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Abstract
An organic dye-doped polymer laser with a novel wavelength-tuning mechanism is proposed. This device is a vertical cavity surface-emitting laser realized by using a couple of distributed Bragg reflectors (DBRs), on which a dye-doped polymer thin film and a polydimethylsiloxane film are deposited individually. One of the DBRs is electrically driven to vary the effective cavity length. Under the optical excitation, tunable lasing operation with high stability can be achieved when immersion oil is used for the refractive index matching in the cavity. Since the device operation mechanism proposed here is quite simple, it is promising for designing a compact laser device with wide, precise, and electrically driven wavelength tunability.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Compact lasers having functions of dynamical wavelength tunability are widely used for many applications [1–3]. Some types of the tunable lasers based on various operation principles have been suggested and demonstrated. For inorganic semiconductor lasers, vertical cavity surface emitting lasers (VCSELs) have been extensively studied because of their advantageous features for integration and packaging. Microelectromechanical systems (MEMS) have been used in the inorganic VCSELs to change the lasing wavelength, especially in the infrared region [4,5]. Micromanipulation of the top or bottom distributed Bragg reflectors (DBRs) enables the precise and dynamical control of the effective cavity length and the resonant wavelength [6,7].
As well as the inorganic semiconductor lasers, solid-state organic lasers, that exhibit attractive functions such as low threshold operation, mechanical flexibility, and various lasing wavebands, are also very promising as the wavelength tunable device [8–12]. Two types of wavelength tunable organic lasers have been studied mainly, distributed feedback (DFB) lasers and VCSELs. In the DFB lasers, the grating pitch or effective refractive index could be parameters for tuning the wavelength of Bragg reflection stop-band. The grating pitch can be mechanically elongated when the device is fabricated on an elastic platform consisting of polydimethylsiloxane (PDMS) [13–16], whereas the effective refractive index can be changed by the spatial index profile of waveguide structure [17–20]. In the VCSELs, on the other hand, the effective cavity length is almost the only parameter for the resonant wavelength tuning [21,22]. Electrical wavelength tuning techniques are important for practical applications, and thus they have also been studied in both DFB lasers [23–25], and VCSELs [26,27]. Those studies presented tuning efficiency for applied voltage of ∼0.17 nm/V (tuning range of 625–642 nm) and ∼1.7 nm/V (627–637 nm) for the DFB and VCSEL, respectively. However these tuning methods are not suitable in principle as widely tunable ones due to the intrinsic material properties such as small electrooptic coefficients and small elastic constants [25,26]. In addition, accuracy of the wavelength tuning must be ensured regardless of the operation principle and/or tunable range.
In this study, we propose a novel design for a wavelength tunable organic VCSEL. As shown in Fig. 1, the VCSEL device has a wedge-shaped cavity structure and consists of a couple of glass plates on which high reflectivity DBRs are deposited. One of the DBR plates is coated with an organic dye-doped polymer film whereas the other one is covered with a PDMS film to avoid physical damaging. The spacing between the two DBRs, namely the cavity length, varies with the mechanical displacement of one of the plates. Tunable laser emission can be obtained under optical excitation. As compared to a similar tuning method using a wedge-shaped cavity structure [21], our method does not require a change of excitation position and can avoid unstable operation due to the spatial inhomogeneity of the active layer. It will be found in this study that the stable laser action is possible when the index matching in the cavity is done by filling immersion oil. The tunable laser system proposed here is based on a very simple strategy, allowing the precise wavelength tuning over a broadband wavelength range.
2. Experimental
The DBRs were prepared by the magnetron sputtering method using glass plates (BK-7, thickness of ∼0.5 mm and area of 10 mm× 10 mm) as substrates. A multilayer of SiO2/TiO2 (nine pairs) with thicknesses of 90/54 nm shows a 99%-reflection band of 550–650 nm. To prepare dye-doped polymer film, we used polyvinylpyrrolidone (PVP) as the host matrix and 4- (dicyanomethylene) -2-methyl-6- (4-dimethylaminostyryl) -4Hpyran (DCM) as the active dopant. The gain band of DCM is ∼600–680 nm, which is relatively wide as an organic luminescent dye [12]. They were dissolved in ethanol solvent with a concentration of 50 mg/mL. Thereafter, the solution was spin-coated on the DBR plate. The spin coated film was dried at 70∼100 °C. The thickness of the polymer film was ∼300 nm. The doping concentration of dye in the resultant PVP film was 6 wt%. To form the PDMS film, the PDMS precursor was diluted with toluene to a concentration of 1.3 wt% and was spin-coated on the other DBR plate. The spin-coating speed was varied from 1,000 to 3,000 rpm for 50 sec. The thickness of the PDMS film after the annealing at 90–110 °C was ∼12 µm. As shown in Fig. 1, these two films were placed face to face. At one end of the device, the top DBR plate was equipped with a manipulation needle driven by a stepper motor or a piezoelectric actuator. Photoexcitation was performed from the top side by using a second harmonic of an Nd: YLF pulsed laser having a wavelength of 527 nm, a pulse width of about 8 ns, and a repetition rate of 1 Hz. Since the absorption band of DCM is around 450–550 nm, this dye molecule can be excited at this pumping wavelength [12]. The emission from the bottom side was collected by an optical fiber and measured with a spectrometer. The size of excitation spot was ∼300 µm. By manipulating the top plate, we changed the effective cavity length and varied the resonant wavelength dynamically.
3. Results
Figure 2(a) shows typical results of steady-state optically pumped emission measurement for a proposed VCSEL device. No temperature or mechanical controls was applied while the system was set on a vibration isolation table during measurements. When emersion oil was introduced to the gap between two DBR plates, a multimode emission spectrum appeared in the emission band of DCM. The lateral multimode issue on output emission spectrum was not observed at this pump spot size (∼300 µm). The emission intensity showed a threshold behavior and increased nonlinearly against the pump fluence (see the inset). The full-width of half-maximum of each emission peak was smaller than the spectral resolution of the system (∼0.2 nm). These features demonstrate the emergence of VCSEL type laser oscillation. As shown in the inset, the lasing threshold is evaluated to be ∼400 µJ/cm2. The above-threshold output pulse energy was around 6 µJ at the input pulse energy of ∼30 µJ (∼10 mJ/cm2). A damage threshold where the DCM dyes were aged was ∼83 mJ/cm2. As shown in Fig. 2(b), the emission wavelength was found to be so stable whereas the half-life in the output intensity was as short as ∼3,000 pulses that is possible value for a conventional organic dye without encapsulation. Importantly, when the emersion oil was not introduced, no lasing was observed. These results show that, because of the wedge-shaped VCSEL device with the resonator mirrors that are not perfectly parallel, the large index mismatch in the cavity causes strong light scattering and large-angle refraction, disturbing the stable lasing operation.
Fig. 2. (a) Optically-pumped emission spectra of organic VCSELs with an active layer of DCM-doped polymer film. The red and black curves show the results with and without emersion oil, respectively. Inset shows fluence dependence of output emission intensity. (b) Temporal stabilities in output intensity (closed circles) and emission wavelength (open triangles) for one of the lasing modes.
Next, we show the dynamical wavelength tunability of the proposed device. As shown in Fig. 1, one end of the top DBR plate was vertically manipulated to vary the spacing between the top and bottom DBR plates. This manipulation method leads to also a change in the tilt angle (θ shown in Fig. 1) forming the wedge-shaped cavity. Therefore the effective cavity length L depends on the horizontal position of active point where the excitation light is irradiated. If we assume that the DBR plates are perfectly flat, the change in effective cavity length ΔL is equal to αΔP, where ΔP is the vertical displacement at the manipulation point and α is a ratio of the horizontal distance between the excitation position and the fulcrum to that between the manipulation point and the fulcrum. Figure 3(a) shows an experimental result of wavelength tuning at the condition of α = 0.2. The ΔP was changed by the stepper motor in 1 µm increments (minimum value in the system). Each emission peak in the multimode lasing spectrum exhibited a systematic change with respect to ΔL, while the mode separation (${\lambda _{sep}}$) was almost constant at ∼4.4 nm. The intensity mismatch between the modes is likely due to the gain competition. Now let us estimate theoretically the lasing wavelength depending on ΔL. The resonance wavelength ${\lambda _m}$ for a Fabry-Pérot cavity is expressed as
with L, the effective refractive index ${n_{eff}}$, and the order of longitudinal mode m. Under the assumption that ${n_{eff}}$ is spatially constant and shows a small variation with λ, the lasing wavelength depending on ΔL is expressed by the following equation.Fig. 3. (a) VCSEL lasing spectra depending on change in cavity length ΔL. (b) Lasing wavelength of VCSEL as a function of ΔL. Results for two devices were presented by circles and triangles. Dashed lines show functions of Eq. (2) where the cavity length L was individually estimated from ${\lambda _{sep}}$.
${\lambda _m}$(0) is the lasing wavelength before manipulating the top DBR. Thus the wavelength shift for one of the modes with arbitrary m is expressed as
Around the emission wavelength, the optical path length ${n_{eff}}$ L [${\sim} {\lambda ^2}/({2{\lambda_{sep}}} )$] is estimated from the ${\lambda _{\textrm{sep}}}$ ∼4.4 nm to be ∼44 µm. ${n_{eff}}$ is roughly expected to be ∼1.5, based on the cavity materials used in the device. Thus, by substituting the estimated L (∼29.1 µm) into Eq. (3), we find that the $\Delta {\lambda _m}/\Delta L\; $ is ∼21.5 nm/µm. The experimental result agrees well with this model, as shown in Fig. 3(b), demonstrating a good stability of this tuning mechanism. We have confirmed the reproducibility of this trend also in another device having a slightly different L [∼29.4 µm, see triangles in Fig. 3(b)]. The same result could be obtained also in a device where another dye molecule (pyrromethene 567) was used as the active dopant. These facts confirm that the tuning principle of our method is working correctly. However, the device is still in the multimode operation with large L. This is certainly an issue and cannot be easily solved. We will discuss on this point later.4. Discussions
Another important point for the stable operation should be mentioned. We examined a slightly different device structure where the DCM-doped polymer films were formed on both the top and bottom DBR plates. However, the observed lasing behavior was quite unstable because the spatial position for excitation in the active layer is changed with the wavelength tuning, i.e. with the displacement of top mirror. These features would be due to spatial inhomogeneity of the fabricated active medium. For organic dye-doped polymer films prepared by the spin coating, the optical constants are not completely homogeneous in-plane due to the segregation of dye and the dewetting of film [28]. Even a slight disorder in the refractive index, gain coefficient, or active layer thickness will cause unstable lasing behavior because, in this configuration, the photo-excited region in the active layer is slightly changed as the top plate is driven. In other words, it can be said that a fabrication technique of the homogeneous active layer is also important. Vapor deposition method might be a possible method to form a homogeneous active layer [21].
We have already shown in Fig. 3 that the ‘broadband’ tunability is available in our device. Next we would like to reveal a potential of the ‘fine’ tunablity. For this purpose, we employed a piezoelectric element that shows a vertical displacement ΔP smaller than 1 µm and used a smaller α value (∼0.012). As shown in Fig. 4(a), a piezoelectric actuator was integrated into the VCSEL for manipulating the top mirror. This piezoelectric actuator has a thickness of ∼2 mm and an area of 3 mm× 3 mm (THORLABS, PA2JEW). The top DBR plate was driven by vertical displacement of the actuator (ΔP ∼ 0–1.0 µm) with the applied voltage of 0–30 V. As shown in Fig. 4(b), this resulted in a ∼0.3 nm blue shift with a tuning response of 0.01 nm/V (at L ∼ 30 µm and α ∼ 0.012), demonstrating the electric fine tuning of lasing wavelength. Note that, in the configuration shown in Fig. 4(a), the manipulation point and the excited position are on different sides with respect to the fulcrum, leading to the blue shift of lasing wavelength with the increased ΔP.
Fig. 4. (a) A schematic of organic VCSEL integrated with a piezoelectric actuator. (b) VCSEL lasing spectra depending on displacement of the piezoelectric actuator ΔP.
As shown above, our device is working still in the multimode regime. Singlemode operation is highly desired in the future study while it is more difficult when the width of gain band becomes broader. For this purpose, it would be first required to remove the PDMS cover layer and to use an emersion oil showing low viscosity, leading to further reduction of L and large ${\lambda _{sep}}$. In essence, however, the cavity length in our device needs to include the thickness of active layer (∼300 nm) and < ∼1 µm of the driving space even though the PDMS cover layer is removed. This means that there is a limitation to enlarge ${\lambda _{sep}}$ (< ∼100 nm). To solve this difficult problem, Curwen et al. recently suggested a very promising strategy [29]. In their study, ‘a gain reflector’ was used for both the light amplification and the optical feedback. This enables a significant reduction of cavity length in a tunable VCSEL with an external cavity. While their study was a demonstration in the THz region using a quantum-cascaded laser, it would be very promising also for the visible laser devices. Furthermore, the reduced L brings another impact on the laser performance; as found in Eq. (3), since $\Delta {\lambda _m}$ is proportional to ${L^{ - 1}}$, the tuning efficiency is largely improved at a smaller L (as well as a larger α). We will be able to optimize the device structure so that the operation of the piezoelectric element is efficiently used.
5. Conclusion
In conclusion, we proposed an organic dye-doped polymer laser based on a novel concept of wavelength tuning. VCSEL type lasing was obtained in a wedge-shaped device consisting of a couple of DBR plates where the immersion oil was used for the index matching in the cavity. The results demonstrated that, by electric driving of the top DBR plate using two types of driving sources, the laser wavelength can be tuned with high accuracy and wide tunability. The tunable lasers operating under this principle are promising for many applications such as analysis, diagnosis, lighting, and remote sensing.
Funding
Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number (18H01476).
Disclosures
The authors declare no conflicts of interest.
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