrical lens. The width and length of the focused pumping line are 100 μm and 2 mm, respectively. On the other hand, the probe signal is output from the tunable laser source (Agilent 8164A). The wavelength of probe signal is set as 1550-nm. The polarization controller is utilized to control the polarization of the probe beam. The probe signal is injected into the Si-QD based waveguide by using the lens fiber, and the mode diameter of the lens fiber is ~ 3 μm. The modulated probe signal is collected by the lens fiber from the waveguide facet. Finally, the probe signal traces are displayed by the digital sampling oscilloscope (HP 83485A). Fig. 1 The pattern generator based pump-probe system for FCA modulation. (PG: electrical pattern generator, LD: 405-nm GaN laser diode, TL: tunable laser source, EDFA: erbium-doped fiber amplifier, PC: polarization controller, DSO: digital sampling oscilloscope.) Inset: The geometric structure of the Si-QD based rib waveguide and the SEM cross-section view. 47 WP7 (Contributed) 18:00 – 18:00 978-1-4673-5804-0/13/$31.00 ©2013 IEEE