Zhongyang Wang

Large-scale fabrication of heavy doped carbon quantum dots with tunable-photoluminescence and sensitive fluorescence detection

Carbon quantum dots (CQDs) have been intensively investigated due to their interesting electrochemical and photoluminescent properties. Here, we report a novel method for large-scale preparation of heavy doped CQDs with tunable photoluminescence. In the present synthetic process, the carbon nanoparticles from Chinese ink were oxidized and cut simultaneously using a mature process to obtain oxidized-CQDs as precursors, and then the heteroatom (N, S or Se) doped CQDs were obtained by a one-step hydrothermal reduction and in situ doping treatment. The heavy doped CQDs are just 1–6 nm size, and have improved photoluminescence with different emission wavelengths, higher quantum yield, longer lifetime and good photostability. Further experiments suggested that these N and S doped CQDs were very sensitive for the detection of Cu2+ and Hg2+, respectively.

Creating diamond color centers for quantum optical applications

Abstract Nitrogen vacancy (NV) centers in diamond have distinct promise as solid-state qubits. This is because of their large dipole moment, convenient level structure and very long room-temperature coherence times. In general, a combination of ion irradiation and subsequent annealing is used to create the centers, however for the rigorous demands of quantum computing all processes need to be optimized, and decoherence due to the residual damage caused by the implantation process itself must be mitigated. To that end we have studied photoluminescence (PL) from NV − , NV 0 and GR1 centers formed by ion implantation of 2xa0MeV He ions over a wide range of fluences. The sample was annealed at 600xa0°C to minimize residual vacancy diffusion, allowing for the concurrent analysis of PL from NV centers and irradiation induced vacancies (GR1). We find non-monotic PL intensities with increasing ion fluence, monotonic increasing PL in NV 0 /NV − and GR1/(NV 0 xa0+xa0NV 1 ) ratios, and increasing inhomogeneous broadening of the zero-phonon lines with increasing ion fluence. All these results shed important light on the optimal formation conditions for NV qubits. We apply our findings to an off-resonant photonic quantum memory scheme using vibronic sidebands.

Space-time profiles of an ultrashort pulsed Gaussian beam

By using a different initial value from the previous treatments, we reveal that the pulsed Gaussian beam in free space can be expressed as a simple wave packet along with the complex time, which gives all the information about the spatial and temporal behaviors of the pulse. Then the space-time profiles of a space-time Gaussian pulse are studied. It is shown that even when the pulse propagates in free space, there exist couplings among the beam parameters in space and time. The spot-size-related couplings enlarge the temporal domain (TD) spatial size of the pulse and lessen the carrier frequency of the pulse of the paraxial points, and the wavefront-related couplings bend the transverse spatial shape of the pulse with the wavefront.

Mapping broadband single-photon wave packets into an atomic memory

We analyse the off-resonant Raman interaction of a single broadband photon, copropagating with a classical `control pulse, with an atomic ensemble. It is shown that the classical electrodynamical structure of the interaction guarantees canonical evolution of the quantum mechanical field operators. This allows the interaction to be decomposed as a beamsplitter transformation between optical and material excitations on a mode-by-mode basis. A single, dominant modefunction describes the dynamics for arbitrary control pulse shapes. nComplete transfer of the quantum state of the incident photon to a collective dark state within the ensemble can be achieved by shaping the control pulse so as to match the dominant mode to the temporal mode of the photon. Readout of the material excitation, back to the optical field, is considered in the context of the symmetry connecting the input and output modes. Finally, we show that the transverse spatial structure of the interaction is characterised by the same mode decomposition.We analyze a quantum optical memory based on the off-resonant Raman interaction of a single broadband photon, copropagating with a classical control pulse, with an atomic ensemble. The conditions under which the memory can perform optimally are found, by means of a `universal mode decomposition. This enables the memory efficiency to be specified in terms of a single parameter, and the control field pulse shape to be determined via a simple nonlinear scaling. We apply the same decomposition to determine the optimal configurations for read-out.

Fluorescent N-Doped Carbon Dots as in Vitro and in Vivo Nanothermometer

The fluorescent N-doped carbon dots (N-CDs) obtained from C3N4 emit strong blue fluorescence, which is stable with different ionic strengths and time. The fluorescence intensity of N-CDs decreases with the temperature increasing, while it can recover to the initial one with the temperature decreasing. It is an accurate linear response of fluorescence intensity to temperature, which may be attributed to the synergistic effect of abundant oxygen-containing functional groups and hydrogen bonds. Further experiments also demonstrate that N-CDs can serve as effective in vitro and in vivo fluorescence-based nanothermometer.

Spectral and temporal properties of ultrashort light pulse in the far zone

The spectral and temporal properties of ultrashort pulses in the far zone are studied by using Rayleigh diffraction integrals. It is shown that the Fourier spectra of the pulse at an observation point in the far zone is different from the spectra in the initial plane. For ultrashort pulses there exists a spectral shift whose magnitude depends on the spectral linewidth, the effective spatial width, and the direction of observation. These results are more similar to that of a partially coherent light beam given by Dacic and Wolf [J. Opt. Soc. Am. A 5 (1988) 1118]. But the spectral shift presented here only arises from the frequency dependence of the propagator that characterizes the propagation of the Fourier monochromatic components of the pulsed field. The shift is remarkable for light pulses with several femtosecond duration, but the pulse shape in the far zone is nearly unchanged.

The emission wavelength dependent photoluminescence lifetime of the N-doped graphene quantum dots

Aromatic nitrogen doped graphene quantum dots were investigated by steady-state and time-resolved photoluminescence (PL) techniques. The PL lifetime was found to be dependent on the emission wavelength and coincident with the PL spectrum, which is different from most semiconductor quantum dots and fluorescent dyes. This result shows the synergy and competition between the quantum confinement effect and edge functional groups, which may have the potential to guide the synthesis and expand the applications of graphene quantum dots.

Diffraction integral formulas of the pulsed wave field in the temporal domain.

To resolve the diffraction problems of the pulsed wave field directly in the temporal domain, we extend the Rayleigh diffraction integrals to the temporal domain and then discuss the approximation condition of this diffraction formula.

A new theory for the treatment of a pulsed beam propagating through a grating pair

A new theoretical model, without the first-order approximation of grating diffraction, has been developed for studying the propagation of a pulsed beam through a grating pair. By using our model, the astigmatic aberration of the grating pair has been analyzed in detail. It was shown that the grating pair may be a good optical element for astigmatic compensation of monochromatic optical beams. But for grating pulse compression, the astigmatic aberration affects the pulse fronts and the effects of finite beam size (FBS). These effects on the compression of ultra-broadband pulses have been studied. It was found that, due to these effects, the space-time shapes of the compressed pulse are severely distorted. The magnitude of this distortion depends not only on the astigmatic aberration, but also on the spectral bandwidth and spatial divergence of the pulse. When the input pulsed beam is collimated, the waveform distortion due to the effects of FBS can be eliminated, but the pulse front distortion remains. In addition, the spatial and temporal properties of the compressed pulse for a single-pass compressor have been studied. An analytical expression without the well-collimated condition was obtained to describe the effect of lateral frequency shift. Also, the effect of third-order dispersion on a single-pass compressor is discussed.

Ultrafast hole burning in intersubband absorption lines of GaN/AlN superlattices

Femtosecond two-color pump-probe experiments on intersubband absorption spectra of electrons in a GaN/AlN superlattice show distinct spectral holes unraveling both the homogeneous broadening contribution and the underlying optical phonon progression.