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Sekine, Norihiko; Hosako, Iwao |
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Intensity modulation of terahertz quantum cascade lasers under external light injection |
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2009 |
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Applied Physics Letters |
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Appl. Phys. Lett. |
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95 |
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201106(1-3) |
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QCL modulation, THz, terahertz |
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We investigated the light-current characteristics of terahertz (THz) quantum cascade lasers under external light injection, which excites interband transitions in the active materials. It was found that the amount of reduction in the THz power was constant for all injection currents above threshold, and the dependence of the reduction amount on the wavelength of the external light was observed to show a resonancelike feature. The dominant intensity modulation mechanism was found to be the loss change caused by interband transitions in the active region. Further, the effective coupling efficiency plays an important role in the intensity modulation. |
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630 |
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Kerman, Andrew J.; Yang, Joel K. W.; Molnar, Richard J.; Dauler, Eric A.; Berggren, Karl K. |
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Title |
Electrothermal feedback in superconducting nanowire single-photon detectors |
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Journal Article |
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2009 |
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Phys. Rev. B |
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Phys. Rev. B |
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79 |
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10 |
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4 |
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SNSPD |
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We investigate the role of electrothermal feedback in the operation of superconducting nanowire single-photon detectors (SNSPDs). It is found that the desired mode of operation for SNSPDs is only achieved if this feedback is unstable, which happens naturally through the slow electrical response associated with their relatively large kinetic inductance. If this response is sped up in an effort to increase the device count rate, the electrothermal feedback becomes stable and results in an effect known as latching, where the device is locked in a resistive state and can no longer detect photons. We present a set of experiments which elucidate this effect and a simple model which quantitatively explains the results. |
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RPLAB @ gujma @ |
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680 |
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Lobanov, Y.; Tong, C.; Blundell, R.; Gol'tsman, G. |
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A study of direct detection effect on the linearity of hot electron bolometer mixers |
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2009 |
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Proc. 20th Int. Symp. Space Terahertz Technol. |
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Proc. 20th ISSTT |
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282-287 |
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HEB mixer, direct detection effect |
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We have performed a study of how direct detection affects the linearity and hence the calibration of an HEB mixer. Two types of waveguide HEB devices have been used: a 0.8 THz HEB mixer and a 1.0 THz HEB mixer which is ~5 times smaller than the former. Two independent experimental approaches were used. In the ΔG/G method, the conversion gain of the HEB mixer is first measured as a function of the bias current for a number of bias voltages. At each bias setting, we carefully measure the change in the operating current when the input loads are switched. From the measured data, we can derive the expected difference in gain between the hot and cold loads. In the second method (injection method [1]), the linearity of the HEB mixer is independently measured by injecting a modulated signal for different input load temperatures. The results of both approaches confirm that there is gain compression in the operation of HEB mixers. Based on the results of our measurements, we discuss the impact of direct detection effects on the operation of HEB mixers. |
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RPLAB @ gujma @ |
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724 |
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Hu, Xiaolong; Zhong, Tian; White, James E.; Dauler, Eric A. Najafi, Faraz; Herder, Charles H.; Wong, Franco N. C.; Berggren, Karl K. |
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Fiber-coupled nanowire photon counter at 1550 nm with 24% system detection efficiency |
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2009 |
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Optics Letters |
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Opt. Lett. |
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34 |
Issue |
23 |
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3607-3609 |
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SNSPD |
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We developed a fiber-coupled superconducting nanowire single-photon detector system in a close-cycled cryocooler and achieved 24% and 22% system detection efficiencies at wavelengths of 1550 and 1315 nm, respectively. The maximum dark count rate was ~1000 counts/s. |
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RPLAB @ gujma @ |
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679 |
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Ryabchun, S. A.; Tretyakov, I. V.; Pentin, I. V.; Kaurova, N. S.; Seleznev, V. A.; Voronov, B. M.; Finkel, M. I.; Maslennikov, S. N.; Gol'tsman, G. N. |
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Low-noise wide-band hot-electron bolometer mixer based on an NbN film |
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Journal Article |
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2009 |
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Radiophys. Quant. Electron. |
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52 |
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8 |
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576-582 |
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HEB mixer, in-situ contacts, noise temperature, conversion gain bandwidth, diffusion cooling channel |
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We develop and study a hot-electron bolometer mixer made of a two-layer NbN–Au film in situ deposited on a silicon substrate. The double-sideband noise temperature of the mixer is 750 K at a frequency of 2.5 THz. The conversion efficiency measurements show that at the superconducting transition temperature, the intermediate-frequency bandwidth amounts to about 6.5 GHz for a mixer 0.112 μm long. These record-breaking characteristics are attributed to the improved contacts between a sensitive element and a helical antenna and are reached due to using the in situ deposition of NbN and Au layers at certain stages of the process. |
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