Femtosecond laser writing shows great potential for novel 3D photonic architectures and high quality NV- quantum emitters in the bulk of diamond. However, the direct writing method cannot achieve nanometric placement of NV- centers near the surface of diamond, which is required for certain quantum sensing tasks. We will demonstrate a hybrid approach where the advantages of 3D optical waveguides by femtosecond laser writing and precise and shallow placement of NV- centers by ion implantation will be combined to form an integrated quantum sensor with record high performance.
SnO2-based glass-ceramics activated by rare earth ions have been extensively investigated because of the need to develop reliable fabrication protocols and clarify some interesting optical, structural, and spectroscopic features of the system. There is one important weakness in glass photonics when the rare earth ions are employed as luminescent sources. This is the low absorption cross section of the electronic states of the rare earth ions. A sensitizer is therefore requested. In the last years, we demonstrated that SiO2-SnO2 glass ceramics, presenting a strong absorption cross section in the UV range due to the SnO2 nanocrystal, are effective rare earth ions sensitizers. Another interesting property of the SiO2-SnO2 system is its photorefractivity. The high photorefractivity of sol-gel-derived SnO2-SiO2 glass-ceramic waveguides has been demonstrated in several papers published by our consortium. It has been shown that the UV irradiation induces refractive index change allowing the direct writing of both channel waveguides and Bragg gratings.
The results presented in this communication not only demonstrate the viability and outstanding properties of the SiO2- SnO2 glass-ceramics for photonic applications but also put the basis for the fabrication of solid state and integrated lasers. The next steps of the research are the fabrication of the channels and mirrors exploiting the photorefractivity as well as to draw glass ceramic fiber, checking the lasing action and corresponding functional characteristics. Finally, it is worth noting that the dynamic of the energy transfer from the nanocrystals to the rare earth ions is still an exciting open question.
Diamond’s nitrogen-vacancy (NV) center has been shown as a promising candidate for sensing applications and quantum computing because of its long electron spin coherence time and its ability to be found, manipulated and read out optically. An integrated photonics platform in diamond would be useful for NV-based magnetometry and quantum computing, in which NV centers are optically linked for long-range quantum entanglement due to the integration and stability provided by monolithic optical waveguides. Surface microchannels in diamond would be a great benefit for sensing applications, where NV centers could be used to probe biomolecules.
In this work, we applied femtosecond laser writing to form buried 3D optical waveguides in diamond. By engineering the geometry of the type II waveguide, we obtained single mode guiding from visible to the infrared wavelengths. Further, we demonstrate the first Bragg waveguide in bulk diamond with narrowband reflection. We show the formation of single, high quality NV centers on demand in ultrapure diamond using a single pulse from a femtosecond laser. With these building blocks in place, we fabricated an integrated quantum photonic circuit containing optical waveguides coupled to NV centers deterministically placed within the waveguide. The single NVs were excited and their emission collected by the optical waveguides, allowing easy interfacing to standard optical fibers. We also report high aspect ratio surface microchannels, which we will integrate with laser-written NVs and waveguides, paving the way for ultrasensitive, nanoscale resolution biosensors.
Diamond’s nitrogen-vacancy (NV) centers show great promise in sensing applications and quantum computing due to their long electron spin coherence time and their ability to be located, manipulated and read out using light. The electrons of the NV center, largely localized at the vacancy site, combine to form a spin triplet, which can be polarized with 532- nm laser light, even at room temperature. The NV's states are isolated from environmental perturbations making their spin coherence comparable to trapped ions. An important breakthrough would be in connecting, using waveguides, multiple diamond NVs together optically. However, the inertness of diamond is a significant hurdle for the fabrication of integrated optics similar to those that revolutionized silicon photonics. In this work we show the possibility of buried waveguide fabrication in diamond, enabled by focused femtosecond high repetition rate laser pulses. We use μRaman spectroscopy to gain better insight into the structure and refractive index profile of the optical waveguides.
Optical waveguides were fabricated with femtosecond pulsed lasers on glass and characterized by transmission measurements. Glass waveguides were later used for excitation of the whispering gallery modes in a silicon microsphere. The coupling between the silicon microsphere and the femtosecond laser inscribed optical waveguide was simulated in both 90° elastic scattering and 0° transmission spectra. The silicon microsphere whispering gallery modes are available for both in the transverse electric and transverse magnetic polarizations with a spectral mode spacing of 0.25 nm. Optical resonances on silicon microsphere integrated with femtosecond laser written optical waveguides may lead to future quantum optical communication devices.
Femtosecond laser microprocessing is a direct, maskless fabrication technique that has attracted much attention in the past 10 years due to its unprecedented versatility in the 3D patterning of transparent materials. Two common modalities of femtosecond laser microfabrication include buried optical waveguide writing and surface laser ablation, which have been applied to a wide range of transparent substrates including glasses, polymers and crystals. In two photon polymerization, a third modality of femtosecond laser fabrication, focused femtosecond laser pulses drive photopolymerization in photoresists, enabling the writing of complex 3D structures with submicrometer resolution. In this paper, we discuss several microdevices realized by these diverse modalities of femtosecond laser microfabrication, for applications in microfluidics, sensing and quantum information.
We have fabricated entirely by femtosecond micromachining a plastic optofluidic chip with integrated microfluidics and
optical excitation/detection. First a microfluidic channel and two fiber grooves were ablated on one surface of the
PMMA substrate. In order to collect and focus the fluorescence signal onto a detector, two binary Fresnel lenses were
micromachined on the back surface of the substrate. The operatio of the integrated optofluidic chip was demonstrated by
filling the channel with different Rhodamine 6G solution, and a limit of detection of 50 nM was achieved.
KEYWORDS: Microfluidics, Optical filters, Two photon polymerization, Optical spheres, Image filtering, Lab on a chip, Chemical elements, Rhodamine, Microscopes, Molecules
We report on the integration of a size-based three-dimensional filter, with micrometer-sized pores, in a commercial
microfluidic chip. The filter is fabricated inside an already sealed microfluidic channel using the unique capabilities of
two-photon polymerization. This direct-write technique enables integration of the filter by post-processing in a chip that
has been fabricated by standard technologies. The filter is located at the intersection of two channels in order to control
the amount of flow passing through the filter. Tests with a suspension of 3-ìm polystyrene spheres in a Rhodamine 6G
solution show that 100% of the spheres are stopped, while the fluorescent molecules are transmitted through the filter.
We demonstrate operation up to a period of 25 minutes without any evidence of clogging. Moreover, the filter can be
cleaned and reused by reversing the flow.
Recent studies have shown that mechanical factors can direct stem cell fate in vitro, even in the absence of biochemical
factors. Two-photon laser polymerization was applied here to fabricate ultra-precise 3D micro-scaffolds with different
architectures and pore sizes able to structurally interact with cells at the single-cell scale. Our experiments have shown
that randomly seeded mesenchymal stem cells systematically colonize the internal volumes of 3D scaffolds and
proliferate, while showing a roundish morphology. Even if stem cell mechanobiology is a very complex field, this study
shows how mechanical interactions studied in a 3D micro-architecture at a single cell scale may influence stem cells
response.
High-repetition rate (>200 kHz) ultrafast lasers drive novel heat accumulation processes attractive for rapid writing of low loss optical waveguides in transparent glasses. Heat accumulation is significant at ~1 MHz when thermal diffusion is insufficient to remove the absorbed laser energy from the focal volume (<10-μm diameter) during the interval between pulses. A novel femtosecond fiber laser system (IMRA, FCPA μJewel) providing variable repetition rate between 100 kHz and 5 MHz was applied to waveguide writing in fused silica and various borosilicate glasses in order to investigate the relationship of such heat accumulation effects. Waveguides were formed with <400-fs pulses at 1045-nm at pulse energies of 2.5 μJ at 100 kHz to >150 nJ at 5 MHz. Wide variations in waveguide properties were encountered, particularly when processing 1737F and AF45 borosilicate, at repetition rates greater than 200 kHz. Waveguide characterization revealed unique material-dependent thresholds for cumulative and single pulse phenomenon. Of these materials, fused silica is unusual in resisting waveguide formation at the fundamental wavelength of 1045 nm, but amenable to waveguide writing at the second harmonic of 522 nm. Laser processing windows are presented for several silica-based glasses for creating symmetric waveguides with low insertion loss when coupled to standard optical fibers. The effects of material and laser parameters on thermal accumulation and waveguide characteristics are discussed.
Direct waveguide writing with femtosecond lasers can be divided into two general categories based upon the type of lasers used: amplified systems that emit high pulse energy (>2 μJ) at low repetition rates (<250 kHz), and oscillators that produce low energy (<200 nJ) at high repetition rates (>1 MHz). In this presentation, we report on waveguide writing with a novel commercial femtosecond fiber laser system (IMRA, FCPA μJewel) that bridges the gap between these two regimes, providing sub-400 fs pulses with pulse energies of >2.5 μJ at 100 kHz and >150 nJ at 5 MHz. The laser repetition rate can be varied from 100 kHz to 5 MHz in 1 kHz increments through a computer controlled user interface. The ability to quickly and easily vary the repetition rate of this laser was critical in identifying and optimizing laser processing windows for different target glasses. An overview of laser processing windows and waveguide characteristics are presented for borosilicate and fused silica glasses exposed to fundamental (1045 nm) and second harmonic (522 nm) laser light.
Lasers microprocessing is attractive for the custom fabrication of novel lab-on-a-chip designs. However, processing of glass biochips is challenging for most lasers because of the weak light interactions inherent in such transparent substrates. The F2-laser generates a high 7.9-eV photon energy that drives strong absorption in glasses, while the short 157-nm wavelength offers high-resolution patterning on the 100-nm scale. With these benefits, F2-laser ablation is well suited to the fabrication of high aspect ratio microfluidic channels and other biochip functions. F2-laser radiation also produces a strong photosensitivity response in fused silica and other glasses that enable the fabrication of buried optical waveguides, Bragg grating filters and other refractive index structures inside the glass. In this paper, we combine laser micromachining and refractive index profiling to enable single-step integration of photonic functions with microfluidic functions on a single chip. Optical waveguides were written to intercept microfluidic channels for optical sensing of cells and other bio-materials. An integrated biophotonic sensor is demonstrated for polystyrene spheres. The sensor is optically characterized for insertion loss, propagation loss, and particle sensitivity. The demonstration and analysis of this simple device offers insight into the capabilities and potential applications for laser fabricated glass lab-on-a-chip devices. Moreover, the groundwork is laid for rapid laser prototyping of custom-designed microfluidic biochips interlaced with integrated-optical circuits to define a new generation of highly functional bio-sensor and lab-on-a-chip devices.
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