Accepted papers to appear in an upcoming issue
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Nonlinear terahertz metamaterial perfect absorbers using GaAs
Richard Averitt, Grace Metcalfe, Jingdi Zhang, Michael Wraback, Kebin Fan, Xiaoguang Zhao, Guangwu Duan, and Xin Zhang
Doc ID: 258715 Received 02 Feb 2016; Accepted 25 Apr 2016; Posted 28 Apr 2016 View: PDF
Abstract: We investigate the nonlinear response of terahertz metamaterial perfect absorbers consisting of electric split ring resonators (ESRRs) on GaAs integrated with a polyimide spacer and gold ground plane. These perfect absorbers on bulky semi-insulating GaAs are characterized using high field terahertz time domain spectroscopy. The resonance frequency redshifts 20 GHz and the absorbance is reduced by 30% as the incident peak field is increased from 30 kV/cm to 300 kV/cm. The nonlinear response arises from THz field driven interband transitions and intervalley scattering in the GaAs. To eliminate the Fresnel losses from the GaAs substrate, we design and fabricate a flexible metamaterial saturable perfect absorber. The ability to create nonlinear absorbers enables appealing applications such as optical limiting and self-focusing.
Time-resolved terahertz spectroscopy of charge carrier dynamics in the chalcogenide glass As30Se30Te40
Peter Jepsen, Tianwu Wang, Elena Romanova, Nabil Abdel-Moneim, David Furniss, Angela Seddon, Trevor Benson, ZHUOQI TANG, Andrei Lavrinenko, and Anna Loth
Doc ID: 259125 Received 08 Feb 2016; Accepted 19 Apr 2016; Posted 22 Apr 2016 View: PDF
Abstract: Broadband (1.6-18 THz) terahertz time-domain spectroscopy (THz-TDS) and time-resolved terahertz spectroscopy (TRTS) were performed on a 54 µm thick chalcogenide glass (As30Se30Te40) sample with a two-color laser-induced air plasma THz system in transmission and reflection modes, respectively. Two absorption bands at 2-3 THz and 5-8 THz were observed. TRTS reveals an ultrafast relaxation process of the photo-induced carrier response, well described by a rate equation model a finite concentration of mid-bandgap trap states for self-trapped excitons. The photo-induced conductivity can be well described by the Drude-Smith conductivity model with a carrier scattering time of 12-17 fs, and we observe significant carrier localization effects. A fast refractive index change was observed 100 fs before the conductivity reached its maximum, with two orders of magnitude larger amplitude than expected for the optically induced THz Kerr effect, indicating that free carriers are responsible for the transient index change.
Photonic Integrated Circuit Components based on Amorphous Silicon-on-Insulator Technology
Timo Lipka, Lennart Moldenhauer, Jorg Mueller, and Hoc Khiem Trieu
Doc ID: 260153 Received 02 Mar 2016; Accepted 07 Apr 2016; Posted 08 Apr 2016 View: PDF
Abstract: We present integrated-optic building blocks and functional photonic devices based on amorphous silicon-on-insulator (a-SOI) technology. Efficient deep-etched fiber-to-chip grating couplers, low-loss single-mode photonicwire waveguides, and compact power splitters are presented. Based on the sub-μm photonic wires 2x2 Mach-Zehnder-interferometers and add/drop microring resonators (MRRs) with low device footprints and high finesseup to 200 were realized and studied. Compact polarization rotators and splitters with ≥10 dB polarization extinction ratio were fabricated for the polarization management on-chip. The tuning and trimming capabilitiesof the material platform are demonstrated with efficient microheaters and a permanent device trimming method that enable the realization of energy-efficient photonic circuits. Wavelength multiplexers in form of cascadedfilter-banks and 4x4 routers based on MRR-switches are presented. Fabrication imperfections were analyzed and permanently corrected by an accurate laser trimming method enabling 8-channel multiplexers with record low metrics of sub-mW static power consumption and ≤1ºC temperature overhead. The high quality of the functional devices, the high tuning efficiency, and the excellent trimming capabilities demonstrate the potential to realize low-cost, densely integrated, and ultra-low-power 3D-stacked photonic circuits on top ofCMOS microelectronics.
Location-dependent metamaterials in terahertz range for reconfiguration purpose
Kejian Chen, Ting Bu, Hong Liu, Jianjun Liu, Zhi Hong, and Songlin Zhuang
Doc ID: 259205 Received 09 Feb 2016; Accepted 06 Apr 2016; Posted 08 Apr 2016 View: PDF
Abstract: We propose and demonstrate a multifunctional location-dependent metamaterial in terahertz (THz) range, in which the unit cell consists of two pairs of coupled resonators. Experimental and simulation results of our devices reveal that both two pairs of the coupled resonators will keep their individual resonance modes when they joint together. Thus the overall transmission spectrum is a combination of frequency response spectra of its corresponding constituent parts. While changing the locations of the inner resonators in our structure, controllable width of transmission window and changeable number of transmission dips can be realized. Our design provides a feasible structure for multifunctional Micro-electromechanical devices.
Modulated Orientation Sensitive Terahertz Spectroscopy
Andrea Markelz, Deepu George, Chejin Bae, Rohit Singh, and Katherine Niessen
Doc ID: 258858 Received 03 Feb 2016; Accepted 31 Mar 2016; Posted 01 Apr 2016 View: PDF
Abstract: Intramolecular vibrations of large macromolecules lie in the THz range. However, for complex molecular systems such as proteins, the is a large vibrational density of states has vibrations closely spaced in frequency, interfering with the identification of the absorption lines. In addition, absorption from the solvent and local relaxational motions can dominant the measurement. One can both remove the isotropic contribution from the relaxational response and simplify the vibrational spectrum using aligned samples and polarization sensitive measurements. Here we show that one can rapidly attain the anisotropic THz absorbance of molecular crystals using terahertz time domain spectroscopy and a spinning sample. The technique, modulated orientation sensitive terahertz spectroscopy (MOSTS) is characterized with sucrose and oxalic acid molecular crystals. We find agreement between the measurements and modeled results. Further we demonstrate that even in the presence of a large relaxational background, the technique can isolate well defined vibrational resonances.
Study of photoexcited-carrier dynamics in GaAs photoconductive switches using dynamic terahertz emission microscopy
Hironaru Murakami, Shogo Fujiwara, Iwao Kawayama, and Masayoshi Tonouchi
Doc ID: 258673 Received 04 Feb 2016; Accepted 29 Mar 2016; Posted 01 Apr 2016 View: PDF
Abstract: We propose dynamic terahertz (THz) emission microscopy (DTEM) to visualize temporal-spatial dynamics of photoexcited carriers in electronic materials. DTEM utilizes THz pulses emitted from a sample by probe pulses irradiated after pump pulse irradiation to perform time-resolved two-dimensional mapping of the THz pulse emission, reflecting various carrier dynamics. Using this microscopy, we investigated carrier dynamics in the gap region of low-temperature-grown GaAs and semi-insulating GaAs photoconductive switches of the identical-dipole type. The observed DTEM images are well explained by the change in the electric potential distribution between the electrodes caused by the screening effect of the photoexcited electron-hole pairs.