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Abstract
A simple one-step approach to producing a distributed feedback (DFB) laser through selective irradiation of the gain medium, MEH-PPV, is presented. Electron irradiation alters the refractive index of MEH-PPV, thus, direct patterning by electron irradiation can be applied to create a periodic diffraction grating. The non-irradiated regions of MEH-PPV serve as the primary gain medium, while the irradiated regions of MEH-PPV provide the refractive index difference required to fabricate a DFB laser. This method was successfully applied to achieve lasing with a relatively low lasing threshold of 3 kW/cm2or 1.8 µJ/cm2 (pulse width: 600 ps). Furthermore, the lasing wavelength can be finely tuned by simply adjusting the grating period. In stark contrast to the simple one-step process described in this work, conventional procedures for the fabrication of DFB lasers involve multiple steps of varying complexity, including mold creation and careful coating of the substrate with the gain medium.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Conjugated polymers have been used for the purpose of fabricating optoelectronic devices since the early 1990s [1–3]. Conjugated polymers emit light with wavelengths that are dependent on the lengths and structures of their chains. The drive to implement polymers into light-emitting devices, such as lasers, stem from the advantages the polymers have over their inorganic counterparts; polymers are cheap and are easy to produce [4]. Additionally, light-emitting conjugated polymers have large stimulated emission cross-sections [5,6], fast response times [7–9] and excellent waveguide properties [10,11], making them a highly appealing gain medium. Lasers can, thus, be based on conjugated polymers, and such lasers can be applied to optical sensing [10,12–15], data relay [16–18], optical diagnostics [10,14,19,20], digital imaging [19,21,22] and interferometry [19,23].
Due to the processing flexibility of polymers, lasers based on them are diverse in the resonator applied; these organic lasers can utilize distributed Bragg reflectors (DBR) [22,24–27], planar microcavities [2,3], microrings [20,28], microbubble cavities [29–31] and others to achieve lasing. Out of the listed, one of the most common resonators used is the DBR. A DBR requires a periodic nano-scale diffraction grating with alternating refractive indices (n1, n2). Such nano-structure causes light within to backscatter and provide the necessary feedback for lasing. The wavelengths that can achieve such feedback within a DBR can be determined with the Bragg’s law:
where m is the diffraction order, λ is the emission wavelength, neff is the effective refractive index of the system and Λ is the grating period. In the case of second order diffraction (m=2), outcoupling can be achieved both perpendicular and parallel to the direction of feedback. Since it is easy to achieve clean outcoupling from the surface, second order operation is commonplace in lasing research [22,24–27].Currently, many works regarding the fabrication of distributed feedback (DFB) lasers involve a similar procedure. A diffraction grating mold is fabricated via irradiation coupled with a positive or negative resist [22,24–26]. The mold is then used to fabricate a diffraction grating on a substrate by either thermal nanoimprinting [22,26] or coating the mold with the substrate then curing and peeling [24,25]. Once the diffracting grating has been made, it is coated with the gain medium [24–26]. The gain medium provides the photon generation and the substrate provides the necessary refractive index difference for feedback. The completed system is then pumped or excited with a power source, a UV laser in many cases, to achieve lasing.
The described fabrication procedure is not simple as it requires multiple steps of varying complexity. The mold fabrication process requires a delicate development process that is highly sensitive to factors such as development-solvent concentration, temperature and development time. Due to the nature of the process, it is quite difficult to make minute adjustments to the grating period without having to undergo the arduous process again. Although the mold has the benefit of being able to be used multiple times, it is confined to being used with a specific gain medium and a substrate as the mold was finely tuned to be compatible only with them; the grating period of a mold is one of the factors that determine the lasing wavelength, and it is important for efficient lasing that the lasing wavelength matches the peak emission wavelength of the gain medium. Furthermore, extra care must be taken to ensure that the gain medium strongly adheres to the diffraction grating as gaps could cause the DBR to malfunction, ultimately causing the entire lasing system to fail.
In this paper, we propose a simple approach to producing a DFB laser by direct electron-beam patterning of the gain medium. Electron irradiation alters the molecular weights of polymers [32], and, in turn, this alteration changes the refractive indices of the polymers, allowing a working diffraction grating to be directly created via electron irradiation. Since the aforementioned sensitive development process can be foregone, minute adjustments to the grating period can be efficiently achieved. By employing this electron irradiation approach, a one-step procedure can be used to fabricate a DFB laser with a highly precise grating period from just a gain medium. There have been other one-step methods requiring only a gain medium such as those involving the process of surface corrugation [33,34]. However, with these conventional one-step approaches, precise control of the grating period is difficult.
2. Experimental section
2.1 Sample preparation
Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) (Mn = 40,000∼70,000) was purchased from Sigma-Aldrich and was utilized as the gain media for lasing. To determine the effect of electron irradiation on the polymer by Fourier-transform infrared (FTIR) spectroscopic assessment, the polymer was first pulverized using a mortar and pestle, and the resulting powder was suspended in deionized water (DI) and sonicated for 20 min. The suspension was drop-casted onto a square metal mold with a side length of 4 cm and was then left to dry, allowing a thin polymer film to form. To achieve thorough irradiation, films were prepared with a thickness of less than 10 µm since the range of 30 keV electrons is near 15 µm. The samples were then homogenously irradiated under vacuum (10−6 Torr) with an electron beam generated using a thermionic electron gun. The energy and the spot size of the electron beam was 30 keV and 5 cm, respectively. To prevent thermal damage to the samples, the current density of the electron beam was set to a low value of 0.5 µA cm−2 and the temperature inside the chamber was maintained at 0 °C using a chiller. The electron fluence delivered to the samples ranged from 4 × 1016 electrons/cm2 to 1.6 × 1017 electrons/cm2. After the irradiation procedure the films were re-pulverized using a mortar and pestle and stored in plastic vials.
To test the patterning precision of the electron-beam lithography system (Vega3 Elphy quantum lithography system, resolution: 2 nm), PMMA (C4 positive resist, Microchem) and MEH-PPV dissolved in benzene were spin-coated (5000 rpm, 4 wt %) to a thickness of 250 nm onto silica substrates coated with silver (≥99.99% purity, Sigma-Aldrich); since electron is negatively charged, the substrates were coated via sputtering (MSP-30 T) to prevent charge build-up during irradiation to mitigate charge-induced repulsion and enhance patterning precision. The PMMA and MEH-PPV samples were then patterned using the electron-beam lithography system and developed in MIBK/IPA and benzene/IPA solutions, respectively, at 5 °C for 5 min.
To develop a DFB laser, MEH-PPV was again spin-coated to a thickness of 250 nm onto a silver-coated silica substrate. The sample was then patterned via electron-beam lithography in the same manner that the PMMA sample was patterned. Figure 1 shows the overview of the fabrication process.
2.2. Characterization
The PL spectra of the samples were measured with a photoluminescence (PL) spectrometer (RAMBOSS-Star) equipped with a Nd:Yag laser (wavelength 355 nm; pulse width: 600 ps). Excitation and detection were done orthogonal to the sample surface with a detection cone angle of 12°. The spot diameter of the excitation laser was set to 250 µm. An ellipsometer (M2000D Woollam) with the spot diameter set to 30 µm was used to measure the refractive indices of pristine and irradiated samples. To assess the effect of electron irradiation on MEH-PPV, a FTIR spectrometer (Thermo Fisher Scientific Instrument, Nicolet IS50 FTIR) was used in attenuated total reflection (ATR) mode to obtain the FTIR spectra of both pristine and irradiated MEH-PPV samples. To investigate the patterning precision of the electron-beam lithography system, the irradiated and developed PMMA and MEH-PPV samples were analyzed using an atomic force microscope (AFM, Park Systems XE-70) in tapping mode. All microscopic images were obtained with an optical microscope (OM, Olympus BX53M).
3. Results and discussion
3.1. Irradiation-induced changes to MEH-PPV
Figure 2 shows the refractive indices of both pristine and electron-beam irradiated MEH-PPV. It was observed that electron irradiation significantly reduces the refractive index of the polymer at wavelengths between 560 and 600 nm; the reduction in refractive index becomes most apparent at the wavelength of 573 nm with the refractive indices of the pristine MEH-PPV sample and the MEH-PPV samples irradiated with fluences of 4 × 1016 electrons/cm2, 8 × 1016 electrons/cm2 and 1.6 × 1017 electrons/cm2 being 2.2, 1.75, 1.68 and 1.64, respectively. A greater refractive index difference leads to a higher feedback efficiency, thus, a significant difference is desirable. The discrepancy in refractive index shown between the pristine and irradiated (4 × 1016 electrons/cm2) MEH-PPV is sufficient as other previous works have achieved efficient lasing with smaller discrepancies [25,26]. Additionally, since delivering a lower dose of radiation is much more economical due to the costly nature of the electron beam lithography process, it was decided that the electron fluence of 4 × 1016 electrons/cm2 is sufficient for the production of a DFB laser.
Electron irradiation not only affects the refractive index of MEH-PPV, but it also affects the PL property of the polymer. Figure 3(a) shows the PL spectra of pristine and irradiated MEH-PPV. It was observed that electron irradiation causes notable reduction in PL emission intensity of the polymer. Slight blueshift and peak broadening can also be seen from the spectra. As stated earlier, the fluence of 4 × 1016 electrons/cm2 was deemed sufficient for DFB lasing, thus, the spectra obtained from the pristine sample and the sample irradiated with that fluence were averaged and shown as a single combined spectrum in Fig. 3(b). Assuming a one to one ratio between pristine and irradiated MEH-PPV, it is very evident from the combined spectrum that the PL peak from the pristine component dominates. In other words, peak emission wavelength is determined by the pristine component and unaffected by the irradiated component. This fact is important to consider for efficient lasing, therefore, will be revisited in a later section.
Fig. 3. (a) PL spectra of pristine and irradiated MEH-PPV and (b) a combined (pristine and 4 × 1016 electrons/cm2) PL spectrum.
The FTIR spectra of pristine and irradiated MEH-PPV are shown in Fig. 4. The spectra show that irradiation causes reduction of peaks corresponding to –CH3 asymmetric stretching at 2953 cm−1, alkyl -C-H stretching at 2925 cm−1, -CH2 symmetric stretching at 2865 cm−1, aromatic C = C stretching at 1502 cm−1 and 1461 cm−1, vinyl C-H bending in plane at 1409 cm−1, symmetrical alkyl CH2 deformation at 1349 cm−1, aromatic ethers stretching at 1251 cm−1, skeletal C-C vibration at 1201 cm−1 and C-O stretching at 1037 cm−1. These peak diminutions indicate that the electron irradiation process causes a large portion of these aforementioned chemical bonds to break, decreasing the overall conjugation length and the molecular weight of MEH-PPV. As the molecular weight decreases, the density and the refractive index of the polymer follow suit [35–37]. In other words, electron irradiation causes the molecular weight of MEH-PPV to decrease, ultimately resulting in the refractive index of the polymer to decrease as well. Furthermore, the reduction in conjugation length leads to the energy difference between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) to increase. This increase manifests itself as the observed emission intensity reduction, blueshift and peak broadening phenomenon in the PL spectrum of MEH-PPV when the polymer is irradiated with electrons.
3.2. Electron-beam patterning
The lasing wavelength can be altered by adjusting the grating period of a DFB laser. As stated previously, peak emission wavelength is a highly important factor in determining the lasing efficiency. For optimal efficiency, it is crucial that the lasing wavelength aligns well with the peak emission wavelength of the gain medium. Assuming that the gain medium used in this study is equal parts pristine and irradiated MEH-PPV, the peak emission wavelength of the medium is near 573 nm as can be seen in Fig. 3(b). The refractive indices of pristine and irradiated MEH-PPV at this wavelength are 2.2 and 1.75, respectively. Inputting these values into Lumerical Solutions simulating a silver/MEH-PPV/air three-layer setup, it was determined that the effective refractive index of the complete system is 2.0 for single mode operation. Additionally, the DFB laser was designed to operate in second order (m = 2). By employing the Bragg’s law, it was found that the grating period of 286 nm produced a lasing wavelength that matched the peak emission wavelength of 573 nm.
It is difficult to determine if a polymer sample has been patterned properly via electron beam lithography. This issue is magnified when nanoscale patterning is concerned as it becomes near impossible to visually verify the veracity of the patterning process; it is not possible to visually identify patterns with lengths less than the wavelengths of visible light. Thus, the electron-beam lithography system used in this study was tested to assess the precision of the device. It was determined from simulating 30 keV electrons with MCNP6 that 30 keV is more than enough energy to allow thorough and precise irradiation at all depths up to 500 nm. Therefore, a PMMA sample was patterned with 30 keV electrons using the electron-beam lithography device and developed in a solvent to dissolve away the irradiated sections of the polymer. The pattern was designed to have a grating period of 286 nm. The AFM images and measurement of the developed PMMA sample are shown in Fig. 5(a). Each bar in the grating structure has a height of approximately 250 nm, indicating that the irradiation procedure is capable of delivering a radiation dose to all depths since PMMA was initially coated onto a substrate to a thickness of 250 nm to create the PMMA sample. Furthermore, the period was determined to be near 290 nm, which is sufficiently close to the desired value of 286 nm.
Fig. 5. AFM images and measurements of (a) a patterned PMMA sample developed in a 1:3 MIBK/IPA solution at 5 °C for 5 min. AFM images and measurements of MEH-PPV samples developed in (b) 1:3.8, (c) 1:3.5 and (d) 1:3.2 Benzene/IPA solution at 5 °C for 5 min.
The aforementioned process performed with PMMA was repeated using MEH-PPV. Although MEH-PPV is not optimal to be used as a positive resist, it was believed that the electron beam irradiation process provided enough contrast in material solubility. The pattern was again designed to have a grating period of 286 nm. Figure 5(b)-(d) shows the AFM images and measurements of the developed MEH-PPV samples. Various solvent concentrations were tested, and, although complete development was not achieved as the height of each bar is less than 250 nm, the grating period was measured to be near 290 nm.
3.3. Fabrication and characterization of DFB laser
A MEH-PPV sample was patterned in the same fashion that the PMMA sample was patterned; the pattern was designed with a grating period of 286 nm. Figure 6(a) shows the OM image of the patterned MEH-PPV sample. Electron irradiation causes MEH-PPV to darken, thus, the darker area is the region where the patterning occurred. As mentioned earlier, due to the grating period being smaller than the wavelengths of visible light, it is impossible to visually observe the grating structure. However, it can be assumed that the patterned area consists of sections that alternate between pristine and irradiated MEH-PPV. When the sample was pumped with an energy source (Nd:YAG laser), lasing was observed. Figure 6(b) shows the PL spectra of the patterned MEH-PPV sample at various pump intensities, and the insets show the brightness and the color of the emitted light. The lasing wavelength was located at 582 nm, which is relatively close to the peak emission wavelength of 573 nm. To determine the lasing threshold, the intensity at this lasing wavelength was plotted as a function of pump intensity (Fig. 6(c)). The plot was linearly fitted, revealing a reasonably low lasing threshold of 3 kW/cm2. Above this lasing threshold, a narrow peak (FWHM of 2 nm), resulting from stimulated emission, begins to manifest in the PL spectrum.
Fig. 6. (a) Microscopic image of electron-beam patterned MEH-PPV sample and an illustration of its structure. (b) PL spectra of the patterned MEH-PPV sample and the (c) corresponding output intensity as a function of pump intensity.
If the need to create a highly efficient lasing system can be forgone, the system can be finely tuned by adjusting the grating period. Unlike with the conventional approaches to fabricating DFB lasers that involve fabricating a mold, such adjustment can be done easily with the electron lithography system used in this work. Figure 7 illustrates the tunability the approach presented in this work offers. The figure shows the PL spectra of MEH-PPV samples patterned to have grating periods of 286 nm, 290 nm and 295 nm. As emphasized in the earlier section, the lasing efficiency is dependent on how well aligned the lasing wavelength is to the peak emission wavelength. Thus, the discrepancies in pump intensity required for comparable lasing intensities between the three samples result from the change in lasing efficiency as the lasing wavelength moves away from the peak emission wavelength.
Fig. 7. PL spectra of MEH-PPV samples with grating periods of (a) 295 nm, (b) 290 nm and (c) 286 nm.
4. Conclusion
We have shown that a DFB laser can be easily fabricated by simply patterning an organic material, MEH-PPV, with an electron beam. Electron irradiation causes chemical bond disassociations in MEH-PPV, leading to an alteration in the refractive index of the polymer. This change in refractive index allows a working DFB laser to be directly created by patterning the polymer via electron irradiation. Since the irradiation process reduces the PL emission intensity of MEH-PPV, the pristine sections of the gain medium serve as the primary light emitter, whereas, the irradiated sections provide the refractive index difference that is necessary in a DBR. By carefully adjusting the grating period, the lasing wavelength can be finely tuned. Furthermore, this process can be applied to other conjugated polymers that show the property of irradiation-induced RI change to achieve lasing. Unlike the many of the approaches employed in other works concerning DFB lasers, our methodology is a single-step process that only requires a gain medium. The proposed method is also different from the other single-step methods requiring only a gain medium in that it possesses the capability of creating a highly precise diffraction grating through the use of electron beam lithography. Therefore, we believe that the work presented here can be of a high value in any fields involving organic lasers.
Funding
National Research Foundation of Korea (NRF- 2020M2D8A206972712).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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