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IEEE Transactions on Applied Superconductivity

Analysis of NbN Hot Electron Bolometer Receiver Noise Temperatures Above 2 THz With a Quantum Noise Model

P. Khosropanah, Wen Zhang, E. Kollberg, K. S. Yngvesson, J. R. Gao, Tarun Bansal, Merlijn Hajenius

Published Jun 1, 2009• 5 citations
Publisher

Abstract

This paper summarizes our receiver noise temperature data of NbN HEB mixers obtained at a number of local oscillator frequencies between 1.9 to 4.3 THz in order to verify the role of quantum noise. The experimental data show that the receiver noise temperature increases roughly linearly with frequency. At 4.3 THz, we measured a receiver noise temperature of 1300 K, which is about 6 times (hf/kB) . The noise data at different frequencies are compared to a prediction of a noise model including the contribution of quantum noise and making use of a hot-spot model for mixing. We draw a preliminary conclusion that at 4.3 THz roughly 30% of the receiver noise temperature can be ascribed to the quantum noise. However, more dedicated measurements are required in order to further support the quantum noise model for HEB mixers.

DOI
10.1109/tasc.2009.2018817

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This paper summarizes our receiver noise temperature data of NbN HEB mixers obtained at a number of local oscillator frequencies between 1.9 to 4.3 THz in order to verify the role of quantum noise. The experimental data show that the receiver noise temperature increases roughly linearly with frequency. At 4.3 THz, we measured a receiver noise temperature of 1300 K, which is about 6 times (hf/kB) . The noise data at different frequencies are compared to a prediction of a noise model including the contribution of quantum noise and making use of a hot-spot model for mixing. We draw a preliminary conclusion that at 4.3 THz roughly 30% of the receiver noise temperature can be ascribed to the quantum noise. However, more dedicated measurements are required in order to further support the quantum noise model for HEB mixers.
Abstract— Superconducting NbN hot electron bolometer (HEB) mixers are so far the most sensitive detectors for\nheterodyne spectroscopy in the frequency range between 1.5 THz and 5 THz. To reach the ultimate receiver noise\ntemperatures in the high end of the THz range (3-6 THz), it is crucial to understand their fundamental noise contribution\nfrom different origins. With increasing frequency, the classical output noise contribution should remain unchanged, but\nthe quantum noise contribution is expected to play an increasing role [1].\nThis paper reports the first dedicated experiment using a single NbN HEB mixer at a number of local oscillator\nfrequencies between 1.6 to 4.3 THz to address and quantify the contribution of the quantum noise to the receiver noise\ntemperature.\nWe used a spiral antenna coupled NbN HEB mixer with a bolometer size of 2 μm×0.2 μm. In order to minimize\nuncertainties in the corrections of the optical losses, we use a vacuum hot/cold load setup [2] to eliminate the air loss, and\nan uncoated elliptical Si lens. Although other components, a 3 μm Mylar beam splitter and a QMC heat filter, also\nintroduce frequency dependent optical losses, they can be accurately calibrated. Furthermore, to reduce uncertainties in\nthe data, we measure Y-factors responding to the hot/cold load by fixing the voltage, but varying the LO power [2]. As\nLO, we use a FIR gas laser.\nWe measure the Y-factor at the optimal point at different frequencies by only varying LO frequencies, but keeping\nthe rest exactly the same. We obtain DSB receiver noise temperatures, which are 842 K (at 1.6 THz), 845 K (1.9 THz), 974\nK (2.5 THz) and 1372 K (4.3 THz). After the correction for the losses of the QMC filter and the beam splitter, the noise\ndata show a linear increase with increasing frequency.\nUsing a quantum noise model [1] for HEB mixers and using a criterion for which the classical output noise must be\nconstant at different frequencies, we analyze the results and find the excess quantum noise factor β to be around 2 and\nthat 24 % of the total receiver noise temperature at 4.3 THz (at the input of the entire receiver) can be ascribed to\nquantum noise. Clearly the quantum noise has a small but measurable effect on the receiver noise temperature at this\nfrequency.\nWe are still analyzing different alternatives of interpretation for the mismatch loss between the bolometer and\nthe spiral antenna.\n[1] E. L. Kollberg and K. S. Yngvesson, “Quantum-noise theory for terahertz hot electron bolometer mixers,” IEEE Trans.\nMicrowave Theory and Techniques, 54, 2077, 2006.\n[2] P. Khosropanah, J.R. Gao, W.M. Laauwen, M. Hajenius and T.M. Klapwijk, “Low noise NbN hot-electron bolometer mixer\nat 4.3 THz,” Appl. Phys. Lett., 91, 221111, 2007.\n
New advances in hot electron bolometer (HEB) mixers have recently resulted in record-low receiver noise temperatures at terahertz frequencies. We have developed quasi-optically coupled NbN HEB mixers and measured noise temperatures up to 2.24 THz, as described in this paper. We project the anticipated future performance of such receivers to have even lower noise temperature and local-oscillator power requirement as well as wider gain and noise bandwidths. We introduce a proposal for integrated focal plane arrays of HEB mixers that will further increase the detection speed of terahertz systems.
To put space-based and airborne heterodyne instruments into operation at frequencies above 1 THz the superconducting NbN hot-electron bolometer (HEB) will be incorporated into heterodyne receiver as a mixer. At frequencies above 1.3 THz the sensitivity of the NbN HEB mixers outperform the one of the Schottky diodes and SIS-mixers, and the receiver noise temperature of the NbN HEB mixers increase with frequency. In this paper we present the results of the noise temperature measurements within one batch of NbN HEB mixers based on 3.5 mn thick superconducting NbN film grown on Si substrate with MgO buffer layer at the LO frequencies 2.5 THz and 3.8 THz.
In this article, we measure the double sideband (DSB) receiver noise temperature (TrecDSB)T_{\text{rec}}^{\text{DSB}})TrecDSB​)of an NbN hot electron bolometer (HEB) mixer at three local oscillator frequencies of 1.6, 2.5, and 5.3 THz. The HEB has cleaned contact interfaces with a 200-nm-thick Au layer. The measuredTrecDSBT_{\text{rec}}^{\text{DSB}}TrecDSB​values are 530 ± 11 K, 640 ± 18 K, and 2190 ±150 K at 1.6, 2.5, and 5.3 THz, respectively, using an air setup with total optical losses of 2.60 ± 0.04, 2.63 ± 0.16, and 4.70 ± 0.24 dB, respectively. We derived low mixer noise temperatures (TmixerDSB)T_{\text{mixer}}^{\text{DSB}})TmixerDSB​)of 240 ± 6 K at 1.6 THz and 290 ± 13 K at 2.5 THz, achieving over 30% improvement compared to published NbN HEB mixers. This enhancement can reduce the integration time of a heterodyne instrument by roughly a factor of 2. At 5.3 THz,TmixerDSBT_{\text{mixer}}^{\text{DSB}}TmixerDSB​is 620 ± 55 K, showing limited improvement due to nonoptimized antenna geometry. These results also contribute to understanding the device physics of a wide HEB (4 μm) at high frequencies. The mixer was developed for the proposed Orbiting Astronomical Satellite for Investigating Stellar Systems and Single Aperture Large Telescope for Universe Studies (concept) missions.
New advances in Hot Electron Bolometer (HEB) mixers have resulted in record low receiver noise temperatures at THz frequencies recently. We have developed quasi-optically coupled NbN HEB mixers and measured noise temperatures up to 1.56 THz, as described in this paper. We project the anticipated future performance of such receivers to have even lower noise temperature and LO power requirement as well as wider gain and noise bandwidths. We introduce a proposal for integrated focal plane arrays of HEB mixers which will further increase the detection speed of THz systems.
We have measured the noise temperature of a single, sensitive superconducting NbN hot electron bolometer (HEB) mixer in a frequency range from 1.6 to 5.3 THz, using a setup with all the key components in vacuum. By analyzing the measured receiver noise temperature using a quantum noise (QN) model for HEB mixers, we confirm the effect of QN. The QN is found to be responsible for about half of the receiver noise at the highest frequency in our measurements. The β-factor (the quantum efficiency of the HEB) obtained experimentally agrees reasonably well with the calculated value.