Comparison of various excitation and detection schemes for dye-doped polymeric whispering gallery mode micro-lasers
Tobias Siegle, Jonas Kellerer, Marielle Bonenberger, Sarah Krämmer, Carolin Klusmann, Marius Müller, and Heinz Kalt
Tobias Siegle,* Jonas Kellerer, Marielle Bonenberger, Sarah Krämmer, Carolin Klusmann, Marius Müller, and Heinz Kalt
Institute of Applied Physics (APH), Karlsruhe Institute of Technology (KIT), Wolfgang-Gaede-Str. 1, 76131 Karlsruhe, Germany
*Corresponding author: tobias.siegle@kit.edu
T Siegle
J Kellerer
M Bonenberger
S Krämmer
C Klusmann
M Müller
H Kalt
Tobias Siegle, Jonas Kellerer, Marielle Bonenberger, Sarah Krämmer, Carolin Klusmann, Marius Müller, and Heinz Kalt, "Comparison of various excitation and detection schemes for dye-doped polymeric whispering gallery mode micro-lasers," Opt. Express 26, 3579-3593 (2018)
Diode pumped lasers
Nd:YVO4 lasers
Purcell effect
Tunable diode lasers
Dye lasers (140.2050)
Optical resonators (140.4780)
Microcavities (140.3945)
Polarization (260.5430)
Optical sensing and sensors (280.4788)
Original Manuscript: August 9, 2017
Manuscript Accepted: December 27, 2017
We compare different excitation and collection configurations based on free-space optics and evanescently coupled tapered fibers for both lasing and fluorescence emission from dye-doped doped polymeric whispering gallery mode (WGM) micro-disk lasers. The focus of the comparison is on the lasing threshold and efficiency of light collection. With the aid of optical fibers, we localize the pump energy to the cavity-mode volume and reduce the necessary pump energy to achieve lasing by two orders of magnitude. When using fibers for detection, the collection efficiency is enhanced by four orders of magnitude compared to a free-space read-out perpendicular to the resonator plane. By enhancing the collection efficiency we are able to record a pronounced modulation of the dye fluorescence under continuous wave (cw) pumping conditions evoked by coupling to the WGMs. Alternatively to fibers as a collection tool, we present a read-out technique based on the detection of in-plane radiated light. We show that this method is especially beneficial in an aqueous environment as well as for size-reduced micro-lasers where radiation is strongly pronounced. Furthermore, we show that this technique allows for the assignment of transverse electric (TE) and transverse magnetic (TM) polarization to the observed fundamental modes in a water environment by performing polarization-dependent photoluminescence (PL) spectroscopy. We emphasize the importance of the polarization determination for sensing applications and verify expected differences in the bulk refractive index sensitivity for TE and TM WGMs experimentally.
Size-optimized polymeric whispering gallery mode lasers with enhanced sensing performance
Sarah Krämmer, Sanaz Rastjoo, Tobias Siegle, Sentayehu F. Wondimu, Carolin Klusmann, Christian Koos, and Heinz Kalt
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» Visualization 1 Polymeric micro-disk laser evanescently excited via a tapered glass fiber with a pulsed laser at 532 nm and a repetition rate of 2 Hz. The lasing emission is blocked with a filter. Scattered excitation light is visible at the resonator rim.
Fig. 1 Excitation (grey color) and collection (red color) methods for active micro-cavities: Excitation is either performed in a free-space configuration or by evanescent field coupling using tapered fibers. Resulting WGM emission is collected with either a microscope or aligned tapered fibers and guided to a detector consisting of a grating spectrometer equipped with a CCD camera.
Fig. 2 Free-space excitation of a micro-laser and free-space collection of the WGM emission perpendicular to the resonator plane. Imperfections at the resonator rim and corresponding alignment with respect to the spectrometer entrance slit greatly enhance the SNR as in case (a) compared to the case of a resonator without imperfections and missing alignment (b).
Fig. 3 (a): Typical PL spectrum of a PM597-doped micro-cavity excited above lasing threshold with fundamental and higher-order modes (recorded with method I). The lasing threshold is determined from the integrated PL intensity of the lasing mode arising at the lowest pump energy (marked in red color). (b): Using fiber excitation and collection results in a significant decrease of the lasing threshold and increase of the collection efficiency; note the dimension on the x-axis changed to pJ (c). Input-output curves for the different measurement configurations demonstrating significant differences in the resulting collection efficiencies (d).
Fig. 4 Transmission spectrum of a tapered glass fiber aligned to the rim of a dye-doped micro-disk. Sharp dips corresponding to the WGM resonances are observed. Q factors as high as 6.5·104 have been determined from the resonance linewidths.
Fig. 5 Fluorescence emission of dye-doped micro-disks under cw excitation: Method I in (a), IV in (b) and III in (c). Whereas in case (a) the expected WGM-modulation is not visible due to the large non-modulated spontaneous dye emission, in (b) and (c) a clear WGM-modulated dye emission with the expected free-spectral range is observed.
Fig. 6 Input-output curves for micro-lasers with diameters of D = 50 µm (a) and D = 20 µm (b) recorded in aqueous environment. In-plane (red color) collection efficiencies are enhanced compared to the collection from the top (black color). As method IV is radiation-based, it is even more advantageous for smaller sized resonators (b) with enhanced radiation losses.
Fig. 7 (a): PL spectrum of a micro-laser in aqueous environment. Fundamental modes separated by the FSR are visible. From polarization-dependent PL spectroscopy we find a 180°-periodic intensity dependence for adjacent modes (b). From the phase shift of 90° and the polarizer's orientation we can identify TE and TM lasing peaks.
Fig. 8 Simulated and experimentally determined BRIS of micro-resonators with D = 20 µm in method IV. Both simulated and experimental data have been fitted with straight lines. The uncertainties of the BRIS values correspond to the uncertainties of the slope of the linear regression for the plot " Δλover Δn". For the linear fits to the data the 95% Confidence Level is shown.
Table 1 Averaged lasing thresholds and collection efficiencies for PM597-doped micro-lasers, investigated with different excitation and collection schemes
View Table | View all tables in this article
Table 2 Averaged lasing thresholds and collection efficiencies for PM597-doped micro-lasers in aqueous environment recorded method I and IV.
Table 3 Measured and simulated BRIS for fundamental TE and TM modes of differently sized resonators. The values are determined at the wavelength where the linear wavelength regressions have the smallest uncertainty. Although the absolute values differ, simulation and measurements show the same trends for the dependencies on radius and polarization.
(1) P th,cal = γh c 0 τ F λ P σ a 2 ( λ P ) n t d ,
(2) F th,cal = γh c 0 τ F λ P σ a 2 ( λ P ) n t d ⋅ τ L ≈30 μJ cm 2 .
(3) F th,exp = E th A res ≈25 μJ cm 2 .
(4) E mode = E th ⋅ C mode ⋅ C abs ≈4.2pJ
Averaged lasing thresholds and collection efficiencies for PM597-doped micro-lasers, investigated with different excitation and collection schemes
Measurement configuration
Lasing threshold Eth (nJ)
Collection efficiency ξ (Counts/nJ)
I Free-space excitation – free space collection (top) 0.50 ± 0.22 (1.14 ± 1.61)·103
II Free-space excitation – fiber collection 0.45 ± 0.14 (1.11 ± 0.54)·105
III Fiber excitation – fiber collection (4.54 ± 1.99)·10−3 (1.79 ± 1.20)·107
IV Free-space excitation – free space collection (in-plane) 0.58 ± 0.07 (1.00 ± 0.71)·104
Averaged lasing thresholds and collection efficiencies for PM597-doped micro-lasers in aqueous environment recorded method I and IV.
D = 20 µm
I Free-space excitation – free space collection (top) 6.16 ± 1.37 1.21 ± 0.33 (2.97 ± 0.79)·102 (4.35 ± 2.60)·102
IV Free-space excitation – free space collection (in-plane) 4.33 ± 0.83 1.26 ± 0.71 (5.84 ± 0.32)·104 (3.22 ± 2.44)·103
Measured and simulated BRIS for fundamental TE and TM modes of differently sized resonators. The values are determined at the wavelength where the linear wavelength regressions have the smallest uncertainty. Although the absolute values differ, simulation and measurements show the same trends for the dependencies on radius and polarization.
Diameter D (µm)
Wavelength λ (nm)
BRIS TE (nm/RIU) 43.4 ± 0.3 52.35 ± 0.01 36.5 ± 0.7 42.78 ± 0.01 19.2 ± 0.3 22.63 ± 0.02
BRIS TM (nm/RIU) 38.0 ± 0.3 46.61 ± 0.01 32.4 ± 0.3 38.96 ± 0.01 19.5 ± 0.3 22.48 ± 0.02
ΔBRIS (nm/RIU) 5.4 ± 0.6 5.74 ± 0.02 4.1 ± 1.0 3.82 ± 0.02 −0.3 ± 0.6 0.15 ± 0.04