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We have demonstrated an optically pumped polymer microring laser fabricated by two photon polymerization (TPP) of SU-8. The gain medium is an organic dye (Rhodamine B) doped in SU-8, and the laser cavity is a double coupled microring structure. Single mode lasing was obtained from the two coupled rings each with 30 µm and 29 µm radii using Vernier effect. Low laser threshold of 0.4 µJ/mm2 is achieved using 1 µm wide polymer waveguides and the quality factor is greater than 104 at 612.4 nm wavelength. The lasing remained stable with pump energies from threshold to energies as high as 125 times the threshold.
© 2015 Optical Society of America
1. Introduction
Integrated photonic elements based on organic materials have tremendous potential because of their low cost, ease of fabrication, mass production capability and biocompatibility. These molecules give us the flexibility to design and engineer them for specific applications [1]. Microcavity organic laser as an essential photonic device can operate at low thresholds and has narrow line width, wavelength tunability as well as a small footprint [2].
Numerous cavities that support stimulated emission in organic based gain materials have been investigated such as grating and photonic crystal cavities [3–6], microcapillary, microdisk, microring and microtoroid cavities [7–10]. In addition to solid state lasers, microfluidic lasers have also been fabricated based on microresonators [11–14]. Amongst the mentioned resonators, microring and microdisk cavities as (whispering gallery mode (WGM)) resonators can be easily integrated on organic microphotonic chips and be coupled into waveguides. High quality factors (Q-factor) and small mode volumes of WGM resonators yield low threshold and narrow line width microlasers [10,15–17].
Microring resonators work at a lower threshold and higher stability due to the WGM confinement and the smaller mode volume [18]. However despite these benefits, single microring dye lasers support many longitudinal modes. To eliminate this drawback, Vernier effect is applied using two adjacent microring cavities [14,19,20].
In this research we have fabricated microring, microdisk and double microring lasers using a femtosecond laser direct-writing (FLDW) on Rhodamine B (RhB) doped SU-8 polymer films. Two photon polymerization (TPP) using FLDW method is an emerging and cost effective technique to easily create 2D and 3D photonic structures [10,15,21,22]. Using this technique, we have successfully fabricated single mode and low threshold dye-doped polymer lasers with high performance based on narrow double microring resonators. The comparison of spectral lasing properties of microdisk, microring and double microring resonators demonstrate reduction in the spontaneous emission and pump threshold of the double microring lasers.
This on-chip double ring polymer laser demonstrates the flexibility of the TPP technique to fabricate active integrated photonic structures. This method can be used to simply and rapidly fabricate single mode micro cavity lasers with low threshold. We have performed a detailed investigation of the Vernier effect in a double ring microlaser. One of the features that stand out in our study is the high Q-factor which is one of the highest obtained in double micro-ring structures.
2. Experiment
To produce the gain medium, we dissolved 1 mg of RhB dye in 1 ml of Gamma-Butyrolactone. The solution was then added to 1.4 mg of SU-8 (Gersteltec GM 1070). Fused silica substrates were cleaned and heated at 200 °C for 10 minutes. The mixture then was spin coated on fused silica substrates at a speed of 1600 rpm for 40 seconds to produce layers with a thickness of 2 ± 0.1 µm. The film was prebaked first at 65 °C for 1 minutes and then at 95 °C for another 1 minutes.
The beam from a tunable femtosecond laser (80 MHz repetition rate, 120 fs pulse width and 730 nm central wavelength) was focused on the film using a 40x microscope objective. To create features with 1 µm width, laser power was adjusted to be 20 mW after the microscope objective. The waveguide width is directly related to the laser power. A computer controlled X-Y stage with a resolution of 50 nm was used to directly write the microring and microdisk patterns on the film. The writing speed was 20 µm/s and it takes less than twenty seconds to write a double ring structure. After writing the features on the film with the femtosecond laser beam, it was post baked at 65 °C for 5 minutes and then at 95 °C for another 5 minutes. The unexposed SU-8 was removed by the developer and the microring and microdisk were obtained as shown in Fig. 1. Finally, the samples were hard baked at 130 °C for 2 hours. Figure 1(a) illustrates double microring structure with r1 = 29 µm and r2 = 30 µm radii. Figure 1(b and c) depict the top view and cross section of the interaction region respectively. The waveguides are 1 µm in width and 2 µm in depth and the separation of the two rings is in the order of lasing wavelength (λ~612.4 nm).
Fig. 1 (a) Scanning electron microscope (SEM) image of double microring laser with 29 µm and 30 µm radii fabricated using femtosecond laser direct writing on dye doped polymer (b) SEM image of the interaction region of the two adjacent microrings (c) Top of view SEM representation of the two microrings at the interaction region. Waveguides with 1 µm width are separated by a 650 nm air gap (d) Schematic of the experimental setup used for laser emission characterization. The green pump pulse passes through a wave plate and polarizer for power adjustment a beam splitter for energy measurement. The sample’s emission and the pump beam collected by a 20x objective passes through a notch filter to filter the pump light, then a 10x objective focused on the sample, couples the emission to a fiber attached spectrometer.
Figure 1(d) shows a schematic diagram of the experimental setup that we use to measure the laser characteristics such as lasing wavelength and threshold. A frequency doubled Nd:YAG pulsed laser (1 Hz, 20 ns pulse width and 532 nm wavelength) excites the sample normal to the substrate surface with an excitation spot of about 1 mm diameter. A 20x microscope objective collects the scattered light from the microring cavity. This light is picked up by a 10x objective lens connected to a multimode fiber which is sent to a diode array spectrometer. The pump light is filtered out by a notch filter.
3. Results and discussion
When pumped by 532 nm pused laser, the double ring microcavity (r1 = 29 µm and r2 = 30 µm) shows a single mode laser emission at the wavelength of 612.4 nm with a full width at half maximum (FWHM) of 0.6 nm, which is the resolution limit of the spectrometer. Figure 2 shows the laser spectra at 1, 2, 4, 5, 8 and 10 µJ/mm2 of the pump intensity. The laser is very stable at pump energies from threshold to as high as 125 times the threshold energy.
Fig. 2 Single mode lasing spectra of the double microring laser with radii of 29 µm and 30 µm for various pump energies. The lasing wavelength is centered at 612.4 nm with 0.6 nm FWHM (0.6 nm spectrometer resolution) and is highly stable with the increasing pump energy. The minor peaks at the wavelengths of 627 nm and 634.5 nm are present at the pump energies higher than 10 times the threshold.
Using the Vernier equation for the calculation of free spectral range (FSR), Eq. (1) [14], for λ = 612.4 nm, neff = 1.56, r1 = 29 µm and r2 = 30 µm, we have calculated the FSR to be 36 nm.
This relatively large FSR leads to presence of only one dominant mode under the gain spectrum. Besides the fundamental lasing mode, there are two minor modes at the wavelengths of 626 nm and 635 nm. These modes that are present at higher pump energies as high as 10 times the threshold energy are attributed to higher order transverse modes.
Figure 3 reports data of spectrally integrated emission intensity as a function of excitation pump energy. The intercept of the fitted line determines the laser threshold to be as low as 0.4 µJ/mm2. This lies among the lowest thresholds achieved for microcavity dye lasers [8,21,25]. We have determined the laser threshold for single microring and microdisk lasers to be 0.8 µJ/mm2 and 1.1 µJ/mm2 respectively. The decrease in threshold energy is mainly due to the reduction in the number of transverse WGMs in microring laser compared to microdisk laser and also fewer numbers of longitudinal modes in double microrings compared to single microring. This is depicted in Fig. 4 which shows the spectra of three microlasers with pump energy of 50 µJ/mm2. It is also clear that the spontaneous emission in the microring structures has decreased significantly compared to microdisk structure as a consequence of mode volume decreasing and mode confinement in 1 µm waveguides. The double microring produces much more stable laser output even at low pumping powers and has a much lower lasing threshold.
Fig. 3 Spectrally integrated emission intensity of double microring laser as a function of pump energy. The solid line gives the lasing threshold to be 0.4 µJ/mm2.
Fig. 4 The spectra of (a) SU-8 doped with RhB microdisk, (b) single microring and (c) double microring lasers at 50 µJ/mm2 of the pump energy. The lasing threshold and spontaneous emission decreases from microdisk to single ring and to double microring. The double ring laser shows a much cleaner spectral behavior. The number of laser modes have been reduced for the double microring laser even at very high pump energies (125 times the threshold).
To theoretically explain lasing characteristics of single microring structure, we use Eq. (2). It describes unidirectional coupling between a waveguide and a ring resonator with radius r [23]. The circulating power I in the ring is given by
Where α is the attenuation coefficient of the guided-wave mode of the ring (zero loss: α = 0), neff is the effective refractive index of the ring modes which is calculated neff = 1.56 (this calculated by beam propagation method for waveguides with 2mµ × 1µm cross section and refractive index of n = 1.59 on fused silica substrate), λ is the vacuum wavelength and κ is the coupling coefficient between microring and waveguide. To explain lasing characteristics of double microring cavity, we make use of the Vernier effect as expressed by Eq. (3).
The circulating power I in a double ring structure with coupling coefficient κ is then given by the following equation which describes bidirectional coupling between two-ring resonator with radii r1 and r2 [24].
Figure 5(a) represents the wavelength-dependent microcavity characteristic for a double ring resonator configuration with radii of r1 = 30 μm and r2 = 29 μm, coupling coefficient and loss coefficient, κ2 = 0.12 and α = 0, respectively and the effective refractive index of fundamental mode equal to neff = 1.559. The dash-dot line shows the spectra of double microring cavity based on Eq. (3). The solid line depicts double microring laser emission spectra.
Fig. 5 (a) ASE spectrum of the RhB doped SU-8 thin film pumped by nanosecond green laser (dotted line) and dash-dot line shows the spectra of double microring cavity based on Eq. (2). The solid line depicts the experimental measurements of the double microring laser emission spectra. (b) The calculated (solid line) and measured (dashed line) spectra of fabricated single mode laser.
Amplified Spontaneous Emission (ASE) spectrum of the dye (RhB) doped SU-8 thin film is shown in Fig. 5(a). The calculated longitudinal resonant peaks (dash-dot line) with a relatively wide spectral separation (about 40 nm) are shown in the figure. Only the mode near the peak of the ASE spectrum is lasing. It satisfies the single mode lasing condition at the wide range of the pump energies even to the extent I pump = 125 I threshold. The experimental output of double ring lasing in the Fig. 5(a) (solid line) is fully supported by the theoretical calculation.
Figure 5(b) is the expanded view of the laser peak. The main longitudinal mode and side modes are clearly visible (dashed line). For theoretical calculation from Eq. (3) the experimental data are in agreement with the calculated results such that the ratio of the first side mode intensity to the main mode intensity is about 0.1 at both curves. This ratio is strongly dependent on coupling coefficient (κ). Since fitting a curve to experimental data gives κ2 = 0.12 (Fig. 5(b)), the Q-factor can be calculated from the Eq. (4) [23,24]:
Thus the Q-factor of our double ring cavity was obtained 1.2 × 104 experimentally. Using beam propagation method (BPM), we have calculated κ2 = 37 × 10−4 for the first order mode. It results the Q-factor 4 × 105 which is bigger one order of magnitude from experimental calculation. It demonstrates the high Q-factor for single mode double ring microlaser which is comparable with the best results achieved from WGM structures produced by other means producing Q-factor of 106 and even greater [15,25]. The Q-factor of WGM can be roughly calculated according to the spectral parameters, Q = λ/δλ where λ is the resonance wavelength and δλ is the FWHM. If FWHM can be measured accurately, then we can simply calculate the Q-factor. But the FWHM of laser is in order of 0.1 nm or smaller in many cases. On the other hand, the resolution of spectrometer device is limited (in our case is 0.6 nm). This limits the Q-factor value to the spectrometer resolution. It means that the calculation of Q-factor according FWHM may be off by as much as several order of magnitude, unless we are sure that the spectrometer resolution is less than FWHM of laser.
4. Conclusion
In conclusion, by use of TPP procedure on dye (RhB) doped SU-8 polymer films, we have been able to fabricate microdisk, microring and double microring resonators. These results are offering single mode and low threshold lasing (0.4 µJ/mm2) from double ring high Q-factor (Q~104) microcavity. The lasing wavelength is highly stable from the threshold pump energy to the energies as high as 125 times the threshold energy. These on-chip double ring polymer lasers demonstrate high flexibility of the TPP technique to fabricate active integrated photonic structures. Single mode and low threshold dye-doped polymer laser is achievable and can used as an on-chip polymer microcavity solid state laser.
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