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Sensors

Towards Measuring Terahertz Photon Statistics by a Superconducting Bolometer

Pavel A. Prudkovskii, A.A. Leontyev, Kirill A. Kuznetsov, G. Kh. Kitaeva

Published Jul 21, 2021• 18 citations
Publisher

Abstract

Statistical distributions of the analog readings of an antenna-coupled THz superconducting bolometer were measured and analyzed under a special type of irradiation by low-energy fluxes of THz photons with Poisson photon statistics and controllable mean photon numbers. The photons were generated via low-gain parametric down-conversion in pulse-pumped Mg:LiNbO3 crystal placed to a cooled cryostat together with the bolometer NbN film. Results of theoretical approximation of experimental histograms reveal the discrete nature of THz detection by superconducting bolometers and open a way for studying their quantum characteristics. It is shown that bolometer readings per pulse consist of discrete counts (“single charges”), with the mean number linearly dependent on the number of input photons. Contributions of single counts to a total analog reading are statistically distributed according to the normal law, with average values slightly depending on the number of counts in each reading. A general formula is proposed to describe the relationship between continuous statistical distribution of the bolometer readings and discrete quantum statistics of the incident photons.

DOI
10.3390/s21154964

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Statistical distributions of the analog readings of an antenna-coupled THz superconducting bolometer were measured and analyzed under a special type of irradiation by low-energy fluxes of THz photons with Poisson photon statistics and controllable mean photon numbers. The photons were generated via low-gain parametric down-conversion in pulse-pumped Mg:LiNbO3 crystal placed to a cooled cryostat together with the bolometer NbN film. Results of theoretical approximation of experimental histograms reveal the discrete nature of THz detection by superconducting bolometers and open a way for studying their quantum characteristics. It is shown that bolometer readings per pulse consist of discrete counts (“single charges”), with the mean number linearly dependent on the number of input photons. Contributions of single counts to a total analog reading are statistically distributed according to the normal law, with average values slightly depending on the number of counts in each reading. A general formula is proposed to describe the relationship between continuous statistical distribution of the bolometer readings and discrete quantum statistics of the incident photons.
Abstract Statistical characteristics of the photoresponse of a superconducting hot-electron bolometer detector were measured under illumination by low-power terahertz pulses of 25 ns duration and 1.5 THz frequency. The pulses were generated via nonlinear-optical effects of parametric down-conversion in Mg:LiNbO 3 crystal, which was cooled to 4.5 K and pumped by a pulsed laser. The asymmetric experimental histograms of the photoresponse values obtained at different levels of incident radiation power were approximated by the Poisson-weighted sum of functions representing the convolutions of varying numbers of Gaussian distributions of elementary photocurrent pulses and the dark current distribution. It was found that the average number, the mean value, and the variance of such elementary pulses change with incident power. Specifically, the mean value and standard deviation of these pulses both grow monotonically with the increase in the mean current response, while their number tends to reach saturation at high powers. Conclusions were made about the fluctuating nature of elementary pulses, which is related to the formation of different configurations of hot spots appearing due to absorption of photons in different areas of the superconducting film. An estimate of the maximum lifetime of a hot spot was made based on the experimental data.
The statistical distributions of output signals and their derivatives have been investigated for a superconducting bolometer exposed to weak photon pulses at a frequency of 1 THz. The terahertz pulses were generated via strongly nondegenerate parametric down-conversion in a LiNbO 3{}_{3}3​ crystal cooled to 4.8 K, under conditions of pumping the crystal with laser pulses of 28 ps duration. Various scenarios of the superconducting bolometer’s response to excitation by a small number of terahertz photons are discussed.
We study the statistical distributions of the output signals of a superconducting hot-electron bolometer (HEB) detector exposed to weak photon pulses of 1 THz (terahertz) frequency. Pulses with variable photon numbers were generated through strongly non-degenerate parametric downconversion (PDC) in a LiNbO3 crystal at 4.8 K. Fundamental differences in histograms are found between two PDC pumping modes, with pulse widths of 28 ps and 10 ns. HEB response to a picosecond THz pulse was detected in the form of a single elementary electrical pulse (SEP), with the average amplitude proportional to radiation intensity and dispersion relative to the bolometer's dark current. HEB responses to extremely weak radiation intensities have been recorded using nanosecond pulsed illumination. We found that nanosecond histograms are asymmetrical and broaden as radiation intensity increases, indicating that the nanosecond response consists of several SEPs. Poisson–Gauss approximation of histograms indicates that not only the average number of SEPs increase but also the average amplitude of a SEP increases with increasing power of incident THz radiation.
The conditions for the generation and detection of quantum-correlated pairs of optical and terahertz photons in the process of strongly frequency-nondegenerate parametric down-conversion of light are investigated. An approach based on increasing the frequency of the optical pump is proposed, whereby the power of a weak flux of terahertz idler photons can be measured for a wide range of parametric gain coefficients, down to the onset of the spontaneous down-conversion regime, using the same terahertz detector with a limited dynamic range. It is shown experimentally that the doubling of the frequency of pulsed laser pump leads to a 5-fold reduction in the lowest value of parametric gain that is still sufficient for detecting terahertz idler photons with a cooled hot-electron bolometer.
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.
We study parametric down-conversion (PDC) of optical laser radiation in the strongly frequency non-degenerate regime which is promising for the generation of quantum-correlated pairs of extremely different spectral ranges, the optical and the terahertz (THz) ones. The possibility to detect tenuous THz-frequency photon fluxes generated under low-gain spontaneous PDC is demonstrated using a hot electron bolometer. Then experimental dependences of the THz radiation power on the detection angle and on the pump intensity are analyzed.