Controllable movement of single-photon source in multifunctional magneto-photonic structures

Abstract

Quantum dot (QD) coupling in nanophotonics has been widely studied for various potential applications in quantum technologies. Micro-machining has also attracted substantial research interest due to its capacity to use miniature robotic tools to make precise controlled movements. In this work, we combine fluorescent QDs and magnetic nanoparticles (NPs) to realize multifunctional microrobotic structures and demonstrate the manipulation of a coupled single-photon source (SPS) in 3D space via an external magnetic field. By employing the low one photon absorption (LOPA) direct laser writing (DLW) technique, the fabrication of 2D and 3D magneto-photonic devices containing a single QD is performed on a hybrid material consisting of colloidal CdSe/CdS QDs, magnetite Fe₃O₄ NPs, and SU-8 photoresist. Two types of devices, contact-free and in-contact structures, are investigated to demonstrate their magnetic and photoradiative responses. The coupled SPS in the devices is driven by the external magnetic field to perform different movements in a 3D fluidic environment. The optical properties of the single QD in the devices are characterized.

Figure 1

Characterizations of the hybrid material. (a) Measured absorption spectra of the hybrid material and separated components at ambient condition. (b) Morphological atomic force microscope (AFM) image of a 10-μm-thick film of the hybrid material. (c) Magnetic force microscope (MFM) image of the corresponding film.

Figure 2

Low-one photon absorption (LOPA) direct laser writing (DLW) technique. (a) Schematic of the LOPA-based DLW set-up combined with the Hanbury Brown and Twiss experiment for both fluorescence characterization and structural fabrication. APD: Avalanche Photodiode. Fluorescence map of (b) individual QDs within the film of the hybrid material and (c) a single QD coupled to a micro-wheel structure after the writing process.

Figure 3

Contact-free devices with incorporated SPS and their movement manipulation in a fluidic environment. Scanning electron microscope (SEM) images of the contact-free structures: (a) multiple and (b) single micro-wheel structure, (c) a micro-arrow structure, with their corresponding fluorescent image in the sub-set. Optical microscope images of (d) the micro-wheel performing translational displacement and (e) the micro-arrow performing rotational movement.

Figure 4

In-contact device and its controllable manipulation. (a) Scanning electron microscope (SEM) images of a micro-spring. (b) Fluorescent map of fabricated micro-spring structures with a integrated QD at the center of the micro-wheel before the development step. (c) The transmission optical microscope image of micro-spring structures corresponding to (b) the fluidic environment. (d) The transmission optical microscope images of a micro-spring at different bending stage when deliberately changing the direction of the magnetic flux in a semicircular track above the sample. (e–h) The orientation of structures toward higher gradient of magnetic field with depicted magnetic field gradient direction.

Figure 5

The elasticity of the micro-spring under the influence of an applied external magnetic field. (a) The dependence of the magnetic field on the distance, d, from a magnet to the sample position. (b) The stretching, recovery, and irreversible performance of a micro-spring structure under the influence of the applied magnetic field, as observed by the transmission microscope. (c) The dependence of spring displacement on the applied magnetic field. The green area denotes for the recoverable zone of the micro-spring and the yellow zone denotes for the irreversible zone of the micro-spring.

Figure 6

Characterization of the coupled QD after the manipulation. (a) Emission spectrum of a coupled QD in the structure fitted with a Gaussian function to extract the emission peak at 620 nm and full-width-half-maximum (FWHM) of around 30 nm. (b) The dependence of emission rate of the coupled QD as the function of the excitation power, excited by the green laser at 532 nm. Green area indicates a region obtaining single photon emission g⁽²⁾(0) ≤ 0.5. (c) Antibunching curve of the coupled QD with the fitted value of g⁽²⁾(0) ≈ 0.1. (d) Time-resolved fluorescence decay of the SU-8/Fe₃O₄ mixture and of a QD in the mixture with fitted lifetime of 3.7 ± 0.1 and 45 ± 1 ns, respectively.