1 INTRODUCTION

It is of current interest to develop methods of creating high-quality photonic components for photonic integrated circuits. Active and passive photonic microstructures are currently fabricated primarily by planar lithography. The transition to three-dimensional (3D) photonic structures that, being integrated to photonic integrated circuits, can serve an interchip junctions, ensure efficient input and output of radiation from components of photonic integrated circuits, and allow the simplification of the topology of circuits and optical logic elements is promising. Additive production methods such as stereolithography make it possible to create fundamentally new 3D optical elements and to exceed the capabilities of planar lithography, where 3D structures are fabricated by their layer-by-layer growth, which increases the time of fabrication of photonic circuits. One of the promising types of stereolithography is two-photon photopolymerization, which is the modification of direct laser writing with a significant increase in the resolution by using the two-photon absorption effect in the waist region of a focused femtosecond laser beam [1, 2]. The capabilities of this method were demonstrated upon fabricating a 3D yablonovite photonic crystal [3] with linear dimensions of elementary structural units no more than 300 nm, efficient connection between photonic circuits and radiation input/output devices [4, 5], and a broadband polarization beamsplitter and a radiation polarization rotator [6]. Important components of photonic integrated circuits are microcavities of whispering gallery modes (WGM), which are transparent dielectric structures with axial or central symmetry (tori, rings, spheres, squares, pentagons, etc.) [7]. Owing to the existence of high-Q cavity modes, such microstructures can be used as filters [8, 9], microlasers with optical [10, 11] or electric pumping [12, 13], sensors [14–16], and modulators [17], and nonlinear optical effects can be enhanced by means of the spatial localization of the electromagnetic field near their surface. It is also of interest to fabricate metal–dielectric nanostructures, where a stronger localization of the field allows the efficient control of light. Such an approach was developed in [18], where it was shown that the combination of two-photon laser lithography and electron beam lithography allows one to fabricate 3D hybrid metal–dielectric metamaterials.

Thus, the development of two-photon laser lithography (TPLL) is promising for the fabrication of 3D microcavities of WGM, including those activated by luminescent particles and materials. This field requires photonic devices combining microwaveguides and active microcavities with minimum optical losses caused by the leakage of radiation to the substrate.

In this work, we report results on the optimization of TPLL to increase the quality of fabricated microstructures, waveguides, and related microcavities. Structures fabricated within the standard and optimized TPLL methods are comparatively examined. It is shown that the proposed TPLL method allows one to fabricate waveguides elevated over the substrate on specially designed optical prism adapters. A method is presented to form combined active and passive photonic microstructures.

2 EXPERIMENTAL RESULTS

2.1 Experimental Technique

Microstructures were fabricated using the TPLL method on an experimental setup described in [7]. Pump radiation was obtained from a Tif-DP Ti:sapphire laser (Avesta Ltd.) with direct diode pumping [19], a wavelength of 780 nm, a pulse duration of 76 fs, and a pulse repetition frequency of 80 MHz. Laser radiation was guided through an acousto-optical modulator and a 5× telescope joined with a spatial filter of the main transverse Gaussian mode of the laser beam to a D1105 Sino-Galvo galvanoscanner used to set the waist of the laser beam in the lateral plane of the working volume. The galvanoscanner was located at the focus of the input lens of a 2× telescope, and the input lens of a Nikon Plan Apo VC 60×/NA 1.40 oil immersion objective was located at the focus of the output lens of the 2× telescope. The waist of the laser beam was fixed along the optical axis in the polymer by means of the displacement of the objective of a piezoelectric translator within the range of 40 μm. The rough positioning of the substrate with the polymer along three axes was ensured by stepper motors. The circuit of the setup is shown in Fig. 1.

Fig. 1.
figure 1

(Color online) Circuit of an experimental setup for two-photon laser lithography: (red) pump beam from a Tif-DP Ti:sapphire laser (Avesta Ltd.), (yellow) illumination channel, and (AOM) acousto-optical modulator.

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Microstructures were prepared primarily from OrmoComp® hybrid photopolymer (Micro Resist Technology) with a refractive index of 1.520 containing diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide as a photoinitiator. To form microstructures using the TPLL method, 5 μL of the photopolymer was deposited on a 170-μm-thick cover glass matched to the immersion objective. In order to obtain active microstructures [20], 0.36 mol/L coumarin-1 (7-diethylamino-4-methylcoumarin) dye, which served as a photoinitiator of polymerization [21], was added to OrmoComp® photopolymer; as a result, the polymerization threshold power was reduced by an order of magnitude. Coumarin-1 dye was dissolved in OrmoDev® developer consisting of methyl isobutyl ketone and isopropanol. The resulting solution was mixed with OrmoComp® photopolymer in a volume ratio of 1 : 1 for better dissolution of dye. The composition with dye was deposited on the cover glass and was kept for at a temperature of 60°C for 60 min to evaporate OrmoDev® developer. The printing rate was 500 μm/s at a laser power of 31 and 3 mW for pure OrmoComp® polymer and its mixture with coumarin-1 dye. According to the performed calibration experiments, these parameters made it possible to obtain photopolymerized structures with the maximum resolution. The presented powers were measured at the input of the galvanoscanner. The optical losses in the setup from the galvanoscanner to the cover glass were 50%. A digital 3D model for printing was formed within the Solidworks 3D CAD software, was then prepared for printing using the Simplify3D software, and was divided into layers parallel to the substrate with unidirectional hatching by the laser waist in each layer. A step between layers of 100 nm and a hatching step of 70 nm, which is approximately one-third of the linear dimensions of the voxel, were determined in test measurements. These printing parameters allowed us to obtain an acceptable geometric stability of fabricated optical microstructures. Printed structures were developed in OrmoDev® developer.

2.2 Test of the TPLL Method. Suspended Polymer Filaments

To determine the maximum resolution of the created experimental setup, we prepared test structures from OrmoComp® polymer with 0.36 mol/L coumarin-1 dye in the form of filaments on two 10-μm-high pedestals 4 × 10 μm in base; their scanning electron microscopy images are presented in Fig. 2. A thin thread was formed between the pedestals in one pass of the laser beam in polymer at different velocities of the waist and different radiation powers. The printing parameters for reaching the minimum size of the voxel in our setup were determined in the printed samples; the optimal velocity of the waist was 500 μm/s, and the optimal power at the output of the objective was 1.5 mW.

Fig. 2.
figure 2

Scanning-electron microscopy image of the calibration structure consisting of two pedestals and a 10-μm bar (filament) between them printed in one pass of the waist of the laser beam in the (a) absence and (b) presence of a spatial filter in the pump beam; the transverse dimensions of the filament (width × height) were 660 × 690 nm and 250 × 325 nm in the former and latter cases, respectively.

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The dimensions were determined from scanning electron microscopy images. Figure 2 shows images of the structures fabricated in the (a) absence and (b) presence of the spatial filter of the fundamental transverse Gaussian mode of the laser beam. The minimum achievable width and height of the filament were 660 and 690 nm, respectively; the spatial filter reduced these parameters to 250 and 325 nm, respectively. Thus, the optimization of the size and shape of the pump beam allows one to significantly reduce the size of the polymerized region and, thereby, to increase the quality of formed polymer structures with different shapes.

2.3 Optical Waveguides and Microcavities

One of the main goals when fabricating photonic integrated circuits is to minimize optical losses caused by the leakage of radiation to the substrate because of the closeness of the refractive indices of polymer (np = 1.520) and glass substrate (ns = 1.523). These losses for microcavities can be minimized using a sufficiently high microstructure [22], while these losses significantly reduce the efficiency of waveguides. One of the methods to solve this problem is the placement of components of the photonic integrated circuit above the substrate [23, 24] with the support on auxiliary elements.

The layout of the created structure implementing a waveguide with prism adapters, where the input and output light beams are codirected, is shown in Fig. 3, where the dimensions are given in microns. Additional tetrahedral elements were used to introduce radiation to the waveguide in the connector structure and extract radiation from it. It was found that the optimal trajectory of the laser waist printing a waveguide is the parallel “hatching” across the waveguide in each layer; this trajectory makes it possible to avoid strong distortions and deformation of the polymerized waveguide. The losses in the printed structure were determined by microscopy on the experimental setup, radiation was focused on the input end of the prism by a Mitutoyo M Plan Apo 100×/NA 0.7 objective, and radiation from the output prism was collected by the same objective from the opposite side. The optical losses in this structure with the waveguide 2 μm in diameter at a wavelength of 405 nm were 1.25 dB per prism adapter, which is comparable with the value for adapters based on diffraction gratings that usually lies [25] in the range from 1.2 dB [26] to 5.1 dB [27] for the telecommunication wavelength range. The efficiency of implemented prism connectors depends slightly on the wavelength and angle of incidence of radiation, which cannot be achieved with adapters based on diffraction gratings.

Fig. 3.
figure 3

(Color online) Three-dimensional model of a waveguide with prism radiation input/output adapters; the dimensions of the structure are given in microns; the laser beam is shown in blue; the height of the structure is 16 μm.

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We also note that this scheme of prism optical radiation input/output adapters is compatible with structures made of various materials, in particular, optical fiber.

Active waveguides optically uncoupled from the substrate were fabricated from OrmoComp® polymer with coumarin-1 dye according to the described scheme using the TPLL method. Figure 4 shows the optical image of photoluminescence of such a waveguide with a diameter of 2 μm detected in the spectral range of 450–600 nm. Pump radiation at a wavelength of 405 nm was focused on the prism optical connector (right in Fig. 3) and excited photoluminescence of dye. The photoluminescence spectrum of coumarin-1 dye in OrmoComp® polymer has a maximum at a wavelength of 439 nm. The presence of dye in the structure makes it possible to visualize pump radiation. In the case under consideration, radiation successfully passes through the connector, enters the waveguide, and transfers to the input prism; comparable brightness of photoluminescence in the input and output prisms indicates a high transparency of the structure. The spatial fluorescence modulation at the end of the output prism is due to the inhomogeneity of TPLL printing and the corresponding nonuniformity of the dye distribution in the prism.

Fig. 4.
figure 4

(Color online) Optical image of the waveguide made of OrmoComp® with 0.36-mol/L coumarin-1 dye in the spectral range of 450–600 nm, where 405-nm laser radiation propagates and is focused on the prism located to the right of the figure.

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The above TPLL method for the fabrication of prism connectors and waveguides separated from the substrate was then be used to form test structures based on the coupled ring cavity and waveguide. Figure 5 shows the scanning electron microscopy image of the 20-μm-long OrmoComp® waveguide about 2 μm in diameter located at a height of 10 μm above the substrate surface with the prism radiation input/output system described above and a hollow cylinder with an outer diameter of 10 μm whose lateral surface is spaced from the waveguide by a distance of about 1 μm. The cavity was activated by coumarin-1 dye. Such a structure was printed using the TPLL method in two stages. First, a pure OrmoComp® polymer layer was deposited and the suspended waveguide structure was printed, and the unilluminated part of the polymer was removed using OrmoDev® developer. At the next stage, OrmoComp® polymer with dye was deposited on the substrate with the waveguide and was dried for 60 min, and a microcavity was printed so as to ensure the required gap between it and the waveguide.

Fig. 5.
figure 5

Scanning electron microscopy image of the waveguide located at a height of 10 μm over the substrate with the prism radiation input/output system and the hollow cylinder 10 μm in diameter whose outer side surface is spaced from the waveguide by 1 μm. The printing rate is 500 μm/s.

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Figure 6а shows the optical image of the structure that consists of the waveguide and active microcavity and is illuminated by white light and ultraviolet light of a 365-nm diode, which induces fluorescence of coumarin-1 dye added to OrmoComp® polymer only in the microcavity. Figure 6b presents the image of the same structure illuminated only by the ultraviolet diode. In this case, the prism connectors and the waveguide, which do not contain dye luminescing under ultraviolet radiation, are not observed in the image. At the same time, a bright photoluminescence signal of coumarin-1 in the microcavity region and the yield of this radiation from the output connector located in the right part of Fig. 6 are observed, indicating the optical coupling between the waveguide and optical microcavity. Any photoluminescence signal in region of the connector on the other side from the microcavity is absent because of the design of the connector: radiation leaving it is directed predominantly outward from the detector (see Fig. 3) and, therefore, its detected intensity is much lower. Similar results were obtained for a whole set of structures consisting of the waveguide and active microcavity with various parameters. Thus, the efficiency of the proposed TPLL method for the fabrication of combined active and passive photonic microstructures has been demonstrated.

Fig. 6.
figure 6

(Color online) Optical image of the waveguide made of pure OrmoComp® polymer and the nearby hollow cylinder 10 μm in diameter made of OrmoComp® with coumarin-1 dye upon illumination by (a) a halogen lamp (white light) and an ultraviolet diode (at a wavelength of 365 nm) and (b) the ultraviolet diode.

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3 CONCLUSIONS

To summarize, the two-photon laser printing method has been developed using femtosecond Ti:sapphire laser radiation with hybrid OrmoComp® photopolymer activated by coumarin-1 dye. The analysis of freely suspended waveguides and the system consisting of a waveguide and a ring microcavity has shown that the spatial filter of the Gaussian mode improves the quality of printed microstructures: the rounding radius decreases, more distinct edges of the structure are formed, and the homogeneity of the structure increases. The functional capabilities of the method for the printing of components of photonic integrated photonics have been demonstrated by the example of a passive waveguide elevated over the substrate coupled to a dye-activated ring cavity. Prism radiation input/output microadapters with losses of no more than 1.25 dB per adapter have been fabricated.