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Herein, we report a straight forward stress probing method based on mechanically tunable organic VCSELs via dual detecting-modes. By designing the active layer thickness, uploaded stress was measured simultaneously by the laser wavelength and mode separations, facilitating highly sensitive stress detection in broad ranges. Single-mode laser emission with low threshold and narrow line-width was characterized, which could be tuned continuously within 8 nm. The probing sensitivity and resolution were estimated to be 60 Pa and 5.6 nm/KPa respectively, which were ~160-folds higher than previous results.
© 2015 Optical Society of America
1. Introduction
Organic tunable lasers, i.e., solid-state laser media activated by dye chromophores, have been demonstrated to be crucially important in a wide variety of novel applications, from spectroscopy to isotopic separation, photochemistry, medical diagnosis and therapy [1–6]. Micro-cavity lasers based on the versatile organic lasers such as distributed-feedback (DFB) lasers [7–9], surface-emitting lasers [10–14], random lasers [15, 16], micro-ring lasers [17], whisper-gallery mode lasers [18, 19], which were tuned commonly by electrical means [7, 14] or sample movement to vary the periodic structures, effective refractive index, and film thickness [9], were investigated extensively because of their distinct advantages including enhanced durability, reduced laser threshold and line-width, easy of fabrication, and integration compatibilities.
Aside from applications mentioned above, other applications of the mechanically tunable lasers were also suggested, such as conformable sensitive optical skins for monitoring the structural health of civil infrastructure [20]. Recently, elastically tunable organic lasers were realized, largely attributed to the adoption of cholesteric liquid crystalline or polydimethylsiloxane (PDMS) elastomer, which were used as the host matrices [21, 22] or substrates of DFB structures [23, 24], respectively. Up to now, most of the mechanically tunable lasers reported were tuned by stretch elongation [21–27] although a few could be tuned by compression, in which liquid solvents were needed [28], bringing vulnerability and inconvenience similar to traditional liquid dye lasers. Moreover, the corresponding probing sensitivity, i.e., the direct responses of laser characteristics to mechanical stress was barely presented. In cases of mechanical tuning by stretch elongation, responses on laser wavelength by strain were normally reported [21, 22], which was somewhat inconvenient due to the elastic modulus being unknown. The elastic polymer PDMS, which was employed in bio fields in lab-on-chip devices [9], has shown stress probing capabilities as substrate for DFB gratings [23, 24]. However, the different stiffness between PDMS and active media, e.g. conjugated polymer, resulted in lack of or non-linear response in low strain regions [23].
In previous studies, for the first time to our knowledge, the feasibility of dye doped PDMS films as the active layer and based on it, a simple stress probing method were demonstrated [29, 30]. By sandwiching the dye doped PDMS film between two Fabry-Perot (F-P) plane mirrors, mechanical stress upon the micro-cavity was measured through variations on the mode separation of output laser emission. The probe sensitivity was estimated to be 0.01 MPa with a resolution of 35.0 nm/MPa. However, problems should still be resolved considering practical employment. Firstly, detecting mechanisms other than mode separations dependence on stress were required for higher sensitivity. Thus, color tuning, i.e. laser wavelength dependence on stress uploaded to VCSELs was proposed because the oscillation wavelength varied within the stop-band of the DFB reflector according to the thickness of the active phase-shift layer, which has not be actually realized yet [10]. Moreover, broad detecting-range was required which was suggested to be resolved by designing the active layer thickness of such mechanically tunable VCSELs, facilitating not only single mode emission, but also multi-mode oscillation.
In this work, we report a straight forward stress probing method based on mechanically tunable organic VCSELs via dual detecting-modes. By elaborately designing the active layer thickness, mechanical stress uploaded onto the micro-cavities could be measured simultaneously by the oscillation wavelength and mode separations, providing high sensitivity in a broad detecting range. Single-mode laser emission with low threshold and narrow line-width was characterized, which could be tuned continuously within 8 nm. The probing sensitivity and resolution were estimated to be 60 Pa and 5.6 nm/KPa respectively, which were ~160-folds higher that the best of previous results.
2. Experimental details
2.1 Materials and fabrication
The laser dye pyrromethene 597 (PM597, laser grade from Exciton), the commercially available silicone, known as SIM-360 (Shinetsu Chem. Co.) mixed with its curing agent CAT-360, and the solvent toluene were used as received without further purification. For the fabrication of high- and low-refractive index layers of distributed Bragg reflection (DBR) mirrors (Ø = 35 mm, 1 mm thick, quartz substrates), the commercially available polymers polyvinylcarbazole (PVK, TCI) and cellulose acetate (CA, Sigma Aldrich) together with their solvents, chlorobenzene and diacetone alcohol respectively, were also used as received. First, PVK and CA were dissolved into their solvents at the concentration of 22.2 and 28.6 mg/ml respectively under continuously vigorous magnetic stirring. After the ultrasonic treatment and filtering (Whatman, PTFE 0.2 μm), the PVK/chlorobenzene or CA/diacetone alcohol solutions were spin-coated alternately onto the quartz substrates (Ø = 35 mm, 1 mm thickness). The spin-coated films were then cured in a 130 °C oven for 5 minute to remove the solvents before the deposition of next layers and the optical thickness of each layer was precisely controlled to be a quarter of the designed Bragg wavelength, at ~620 nm for an instance. After the fabrication of DBR resonators comprising 14.5 pairs of PVK/CA layers, a PM597 doped PDMS film was spin-coated as the active layer onto one DBR mirror which was then sandwiched with another counterpart for laser characterization. PM597 was dissolved into SIM-360 with the help of a small amount of toluene and de-bubbling was then accomplished in a vacuum chamber after mixed with the curing agent. After spin-coating onto DBR mirrors, the PM597/PDMS films were then cured in a 50 °C oven for 30 min before sandwiching. The concentration of PM597 in the PDMS host was fixed at 2.9 mM. With the strong adhesion between the two DBR mirrors provided by the PDMS layer, the micro-cavity lasers were thus built. Details for the fabrication could also be found elsewhere [30, 31].
2.2 Characterization
The UV-vis absorption and fluorescence spectra of the PM597/PDMS films were measured by a Perkin-Elmer Lambda 20 and a Hitachi 850 fluorescence spectrophotometer, respectively. The scan speed and the slit width were kept at 60 nm/min and 1.0 nm, respectively. The refractive indexes of the PVK and CA films were ~1.675 and ~1.483 respectively, determined by a commercial prism coupler (Metricon model 2010) at 633 nm. The refractive index and thickness of the PVK and CA layers were also measured by a scanning ellipsometer (Semilab, GES 5E). The morphology of the films was measured by atomic force microscopy (Bruker, Dimension Edge). The refractive index of the PM597/PDMS film was measured to be ~1.408 at 633nm. For characterization of laser properties, the second harmonic output from a Q-switched Nd:YAG laser with pulse width of 3~5ns (FWHM) was used as the pump source and the pumping beam was focused by a spherical focal lens (f = 300 mm) to form the pumping spot of ~80 μm in diameter on the sample surface. The micro-cavity lasers were pumped longitudinally and the output lasers were monitored perpendicular to the sample surface by an optical fiber spectrometer (Ideaoptics, FX4000) with a spectral resolution of 0.35 nm. The laser output energy was measured by a laser energy meter (Ophir, Nova II, PD10) and a narrow-band filter was also applied to avoid any possible interference caused by the transmitted 532 nm laser pulses. Mechanical stress was uploaded onto the micro-cavity by putting metal rings in various weights onto the sample surface.
3. Results and discussions
As shown schematically in Fig. 1, the VCSELs were established by sandwiching the gain media between two organic DBR mirrors, onto one of which the thin film of PM597 doped PDMS was spin-coated, providing the strong adhesion between two couplers. The laser dye, PM597 was selected as the dopant for its high efficiency and solubility in host matrices. The absorption and fluorescent peaks of PM597 in PDMS films were ~527 and 565 nm, respectively (Fig. 2). The transmittance spectrum of the organic DBR reflector was also shown in Fig. 1, with a relatively narrow stopband of ~60 nm (FWHM) centered at ~611 nm. Under pumping with laser pulses at 532 nm, output laser beam was observed with pump energy higher than 0.1 μJ, corresponding to an energy density of ~2.0 mJ/cm2. As shown in Fig. 2 and 3, with the active layer thickness of 17.0 μm, multi-mode laser oscillation was observed. The line-width of the multi-mode laser emission was ~0.35 nm, which was actually the spectral resolution of the fiber spectrometer used in this work. The mode separation (Δν) between the laser modes can be given according to the equation below,
where Δν, c, n, and Leff were the mode separation in wavelength or frequency, light speed in vacuum, refractive index and cavity length, respectively. It should be mentioned that the cavity length (Leff) was given as the sum of active layer thickness (L) and the effective reflector length (LDBR), according to the equation below, where κ, and D were the coupling coefficient and thickness of the DBR mirrors, respectively. With the κ of 9.54 × 105 m−1 and D of 2.90 μm, the LDBR was estimated to be ~0.43 μm. With the Δν of 197.7 cm−1 given in Fig. 3, and n of 1.408 for PM597 doped PDMS films, the cavity length (Leff) was calculated as 17.96 μm, which was in high accordance with the active layer thickness calculated from the linear absorption spectrum. Thus, minor variation on the active layer thickness under mechanical stress might be detected not only by the laser mode separation, but also by the oscillation wavelength.
Fig. 2 Absorption, PL, and laser output spectra of the PM597 doped micro-cavity laser (active layer thickness: 17.0 μm, the black solid line shows the transmittance spectrum of the DBR reflector).
To demonstrate the stress detection by oscillation wavelength, the active layer thickness was reduced for single-mode emission for simplicity. The laser emission spectra of the organic VCSEL with the PM597 doped PDMS film thickness of 10.5 μm under various mechanical stresses is presented in Fig. 4. With the increase of stress from 0 to 0.74 KPa, the single-mode laser emission could be tuned continuously from 629.02 to 624.91 nm. The laser slope efficiency and threshold of the mechanically tunable VCSEL under various stresses are also shown in Fig. 5, in which these two parameters demonstrate distinct tendencies. The highest slope efficiency (2.9%) together with the lowest threshold (0.5 μJ, ~9.9 mJ/cm2) was obtained at the laser wavelength of 627.99 nm, corresponding to the stress of 0.18 KPa. The appearance of maximum slope efficiency and minimum laser threshold with the increase of mechanical stress could be ascribed to the deviation of the active layer thickness (L) from the mλ/2 condition, in which m represents an integer. In this concern, the elastic deformation (ε) and Young's modulus (E) of the dye doped PDMS films under mechanical stress, together with the oscillation wavelength (λ’) of the VCSELs after compression could be given by the equations below,
where L0 and L’ were the active layer thickness before and after mechanical compression respectively, and σ was the mechanical stress. Thus, by detecting the oscillation wavelength λ’, the mechanical stress σ could be monitored instantly with high accuracy and sensitivity. As shown in Fig. 6, the plotted λ’ as a function of σ could be well fitted linearly, accordingly with Eq. (6), demonstrating the feasibility of employing the oscillation wavelength of VCSELs as a novel stress probe. According to the spectral resolution (0.35 nm) of the fiber spectrometer used in this work, the probing sensitivity and resolution were estimated to be ~60 Pa and 5.6 nm/KPa respectively, which were improved by 160-folds in comparison with the best of our previous results [30]. Some piezoelectricity, triboelectricity, and capacity-based stress sensors with ultra-high sensitivity of less than 10 Pa had been reported recently [32–34]. It is expected that by employing spectrometers with higher spectral resolution and PDMS matrices with lower Young's modulus, stress probing sensitivity of several Pa might also be possible with the VCSELs-based sensors.Fig. 5 Laser output energy of the VCSELs as a function of the input energy at various oscillation wavelength (active layer thickness: 10.5μm).
Fig. 6 The plotted oscillation wavelength as a function of mechanical stress (active layer thickness: 10.5μm).
It should be noted that the oscillation wavelength jumped to another continuous tuning range (630.14~625.56 nm) while the mechanical stress increased further from 0.94 to 1.76 Kpa, as shown in Fig. 7. Such mode-hopping was suggested to be ascribed to the mλ/2 condition for active layer thickness (L), which decreased with the mechanical stress, resulting in the replacement of m by (m-1). The mode hopping observed in Fig. 4 and 7 could also be explained by the fact that lasing primarily occurred on the red edge of the emission spectrum, at wavelengths farthest away from the absorption spectrum. Wavelengths closer to the absorption spectrum did not lase because of increased losses due to self-absorption. At redder wavelengths the gain was too low. Therefore one mode moved outside of the sweet spot while another mode moved into it, resulting in the mode hopping. And it should also be mentioned that in the second and even third cycle of mechanical tuning, the probing resolution was kept almost constant while the tuning range and laser threshold increased (Fig. 8), as predicted by numerical simulations [10].
Fig. 7 Laser spectra of the VCSEL under various mechanical stresses (active layer thickness: 10.5μm).
Fig. 8 Laser output energy of the VCSELs as a function of the input energy at various oscillation wavelengths (active layer thickness: 10.5 μm).
In order to introduce detection by mode separation, the active layer thickness was increased to 15.3 μm. The mode separation (Δν’) of the VCSEL after compression could be given by the equation below,
The laser emission spectra of the VCSEL with 15.3 μm of active layer thickness under various stress are shown in Fig. 9. With the increase of mechanical stress from 0 to 0.15 MPa, the mode separation increased steadily from 7.11 to 8.33 nm. It should be noted that after the removal of mechanical stress, the Δν of the relaxed VCSEL returned to its initial value, indicating the elastic deformation of the active layer. As shown in Fig. 10, the plotted Δν as a function of σ could be well fitted by the Eq. (7), demonstrating the feasibility of using the mode separation of such mechanically tunable VCSELs as the stress probe in relatively larger detecting range in comparison with that based on oscillation wavelength shift. According to the spectral resolution (0.35 nm) of the fiber spectrometer used in this work, the sensitivity and resolution of the stress probe based on mode separation variation were estimated to be ~0.01 MPa and 35.0 nm/MPa respectively. It should also be mentioned that the Young's modulus E of the dye doped PDMS films calculated by Eq. (7) as shown in Fig. 10 was ~0.7 MPa, which was close to that obtained by Eq. (6) as shown in Fig. 6, indicating the high accordance between results get by the two probing methods, although their probing sensitivity and ranges were distinct.
Fig. 9 Laser spectra of the VCSEL under various mechanical stress (active layer thickness: 15.0 μm).
Fig. 10 Mode separations of the VCSEL as a function of the mechanical stress (colored solid lines: theoretical simulation of sample with various Young's modulus; dashed line: plots fitted by the equation; active layer thickness: 15.0 μm).
Thus, as mentioned above, simultaneous probing of mechanical stress by oscillation wavelength and mode separation, i.e. the dual-mode detection might facilitate the optimization of sensitivity and range. As shown in Fig. 11, with the active layer thickness of 15.0 μm, stress probing was also feasible by the oscillation wavelength method beside the mode separation one. With the increase of mechanical stress from 0 to 6.7 KPa, the oscillation wavelengths could be tuned continuously from 615.37 to 608.16 nm (Fig. 12) while the mode separation was kept unchanged. The variation tendencies on oscillation wavelength and laser threshold with the increase of mechanical stress were similar to those revealed in Fig. 5 and 8. Although the mode-hopping could still be observed with the further increase of stress, it was then detected by the slight variation on mode separations. In such a way, high probing sensitivity was kept in the whole mechanical stress range of 0~0.15 MPa by monitoring the laser emission wavelength (λ’) with the help of the mode separation (Δν’).
Fig. 11 Laser spectra of the VCSEL under various mechanical stress (active layer thickness: 15.0 μm).
Fig. 12 Oscillation wavelengths of the VCSEL as a function of the mechanical stress (active layer thickness: 15.0 μm).
For practical application, the photostability and homogeneity of the organic VCSELs were also characterized. Under the pump repetition rate of 10 Hz and pump energy 10 times higher than the threshold, the laser output presented no sign of degradation after 120,000 pulses (Fig. 13). The homogeneity of the dye-doped PDMS film was verified by measuring the laser output spectra at various pumping positions on the samples and no obvious variation on the oscillation wavelength was found in this work. It is expected that the mechanically tunable VCSELs might be employed not only as highly sensitive stress probes, but also as cost-effective excitation sources on integrated flow-cytometry chips, which might be fabricated in the form of micro-cavity laser arrays by inkjet methods. This work is now underway.
Fig. 13 The normalized laser output energy of the VCSEL as a function of pump pulses under the pump energy 10 times higher than the threshold and repetition rate of 10 Hz. (active layer thickness: 17.0 μm).
4. Conclusion
In conclusion, a straightforward stress probing method was demonstrated based on mechanically tunable organic VCSELs via dual detecting-modes. Single-mode laser emission with low threshold and narrow line-width was characterized, with continuous laser tuning within the range of 10 nm. The probing sensitivity and resolution were estimated to be 60 Pa and 5.6 nm/KPa respectively, which was ~160-folds higher that the best of previous results. By designing the active layer thickness, the mechanical stress was measured simultaneously by the laser wavelength and mode separation methods, facilitating highly sensitive stress probing in broad ranges.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (Nos. 51272231, 51229201, 51372221, and 51472067), Program for Innovative Research Team in University of Ministry of Education of China (IRT13R54), and Natural Science Foundation of Zhejiang Province (No. LY12E02004).
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