This research project has two main objectives: firstly, to achieve the realisation of a solid-state device capable of single photon emission and, secondly, to implement a detection system based on a single photon avalanche diode (SPAD) array detector.
For the first objective, the ER had to perform two main types of technical activities: one related to the implementation of a custom experimental setup for performing time-resolved photoluminescence (PL) spectroscopy and photon correlation spectroscopic measurements, and the other related to the fabrication of a solid-state device.
The ER, with the support of the Photonic Nanomaterials group, worked on the assembly and installation of an optical microscope coupled to a picosecond pulsed diode laser. In addition, the ER connected this system to a Hanbury-Brown and Twiss (HBT) interferometer consisting of two single photon avalanche diode (SPAD) detectors coupled to a time-correlated single photon counting (TCSPC) module equipped with two identical synchronised but independent input channels, allowing spectroscopic measurements of the photon correlation. This optical setup also has the capability to perform PL measurements resolved in wavelength by the addition of a Gemini interferometer (Nireos) and also in time by using only one SPAD detector with the TCSPC module. In addition, the ER added a Keithley 2636 source-measurement unit to the setup to perform optical and electrical measurements simultaneously. Most of this work was carried out during the first part of the project, but the setup required optimisation and adjustment throughout the project.
For the fabrication part, the ER was trained in the Nanochemistry laboratory facility for the synthesis of CsPbBr3 perovskites (PVKs) QDs and in the clean room facility for various deposition techniques (in particular atomic layer deposition (ALD), thermal evaporation system and sputter coater system).
The fabrication of the proposed device requires several steps, the substrates used are glass coverslips with a thickness of 170 µm (this is necessary for the optical measurements performed with an oil immersion objective), then the ER followed these steps:
1. Deposition of ITO (Indium Tin Oxide) by sputtering.
2. Deposition of PVKs QDs by spin coating from a dilute solution.
3. Deposition of Al2O3 thin film (30 nm) by ALD
4. Deposition of a thicker Al2O3 layer (200 nm) by e-beam evaporation
5. Deposition of an Al layer (100 nm).
The ER has produced a first series of devices with a continuous film of PVKs QDs characterised using the setup described above. By applying a voltage bias of up to 40 V to the devices, the ER observed a decrease in the lifetime of about 20% of the PVKs QDs film, which is reversible by returning to 0 V. These preliminary results were promising for moving on to the study of individual QDs.
For this task, the solution of PVKs QDs used for spin coating was further diluted and mixed with polystyrene to encapsulate and isolate the individual PVKs QDs. The ER then measured the second-order correlation function (g2(τ)) of several individual PVKs QDs, confirming that they behave as single photon emitters at 0 V. However, by applying a voltage bias to the sample, the ER found that the photon emission decreased, and the measurement of the second-order correlation function was no longer possible. The ER, with the support of the Photonic Nanomaterials Group, concluded that the application of a voltage bias caused the degradation of individual PVKs QDs and therefore the observation of the change in lifetime with voltage application was not possible. The ER then produced a new series of samples using core-shell CdSe/CdS single QDs. The results were similar to those observed for PVKs QDs.
Due to the versatility of the optical setup, the ER was able to perform measurements on other materials provided by colleagues in the Photonic Nanomaterials group, such as photoluminescence spectra measurements of InAs/ZnSe core/thick shell QDs (published in Advanced Science, vol. 11, 2400734, 2024) and blue CdSe/CdS core/crown nanoplatelets (published in Nanoscale, 2024).
In addition, the ER has performed measurements on several samples, the results of which have not yet been published, but the corresponding manuscripts are in preparation.
The second objective of the project was to develop a new detection system for the measurement and analysis of single photons emitted by colloidal QDs based on the use of a SPAD array detector. This work is in the field of quantum optics, where the ability to analyse the quantum signatures of the emitted light from single photon sources is essential, and current experimental configurations offer limited possibilities for the metrology of quantum light sources. To date, the primary method for probing non-classical properties of light has been to measure the second-order correlation function with an HBT intensity interferometer. Most of these experiments use single photon counting detectors based on avalanche photodiodes or superconducting nanowires. These are called "click detectors" because the arrival of a single photon causes the detector output state to change from 0 to 1, followed by a dead time during which no photon detection is possible. Therefore, click detectors cannot discriminate the number of photons present in a field of light at any given time, and offer limited capabilities for the metrology of quantum light sources. Therefore, the development of detectors capable of resolving the number of incident photons (so-called photon number resolving (PNR) detectors) is crucial for many applications in quantum information science. SPAD array detectors provide PNR functionality by having the incoming photons strike an array of parallel SPADs.
The ER has therefore started to explore this technique using a custom-built optical setup already implemented by the Molecular Microscopy and Spectroscopy group for confocal-based fluorescence fluctuation spectroscopy and imaging. This setup was modified by the ER to perform spectroscopic measurements of the photon correlation. For this task, ER used a sample consisting of an array of 50x50 holes filled with single QDs (this sample was provided by a colleague in the Photonic Nanomaterials group). The same sample was first measured with the optical setup described above (standard click detectors in HBT configuration). Then the ER measured the photon correlation for several QDs and by analysing the data the second order correlation was extracted. However, this type of measurement is extremely challenging, and further work is required before single photon emission from QDs can be assessed. However, this work has the potential to lead to a new technological platform for the characterisation of single photon sources that overcomes the limitations of conventional standard click detectors in HBT configuration.
