Continuous-wave biexciton lasing at room temperature using solution-processed quantum wells

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1 CORRECTION NOTICE Continuous-wave bieciton lasing at room temperature using solution-processed quantum wells Joel Q. Grim, Sotirios Christodoulou, Francesco Di Stasio, Roman Krahne, Roberto Cingolani, Liberato Manna and Iwan Moreels Nature Nanotechnology 9, (2014) In the version of the Supplementary Information originally published, ref. 3, A. Naeem, F. Masia, S. Christodoulou, I. Moreels, P. Borri, and W. Langbein, arxiv: (2014), was not included, and the first sentence under the heading Calculation of the eciton binding energy should have read The eciton binding energy was determined by fitting the etinction spectrum in Fig. 1a (see main tet) with a quantum-well absorption model (1,2), first applied to CdSe colloidal quantum wells by Naeem et al.(3). These errors have been corrected in this file 27 February 2015.

2 Continuous-wave bieciton lasing at room temperature using solution-processed quantum wells Authors Joel Q. Grim, 1 Sotirios Christodoulou, 1 Francesco Di Stasio, 1 Roman Krahne, 1 Roberto Cingolani, 1 Liberato Manna, 1 Iwan Moreels 1 Affiliation 1 Istituto Italiano di Tecnologia, via Morego 30, IT Genova, Italy. 1. Size Histograms of Transmission Electron Microscopy Images Figure S1 shows the size distribution of the CdSe CQwells used in this work, obtained by measuring the width and length of 100 particles in transmission electron microscope images. Averages are determined directly from the data, with a Gaussian distribution shown for comparison. Figure S1. Histograms of the distribution of the CQwell (a) length and (b) width. The average length of 35.3 nm and width of 8.5 nm result in an CQwell area of 300 nm Calculation of the Eciton Binding Energy The eciton binding energy was determined by fitting the etinction spectrum in Fig. 1a (see main tet) with a quantum-well absorption model (1,2), first applied to CdSe colloidal quantum wells by Naeem et al.(3) NATURE NANOTECHNOLOGY 1

3 A ( EE ) E C 0 b ( ) px erf 1 2 C pe (S1) p is the absorption line shape for a quantum well eciton with asymmetric broadening 2 1 EE erf 0 EE 1 ep 0 p (S2) E b are the absolute eciton energy and binding energy, respectively. is the line width E0 and of the absorption peak. The continuum edge has a step height absorption spectrum is fit with A C and width C. The full ( E) A p ( E) A p ( E) (S3) HH HH A HH and A LH are the weights of the heavy and light hole bands. The parameter values determined from the fit are listed in Table S1. HH LH E b (mev) 132 ± ± 2 E 0 (mev) 2406 ± ± (mev) 30.4 ± ± 0.1 A C ( mev) 5734 ± ± 111 C (mev) 79.9 ± ± 3.9 (mev) 47.6 ± ± 0.01 A 0.09 ± ± 0.01 Table S1. Parameters and corresponding errors from fitting Eq. S3 to the absorption spectrum in Fig. 1b (see also main tet). Components of the fit are shown in Fig. S2, with line shapes p HH and p LH corresponding to the heavy and light hole absorption peaks, and continuum edges C HH and C LH. LH LH 2 NATURE NANOTECHNOLOGY

4 SUPPLEMENTARY INFORMATION Figure S2. Measured absorption spectrum (circles) and fit (black line) with labeled individual components. Due to a minor contribution of 4ML CQwells, there are small peaks in the absorption spectrum around 2.7 ev and 2.95 ev, corresponding to the HH- and LH- transition, respectively. Therefore, the regions between 2.66 ev and 2.74 ev, and 2.90 ev and 3.0 ev are omitted from the fit. (1) R. F. Schnabel, R. Zimmermann, D. Bimberg, H. Nickel, R. Lösch, and W. Schlapp, Phys. Rev. B 46, 9873 (1992). (2) K. Leosson, J. R. Jensen, W. Langbein, and J. M. Hvam, Phys. Rev. B 61, (2000). (3) A. Naeem, F. masia, S. Christodoulou, I. Moreels, P. Borri, and W. Langbein, arxiv: (2014). NATURE NANOTECHNOLOGY 3

5 3. Fluence-dependence of the Bieciton/Eciton PL The development of the red-shifted bieciton band that rises quadratically with intensity (Fig. 2 in main tet) is more clearly visible in the peak-normalized intensity-dependent spectra shown in Fig. S3. Further confirmation comes from measurements at 4 K as the emission line widths are reduced. Again, a red-shifted band emerges with a superlinear intensity dependence. Figure S3. Development of the bieciton band as a function of fluence (fs ecitation) at (a) room temperature and (b) 4 K. 4. Fitting of the Eciton/Bieciton Spectra The eciton and bieciton spectra were fit by a pseudo-voigt function of the form V 2 4ln(2) 4ln(2) (1 ) ep ( EE ) 4( E L E0 ) L G G (S4) The left and right terms are the Lorentzian and Gaussian line shapes with relative contributions and 1 and widths L and G, respectively. The parameters obtained from fitting the eciton PL shown in the top panel of Fig. 2b (see main tet) are held constant for the fit of the spectrum in the bottom panel, and the broadened PL is fit with Vtotal AV AV, where V and V are Voigt functions for the eciton and bieciton contributions with amplitudes A and A. 4 NATURE NANOTECHNOLOGY

6 SUPPLEMENTARY INFORMATION 5. Power intensity and Variable Stripe Length Measurements We determined the SE threshold and gain coefficients with fluence-dependent (Fig. S4) and variable stripe length measurements (Fig. S5). For the fluence-dependence, a threshold is observed, with the PL giving way to SE, at a fluence of merely 6 µj/cm 2. Figure S4. Fluence dependence of the spectrally integrated PL. At 6 µj/cm 2, a clear threshold is observed. For both fs- and c.w.-pumped variable stripe length (VSL) measurements, a cylindrical lens was used to focus a laser stripe on a thin film of CdSe CQwells. The SE signal was collected with a streak camera (in focus mode) with a 7.5 cm focal length lens at 90 o with respect to the ecitation beam. Using the streak camera for spectrum collection allowed switching to operate (streak) mode to measure the decay dynamics without changing alignment. For both c.w. and fs measurements, the sample surface was 0.5 cm in front of the beam focus (i.e. 9.5 cm from the cylindrical lens) to avoid power densities that would damage the sample. Figure S5. (a) Schematic of the VSL measurement used to determine the gain coefficient. (b) Integrated intensity of the ASE peaks for fs and c.w. ecitation vs. stripe length. NATURE NANOTECHNOLOGY 5

7 Details of the VSL measurements: Amplified femtosecond ecitation measurements. The average power was 200 µw and the stripe dimensions were µm, resulting in a full-stripe fluence of 88 µj/cm 2. Continuous wave measurements. The power was 18 mw for 444 nm beam. The stripe dimensions were µm, resulting in a power density of 14 W/cm 2. Thin film preparation: For both general spectroscopy measurements (PL, stimulated emission) and lasing measurements, 5 µl drops of concentrated CdSe CQwell solution was drop cast sequentially, allowing the previous drop to dry before adding the net. We were able to create homogeneous films (on the scale of the ecitation spots used), so spin-coating was not used. The thickness of the drop-casted films for spectroscopy measurements (ASE and PL) were 4 ± 1 µm as determined using a Veeco Dektak150 profilometer. 6. Photoluminescence Ecitation Spectroscopy A PLE spectrum was measured on the CdSe CQwells for an emission wavelength of 515 nm. To avoid instrumental line broadening, emission and ecitation slits corresponding to a resolution of 0.3 nm and 0.5 nm, respectively, were used. The line width of the first PLE peak equals 34.8 ± 0.4 mev, in good agreement with the line width of the first absorbance peak (35.8 ± 0.5 mev). This shows that at room temperature, all CQwells share the same emission feature and there is no heterogeneous line broadening. Figure S6. PLE (full line) and absorbance (dashed line) spectrum of the CdSe CQwells. The PLE spectrum is collected for an emission wavelength of 2.41 ev (515 nm). 6 NATURE NANOTECHNOLOGY

8 SUPPLEMENTARY INFORMATION 7. Scanning Electron Microscopy of the CdSe CQwell Films Scanning electron microscope images were collected from a CdSe CQwell film drop casted onto a silicon substrate (Fig. S7). The film was prepared using the same deposition and drying conditions as the one enclosed into the optical cavity to guarantee comparable morphologies. Cross-sections of the CdSe CQwells film are shown in Fig. S7a and Fig. S7b, and yield a film thickness of about µm. Fig. S7c shows a top view of the close-packed thin film. Figure S7. Scanning electron microscope images of a CdSe CQwell film drop casted onto a silicon substrate. Cross-sections are shown in A and B, with a top view shown in C. NATURE NANOTECHNOLOGY 7

9 8. Bragg Reflectors and Microcavity The optical cavity was prepared by enclosing a CdSe CQwell film in between two Bragg reflectors that are 99.5% reflective at 530 nm (FD1B, Thorlabs, Inc.). The transmission (T) spectrum of the BRs is shown in Fig. S8. It shows T = 90% at 2.79 ev to allow the pump beam to ecite the gain material. Figure S8. Bragg reflector transmission compared with the c.w. lasing emission peak. The CdSe CQwell film was obtained by drop casting a concentrated toluene solution onto the ecitation BR. The solution was dried in air at room temperature before placing the eit BR on top. As observed in the SEM images (Fig. S7), the CdSe CQwell film is about µm thick. 9. Observation of Individual Lasing Modes under c.w. Ecitation Using a 600 gr/mm grating in the spectrometer, we can clearly distinguish the different lasing modes under c.w. pumping conditions. In Fig. S9, a modal frequency of about Hz can be estimated. Assuming the CdSe CQwells plus organic ligands have a refractive inde of about n = 1.7 and that the cavity is completely filled with the CQwell medium, the cavity length can be calculated as: c L 40 μm 2n This value agrees well with the µm CdSe film thickness determined from SEM. 8 NATURE NANOTECHNOLOGY

10 SUPPLEMENTARY INFORMATION Figure S9. Lasing modes with Hz from c.w. ecitation. 10. Observation of Individual Lasing Modes under Femtosecond Ecitation Time-resolved measurements using fs pulsed ecitation confirm both the ultrafast nature of the lasing peak and that it is a multi-mode envelope. The spectrum of the image in Fig. 4b (see main tet) reveals more clearly defined lasing modes with a regular spacing of about Hz (Fig. S9). This results in a cavity length of about 60 µm. The small difference between the c.w. and fs mode spacing can be attributed to the fact that the mode spacing is close to the 0.3 nm spectral resolution of the spectrometer, and that we probe different areas of the cavity. Figure S10. Lasing from fs ecitation lasing modes. Blue dashed lines are spaced at Hz intervals, coinciding with lasing modes. Inset: Streak camera image of the lasing peak under fsecitation. The background is shown in blue. NATURE NANOTECHNOLOGY 9

11 Additional Control Eperiments: 11. Eclusion of Single-Pass Amplified Spontaneous Emission as the Source of the Superlinear, Narrowband Emission Prior to adding the second BR to enclose the CQwell film, the spectrum was collected under identical ecitation conditions used for the c.w. lasing eperiments (i.e. eciting through the first BR to the CQwell film on the other side). The resulting spectrum is consistent with eciton and bieciton PL. The addition of the second BR results in narrowband emission shown in Fig. 4. Figure S11. Measurements with and without the second Bragg reflector to rule out ASE emission. 10 NATURE NANOTECHNOLOGY