Optical Interface for Superconducting Digital Circuits using Low temperature Photon Detectors

利用Ar+N_2混合气体，在不加热衬底的情况下，采用直流磁控溅射方法在MgO(100)衬底上外延生长了NbN薄膜。NbN薄膜的T_c为14K，厚度为8 nm，T_c为10K，T_c为4 nm。采用200 nm厚的电子抗蚀剂，采用电子直接拉伸法制备了NbN纳米线。为了增加光敏区，我们用NbN纳米线制作了50×50μm^2曲折图案。纳米线的线宽从200 nm到500 nm不等。在4.2K下，NbN纳米线的临界电流密度约为3×10~(-6)A/cm~2。重复频率和脉冲宽度可由脉冲图形发生器控制。最大激光脉冲功率约为1 mW。使用200 nm宽的NbN纳米线，1 mW连续激光照射可以抑制三分之一的临界电流。对重复频率为1 MHz、脉宽为2 ns的激光脉冲进行了高速响应。另一方面，假设NbN纳米线开关产生60μA的电流脉冲，设计了一种基于约瑟夫森传输线的单频Q产生电路。所设计的电路可以产生10μA、25 GHz的电流脉冲。对于60μA的电流脉冲，该电路的直流偏置裕度为+-15%。

We investigated superconducting NbN nanowire as a photo switch for generating single flux quantum (SFQ) pulses.Epitaxially grown NbN thin films were prepared on MgO (100) substrates by a dc magnetron sputtering method using gas mixture of Ar + N_2, without substrate heating. NbN thin films showed Tc of 14K for 8 nm, and 10K for 4 nm.NbN nanowires were patterned using a electron direct drawing method with 200 nm thick electron resist. To increasing photo-sensitive regions, we fabricated 50 x 50 μm^2 meander patterns with NbN nanowires. Line width of nanowires was varied from 200 nm to 500 nm. Critical current density of NbN nanowires was about 3x10^6A/cm^2 at 4.2 K.We irradiated 850-nm laser pulses via 50-mm multi-mode optical fibers. Repetition frequency and pulse width could be controlled by a pulse pattern generator. The maximum laser pulse power was about 1 mW. Using 200-nm-wide NbN nanowires, one third of critical current was suppressed by 1 mW CW laser irradiations. High speed responses were observed for laser pulses with repetition frequency of 1MHz and pulse width of 2 ns.On the other hand, we designed an SFQ generation circuit using Josephson transmission lines assuming NbN nanowire switch generates 60 μA current pulses. The designed circuit could generate SFQ pulses by 10 μA, 25 GHz current pulses. Dc bias margin of the circuit was +- 15% for 60 μA current pulses.