1. Introduction

Surface plasmons (SPs), which are excited at the interface between the dielectric and the metallic surface, effectively transform electromagnetic energy into surface plasmon polaritons and result in a localized field enhancement when the introduced frequency matches the plasmon frequency [1]. In recent years, SPs have attracted significant attention because of their remarkable properties. Their most important property is their capacity for subwavelength confinement, thereby enabling the miniaturization of photonic cavities [2–7,29]. Further, the localized field enhancement they cause can notably increase light emission [8–10]. Metal structures have been successfully introduced in nanocavity lasers by means such as surface plasmon amplification by stimulated emission of radiation involving a flat metal surface [11], a metallic grating layer [1], and a quantum cascade laser with a double-metal waveguide [12,13]. Additionally, polymer distributed feedback (DFB) lasers have attracted extensive interest owing to their broad emission spectra, low threshold values, and compactness [14–19]. Different fabrication techniques can be used to create DFB polymer lasers such as nanoprinting [20–22], electron beam lithography [23], reactive ion etching [24], direct writing [25, 26], and ultraviolet exposure [27]. As polymer DFB lasers are restricted by the diffraction limit, the cavity length (L) must be larger than half the emission wavelength as given by L>λ/2n_eff, in which n_eff is the effective refractive index. In contrast, SP lasing is not restricted by the diffraction limit and provides a new way for researchers to explore SP-enhanced DFB lasers. Moreover, the plasmonic coupling structure influences the emission properties of plasmonic lasers. However, the interaction between the DFB cavity and the metallic cavity has not been widely investigated.

In this work, the hybrid plasmonic cavity consisted of a four-layer structure that included two metal films, a grating, and a polymer film. The cavity simultaneously supported SP-enhanced DFB lasing and surface plasmon lasing; it thus had an ideal structure to localize and enhance the electromagnetic field. At the same time, the Joule losses of the metal films were decreased significantly with this structure. The influence of different SP-enhanced modes (SP mode between the two metal films and SP-enhanced waveguide mode) on the laser performance was investigated experimentally and with simulations. The experimental and simulation results show that the lasing threshold of the DFB laser decreased significantly owing to SP-enhancement.

2. Fabrication of the plasmonic cavity

Figure 1 shows a scanning electron microscopy image and a diagram of the plasmonic cavity's structure. Both interference lithography and thermal evaporation techniques were used for the fabrication of the plasmonic cavity. A flat Ag film with a thickness of 25 nm was prepared on a glass substrate. Then a grating structure with a period of 350 nm was fabricated on the Ag film. A 25-nm-thick Ag film was evaporated on top of the grating structure. The grating structure was employed for two reasons: 1. to act as a distributed feedback cavity and 2. to excite the SP modes. The gain material layer (poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-{2,1’,3}-thiadiazole)] (F8BT, American Dye Source, USA)) was attached on the ridge of the metallic grating. The polymer film was deposited using a two-step technique. First, a water-soluble polyvinyl alcohol (PVA 107, Celanese Chemicals, Germany) was spin-coated onto a glass substrate at 3000 rpm, forming a 200-nm-thick film. Then an F8BT solution in xylene with a concentration of 23 mg/ml was spin-coated onto the PVA film at 1500 rpm, forming a 150 nm film. Second, the bilayer was steeped in deionized water for 30 min to obtain a free-standing polymer membrane. The polymer film was then smoothly attached to the metallic grating via the surface tension effect [30]. The air gaps between the grating's valleys and the active waveguide act as a low-index spacer. The fluorescence can be effectively localized in the waveguide owing to the refractivity difference. The air gaps decrease the contact area between the active layer and the metal film, which reduces the metal losses significantly. As shown in Fig. 1, the metallic DFB cavity consists of this grating and the silver film covering it. The Ag film on the grating is responsible for SP-enhanced DFB lasing. Simultaneously, the SP lasing is attributed to a plasmon resonance between the two silver films on both sides of the grating.

 

Fig. 1 Polymer laser based on the proposed plasmonic cavity. (a) Scanning electron microscopy image of the cross-section of the polymer laser in (b). The black arrows indicate corresponding structures between the right and left images. (b) Schematic diagram of the plasmonic cavity. The green arrow denotes the DFB lasing. The yellow-green arrows indicate the SP lasing. The purple arrow corresponds to the pump beam. The angle between the directions of the DFB lasing and the SP lasing is about 10°.

Download Full Size | PDF

3. Spectra characterization of the plasmonic laser

Figure 2(a) illustrates the extinction spectra of the plasmonic cavity. The high-energy peak at 491 nm corresponds to the plasmon resonance of the Ag film. The peak at 566 nm arises from the waveguiding mode in the plasmonic cavity. The peak splits into two as the detection angle changes from 0° to 20° (black line in Fig. 2(a)). Figure 2(b) shows a photograph of the laser spot from the plasmonic cavity. The laser was excited by a frequency-doubled Ti:sapphire laser with a wavelength of 400 nm, a repetition rate of 1 kHz, a pulse duration of 200 fs, and a pulse energy of up to 1 mJ. The pump power was altered by a neutral density filter. The area of the pump spot was approximately 3 mm², which was adjusted by a lens with a focal length of 15 cm. The pump beam impinged on the sample at an angle of 20° and the emission spectra was measured by a spectrometer (Maya 2000 Pro, Ocean Optics). In the experiment, the wavelength of the SP lasing was found to be located at 571 nm and the full width at half maximum (FWHM) of the emission peak was less than 1 nm (see Fig. 2(c)). Note that there was a 557 nm emission peak with a FWHM of less than 1 nm in Fig. 2(c), which originates from the DFB mechanism of the cavity. The emission is governed by the formula 2n_effΛ = mλ, where n_eff is the effective refractive index of the DFB mode, Λ is the grating period, m is the diffraction order, and λ is the emission wavelength.

 

Fig. 2 The SP lasing and SP-enhanced DFB lasing from the plasmonic cavity. (a) Extinction spectra of the plasmonic cavity at different angles. (b) Photograph of the emission spot from the plasmonic laser. (c) Emission spectra from the plasmonic laser with different pump intensities. (d) The relationship between the output intensity and pump intensity, indicating that the thresholds of SP and DFB lasing are 3.2 μJ/cm² and 3.5 μJ/cm², respectively.

Download Full Size | PDF

The SP lasing and the DFB lasing were measured under different pump intensities as shown in Fig. 2(c). The PL of F8BT was measured by a fluorescence spectrometer (F-4600, Hitachi, Japan). Figure 2(d) shows the relationship between the output intensity and the pump power. The threshold for DFB and SP lasing was 3.5 and 3.2 μJ/cm², respectively. The lasing threshold of the DFB laser was 6.7 times lower compared with that of a free-standing membrane polymer DFB laser [28]. This can be attributed to air gaps in the cavity and to the SP enhancement, which will be discussed in detail later. The air gaps formed by the two-step technique effectively reduce the contact between the polymer and the metal film, thus potentially reducing the Joule losses and lowering the lasing threshold. The effect of the SP on the laser performance was investigated by a numerical simulation.

The COMSOL software package was employed to simulate the electric field distribution of the plasmonic cavities. To investigate the influence of the two Ag layers on the laser performance numerical simulations were performed based on the finite element method. The COMSOL simulation parameters were identical to the structure parameters in Fig. 1. The non-periodic boundary above the device was added to avoid boundary conditions interfering with evanescent fields. The distance between non-periodic boundary and the upper surface of sample is larger than 300 nm. At λ = 557 nm, the refractive indices of the F8BT and Ag are 2.04 and 0.06 + 3.586i, respectively. All refractive indices were measured by a spectroscopic ellipsometer (ESNano, Ellitop). At λ = 571 nm, the refractive indices of the F8BT and Ag are 1.94 and 0.05 + 3.858i, respectively. Figures 3(a) and 3(b) show the electric field distributions at 557 nm without and with a silver layer under the grating, respectively. When the Ag layer was below the grating in Fig. 3(b), it can be seen that the SP-enhancement of the Ag layer increased the electric field. When the Ag layer was above the grating, the electric field was localized in the grating and the active material, as can be clearly seen in Fig. 3(c). In this case, the electric field is localized in the ridge of the grating and the waveguide. The structure in Fig. 3(c) was investigated in our previous work [30], where only SP-enhanced DFB lasing could be observed.

 

Fig. 3 Electric field distribution of the plasmonic cavity with different Ag film locations and the transmittance simulation. (a) Free-standing membrane polymer DFB laser without the Ag film at 557 nm. (b) Ag film at the bottom of the grating at 557 nm. (c) Ag film at the top of the grating at 557 nm. (d) Ag films at the top and bottom of the grating at 557 nm. (e) Ag films at the top and bottom of the grating at 571 nm. (f) Transmittance simulation of the cavity in (e).

Download Full Size | PDF

Figures 3(d) and 3(e) demonstrate the electric field distributions of the plasmonic cavity with two Ag layers at 557 nm and 571 nm, respectively. Compared with the result in Fig. 3(a), the electric field in the waveguide in Fig. 3(d) is enhanced when introducing the Ag films. The maximums of the electric field in the waveguides are 4 and 50 for Figs. 3(a) and 3(d), respectively. The electric field increases by 12.5 times. This is why the lasing threshold of the SP-enhanced DFB laser is much lower than that of a DFB laser without metal films.

When the full structure with both silver layers is modeled at 571 nm, the electric field is mainly localized in the gap between two Ag films. Strongly localized electric fields are supported between each gap of the two Ag films because of the resonance effect, as showed in Fig. 3(e). To further understand the resonance effect, the transmittance spectrum of the cavity in Fig. 3(e) is calculated by using the COMSOL software, as shown in Fig. 3(f). The dip at about 571 nm corresponds to the plasmon resonance of the plasmonic cavity. It overlaps excellently with the peak of SP lasing at 571 nm. So, it is a direct identification of the SP lasing.

Stimulated photons from the gain material are able to couple to the SP mode while the PL spectrum of the gain material overlaps with the SP spectrum. The localized SP can be decoupled from the grating structure to the waveguide modes [11]. Then the waveguide grating which out couples the 0th order in-plane SP lasing at 10°.

The polarization response of the hybrid lasing was characterized as shown in Fig. 4. After passing through a half-wave plate, the pump beam illuminates on the sample surface at an angle of 20°. The laser emission is collected after the polarizer by a fiber optic spectrometer. Figure 4(a) illustrates the schematic of the optical layout for measuring the polarization. Figure 4(b) depicts the impact on hybrid lasing when the polarization of the pump beam is changed (no polarizer after the sample). When the half-wave plate is rotated by α degrees along the axis of the incident light, the pump polarization is rotated by 2α degrees, where α is the angle between the polarization of the pump beam and the direction of the half-wave plate. Initially, the pump polarization is parallel to the grating lines. The DFB lasing is more sensitive to the polarization of the pump compared with SP lasing, as shown in Fig. 4(b). This is because the SP-enhancement of the fluorescence is isotropic. Therefore, SP lasing decreases slowly when the polarization of pump beam is changed from 0° to 90° comparing with for DFB lasing. When the polarization is parallel to the grating lines, i.e. α = 0°, the maximum transformation efficiency of the pump beam is obtained and the strongest hybrid lasing emission intensity is observed. When the pump polarization is vertical to the grating lines, i.e. α = 90°, we receive the minimum transformation efficiency of the pump beam and the weakest plasmonic laser emission. Figure 4(c) presents the polarization properties of the output lasing in the plasmonic cavity when the pump polarization is parallel to the grating lines (no half-wave plate before the sample). Thus both SP and DFB lasing here are linearly polarized.

 

Fig. 4 Polarization properties of the plasmonic cavity. (a) Schematic of the optical setup for measuring the polarization dependency of the plasmonic laser. (b) Output emission as a function of the pump polarization. (c) Polarization properties of the output emission.

Download Full Size | PDF

4. Conclusion

By employing two Ag layers in a grating structure, both DFB lasing and SP lasing were achieved in a single laser device. SP lasing is achieved via a pump laser under resonance conditions in the gaps between the two metallic films. The lasing threshold of the hybrid lasing is significantly decreased via SP enhancement and the free-standing membrane structure. These results may have potential applications for devices and structures such as biosensors, all-optical circuits, and compact light sources.