PhD in Physics Amr Farrag

Toward Ultrafast Detection of Quantum Emitters at a High Repetition Rate

The modern information era is built upon technologies such as telecommunications, cryptography, high-performance computing, and, more recently, artificial intelligence (AI). At the core of all these systems are electronic circuits, whose performance is fundamentally limited by the physical speed of electrons. Photonic technologies, by contrast, rely on light quanta—single photons—that propagate orders of magnitude faster and offer intrinsically higher bandwidth and enhanced security. Replacing electronic components with optical counterparts promises major advances in communication speed, data transfer, secure information processing, and quantum technologies. Central to these developments are ultrabright single-photon sources (SPSs), including quantum dots and color centers in solid-state hosts. However, the brightest SPSs currently require cryogenic temperatures, limiting their practicality in real-world applications.

This dissertation proposes an alternative route toward achieving bright, room temperature single-photon emission by exploiting high-repetition-rate excitation. Advances in laser technology now allow solid-state lasers to operate at gigahertz repetition rates. By combining such excitation with hybrid quantum systems—where a quantum emitter is coupled to a nanophotonic structure to enhance its radiative emission through the Purcell effect—ultrabright SPSs can, in principle, be realized at room temperature. At these high radiative rates, however, the fluorescence lifetime becomes extremely short, well below the temporal resolution of conventional single-photon detectors, which are further constrained by cost, cryogenic requirements, bulkiness, and limited synchronization capabilities at gigahertz rates.
 

To address this limitation, this dissertation demonstrates an optical Kerr gate (OKG) operating at a 1-GHz repetition rate with sub-picosecond temporal resolution. The OKG is a third-order (χ⁽³⁾) nonlinear optical process that does not require phase matching conditions and is therefore suitable for broadband fluorescence detection. Kerr gating was achieved using two distinct Kerr media: bulk bismuth-borosilicate (BBS) glass and two-dimensional thin graphite films. In both cases, OKG signals were generated using sub-nanojoule gate-pulse energies—remarkably low compared with traditional OKG implementations that rely on millijoule-level amplified pulses, more than six orders of magnitude higher. The temporal resolution achieved was 175 ± 1 fs with BBS glass (∼ 3% gating efficiency) and 141 ± 6 fs with thin graphite films (∼ 10% efficiency). This compact, room-temperature technique requires no cryocooler, vacuum system, or bulky amplification stages. The demonstrated OKG platform can be directly integrated into a standard confocal fluorescence microscope, enabling ultrafast fluorescence measurements from quantum emitters and paving the way for practical room-temperature ultrabright SPSs.