Integrating Fluorescent Nanodiamonds into Polymeric Microstructures Fabricated by Two-Photon Polymerization

At a Glance

Metadata	Details
Publication Date	2023-09-16
Journal	Nanomaterials
Authors	Filipe A. Couto, Marcelo B. Andrade, Adriano J. G. Otuka, Sebastião Pratavieira, Sérgio Ricardo Muniz
Institutions	Universidade de São Paulo
Citations	4
Analysis	Full AI Review Included

Executive Summary

This research successfully demonstrates a robust method for integrating fluorescent nanodiamonds (NDs) containing Nitrogen-Vacancy (NV) color centers into complex polymeric microstructures using Two-Photon Polymerization (2PP).

Integration Success: NV-center NDs (40 nm diameter) were successfully embedded into acrylate polymeric microcylinders via 2PP, paving the way for integrated solid-state quantum emitters in photonic devices.
Critical Concentration Limit: Successful fabrication requires ND doping concentrations strictly less than 0.01 wt%. Higher concentrations (e.g., 0.5 wt%) cause excessive light scattering, attenuating the laser below the polymerization threshold.
Optimal Doping: Concentrations between 0.002 wt% and 0.005 wt% provide the best trade-off, ensuring excellent structural quality while reliably embedding one to three localized fluorescent emitters per structure.
Photonic Viability: Absorbance measurements confirmed that scattering losses at 0.01 wt% are manageable, reducing the estimated Quality (Q) factor of microresonators from 10⁵ (pure) to 3 x 10⁴, making the structures suitable for quantum optics.
Confirmation: Raman spectroscopy confirmed the presence of the characteristic pristine diamond lattice peak (1332 cm^-1) specifically at the localized fluorescent spots identified by confocal microscopy.
Platform Versatility: The 2PP technique offers a versatile 3D fabrication platform applicable to integrating various other solid-state emitters (e.g., SiV centers) into designed polymeric photonic circuits.

Technical Specifications

Parameter	Value	Unit	Context
Optimal ND Concentration Range	0.002 to 0.005	wt%	Yields 1-3 emitters per structure with high structural quality.
Maximum Viable ND Concentration	< 0.01	wt%	Upper limit before excessive scattering and agglomeration.
Nanodiamond Mean Diameter	40	nm	Fluorescent NDs containing NV color centers.
2PP Laser Wavelength	780	nm	Ti:Sapphire oscillator center wavelength.
2PP Pulse Duration	100	fs	Used for nonlinear absorption polymerization.
2PP Repetition Rate	86	MHz	Laser system specification.
Photoresist Monomer Ratio	90/10	N/A	Dipentaerythritol pentaacrylate (SR399) / tris(2-hydroxy ethyl)isocyanurate tryacrylate (SR368).
Photoinitiator Loading	3	wt%	Lucirin TPO-L.
Estimated Q Factor (Pure Resin)	10⁵	N/A	Typical non-doped microresonator Q factor.
Estimated Q Factor (0.01 wt% ND)	3 x 10⁴	N/A	Reduced Q factor due to scattering losses.
Characteristic Diamond Raman Peak	1332	cm^-1	Confirms presence of pristine diamond lattice (C-C sp³ bond).
Extra Material Absorption Offset	~0.4	cm^-1	Measured offset in absorption coefficient at 0.01 wt% doping (primarily scattering loss).

Key Methodologies

Photoresist and Nanodiamond Doping

Monomer Mixing: Dipentaerythritol pentaacrylate (SR399) and tris(2-hydroxy ethyl)isocyanurate tryacrylate (SR368) were mixed at a 90/10 ratio, with 3 wt% Lucirin TPO-L photoinitiator added.
ND Incorporation: A 1 mg/mL solution of 40 nm fluorescent NDs in deionized water was introduced into the mixture.
Homogenization: The entire mixture was magnetically stirred at 50 °C until complete homogenization and full evaporation of the deionized water was achieved.

Two-Photon Polymerization (2PP) Fabrication

Laser Setup: A Ti:Sapphire oscillator delivered 100 fs pulses centered at 780 nm with an 86 MHz repetition rate.
Focusing: The beam intensity was controlled via a half-wave plate and polarizer, then focused onto the sample using a 0.25 NA microscope objective.
Structure Design: Cylindrical microstructures (50 µm high, 15 µm to 30 µm radius) were fabricated, testing various ND concentrations (0.002 wt% to 0.5 wt%).

Characterization and Loss Analysis

Structural Analysis: Scanning Electron Microscopy (SEM) was used to qualitatively assess the structural quality and identify agglomerates.
Emitter Localization: Commercial Confocal Laser Scanning Microscopy (LSM, 543 nm excitation) was used to sweep the structures’ volume and map the 3D positions of the fluorescent NV centers.
Fluorescence Spectroscopy: A homemade confocal setup (532 nm diode laser excitation) was used to measure the characteristic NV center emission spectra (ZPL and phonon sideband).
Material Confirmation: Raman and Photoluminescence (PL) measurements (532 nm and 633 nm excitation) confirmed the presence of the diamond lattice peak (1332 cm^-1) at the fluorescent spots.
Absorption Loss: Absorbance measurements were performed on 400 µm cured films (0.01 wt% doping) to quantify scattering losses using the Beer-Lambert law, estimating the impact on resonator Q factor.

Commercial Applications

The integration of NV-NDs into 3D polymeric structures via 2PP creates a versatile platform for advanced quantum and photonic technologies.

Integrated Quantum Sensors:
- Development of compact, high-sensitivity magnetic field, temperature, and strain sensors leveraging the NV center’s optically addressable spin state.
- Applicable in biological environments where non-photobleaching, biocompatible markers are required.
Quantum Photonic Circuits:
- Fabrication of functionalized waveguides and interferometers capable of launching and guiding single-photon emission from NV centers into integrated photonic circuits.
- Essential for scalable quantum information processing (QIP) and quantum communication systems.
High-Q Microresonators:
- Creation of cylindrical cavities and photonic crystals where the sharp resonances can be tuned by geometric parameters to enhance (Purcell effect) and spectrally filter single-photon emission.
Versatile Emitter Integration:
- The methodology is directly transferable to integrating other solid-state emitters (e.g., Silicon-Vacancy (SiV) centers in NDs, or emitters in SiC and GaP) into complex 3D architectures, expanding the range of functional devices.

View Original Abstract

Nitrogen-vacancy (NV) and other color centers in diamond have attracted much attention as non-photobleaching quantum emitters and quantum sensors. Since microfabrication in bulk diamonds is technically difficult, embedding nanodiamonds with color centers into designed structures is a way to integrate these quantum emitters into photonic devices. In this study, we demonstrate a method to incorporate fluorescent nanodiamonds into engineered microstructures using two-photon polymerization (2PP). We studied the optimal concentration of nanodiamonds in the photoresist to achieve structures with at least one fluorescent NV center and good structural and optical quality. Fluorescence and Raman spectroscopy measurements were used to confirm the presence and location of the nanodiamonds, while absorbance measurements assessed scattering losses at higher concentrations. Our results show the feasibility of fabricating microstructures embedded within fluorescent nanodiamonds via 2PP for photonics and quantum technology applications.

Tech Support

Original Source

DOI: https://doi.org/10.3390/nano13182571

References

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