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Diamonds with their excellent chemical stability and large bandgap are able to host various types of atoms. The combination of these atomic impurities and "vacancy" centres, known as colour centres, gives rise to interesting spectroscopic properties. Due to their potential applications in quantum communication and biosensing, these diamond-based emissive colour centres are the subject of intense research. Nitrogen based colour centres are the most well studied, and have been exploited as a source of single photons. Recently, interest has turned to silicon-vacancy (SiV) colour centres due to their unique emissive features, which make them particularly suitable for use as single-photon sources.
Our research is at the forefront of nanoscale optics and photonics, where we combine advanced spectroscopy, materials science, and nanofabrication to explore the unique properties of light-emitting materials. Our work focuses on developing fundamental insights into charge carrier dynamics and manipulating light at the nanoscale to create robust platforms for future optoelectronics, sensing, and quantum technologies.
Key Research Pillars and Contributions
I. Fundamental Properties of Silicon Vacancy Centers
Our group has built and optimized a range of ultrafast optical setups, including low-temperature time-resolved photoluminescence excitation microscopy, confocal microscopy, and transient absorption spectroscopy. Using these tools, we conduct fundamental research into silicon vacancy (SiV) centers, particularly within nanocrystalline diamond—a promising, low-cost material for large-scale fabrication.
Our work addresses key challenges in using SiV centers in these materials, such as strain-induced broad linewidths, background signal from surface defects, and light absorption by non-diamond carbon. We have developed a strategy to overcome these limitations using high-energy femtosecond laser pulses to precisely repair the polycrystalline diamond lattice. Our results show that the linewidths of SiV centers in irradiated polycrystalline diamond layers almost match those found in single-crystal diamond, paving the way for potential sensing applications.
A) Polycrystalline diamond layers are composed of diamond grains with SiV centers. Non-diamond phases, such as carbon-like diamond sp2 and transpolyecythelene, are located between the grains. These non-diamond phases hinder the optical properties of polycrystalline diamond because they introduce absorption. We have found that irradiating the polycrystalline diamond with short laser pulses of appropriate power leads to the burning of the non-diamond phases, which is manifested by Raman measurements shown in B). The non-diamond Raman signal decreases in comparison to the sp3 diamond peak (inset of b). This leads to an increase in the intensity of the photoluminescence signal from SiV centers, with the peak located around 740 nm. C) The burning of the non-diamond phases is also visible in the SEM images in C, where small voids arise between the grains when a proper laser power is employed. If the laser power is too high (above 2 mW), the diamond itself starts to burn, which is not desired.
We have also investigated the role of defects as potential traps for charge carriers. By developing and enhancing transient absorption methods with highly stabilized lasers, we successfully differentiated nanosecond recombination dynamics from picosecond decay dynamics linked to defects. Our findings suggest that most SiV centers do not have direct connections to defects in the diamond film, indicating that excited carriers do not migrate away from the centers. These insights into charge carrier dynamics are critical for optimizing the performance of color centers, and we are currently extending the pump and probe method into spectrally-, temperature-, and spatially-resolved frameworks.
We used a transient transmission setup to measure fast dynamics of the SiV centers in polycrystalline diamond.
II. Diamond Photonic Structures for Light Control
Combining diamond color centers with 2D photonic crystal (PhC) slabs and PhC cavities is essential for enhancing their emission properties via improved light extraction efficiency and Purcell enhancement. Our group has developed innovative protocols for fabricating these structures in polycrystalline diamond.
Pioneering Integration: Our early work pioneered the fabrication of high-quality PhC structures in polycrystalline diamond, demonstrating an 8-fold enhancement in light extraction efficiency and directional control of emitted light. This innovative integration unlocks significant potential for applications in optoelectronics and high-sensitivity optical sensors, particularly in biological contexts.
A) Scanning electron microscope (SEM) images show the photonic crystal fabricated within the polycrystalline diamond layer. The crystal is composed of columns etched into the diamond. B) An atomic force microscope (AFM) image shows the topography of the photonic crystal and the surrounding diamond surface. C) The bright-field images of the photonic crystal and the randomly patterned reference are compared with D) the fluorescence images. These fluorescence images were obtained by exciting the sample with green light and detecting the resulting signal through a red-transmitting filter. We can clearly see that the photoluminescence of the planar original layer and the randomly patterned reference is guided toward the edges. In contrast, the photonic crystal structures shine brightly with a red color, demonstrating the light extraction phenomenon.
The extraction of light emitted by the diamond color centers is realized via leaky modes of the photonic crystal. The leaky modes can be mapped via angle-resolved transmission measurements and calculated via rigorous coupled wave analysis approach.
A) A schematic of the sample within the angle-resolved transmission setup. B) Angle-resolved transmission measurements are shown as a function of wavelength and incident angle, revealing the photonic band diagram of the leaky modes in both s- and p-polarization. These spectra are directly compared with theoretical simulations. The strong agreement with theory is a direct result of the high structural quality of the fabricated devices and the precision of the measurement setup.
Leaky modes tuning: Our research has involved designing and fabricating 2D PhC structures to enhance the light extraction efficiency from SiV centers. This work begins by creating a periodic pattern with a desired lattice constant in a silica substrate. A diamond layer containing the SiV centers is then deposited on the patterned silica. The thickness of this diamond layer defines the position of the leaky modes. In this way, the position of the leaky modes can be very precisely controlled.
A) A schematic diagram shows the principle of fabricating photonic crystals. First, the silica substrate is patterned, and then a diamond layer is grown on top of it. B) SEM images show the silica substrate and the grown diamond layer. C) Simulated transmission spectra show the spectral shift of the leaky mode resonances as the diamond thickness changes.
Resonant Excitation: By developing a resonant excitation and extraction scheme in 2D PhCs, we achieved a 100-fold enhancement of the directional light emission efficiency from SiV centers embedded in polycrystalline diamond. This breakthrough enables us to overcome high absorption losses and provides new design principles for future photonic crystals in various optoelectronic and sensor devices.
A) The schematic illustrates the principle of the resonant excitation and extraction scheme. The excitation laser (442 nm) is directed at the surface of the diamond photonic crystal at a resonant angle, which enables its efficient coupling into the layer. This resonant angle varies depending on the polarization of the mode we wish to couple the excitation light into (53 degrees for TE0 or 38 degrees for TM0). The extraction of light at the SiV center's emission wavelength (740 nm) is then achieved via a different mode, in a direction normal to the sample plane. B) This panel shows the electric field distribution of the modes used for both the excitation and extraction processes.
Enhanced Light Control: Our experimental work demonstrates how to use photonic crystal cavities to significantly increase the brightness and efficiency of dense color center ensembles, laying the groundwork for more powerful on-chip light sources. We have successfully coupled SiV centers in polycrystalline diamond to a photonic crystal cavity, achieving a significant Purcell enhancement comparable to that observed in single-crystal diamond. This finding, despite the lower optical quality of polycrystalline diamond, highlights its potential for photonic applications, expanding the possibilities for this versatile material platform.
A) The Cavity Structure: This scanning electron microscope (SEM) image shows a photonic crystal cavity we fabricated. The cavity itself is located in the middle of the periodic pattern, created by simply leaving out three holes. The surrounding array of holes acts like a highly reflective mirror, creating a tiny optical resonator that traps light. B) Enhanced Light: This image shows the experimental result. The photoluminescence intensity from the SiV centers located inside the cavity is significantly enhanced. This is due to the Purcell effect, where the confinement of light in the cavity dramatically boosts the light-emitting properties of the SiV centers. C) Theoretical Design: To understand and optimize our design, we use computer simulations. This image shows a simulation of the electromagnetic field distribution for one of the specific modes within the cavity, confirming how the light is trapped and concentrated.
III. Advanced Material Synthesis and Techniques
Our work extends beyond diamond, and we actively collaborate with other groups to explore new materials and techniques that inform our core research.
Physical Vapor Deposition (PVD): Through collaborations with the group of Andrei Choukourov (Charles University), we have gained hands-on experience with PVD magnetron sputtering. We have measured quantum efficiency and time-resolved photoluminescence of samples fabricated by this method and developed models to describe excited carrier processes. This collaboration inspired us to propose using PVD magnetron sputtering as a source for diamond color centers, as described in our current projects.
Silicon Quantum Dots (Si QDs): We have a strong background in studying Si QDs. By combining various spectroscopic techniques and mathematical modeling, we were the first to unambiguously identify rapid charge separation in QD-sensitized solar cells on a picosecond timescale. Our low-temperature and time-resolved measurements also helped identify the microscopic origin of the fast-decaying radiative channel in silicon nanocrystals.
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