Deepu Koshy George
K. A. Niessen
A. G. Markelz
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Intramolecular vibrations of large macromolecules reside in the terahertz range. In particular, protein vibrations are closely spaced in frequency, resulting in a nearly continuous vibrational density of states. This density of vibrations interferes with the identification of specific absorption lines and their subsequent association with specific functional motions. This challenge is compounded with the absorption being dominated by the solvent and local relaxational motions. A strategy for removing the isotropic relaxational loss and isolating specific vibrations is to use aligned samples and polarization-sensitive measurements. Here, we demonstrate a technique to rapidlyattain the anisotropic resonant absorbance using terahertz time domain spectroscopy and a spinning sample. The technique, modulated orientation-sensitive terahertz spectroscopy (MOSTS), has a nonzero signal only for anisotropic samples, as demonstrated by a comparison between a silicon wafer and a wire grid polarizer. For sucroseand oxalic acid molecular crystals, the MOSTS response is in agreement with modeled results for the intermolecular vibrations. Further, we demonstrate that, even in the presence of a large relaxational background, MOSTS isolates underlying vibrational resonances.
PDF全文 (下载：105) Photonics Research ，2016年第4卷第3期 pp.00A1-00A8
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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.
PDF全文 (下载：70) Photonics Research ，2016年第4卷第3期 pp.00A9-0A15
Grace D. Metcalfe
Richard D. Averitt
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We investigate the nonlinear response of terahertz (THz) metamaterial perfect absorbers consisting of electric split ring resonators on GaAs integrated with a polyimide spacer and gold ground plane. These perfect absorbers on bulk semi-insulating GaAs are characterized using high-field THz 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 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 metamaterialsaturable perfect absorber. The ability to create nonlinear absorbers enables appealing applications such as optical limiting and self-focusing.
PDF全文 (下载：80) Photonics Research ，2016年第4卷第3期 pp.0A16-0A21
Elena A. Romanova
Peter Uhd Jepsen
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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 and 5–8 THz were observed. TRTS reveals an ultrafast relaxation process of the photoinduced carrier response, well described by a rate equation model with a finite concentration of mid-bandgap trap states for self-trapped excitons. The photoinduced 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 2 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.
PDF全文 (下载：82) Photonics Research ，2016年第4卷第3期 pp.0A22-0A28
Edward P. J. Parrott
Benjamin S. Y. UngDepartment of Electronic Engineering
The Chinese University of Hong Kong
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We introduce a novel method to accurately extract the optical parameters in terahertz reflection imaging. Our method builds on standard self-referencing methods using the reflected signal from the bottom of the imaging window material to further compensate for time-dependent system fluctuations and position-dependent variation in the window thickness. Our proposed method not only improves the accuracy, but also simplifies the imaging procedure and reduces measurement times.
PDF全文 (下载：96) Photonics Research ，2016年第4卷第3期 pp.0A29-0A35
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Photoconductive switches were the key components that allowed the generation and detection of coherent broadband electromagnetic pulses at terahertz frequencies, opening the possibility for performing spectroscopy and, therefore, measuring complex dielectric properties of materials in this band, which was mostly unexplored. In this paper, we present a brief introduction to the operation principles of these devices. Subsequently, we present a review of the current state-of-the-art in this field and discuss the challenges to be faced in future development of these devices.
PDF全文 (下载：97) Photonics Research ，2016年第4卷第3期 pp.0A36-0A42
S. C. V. Latas
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This publisher’s note reports corrections to three of the figures in [Photon. Res. 4, 49 (2016)].
PDF全文 (下载：61) Photonics Research ，2016年第4卷第3期 pp.0101
W. L. Zhang
Y. B. Song
X. P. Zeng
Z. J. Yang
Y. J. Rao
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In this paper, we proposed a way to realize an Er-doped random fiber laser (RFL) with a disordered fiber Bragg grating (FBG) array, as well as to control the lasing mode of the RFL by heating specific locations of the disordered FBG array. The disordered FBG array performs as both the gain medium and random distributed reflectors, which together with a tunable point reflector form the RFL. Coherent multi-mode random lasing is obtained with a threshold of between 7.5 and 10 mW and a power efficiency between 23% and 27% when the reflectivity of the point reflector changes from 4% to 50%. To control the lasing mode of random emission, a specific point of the disordered FBG array is heated so as to shift the wavelength of the FBG(s) at this point away from the other FBGs. Thus, different resonance cavities are formed, and the lasing mode can be controlled by changing the location of the heating point.
PDF全文 (下载：128) Photonics Research ，2016年第4卷第3期 pp.0102-0105
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Optical simulations of GaAs/AlGaAs thin-film waveguides were performed for investigating the dependence of the modal behavior on waveguide geometry and the resulting analytical sensitivity. Simulations were performed for two distinct mid-infrared wavelengths, thereby demonstrating the necessity of individually designed waveguide structures for each spectral regime of interest. Hence, the modal behavior, sensitivity, and intensity of the evanescent field were investigated via modeling studies at 1600 and 1000 cm?1, thereby confirming the utility of such simulations for designing mid-infrared sensors based on thin-film waveguide technology.
PDF全文 (下载：96) Photonics Research ，2016年第4卷第3期 pp.0106-0110
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Tungsten disulfide (WS2) is a type of anisotropic-layered compound and has broadband saturable absorption features as saturable absorbers (SAs). With WS2-based SAs, dark solitons in erbium-doped fiber (EDF) lasers are first obtained. For the generated dark solitons, the center wavelength is measured to be 1530 nm, and the repetition rateis about 116.5 MHz. A series of optical spectra is exhibited. The electrical signal-to-noise ratio is better than 94 dB. Results in this paper demonstrate that WS2-based SAs are the promising SAs for generating dark solitons in EDF lasers.
PDF全文 (下载：149) Photonics Research ，2016年第4卷第3期 pp.0111-0114
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By studying the traditional spectral reflectance reconstruction method, spectral reflectance and the relative spectral power distribution of a lighting source are sparsely decomposed, and the orthogonal property of the principal component orthogonal basis is used to eliminate basis; then spectral reflectance data are obtained by solving a sparse coefficient. After theoretical analysis, the spectral reflectance reconstruction based on sparse prior knowledge of the principal component orthogonal basis by a single-pixel detector is carried out by software simulation and experiment. It can reduce the complexity and cost of the system, and has certain significance for the improvement of multispectral image acquisition technology.
PDF全文 (下载：95) Photonics Research ，2016年第4卷第3期 pp.0115-0121
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We propose and demonstrate a multifunctional location-dependent metamaterial in the 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 pairs of the coupled resonators will keep their individual resonance modes when they join 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 feasiblestructure for multifunctional microelectromechanical devices.
PDF全文 (下载：72) Photonics Research ，2016年第4卷第3期 pp.0122-0125
Hoc Khiem Trie
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We present integrated-optic building blocks and functional photonic devices based on amorphous siliconon-insulator technology. Efficient deep-etched fiber-to-chip grating couplers, low-loss single-mode photonic wire waveguides, and compact power splitters are presented. Based on the sub-μmphotonic wires, 2 × 2 Mach–Zehnder interferometers and add/drop microring resonators (MRRs) with low device footprints and high finesse up 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 capabilities of the material platform are demonstrated with efficient microheaters and a permanent device trimming method, which enabled the realization of energy-efficient photonic circuits. Wavelength multiplexers in the form of cascaded filter banks and 4 × 4 routers based on MRR switches are presented. Fabrication imperfections were analyzed and permanently corrected by an accurate laser-trimming method, thus enabling eight-channel multiplexers with record low metrics of sub-mW static power consumption and ≤1°C 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 ultralow-power 3D-stacked photonic circuits on top of CMOS microelectronics.
PDF全文 (下载：188) Photonics Research ，2016年第4卷第3期 pp.0126-0134
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PDF全文 (下载：53) Photonics Research ，2016年第4卷第3期 pp.2