1 Introduction
Optical microcavities with small mode volume and high quality (Q) factors have been considered as the basic building blocks of integrated optoelectronic devices, and may have board applications in optical communications, photonic circuits and optical sensors [1–9]. Microcavity lasers based on whispering-gallery-mode (WGM) typically refer to microdisks, microrings, microcylinders and microspheres cavity [10–18], and generally possess some unique lasing features including ultrasmall mode volume and ultrahigh Q factors. Light can be efficiently confined within such WGM microcavities via the total internal reflection of photons at the surface of rotational symmetric cavities, which enhances the interaction of light and matters and improves the lasing performances.
Lots of efforts have been devoted to the tremendous progress in the development of microlaser processing technologies such as photolithography, proton beam writing and reactive ion etching [19–21]. Recently, femtosecond laser processing technique has attracted much attention because of its intrinsic three-dimensional (3D) prototyping capability with ultrahigh spatial resolution [22,23]. Meanwhile the unique features of ultrashort pulse duration of femtosecond laser with high peak power reduce the heat diffusion from the interaction area to surrounding zones in material processing, making the femtosecond laser processing an ideal tool for fabrications of both soft and hard materials such as polymer, protein, glass and crystal [24–29]. Thus a variety of devices including micro-optics, micro-fluidics, micro-electronics and micro-sensing [30–33] have been fabricated in optical transparent materials by femtosecond laser processing [34].
In this paper, we give an overview of our latest progress in the fabrication of high-Q microlasers in soft materials of polymer and protein by the femtosecond laser processing. We begin with a brief description of the homebuilt femtosecond laser processing system, which is followed by presenting several examples for fabricating dye-doped polymer and protein microlasers, whose lasing performances is at room temperature. Subsequently, one example showing the applications of the protein microlasers in environmental sensing is introduced. Lastly, a summary and a future outlook are given.
2 Femtosecond laser processing system of 3D microcavity lasers
A homebuilt femtosecond laser system is shown in Fig. 1 [35], in which a Ti:sapphire femtosecond laser system (Spectra Physics 3960-X1BB) with a repetition rate of 80 MHz, a pulse width of 120 fs, and a central wavelength at 800 nm was used as a light source. The laser beam was tightly focused, by a high-numerical-aperture (NA= 1.35) oil-immersion objective lens (100×), on the interface between materials and substrate or inside the materials. To achieve 3D micro/nano fabrications, a piezo stage with 1-nm precision (PI P-622 ZCD) was used to control the sample’s vertical movements, and meanwhile a two-galvano-mirror set enabled the beam’s horizontal scanning. To control 3D point-by-point scanning, the complicated geometries of micro/nano-structures were designed by softwares including “Visual Basic” and “3D Studio Max”, and then converted to computer processing data. The power energy of the laser beam, which is key factor in fabricating process, could be varied continuously by using an attenuator before the oil-immersion objective lens. The host materials were rinsed to remove the unsolidified parts after laser fabrication. As a result the produced 3D microcavity lasers were left on the substrates.
3 Micro-lasers fabricated by femtosecond laser processing
3.1 Polymer WGM micro-lasers
The fabrications of passive and active microcavities with femtosecond laser processing appeared only at early 2010s [36−38]. In 2011, we fabricated a WGM microdisk laser with the diameter of 20 mm by femtosecond laser direct writing of dye-doped resins [38]. Laser dye of RhB was selected as the gain medium, which was doped into SU-8 negative photoresist with an overall concentration of 1 wt%. As shown in Fig. 2(a), the absorption peaks of RhB-doped SU-8 is centered at around 550 nm, which guarantees the fabrication of the resin materials by two-photon polymerization, but not by the linear absorption. As shown in Figs. 2(b)–2(d), both the existence of the significant threshold (Figs. 2(b) and 2(c)) and the stronger peaks in the spectral range of 610–650 nm (Figs. 2(d) and 2(e)) show the fingerprint of the lasing action at room temperature, when pumped by a frequency doubled Nd:YLF picosecond laser (532 nm, 15 ps, 50 KHz). It was well known that the free spectral range (FSR) is strongly dependent on the dimensions of the microcavity, basing on the WGM fundamental relation of Dl = l2/(npd), where n and d is the effective index and the diameter of the microcavity, respectively. For a fabricated microdisk with a diameter of 30 mm, we measured the FSR, which was 2.635 nm. This agrees well with the theoretically calculated value of ~ 2.602 nm, showing the WGM feature of the fabricated microcavity.
Fig.2 (a) Absorption spectra of RhB (circle), RhB-doped SU-8 (square) and photoluminescence (PL) spectrum of the mixed resin (diamond); (b) light output versus pumping laser intensity; (c) zoomed-in light output versus pumping laser intensity; (d) emission spectrum in the spectral range of 560– 680 nm; (e) emission spectra of microdisk laser at different pumping intensities [38] |
It is well known that Q factor is a typical factor used to evaluate the energy store capability of a cavity, with the equation that Q - 1 = Qabs- 1 + Qscat- 1 + Qcav- 1, where the inverse of Qabs-1, Qscat-1 and Qcav- 1 correspond to the absorption loss, scattering loss and cavity finesse, respectively [39]. Scattering loss caused by the surface roughness and shape deformation is a key factor in determining the Q value of the microdisk laser. According to Q = l/dl, where l is the resonance wavelength and dl is the full width at half maximum (FWHM), the Q value of the microcavity is estimated to be ~ 1.6 × 103, which is not satisfying. However, the Q factor of the active microcavity may be underestimated when compared with that of a passive one, which normally gives a Q factor at the level 106 and even greater [40].
3.2 Unidirectional lasing emission from polymer micro-lasers
Since the isotropic lasing emission of WGM microrelasers with rotational symmetry can result in unavoidable low collection efficiency by lens coupling [41], polymer microlasers providing unidirectional emissions have been recently developed, for example, by the photolithographic method [42]. Recently, we have demonstrated unidirectional emissions from a spiral-shaped polymer microdisk by femtosecond laser processing via two-photon polymerization [43]. In this case, the boundary of the disk could be defined in polar coordinates (r, j) as r(j) = r0{1+ ej/(2p)}, with e being the deformation parameter and r0 the radius at j = 0. A meaningful notch part will be created as the radius r(j) jumps back to r0 at j = 2p. A typical sample with a diameter of ~30.1 mm and a notch size of 1.6 mm is designed to examine the directional emission property of the spiral-shaped microcavity laser. The thickness of the microdisk is 2 mm.
Fig.3 (a) Room temperature emission spectra of the spiral-shaped disk microlaser. Inset: lasing intensity distribution measured with the signal emitted from different angles; (b) FDTD simulation results of spectral mode structure with the intensity distribution of the mode at 631.65 nm (inset) [43] |
As shown in the inset of Fig. 3(a), the emission intensity distribution pattern of the spiral cavity is plotted in a polar coordinate and it can be found that the lasing intensity from 0° is much stronger than those measured from other angles, where 0° refers to the direction facing the notch section. It can be seen that the spiral shaped microcavity laser exhibited a good unidirectional lasing emission property with a far field divergence of about 40°. Meanwhile, the peak position of lasing modes emitted from the spiral shaped microcavity shifts with the collection angles, and that the measured lasing thresholds are slightly varied with different angles. This might be due to the chromatic dispersion when light hits the boundary with different incident angle and escapes from the cavity via tunneling. The unidirectional emission is verified (see Fig. 3(b)) by a 2D finite difference time domain (FDTD) numerical simulation, in which clockwise propagating high-Q whispering-gallery like modes might couple to the counterclockwise modes, giving a unidirectional output. The realization of the creation of unidirectional emission lasers in polymer provides an important step toward fabricating functional integrated organic optoelectronic devices by femtosecond laser processing.
3.3 Photonic-molecule polymer microlasers
To investigate the cavity-cavity interactions, photonic-molecule (PM) lasers are good candidates [44,45]. Thus, we fabricated polymer dye-doped PM microcavities by femtosecond laser processing in order to reveal the coupling effects of microcavities [35]. Figure 4 shows the scanning electron microscope (SEM) images of four different planar PM microcavity lasers. The two cavities have the same diameters and thinknesses which are 20 and 2 mm, respectively. The edge distances between the two microdisks are 0.79 mm (Fig. 4(a)), - 0.21 mm (Fig. 4(b)), - 1.25 mm (Fig. 4(c)) and - 2.23 mm (Fig. 4(d)), respectively. By using a frequency doubled Nd:YLF picosecond laser (532 nm, 15 ps, 50 KHz) as the excited source, the lasing spectrum in the range of 615–650 nm are obtained for all the PM microcavity lasers at room temperature. The irregular and multiple modes are observed, which is ascribed to the mode coupling between the two disks, leading to the enhancing or suppressing of the modes through Vernier effect due to the slight structural difference between the two disks. It is shown that when the overlapping between the two cavities becomes large (see Fig. 4(d)), the intersection of the two disks may destroy the WGM microcavity property. With destroying the circular symmetry of the microcavity in Fig. 4(d), it is found that single mode lasing can be obtained although the threshold energy to achieve lasing becomes much larger in this case.
To further show the single mode lasing from PM lasers, we fabricated a 3D stacked polymer PM dye-doped microcavity lasers [46]. The 3D PM polymer microlaser composes of two identical microdisks stacked with edge alignment, Figs. 5(a)–5(c) show the coupling principle of the two stacked microdisks by Vernier effect. As the excited energy is distributed at the disk periphery and the couple effect happens at the overlapping edge, the intensity of the modes that resonate in both disks could be enhanced largely, while others would be strongly suppressed, providing the possibility to achieve a single-mode lasing output.
Fig.5 (a) Schematic diagram of 3D PM microcavity lasers; (b) principle of single-mode lasing via Vernier effect; (c) three lasers are shown by simply changing a small smout of size, temperature, etc. The lasing wavelength can be modulated in a wide range; (d) emission spectrum of 3D PM lasers at different pumping intensities with diameters of 18 and 30 mm. Inset: light output versus the pumping intensity; (e) spectrum with more modes is shown, indicating the Vernier effect [46] |
Under the optical pump by a frequency doubled Nd:YLF picosecond laser (532 nm, 15 ps, 50 KHz), the spectra of a 3D PM microlaser obtained with different power densities were obtained at room temperature, as shown in Fig. 5(d). It can be seen that the single mode lasing at ~ 642 nm is achieved with a low lasing threshold of 13 W/cm2. To further reveal the underlying mechanism, the spectrum of another PM microlaser with the diameters of 36 and 60 mm was shown in Fig. 5(e). The intensities of the peaks at 639.1 and 645.3 nm were stronger than others, which indicated that with the mode coupling and competition, these two modes were on resonance simultaneously for both microdisks. According to the WGM relation mentioned above, the FSR12 was about 6.2 nm which was five times of FSR1 obtained from disk diameter of 60 mm, and three times of FSR2 obtained from the disk diameter of 36 mm. That is, FSR12 = 5FSR1 = 3FSR2. This result agrees well with the Vernier effect theory, and further confirm that the coupling of the disks via Vernier effect is the main reason for generating single mode lasing.
3.4 Protein WGM microlasers
To show the ability of femtosecond laser processing for fabricating different soft materials, we used proteins as the host material to produce 3D WGM microlasers [47]. The promising biocompatible and functionality designable biomacromolecule material, protein (BSA) was photo-cross-linked with the help of self-photosensitization and probable RhB photosensitization without any specific photosensitizers added. In the femtosecond laser processing, the high-viscosity BSA/RhB aqueous ink was well sealed in a small polydimethylsiloxane (PDMS) based chamber to avoid evaporation. After being rinsed in water, the produced protein-based 3D WGM microlasers were left on the substrate.
Fig.6 (a) PL spectra from a 60-mm-diameter protein-based 3D WGM microlaser in pure water. Inset: optical microscopic image; (b) lasing spectrum of test number 7 at room temperature; (c) peak values of PL spectra from the protein-based 3D WGM microlaser versus different pumping intensities. Insets: optical microscopic fluorescent images of samples; (d) 3D-waterfall arranged lasing spectra from the protein-based 3D WGM microlaser in aqueous solutions with increasing Na2SO4 concentrations with pumping intensity of ~ 2.0 mW/mm2; (e) 2D stacked lasing spectra in (d); (f) wavelength blueshift of a particular peak in lasing spectra along with Na2SO4 concentration increasing [47] |
Under optical pumping of a 532 nm Nd:YLF laser (15 ps, 50 KHz), a laser dye (RhB) doped protein WGM microlaser with the diameter of 30 mm exhibits good lasing action in air without annealing processing. The lasing threshold was ~ 0.57 mW/mm2 and the FWHM of the lasing line was ~ 0.26 nm at ~ 624.1 nm, which gave rise to the calculated Q factor of ~ 2400 at room temperature. The protein-based WGM microlasers also exhibit valuable steady operation performances in aqueous environments. However, it was found that it was harder to achieve lasing in aqueous environment than in air, we thus fabricated a protein-based 3D WGM active microlaser with the diameter of 60 mm, which gave rise to a Q factors of around ~ 2000 to ~ 3000 in pure water. It should be pointed out that as the equilibrium swelling effect, the diameter of the microcavity in pure water would become a little larger than that in air and the refractive index of the protein hydrogels would decrease slightly. Since the equilibrium-swollen state of protein hydrogels inherently and sensitively responds to the changes of ionic concentration in water, we examined the potential application of the protein-based 3D WGM microlasers in sensing ionic concentration in water, and stimulus-responsive adjustment of lasing actions in aqueous environments is investigated by merely changing Na2SO4 concentration of aqueous solution.
By gradually increasing the concentration with a step of 8.33 × 10-4 mol/L, the central lasing peak of the protein-based microlaser express a linear blue-shift from ~ 611 to ~ 608.5 nm, as shown in Figs. 6(d)–6(f). That is, the lasing spectrum is responsively blue shifted 2.59 nm when the concentration of the Na2SO4 changed (from 0 to 5 mmol/L) by a step of ~ 0.4 nm per 0.83 mmol/L. This reflects that the diameter of microlaser becomes smaller and the effective refractive index increases slightly by hydrogel shrinkage and water extrusion out. It should be noted that the FSR of the coupled disks will be also affected by both the diameter and the effective refractive index of the host materials, and it is found that FSR stays at ~ 1.31 during the test processing with FWHM keeping around 0.2 nm and Q factor remaining at ~ 2000 to ~ 3000. Thus the stimulus-responsive “smart” adjusting with other main performance parameters remained fairly stable in the protein-based 3D WGM microlaser opens new opportunities for biologic and medical applications such as optical biosensing, diagnosis, tunable “smart” biophotonics.
4 Summary and outlook
In summary, we have introduced a homebuilt femtosecond laser processing system and showed our results of fabricating dye-doped microlasers with this homemade system in soft materials including polymer and protein. The oscillations have been achieved in a circular disk and a spiral-shaped disk that produces unidirectional lasing output with a low lasing threshold. Single-mode lasing have also been realized in the stacked PM microlaser by Vernier effect. The cavity-cavity interaction between the two disks in the PM-type microlasers has also been investigated systematically. In addition, we have presented a specific case of the produced protein microlasers operating well both in air and in aqueous solution at room temperature, but showing sensitive response to the Na2SO4 concentration. Our demonstrations of fabricating high-Q microlasers by the femtosecond laser processing technique will benefit the potential applications of microlasers in functional integrated optoelectronic and ultrahigh sensitive bio-sensing systems.