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Abstract
The provision of a coherent light source is a prerequisite for a variety of photonic integrated circuits. The integration of semiconductor laser diodes in disposable photonic devices in fields such as biosensing is, however, impeded by the competitive pricing in this application area. In this work, we demonstrate lasing of an alternative laser light source, namely an integrated hybrid organic solid-state distributed feedback laser for a silicon nitride photonic platform. The laser is optically pumped with a high power 450 nm laser diode and emits in the visible at 630 nm into a waveguide taper to reduce the cross-section to a single mode geometry. Inkjet printing of the organic gain medium enables a local, cost-effective, and flexible processing technology. The fabrication of the presented coherent light source is CMOS compatible and therefore highly interesting for co-integrated sensing platforms.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Photonic integrated circuits (PICs) have been successfully employed to implement advanced active and passive functionalities in data- and telecommunication [1,2], and have generated much interest in new fields such as optical coherence tomography [3], quantum computing [4], and chemical sensing [5]. The replacement of bulk optics with compact integrated components provides a photonic platform for a multitude of applications. Among these, the sensing of biomolecules in life-science and point-of-care diagnostics is an emerging use case [6]. These applications require a highly cost-optimized PIC, which holds especially true for single-use devices. Even though the possibility to multiplex the detection of many targets on a single lab-on-chip [7,8] is envisaged, which results in an immediate reduction of relative costs, a photonic integrated measurement platform has to compete against well-established alternative detection methods [9]. A major challenge of PICs is the cost-effective provision of an ultra-low cost integrated laser light source. The hybrid integration of a semiconductor laser diode with a PIC requires advanced fabrication steps. An alternative are organic solid-state lasers (OSSLs), which provide laser light emission in the visible to near-infrared [10,11]. Furthermore, the flexibility in the selection of the emission wavelength arising from the wide gain bandwidth of organic dyes is a highly interesting property, because it facilitates the integration of multiple laser sources with different emission wavelengths on a single PIC. Ultra low threshold OSSLs have successfully been demonstrated employing mixed order Bragg grating designs in a surface emitting configuration [12,13]. However, the application of OSSLs in PICs requires in-plane emission of the laser light directly into the waveguides. Moreover, bulky and costly research grade pump lasers to excite the OSSLs are not feasible for point-of-care or hand-held devices. OSSLs employing alternative pump sources such as laser diodes (LDs) [14] and light emitting diodes (LEDs) [13,15] have been demonstrated.
A particular PIC platform providing low loss in the visible to near-infrared wavelength region is based on silicon nitride ($\textrm {SiN}$), a well-established dielectric in semiconductor fabrication. SiN allows for co-integration of optical waveguides with electronics through a low-temperature plasma enhanced chemical vapor deposition (PECVD) process [16–18]. The distinct refractive index contrast to a silicon dioxide ($\textrm {SiO}_2$) cladding enables a high integration density with low bend-radii and compact photonic components. The hybrid integration of a multimode emission OSSL on a SiN photonic platform pumped by a frequency-doubled Nd:YLF pulsed laser has been demonstrated by Kohler et al. with a spiral resonator design [19].
In this work we present an integrated silicon nitride organic hybrid ($\textrm {SiNOH}$) distributed feedback (DFB) laser with in-plane emission into a single mode optical waveguide. This SiNOH DFB laser is implemented with a second order Bragg grating to facilitate cost-efficient high volume production. The DFB structure ensures single mode lasing. An organic dye embedded in a polymer host is locally applied to the lasing structure through inkjet printing technology. The gain medium is optically excited by means of a high power blue laser diode.
2. Design and fabrication of the SiNOH laser
2.1 Design of the DFB cavity
The grating period $\Lambda$ of a DFB laser of order $N$ showing resonance at $\lambda _0$ and effective refractive index $n_{\textrm {eff}}$ of the guided mode is given through the Bragg condition [20]
A second-order Bragg grating ($N=2$) for the employed SiN photonic platform has a grating period of $\Lambda =415\;\textrm{nm}$ and a duty cycle of $25\%$ to enable lasing around 630 nm in the TM-like waveguide mode. The dimensions of the Bragg grating were set to 300 µm×50 µm. The elongated laser diode pump spot was adjusted to match the dimensions of the grating. Figure 1(a) depicts the geometry of the demonstrated SiNOH DFB structure. The top $\textrm {SiO}_{2}$ layer has a local opening of 300 µm×75 µm and provides a well for inkjet printing. The SiNOH DFB output waveguide is linearly tapered from 50 µm down to 400 nm to ensure fundamental waveguide mode operation. The condition for single mode operation around the lasing wavelength was verified with an eigenmode solver (PhotonD, FIMMWAVE). According to these simulations waveguide widths below 450 nm ensure single mode operation for both polarizations. The laser emission is routed to the sample edge for edge-coupling into a fiber waveguide. An S-bend structure is used to generate a lateral offset of 600 µm. This lateral offset ensures that only guided light confined in the single mode waveguide is coupled into the fiber and spurious background from the laser is minimized.
Fig. 1. (a) Geometry of SiNOH DFB laser. In-plane laser light emission is coupled into a waveguide taper ($L_{\textrm {taper}}=1000$ µm) to convert the waveguide mode to a single mode cross-section. The optical signal is routed to the edge of the sample where the emission is coupled to a fiber. The routing includes a lateral offset ($\Delta x=600$ µm and $\Delta z=1400$ µm) with an S-bend structure to avoid spurious detection of unguided laser light. An opening of the silicon dioxide top oxide ($h_{\textrm {TOX}}=4$ µm) cladding provides a well to deposit the dye-doped polymer through inkjet printing. (b) Microscope image and (c) fluorescence image of a SiNOH DFB laser with five layers inkjet printed dye-doped polymer films in the well.
Radiative losses from first-order diffraction of a second-order DFB grating lead to an intrinsic longitudinal mode discrimination [21,22]. In contrast to first-order gratings, no phase-element for single-mode operation is needed. Even though second order gratings are commonly employed in surface emitting devices, the in-plane laser light emission is highly efficient since the gain threshold is asymmetric around the band gap [22].
2.2 Device fabrication
The device stack of the laser PIC on top of a silicon substrate consists of three layers, i.e. $\textrm {SiO}_{2}$ buried oxide (BOX), silicon nitride, and $\textrm {SiO}_2$ top oxide (TOX). The BOX and TOX layers have a thickness of 5 µm and 4 µm, respectively. The SiN layer has a thickness of $h_{\textrm {WG}}=160\;\textrm{nm}$. The deposition of the BOX and waveguide layer are based on a low temperature PECVD process. For the TOX cladding a sputtering process was employed. All processes are CMOS compatible for future co-integration with electronics. The waveguide structures are defined by a 248 nm deep-UV lithography scanner production line and subsequent reactive ion etching (RIE) [23]. Local windows in the TOX are generated by a lift-off process. Scanning electron microscopy images revealed that the RIE process leads to a sample edge tilted by 5°. The refractive indices of the $\textrm {SiO}_{2}$ cladding and $\textrm {SiN}$ waveguide layer at a wavelength of 630 nm were measured by ellipsometry resulting in $n_{\textrm {SiO2}}=1.48$ and $n_{\textrm {SiN}}=1.93$, respectively.
The organic dye 2-(4-(bis(4-(tert-butyl)phenyl)amino)benzylidene)malononitrile (PMN) [24] and optical quality poly(methyl methacrylate) (PMMA) (Polycasa GmbH, Acryl G77, $n_{\textrm {PMMA}}\approx 1.49$) were dissolved in 1,4-dioxane (Roth, $\geq 99.5\%$). The dissolving of the polymer was conducted at 50 °C under continuous stirring for 12 h. The PMMA solution and the organic dye solution were mixed to result in a total concentration of $4.0$ wt%, while the content of the organic dye in the solution was selected to reach a mass ratio in the final thin film of $5$ wt% organic dye mass to PMMA mass. This doping level of PMN was found to provide high material gain [24]. Higher levels of dye molecules lead to concentration quenching, which ultimately prohibits lasing. The solution was filtered with a 200 nm pore size filter directly into the print cartridge of the material printer (Fujifilm, Dimatix DMP-2850). The native drop volume of the cartridge is 10 pL. To fill the TOX windows, single rows of drops were printed along the center line length of each window at a drop spacing of 10 µm. As a single printed layer was not sufficient to achieve lasing a total number of $5$ layers were stacked to reach a suitable thickness. The printing process was conducted under cleanroom conditions. The residual solvent was removed by heating the samples on a hotplate to 110 °C for 1 min. The fluorescence microscope image in Fig. 1(c) shows the SiNOH laser with the TOX windows filled with the dye-dope polymer film employing the inkjet printing process. Two types of artifacts from the inkjet printing process have been observed, i.e. small particles and splashes of larger diameter. Both print artifacts should be taken into account in high density designs to avoid contamination of other components, e.g. sensing devices. Since the waveguides are buried in the surrounding cladding material, these printing defects do not deteriorate the performance of the PIC. The polymer is not homogeneously distributed inside the well and therefore the layer thickness cannot be consistently specified over the whole grating area. Measurements with a stylus profilometer (KLA-Tencor, Alpha-Step IQ) showed an average polymer height of approximately 350 nm in the center of the well.
To investigate the tuning range of the SiNOH DFB laser, Bragg gratings with grating periods $\Lambda$ between 363 nm to 428 nm have been fabricated. In this configuration the gain material was spin-coated onto the sample to create a uniform film of 500 nm thickness. The optimal thickness of the gain layer has been determined by eigenmode calculations. In these calculations, the pump light was assumed to decay exponentially inside the gain material layer due to absorption following the Beer-Lambert-Bouguer law. This creates a gain profile in vertical direction inside the gain layer. The local gain was assumed to be proportional to the local pump intensity, i.e. saturation effects were neglected. Thin film transmission measurements (Perkin Elmer, Enspire 2300) on glass slides with a dye-doped polymer layer revealed an attenuation coefficient of 1.151 µm−1. Figure 2 depicts the normalized modal gain derived from the eigenmode calculations as a function of the thickness of a $5$ wt% PMN-doped PMMA layer. The modal gain was normalized to the maximum gain value. For a film thickness larger than 500 nm the modal gain starts to decrease because the pump light is absorbed far away from the waveguide surface, which reduces the gain experienced by the evanescent tail of the guided mode in the SiN waveguide.
Fig. 2. Numerically determined dependence of the modal gain on the thickness of a $5$ wt% PMN-doped PMMA layer. The modal gain is normalized to the maximum gain.
2.3 Optical setup
Optical excitation of the dye-doped polymer was achieved by a blue 450 nm multimode high power TO-can laser diode (Nichia, NDB7K75). The emission characteristics of the laser diode leads to a pump stripe on the sample surface of 415 µm × 45 µm without the need for a cylindrical lens. The LD was driven by a pulsed current source (PicoLAS, LDP-V 50-100) to generate optical pulses of 35 ns at a repetition rate of 3 Hz. The pump intensity was varied with a combination of a $\lambda /2$-plate and a linear polarizer reaching up to 50 kW cm−2. The optical setup allows for a rotation of the pump light polarization to investigate the influence of the pump polarization on the lasing properties. Visual inspection and alignment of the pump stripe with the SiNOH DFB cavity was achieved through a digital microscope. The details of the optical setup are described in [24]. The laser light emission is routed to the sample edge and coupled into a multimode fiber (NA $0.22$, 50 µm core). The output emission was analyzed by a fibre-coupled CCD-spectrometer (Ocean Optics, FLAME-S-VIS-NIR-ES). To measure the polarization of the laser light the slow axis of a polarization maintaining fiber (NA $0.12$, 3.3 µm) was aligned with the sample edge. With a fiber bench and polarization filter the SiNOH laser light polarization was determined.
3. Results and discussion
3.1 Laser light emission
Figures 3(a) and 3(b) summarize the emission properties of the SiNOH DFB laser with an inkjet printed gain medium. The emission wavelength is 629.9 nm with a full-width-at-half-maximum (FWHM) of better than 1.5 nm, limited by the spectrometer resolution. No spurious pump light at the emission wavelength of the pump source at 450 nm was observed. The lasing threshold depends on the pump light polarization. For the pump light polarization perpendicular to the long axis of the grating it amounted to 11 kW cm−2 and for the pump light polarization parallel to the long axis of the grating to 15 kW cm−2. The laser light emission was in the TM-like waveguide mode arising from the larger overlap of the evanescent field with the gain material for this polarization mode. For the perpendicular pump polarization, the pump intensity was limited to 40 kW cm−2 to avoid saturation of the CCD spectrometer by the emitted laser light. At a pump intensity of 40 kW cm−2 with perpendicular pump light polarization the fluorescence background was more than 20 dB lower than the laser light emission. The collection of the laser light at the end-facet was highly sensitive to the lateral position of the fiber with respect to the single mode SiN waveguide. This verifies that only light guided in the single mode waveguide was coupled to the fiber. The fact that no higher order longitudinal Fabry-Pérot modes were observed indicates that back reflections potentially arising either from the sample end facet or the transition between the polymer and the $\textrm {SiO}_{2}$ covered waveguide section did not affect the DFB lasing properties.
Fig. 3. (a) Output power characteristics to determine lasing threshold and (b) emission spectrum of SiNOH DFB laser with inkjet printed gain medium. The doping-level of the organic dye in the PMMA layer was set to $5.0$ wt%. The optical pulse width of the 450 nm laser diode pump source was set to 35 ns at a repetition rate of 3 Hz.
The optical excitation of the gain material with the high intensity blue laser light leads to a photoinduced degradation. Therefore, the OSSL emission power gradually decreases. At a pump intensity two times above the lasing threshold, the SiNOH DFB laser emission drops to half its initial value after approximately 25 000 pulses. It is expected that encapsulation of the SiNOH OSSL could significantly prolong the lifetime by creating an oxygen barrier [25].
Figure 4(a) shows normalized emission spectra of SiNOH DFB laser structures with different grating periods. The SiNOH DFB laser design together with the employed small molecule dye PMN embedded in PMMA allows for an emission wavelength from 572 nm up to 646 nm. Lasing was not achieved for grating periods below 376 nm. A linear relationship between the grating period and Bragg wavelength in Eq. (1) was observed. Figure 4(b) summarizes the emission wavelength and lasing threshold for different grating periods with a perpendicular pump polarization. The lasing threshold for a SiNOH DFB laser with a uniform spin-coated film at the same grating period ($\Lambda =415\;\textrm{nm}$) as the inkjet printed gain material is 5 kW cm−2. The lower lasing threshold observed for the spin-coated film can be attributed to the better film quality and the optimal film thickness.
Fig. 4. (a) Normalized spectra, (b) corresponding laser light emission wavelength and lasing threshold of SiNOH DFB laser of different grating periods. The gain medium PMN:PMMA allows for a wide tuning range from 572 nm to 646 nm. The gain medium was spin-coated onto the sample to achieve a uniform thin film. Error bars indicate the standard deviation from five samples.
4. Conclusion
In this work, we demonstrated an integrated silicon nitride organic hybrid DFB laser for low-cost sensing devices. The DFB laser operates at a single cavity mode in TM-like polarization, enabling the application in highly sensitive integrated interferometric sensors, i.e. Mach-Zehnder interferometers. The emission wavelength was 630 nm. The dye-doped polymer gain medium was locally applied by an inkjet material printer. Inkjet printing offers both minimal material usage and additional flexibility compared to an all-over spin-coating. Excitation was achieved by a low cost 450 nm laser diode, replacing expensive and bulky pump sources not suitable for real world applications. The lasing threshold was 11 kW cm−2. For the employed PMN:PMMA gain material the range of achievable emission wavelengths obtained from SiNOH DFB laser structures with different grating periods was larger than 74 nm.
The lasing threshold of the inkjet printed OSSL was more than two times higher than that of the OSSL fabricated with spin-coating. An optimization of the inkjet printing process could potentially lower the lasing threshold. This optimization should investigate other solvents and host polymers, surface pre-treatment, and environmental parameters which influence the quality of the polymer film during inkjet printing.
Funding
Österreichische Forschungsförderungsgesellschaft (850649).
Disclosures
The authors declare no conflicts of interest.
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