Performance Research On High-T_c Superconducting Terahertz Sources

Superconducting terahertz(THz)sources fabricated with high-Tc superconductor Bi2Sr2CaCu2O8(BSCCO)intrinsic Josephson junction(IJJ)have a series of advantages such as frequency tunability,coherent radiation,and easily fabricated.Therefore,being new THz solid-state sources,they speeded up remarkably the development of THz science and technology.While carrying out research on high-Tc superconductor THz sources,we have found that the performance of sample radiation is strongly affected by its DC bias.The electrical transport characteristics,stability of radiation,frequency linewidth and other features are greatly different under "high bias" and "low bias" condition.The radiation mechanism under these two different bias conditions has yet to be studied.In this thesis,I report on several results about the mechanism of high-Tc superconductor BSCCO THz sources when they are under different bias condition.The main results are as follows.Firstly,we investigate the relationship between the THz emission properties and their resonant cavity modes under different bias conditions.At high bias,with the presence of a "hot spot",the emission frequency fe seems to be continuously tunable by changing the bias current and the bath temperature.In contrast,at low bias the emission frequencies are remarkably discrete.The values of fe correspond to the formation of(0,m)cavity modes with m=3 to 6.In other words,over a wide range of voltages,the radiation of the sample is locked to one frequency point.According to the ac Josephson relation,there must be an extra voltage in the stack,which is not involved in the coherent radiation of the sample.Secondly,for the above-mentioned experimental phenomena,we perform numerical simulations based on coupled sine Gordon equations combined with heat diffusion equations.In simulations the emission frequency in high-bias regime is continuous,in good agreement with the experiment result.The reason for the extra voltage is illucidated by the numerical calculation as well.We find that several IJJ located at both the top and the bottom of the stack are not locked to the resonance,but have developed a higher dc voltage.With the change of bias voltage,the number of the IJJ in phase can be adjusted to satisfy the Josephson Relation.Thirdly,research on the spatial distribution of radiation can help to understand the radiation mechanism.We set up a rotatable cryogenic measuring system to probe the angular distribution of the radiation.The spatial distribution of the radiated power can be accurately measured by this system.The sample stage can totally rotate 320°in X-Y plane under low temperature(accuracy up to 0.5°).With thhis system we have systematically measured the spatial distribution of the radiated power of the samples.Some details have been found in addition to the known angular dependence of radiated power under different bias conditions.This experimental system turns out to be very useful for investigating the radiation mechanism.Finally,a Low Temperature Scanning Laser Microscope is extended for measuring silicon nanowire solar cells.Using this system,we can resolve the local information of a single nanowire solar cell with a diameter of about 200 nm,and distinguish the defects of the solar cell.