near-infrared luminescent polymer waveguides and microlasers

Near-infrared luminescent polymer waveguides and microlasers

Martin Djiangoa, Takeyuki Kobayashi*a, Werner J. Blaua, Bin Caib, Kyoji Komatsub, Toshikuni Kainob

aMaterials Ireland Polymer Research Centre, School of Physics, Trinity College Dublin, Dublin 2, Ireland

bInstitute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

ABSTRACT We have fabricated and characterized polymeric slab asymmetric waveguides doped with a near-infrared-emitting dye, 2-(6-(p-dimethylaminophenyl)-2,4-neopentylene-1,3,5-hexatrienyl)-3-ethylbenzothiazolium perchlorate. Upon nanosecond photopumping, the waveguides have shown a small-signal gain coefficient of 37.2 ± 2.1 cm-1 at 820 nm for a pump fluence of 1.57 mJ/cm2 (314 kW/cm2). The loss coefficient and transparency fluence have been found to be 7.3 ± 1.0 cm-1 at 820 nm and 0.14 mJ/cm2 (28 kW/cm2), respectively. It is shown that a small-signal gain of 19.7 ± 2.3 dB is achievable in a 1.2-mm-long waveguide. Furthermore, near-infrared laser emission from self-assembled luminescent polymer microcavities has been demonstrated. The microrings are formed around silica optical fibers of varying diameters (80, 125, and 200 µm) and the larger microresonators have an overall quality factor of ~2 × 103, which is limited by surface roughness and scattering. We illustrate how the laser threshold varies inversely with both the quality factor and the inner diameter of the microrings. The free spectral range and the intensity variation of the laser output are also presented. Keywords: Amplified spontaneous emission, microcavities, near-infrared region, organic dye, optical gain, polymer lasers, waveguide amplifiers

1-INTRODUCTION In recent years, intense research efforts have been focused on developing organic modulators, switches and amplification devices for optical communication networks. Organic materials draw a keen commercial interest due to their cost-effectiveness, ease of processing and mass-production. The majority of the previous works on polymeric gain media focus on amplification and lasing in the visible region of the spectrum.1-3 However, new materials such as fluorinated polymers have been synthesised and possess a low-loss window which extends well into the near-infrared region (800-1550 nm).4 Because of the remarkable progress made in the field, such materials have started to replace coaxial cables and twisted pairs in short-distance communication networks. Optical amplifiers and sources operating in the near-infrared may find applications in such optical networks.5,6,7

In this Paper, we report optical gain in polymer slab asymmetric waveguides and laser emission from self-assembled polymer microrings. We chose 2-(6-(4-Dimethylaminophenyl)-2,4-neopentylene-1,3,5-hexatrienyl)-3-methyl-benzothiazolium perchlorat as the active dopant (available as LDS821 from Exciton, Inc.) and Poly(1-Vinyl-2-Pyrrolidone) (PVP) and Poly(4-Vinyl)-Phenol (PVPh) as the host polymer matrices. Electroluminescent devices using this organic dye have been previously reported.8 LDS821 has its absorption peak at approximately 600 nm and has a moderate absorption cross-section of 4 × 10-17 cm2 at 532 nm in PVP. The material fluoresces in the near-

* [email protected]

Organic Photonic Materials and Devices X, edited by Robert L. Nelson, Francois Kajzar, Toshikuni Kaino Proc. of SPIE Vol. 6891, 68910Y, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.763225

Proc. of SPIE Vol. 6891 68910Y-12008 SPIE Digital Library -- Subscriber Archive Copy

infrared region with a maximum peak at approximately 800 nm. The resulting large spectral shift of ~200 nm between absorption and fluorescence maxima is indicative of low emission reabsorption.

2-SAMPLE PREPARATION

Films of good optical quality were obtained from spin-casting a cyclohexanone solution containing PVPh and the LDS821 molecules onto pyrex substrates. After evaporation of the solvent, these films constitute asymmetric slab waveguides with the dye-doped polymer layer (dye concentration: 0.4 wt%; refractive index: 1.57 at 839 nm) sandwiched between the pyrex glass (refractive index is 1.47 at 588 nm) and air. The polymer waveguide layers were found to be 1.7 µm thick by white light interferometry. The absorption cross-section (peak is at 621 nm) and the steady-state photoluminescence (peak is at 798 nm) spectra are shown in Fig. 1. The photoluminescence spectrum was taken with a charge-coupled device (CCD) camera when the thin film was photoexcited with a cw Ar-ion laser. The wide spectral width of ~100 nm (~50 THz) at FWHM has favorable implications for broadband amplification. To fabricate the microcavities, we removed the cladding of commercially available plastic-cladding silica-core optical fibers by sulfuric acid and dipped the silica fiber in a chloroform solution containing specified amounts of PVP and the organic dye. After the silica fibers were pulled out of the solution, thin microrings were formed on the surface of the silica fiber owing to adhesion effects and surface tension.9-12 After evaporation of the solvent, the resulting polymer microrings contained 0.5 % of the active dopant. A selection of microrings with different inner diameters (80, 125, and 200 µm) were fabricated.

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Fig. 1. Absorption cross-section (solid curve) and steady-state photoluminescence (short-dashed curve) spectra of

LDS821 in Poly(4-Vinyl)-Phenol (the dashed arrow indicates the pump wavelength of 532 nm) together with amplified spontaneous emission spectrum (dashed curve) taken at 0.79 mJ/cm2 under nanosecond photopumping.

Proc. of SPIE Vol. 6891 68910Y-2

3-SLAB ASYMMETRIC WAVEGUIDES

The second-harmonic output from a Nd: yttrium aluminum garnet laser (532 nm, 5 ns pulse duration, 10 Hz repetition rate) was passed through cylindrical lenses and illuminated a 50 µm narrow stripe on the waveguide. The stripe length was varied from 0-2.5 mm in steps of 50 µm by a slit mounted on a computer-controlled stepping motor. Light emitted from the waveguide edge was collected through a microscope objective and a 532 nm-cut filter, and subsequently analyzed with a spectrograph attached to a CCD camera. For the better signal-to-noise ratio, integration was taken over 20 shots for all the data presented here. All of the measurements were performed under ambient conditions.

Upon photoexcitation, the photoluminescence spectrum shows a dramatic narrowing. The initially broad photoluminescence spectrum (~100 nm at FWHM) collapses into a much narrower band of approximately 30 nm when the film was pumped at a fluence of 0.79 mJ/cm2 (158 kW/cm2) with a 2-mm-long stripe (dashed curve in Fig. 1). Figure 2 shows typical plots of the output intensity at 820 nm as a function of pump stripe length at four different pump fluences. The gain narrowing and superlinear intensity increase are indicative of the occurrence of amplified spontaneous emission (ASE).

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102

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106

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.)

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Fig. 2. Output intensity as a function of excitation stripe length for a series of pump fluences: 0.20, 0.79, 1.18, and 1.57 mJ/cm2 from bottom to top, respectively. The solid curves are the best fit in the unsaturated regime.

Among the important parameters that govern the performance of an amplifier are the small-signal gain coefficient, the loss coefficient, and the unsaturated small-signal gain. Here we used the variable stripe length (VSL) method13 in which ASE from the waveguide is monitored when the gain layer is transversally photopumped with a narrow stripe geometry. The ASE intensity ( ),I Lλ collected at the waveguide edge varies with the pump stripe length (L) as:


( ) ( ) 1,( )

LAI L eγ λλγ λ

−= (1)

where A and ( )γ λ are a constant proportional to the excited state population and the net gain coefficient, respectively, and are used as fitting parameters for the extraction of the net gain coefficient from the experimental data. The curves in the figure are the best fit of each data set to Eq. 1. By repeating the measurement on several samples, the average gain spectra were obtained (Fig. 3). For the low pump fluence of 0.20 mJ/cm2 (40 kW/cm2), the gain peak lies at ~830 nm and the gain coefficient has a value of 6.4 ± 0.7 cm-1. For higher pump fluences, the gain peak shifts towards shorter wavelengths (~820 nm). With increasing pump fluence, the number density of molecules in the ground state, which are responsible for reabsorption of ASE, decreases and, consequently, the gain peak shifts toward blue. For a pump fluence of 1.57 mJ/cm2 (314 kW/cm2), the gain coefficient reaches a value of 37.2 ± 2.1 cm-1. This is the largest reported to date for organic gain media operating in the near-infrared region of the spectrum.

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-1)

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Fig. 3. Gain spectra for a series of pump fluences: 0.20, 0.79, 1.18, and 1.57 mJ/cm2 from bottom to top, respectively. The gainpeak shifts to shorter wavelengths for higher pump fluences.

We measured the waveguide loss coefficient ( )α λ employing a method similar to the gain coefficient extraction. The pump stripe length is kept constant while the excited area is displaced horizontally relative to the sample edge. The spontaneous emission that is generated in the photoexcited region is attenuated as it propagates through the unpumped region, following the Beer-Lambert law (inset of Fig. 4):

( )( ) LoI I e α λ

α λ −= (2) where ( )α λ is the loss coefficient, L is the displacement of the photoexcited region, and Iα is the intensity at the sample edge of an ASE signal of initial intensity oI . By use of oI and ( )α λ as fitting parameters, the data are fitted to Eq. (2) to give a loss coefficient of 7.3 ± 1.0 cm−1 at 820 nm. In Fig. 4, the net gain coefficient is plotted against the pump fluence. For large pump fluences, as the ground-state population is depleted and less molecules are available for pump absorption, the excited state population


saturates. This saturation effect can be seen in Fig. 4, where the net gain coefficient value deviates from a linear increase for the pump fluence above 1.57 mJ/cm2. It is also found that the transparency pump fluence, for which gain and loss inside the waveguide are equal, is 0.14 mJ/cm2 (28 kW/cm2), which corresponds to an excited-state population of 3.8×1016 cm-3 or a fractional population of 0.7 %.

Fp (mJ/cm2) γ (λ) (cm-1) Ls (×10-1 cm) G (dB)

0.20 5.1 ± 0.6 - -

0.79 19.0 ± 1.5 1.98 ± 0.17 16.3 ± 2.7

1.18 27.5 ± 1.9 1.63 ± 0.12 19.4 ± 2.8

1.57 37.2 ± 2.1 1.22 ± 0.07 19.7 ± 2.3

Table 1. The net gain coefficient γ (λ), saturation length Ls, and small-signal gain G (= 10 log[exp[γ(λ)Ls]])

values for a series of pump fluences. To gain further insight into the behavior of the amplifier, we characterized the small-signal gain,

( ) 10log[exp[ ( ) ]]sG Lλ γ λ= , where SL is the unsaturated excitation stripe length. The net gain coefficient, saturation length, and corresponding small-signal gain are given in Table I. It is found that an unsaturated small-signal gain of 19.7 ± 2.3 dB is achievable in a 1.2-mm waveguide.

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Fig. 4. Net gain coefficient plotted as a function of pump fluence. The transparency pump fluence is

found to be 0.14 mJ/cm2, which corresponds to the excited population of 3.8×1016 cm-3. Inset: Emission intensity as a function of pump stripe displacement. Solid curve is fit to Eq. (2), giving a loss of 7.3 ± 1.0 cm-1.

Yamashita et al.14 reported a low ASE threshold from poly(vinyl-pyrrolidone) waveguides doped with the near-infrared dye 4-[4-[4-(dimethylamino)phenyl]-1,3-butadienyl]-1-ethylquinolinium perchlorate (commercially available as LDS 798). These waveguides had their ASE peaks at approximately 800 nm and they exhibited a


maximum net gain coefficient of 35 cm-1, which is close to our value of 37.2 cm-1, albeit for a lower pump fluence of 74 µJ/cm2 (93 kW/cm2). This difference in pump fluence could be attributed to a few factors. First, a much shorter pump pulse duration of 800 ps is used in [14]. In general, the pump duration strongly affects the efficiency of the creation of excited species. The number density of the excited state is given by N2=σpNEpτf/ћωτp[1–exp(–τp/τf)], where σp is the absorption cross section at the pump wavelength, N is the total dye concentration, Ep is the pump fluence, ω is the angular frequency of the pump light, τf is the metastable lifetime, and τp is the pump pulse duration. Here the quantity in the bracket, which is a function of the ratio of the metastable lifetime and pump pulse duration, infers that the subnanosecond pumping is much more efficient than use of 5-ns pump pulses for a gain medium with a nanosecond metastable lifetime. Secondly, The dye concentration used in our study was 0.4 wt% owing to the limited solubility of the dye in the polymer solution as opposed to 2 wt% in [14]. Lastly, the absorption cross-section of LDS821 at the pump wavelength of 532 nm (0.7×10-16 cm2) is somewhat smaller than that of LDS798 at 565 nm (1.0×10-16 cm2).15

4- MICROLASERS

The microcavities were transversely pumped at 532 nm with a Nd:YAG laser (10 Hz repetition rate, 5 ns pulse width). The excitation beam was tightly focused by two cylindrical lenses into a stripe shape perpendicular to the axis of the optical fiber supporting the microrings.12 The polarization of the pump beam was kept parallel to the fiber axis. Emission from the microrings was detected by a spectrograph with a resolution of ~0.25 nm. All the measurements were performed under ambient conditions.

Light in the cavity modes is coupled out via imperfect reflection at the polymer-air interface. Figure 5 shows typical emission spectra from a microring with a diameter of 125 µm for a series of pump fluences above laser threshold. The spectra feature several evenly spaced sharp peaks distributed over a range of 15 nm at approximately 820 nm. Figure 6 shows the spectrally integrated emission intensity plotted as a function of pump fluence for the same microring as in Fig. 5. The output shows a linear increase, as expected for lasing, above threshold, which was found to be ~650 µJcm-2 from the intersection of two straight lines fitted to the data points. The inset in Fig. 5 shows the spectra of two cavity modes at 818 and 822 nm, which were taken at 720 µJcm-2, a fluence slightly above threshold.

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(2.15 mJ/cm2)

(1.59 mJ/cm2)

Em

issi

on In

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ity (a

.u.)

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(0.95 mJ/cm2)

816 820 824

Fig. 5. Emission spectra for a microring with an inner diameter of 125 µm, taken at increasing pump

fluences. From bottom to top, the pump fluences are respectively 0.95, 1.59 and 2.15 mJ/cm2. Inset: Emission spectrum at 0.71 mJ/cm2, just above laser threshold.

Each self-assembled microring has slightly varying dimensions, which could result in a certain variation in the threshold values. Therefore average was taken over threshold values measured for several different microrings.


The result is presented in the inset of Fig. 6.16 The average threshold decreases with the increasing inner diameter of the microrings, ranging from 80 to 200 µm. Here the microrings with a larger diameter result in a longer gain path length and hence show lower threshold values.

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Fig. 6. Input-output characteristic corresponding to the spectra given in (a). From the figure we read a

laser threshold of 641 µJ/cm2. Inset: Laser threshold depicted as a function of microring inner diameter

The lower limit of the overall cavity quality factor (Q) can be estimated directly from the free spectral range.10 The Free Spectral Range (FSR) is given by:

2 2

eff eff

λ λFSR = =2n L πn D

(3)

where D is the diameter of the microring and neff is the effective refractive index. The measured mode spacing for various diameters in the inset of Fig. 2 is in good agreement with the values predicted by Eq. (3) with neff = 1.518. The lower limit of the quality factor Q is given by Q = 2 /λ FSR . In Fig. 7 are plotted the cavity quality factors as a function of microring diameter. The polymer microrings show larger Q with increasing diameter, ie. longer gain path. The total quality factor Q is given by taking summation over three major contributions:10

-1 -1 -1 -1abs scat cavQ = Q +Q +Q (4)

Here Qscat-1 describes the losses due to light scattering by surface roughness and inhomogeneities in the film,

Qcav-1 corresponds to the intrinsic diffraction leakage associated with the curvature of the cavity, and the

absorption limited quality factor Qabs is given by

effabs

2πnQ =

αλ (5)

where α is the absorption coefficient of the polymer gain medium. The absorption edge of the guest dye-molecules extends to 850 nm and the absorption coefficient α of the dye in solid-state form was measured to be α = 4 cm-1 at 820 nm. Using Eq. (5), we obtained a value of 3 × 104 for Qabs at 820 nm. Since the leakage of photons due to imperfect reflection is so small, Qcav is several orders of magnitude higher than Qabs and can


therefore be neglected. A device with an inner diameter of 200 µm has an average Q of 2.3 × 103. Using Eq. (4), we calculated Qscat to be 2.5 × 103, which is small compared to the value of 3 × 104 for Qabs. Thus we conclude that the upper limit for Q is set by Qscat rather than by Qabs.

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R ing Inner D iameter (µm )

Fig. 7. The average Q-factor is plotted for microrings with various inner diameters. Inset: The average FSR is also plotted as a function of inner diameter. Comparing the two plots, it can be deduced that a small FSR corresponds to a high Q-factor.

We also characterized the intensity distribution of the laser output using the method described in [10]. A 1-mm pinhole was positioned in front of the light guide used to detect the emission from the microring. By varying the height of the pinhole and light guide simultaneously (parallel to the length of the fiber), we detected the emission along the microring, in steps of 0.25 mm. Figure 8 shows the measured intensity distribution. A Gaussian curve was fitted to the data points to give a FWHM of 3 mm. The emission from the microring sustained an angle ∆θ of 0.5 rad. From ∆θ we can estimate the lateral size ζ of the lasing mode, where ∆ς λ θ µ= / =1.6 m . This value is smaller than the width of the pump stripe on the microring, which is ~50 µm. The smaller lateral size indicates that the microring suffers from light scattering due to inhomogeneities and surface roughness of the polymer layer. This result confirms that losses due to light scattering make a major contribution to the reduction of the overall quality factor.

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Microring

PumpBeam Y

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(a.u

)

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Fig. 7. Emission distribution of a microring with an inner diameter of 200 µm. The data points were well-fitted with a Gaussian curve (FWHM: 3 mm). Top Left corner: Set-up used to measure the emission distribution.


5- CONCLUSIONS

In summary, a large gain coefficient of 37 ± 2.1 cm-1 and the loss coefficient of 7.3 ± 1.0 cm-1 at 820 nm have been measured in luminescent polymer waveguides under nanosecond photopumping. From the measured net gain coefficient and unsaturated excitation stripe length, it has been shown that a small-signal gain of 19.7 ± 2.3 dB is achievable in a 1.2-mm waveguide amplifier. Furthermore, self-assembled luminescent polymer microrings have been fabricated and characterized. Through the laser threshold determination of the devices with varying cavity diameters, we have shown that the laser threshold is inversely proportional to the inner diameter of the microrings. Our results will provide an impetus for the development of compact photopumped amplifiers and lasers operating in the 0.8-µm communication band, which is currently inaccessible with other organic gain media. The authors are grateful to Y. Rakovich for the excellent technical assistance in the metastable lifetime measurement.

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12 Ben-Messaoud T., Dou S. X., Toussaere E., Potter A., Josse D., Kranzelbinder G., and Zyss J, "Longitudinal pumping of polymer microring lasers." Synth. Met. 127 (1-3), 159 (2002).


13 Shaklee K. L., Nahory R. E., and Leheny R. F., "Optical gain in semiconductors," J. Lumin. 7, 284 (1973).

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15 Djiango M., Kobayashi T., and Blau W. J., unpublished data. 16 There, is no noticeable difference in the laser threshold when the pump beam polarization is

orthogonal, instead of parallel, to the optical fiber axis.


near-infrared luminescent polymer waveguides and microlasers

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