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AIP Advances

A study of thermal effects in superconducting terahertz modulator by low temperature scanning laser microscope

Chao Han, Chun Li, Jingbo Wu, Xianjing Zhou, Jun Li, Biaobing Jin, Huabing Wang, Peiheng Wu

Published Jun 1, 2018• 4 citations
Publisher

Abstract

We use low temperature scanning laser microscope (LTSLM) to study the Joule power distribution of superconducting (SC) terahertz (THz) modulator. The LTSLM scanning images record the SC state transformation process under different DC bias voltages. The change of THz transmission spectra can be well explained by the thermal effect in the devices observed by LTSLM. Hotspots are present in one THz modulator and the transmission spectra changes a lot after the hotspots show up. According to theoretical analysis, the appearance of hotspot may be helpful for improving the modulation speed. These results will be useful to understand the mechanism of SC THz modulator and design higher performance THz moduators.

DOI
10.1063/1.5035406

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IEEE International Superconductive Electronics Conference (ISEC)•Jul 1, 2019•0 citations
We use low temperature scanning laser microscope (LTSLM) to study the Joule power distribution of superconducting (SC) terahertz (THz) modulator. The LTSLM scanning images record the SC state transformation process under different DC bias voltages. The change of THz transmission spectra can be well explained by the thermal effect in the devices observed by LTSLM. Hotspots are present in one THz modulator and the transmission spectra changes a lot after the hotspots show up. According to theoretical analysis, the appearance of hotspot may be helpful for improving the modulation speed. These results will be useful to understand the mechanism of SC THz modulator and design higher performance THz moduators.
Low temperature scanning laser microscopy (LTSLM) is demonstrated to be capable of imaging sub-THz electromagnetic fields in cryoelectronic integrated structures. This method allows one to evaluate the spatial distribution of time-averaged field amplitudes with a resolution of about one micrometer for samples with characteristic dimensions of order millimeters. Using LTSLM, cryoelectronic devices with both passive and active superconducting elements can be characterized. Local heating of superconducting structures by a laser beam introduces extra loss for the propagating and standing sub-millimeter waves. We present LTSLM images of two-dimensional 400 to 500 standing GHz wave patterns in integrated superconducting receiver chips.
This work presents new experimental results on low-temperature (LT) characterization of local RF properties of passive superconducting (SC) microwave devices using a novel laser scanning microscope (LSM). In this technique, a modulated laser beam is focused onto and scanned over the surface of a resonant SC device to probe the spatial distribution of RF current. The highly localized photo-induced change of the kinetic inductance of the SC device produces both a shift of the resonant frequency f/sub 0/ and change of the quality factor Q. An image of these changes is recorded as the laser spot is scanned over the device. We present the first measurements of spatially resolved intermodulation response in a high temperature superconducting (HTS) co-planar waveguide resonator, opening up a new window into the local origins of nonlinearity in the HTS materials.
Terahertz (THz) direct detectors based on superconducting niobium nitride hot electron bolometers with thermal, microwave and THz biasing methods have been studied at a frequency of 0.65 THz and at an operating temperature of 4.2 K. A low noise amplifier at room temperature is used to read out the current change of the detectors modulated by the chopping frequency at 732 Hz. The noise spectral density, current responsivity and noise equivalent power (NEP) have been measured and compared respectively. The detectors with the microwave and THz biasing methods have about one order higher current responsivity than that of the thermal one. With the microwave biasing, an optical NEP at the order of pW/√Hz which shows no deterioration is obtained by choosing the microwave frequency and power appropriately.
Abstract Relaxation oscillations related to terahertz (THz) power detection in superconducting niobium nitride hot electron bolometers (HEBs) have been studied using microwaves as a probe. The reflected microwave signal, which is related to the impedance changes of the HEB irradiated by an incident THz signal, can be used to characterize relaxation oscillations. The frequency of relaxation oscillation switching between the superconducting and resistive states increases linearly with incident THz power but nonlinearly with bias voltage or bath temperature. The observed strong periodic pulse trains of relaxation oscillations have a maximum duty cycle of ∼50%. These strong switching pulses indicate that DC Joule heating, THz radiation or thermal energy is absorbed in the bi-stable region of the HEB intermittently. In addition to relaxation oscillations, we also see weaker oscillations when the device is in the resistive state, which we call intrinsic oscillations. These have a constant frequency at the order of 10 kHz which is independent of external stimuli. The weak periodic pulse trains at a constant frequency can be divided into mainly two types—one is the pulse train with a small duty cycle and the other is the quasi-square wave. Measurement of these oscillations in HEBs has some practical applications. Digital THz power measurement with frequencies below and above the gap frequency can be made based on the linearity between relaxation oscillation frequency and incident THz power. The relaxation oscillation frequency can be used to estimate the maximum speed of a THz direct detection system especially for a multi-pixel HEB array. In particular, extra noise induced by relaxation oscillations and intrinsic oscillations can be determined for the HEB operating as a THz direct detector.
Recently, it has been shown that large stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8 emit synchronous THz radiation, the synchronization presumably triggered by a cavity resonance. To investigate this effect we use low temperature scanning laser microscopy to image electric field distributions. We verify the appearance of cavity modes at low bias and in the high input-power regime we find that standing-wave patterns are created through interactions with a hot spot, possibly pointing to a new mode of generating synchronized radiation in intrinsic Josephson junction stacks.
The properties of terahertz (THz) power detectors based on Superconducting NbN hot electron bolometer (HEB) with microwave (MW) readout are reported here. Features of relaxation oscillations probed with MW when the HEB is not pumped, pumped by THz source or heated by raising the bath temperature are studies. The periodic pulse in the reflected MW signals both in time and frequency domains show that the relaxation oscillation frequency increases with the bias voltage, incident THz power and/or bath temperature. The frequency count forms a quadratic polynomial fit to the bias voltage, a linear fit to the incident THz power and an exponential fit to the bath temperature. Based on the above results, incident THz power between 8-90 nW is measured using the HEB.