# Charge-to-heat transducers exploiting the Neganov-Trofimov-Luke effect for light detection in rare-event searches

## TL;DR

This paper presents the development and testing of large-area germanium bolometric photo-detectors exploiting the Neganov-Trofimov-Luke effect, achieving ultra-low noise levels for detecting few optical photons in rare-event searches.

## Contribution

It introduces a novel fabrication and operation method for large-area NTD-Ge bolometric detectors with enhanced performance using the NTL effect, suitable for rare-event experiments.

## Key findings

- Achieved sub 10-eV baseline noise in large-area detectors.
- Demonstrated high voltage bias operation up to 90V without leakage.
- Enhanced detector performance through improved radiation-tight environment.

## Abstract

In this work we present how to fabricate large-area (15 cm2), ultra-low threshold germanium bolometric photo-detectors and how to operate them to detect few (optical) photons. These detectors work at temperatures as low as few tens of mK and exploit the Neganov-Trofimov-Luke (NTL) effect. They are operated as charge-to-heat transducers: the heat signal is linearly increased by simply changing a voltage bias applied to special metal electrodes, fabricated onto the germanium absorber, and read by a (NTD-Ge) thermal sensor. We fabricated a batch of five prototypes and ran them in different facilities with dilution refrigerators. We carefully studied how impinging spurious infrared radiation impacts the detector performances, by shining infrared photons via optical-fiber-guided LED signals, in a controlled manner, into the bolometers. We hence demonstrated how the radiation-tightness of the test environment tremendously enhances the detector performances, allowing to set electrode voltage bias up to 90 volts without any leakage current and signal-to-noise gain as large as a factor 12 (for visible photons). As consequence, for the first time we could operate large-area NTD-Ge-sensor-equipped NTL bolometric photo-detectors capable to reach sub 10-eV baseline noise (RMS). Such detectors open new frontiers for rare-event search experiments based on low light yield Ge-NTD equipped scintillating bolometers, such the CUPID neutrinoless double-beta decay experiment.