Single-Mode Lasing from a Monolithic Microcavity with Few

Jul 14, 2016 - Single-Mode Lasing from a Monolithic Microcavity with Few-Monolayer-Thick Quantum Dot Films. Hyochul Kim, Kyung-Sang Cho, Heejeong Jeon...
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Letter pubs.acs.org/journal/apchd5

Single-Mode Lasing from a Monolithic Microcavity with FewMonolayer-Thick Quantum Dot Films Hyochul Kim, Kyung-Sang Cho, Heejeong Jeong, Jineun Kim, Chang-Won Lee, Weon-kyu Koh, Young-Geun Roh, Sung Woo Hwang, and Yeonsang Park* Samsung Advanced Institute of Technology, 130, Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 16678, Korea S Supporting Information *

ABSTRACT: We monolithically fabricated vertical cavity lasers with densely packed colloidal quantum dot films and demonstrated single-mode lasing operation using nanosecond optical excitation. Due to the accurate spectral and spatial alignment of the cavity optical modes and the quantum dot gains, in addition to the high quality factors of our devices, we observed lasing from a colloidal quantum dot gain medium only 35 nm thick (6 or 7 monolayers thick) with a threshold of 20 mJ/cm2. We fabricated a laser with more numerous quantum dot layers as well, which exhibited a lower lasing threshold of 9 mJ/cm2 due to the enhanced modal gain. This work represents an important step toward fabricating monolithic colloidal quantum dot lasers for practical photonic devices. KEYWORDS: colloidal quantum dots, vertical cavity surface-emitting laser, quantum dot laser, amplified spontaneous emission, microcavity

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On the other hand, monolithic CQD laser fabrication will also be important in practical applications. In order to integrate them with developed photonic components and in a dense footprint, CQD lasers must be fabricated on substrates on the chip scale. The monolithic integration of CQD films with cavities to produce DFB lasers has been demonstrated by spincoating of CQD films on prefabricated DFB grating structures. Application of CQDs in VCSEL structures is also important, as VCSELs typically have good beam quality, high power efficiency, low threshold, and small form factors compared to DFB lasers. For monolithic fabrication of CQD VCSELs, careful film thickness calibration and uniform CQD layer deposition are required to tune the cavity resonance wavelength and to obtain cavities with high Q-factors. In addition, monolithic fabrication of short-cavity VCSEL enables singlemode lasing. Single-mode lasers are important in the application where a specific lasing wavelength is required without noise from other lasing modes and typically show lower lasing threshold compared to multimode lasers. Previously, CQD VCSELs were fabricated by either injecting liquid solutions of CQDs7 or drop-casting several-micrometerthick CQD films13 between two pre-existing mirrors, typically resulting in bulky devices and multimode lasing. In these cases, the thermal and mechanical stabilities of the CQD materials were not significant issues, as they did not require additional

olloidal quantum dots (CQDs) are promising for use as laser gain media due to their high photoluminescence (PL) quantum efficiencies, tunable emission wavelengths, and simple and cost-effective fabrication.1 CQDs can easily be integrated into various base substrates on demand and thus can have the potential to be used to fabricate unique photonic integrated circuit systems. They have been incorporated into several types of cavities to achieve lasing,2−14 and CQD lasers emitting red, green, and blue light were recently demonstrated in vertical cavity surface-emitting laser (VCSEL) and distributed feedback (DFB) laser structures.7,12 With the objective of developing practical CQD lasers that can operate with continuous wave excitation and eventually with electrical pumping, several studies have been conducted on CQD synthesis to overcome fast Auger recombination.15 For this purpose, giant-shell CQDs,16 type II CQDs,17 and gradually interfaced core/shell CQDs18,19 have been introduced to adjust the exciton wave functions of CQDs. In addition, different types of CQDs including quantum rods20,21 and nanoplatelets22−24 have been studied, and the results have shown that they are promising for use in low-threshold lasing operations. Auger recombination can also be controlled by optimizing cavity structures and their interactions with CQDs. Coupling CQD layers to microcavities with high quality factors (Q-factors)25,26 and small mode volumes, such as photonic crystal, micropillar, and microdisk cavities, can enhance the spontaneous emission due to the Purcell effect,27 thereby providing a means of overcoming fast Auger recombination. © XXXX American Chemical Society

Received: May 11, 2016

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Figure 1. (a) ASE measurement setup. Pump beam is focused onto a bare CQD sample spin-coated on glass by the cylindrical lens (CL). Pump beam length is adjusted by a slit, and signal from the device is measured in transmission geometry. LP: long-pass filter. (b) Measured PL spectra of a 135-nm-thick CQD film at various pumping energy densities. (c) Measured PL intensity versus pumping energy density for 95-, 110-, 120-, and 135nm-thick CQD films. ASE is observed for CQD films thicker than 120 nm. Inset: Modal gain obtained by the VSL method for a 135-nm-thick CQD film at a pump energy density of 28.9 mJ/cm2. Black points are measured PL intensities, and the red curve is I = A + B(egx − 1)/g fit, where g is the modal gain and x is the slit width.

realization of electrically driven CQD lasers, which is a longterm goal. We first investigated the optical properties of a bare CQD film. The CQD was composed of CdSe/CdS/ZnS core/ multishell structures that emitted red. Details about the CQD synthesis procedure can be found in the references.30 The CQD films were deposited on the glass by spin-coating, and the solution concentrations and spin speed were adjusted to obtain CQD layers of uniform thickness and quality. An optical parametric oscillator (OPO) was used as a pump beam, and the wavelength was set to 500 nm. The pulse width and repetition rate were 9 ns and 100 Hz, respectively. We focused the pump beam into a stripe by a cylindrical lens (Figure 1a). The beam length was determined to be 1 mm by using a slit placed before the lens, and the width of the focused beam stripe was measured to be 90 μm at the sample position. Here, the beam width was defined as the full width at half-maximum (fwhm) of the measured power of the focused laser spot, where the power distribution was determined by knife-edge measurement. The transmitted beam was filtered by a long-pass filter, and the signal was collected by an optical fiber and sent to a spectrometer (Ocean Optics QE65000). Figure 1b shows the measured PL spectra for a 135-nm-thick CQD film at various pumping energy densities. At a low pumping energy density of 3.3 mJ/cm2 (black curve), the PL emission is centered at 607 nm with a fwhm of 34 nm. With increasing pump power, the spectra develop an additional peak at 618 nm due to amplified spontaneous emission (ASE). The

high-temperature fabrication. However, the uniform layering of top distributed Bragg reflectors (DBRs) after CQD deposition and the monolithic chip-scale device fabrication require the thermal stability of the underlying CQD film. CQD films with trioctylphosphine oxide ligands possess excess ligands and therefore are not suitable for use in fabrication processes conducted at temperatures above 100 °C. On the other hand, CQD films with oleic acid ligands are usable in monolithic VCSEL fabrication because they are stable at temperatures up to 200 °C due to their minimal content of extra ligands.28 In this study, we fabricated monolithic VCSELs using dense CQD films with oleic acid ligands and demonstrated singlemode lasing by using nanosecond optical pumping pulses. Due to the uniformity and thermal stability of the thin CQD films used in our devices, the top DBRs could be deposited at temperatures above 150 °C, which ensured the fabrication of high-Q-factor VCSELs. Due to its Q-factor above 300, we observed a lasing threshold of 20 mJ/cm2 for the device with a 35-nm-thick CQD layer, which is the thinnest CQD film yet reported to have been used as a gain medium in a VCSEL. We were also able to deposit multiple CQD layers uniformly in a cavity and demonstrated lasing with a lower threshold of 9 mJ/ cm2 using a CQD VCSEL with three CQD layers (total thickness of ∼300 nm). It is worth noting that the CQD films in our devices were as thin as those used for electrical operation of CQD light-emitting diodes.28,29 Further cavity optimization in addition to CQD gain improvement will enable the B

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Figure 2. (a) TEM image of CQD VCSEL (top panel) and magnified image of a cavity (bottom panel) with a 35-nm-thick (6−7-monolayer-thick) CQD layer. Bars indicate 0.5 μm and 100 nm in top and bottom panels, respectively. (b) Reflection spectrum (black) of the same device shown in (a). Transmission spectra of bottom (red) and top DBRs (blue) are shown as well. (c) Spectra measured at a pumping energy density of 1.36 mJ/ cm2 for VCSEL1 (black), VCSEL2 (red), and VCSEL3 (blue), which have 35-, 40-, and 50-nm-thick CQD layers and exhibit cavity modes at 616, 624, and 640 nm, respectively.

of the mirrors (Figure 2b). The CQD films were carefully deposited at the antinode of each cavity field to maximize the modal gain and to achieve high cavity Q-factors. For the λ/2thick SiO2 cavity, we deposited the CQD film near the middle of the cavity, which was the antinode of the optical field. The CQD films used in our devices were thermally stable up to 200 °C, so the bottom and top DBRs were deposited at elevated temperatures of 250 and 150 °C, respectively, to ensure uniform film quality and to obtain TiO2 layers with high refractive indices. As shown in the TEM image in Figure 2a, the film quality in each fabricated device was highly uniform, resulting in high cavity Q-factors. Figure 2b shows the reflection spectrum of the same device, which exhibits a cavity dip with a wide range of photonic band gaps. To achieve sufficient gain to enable lasing, the cavity mode of each device must be spectrally aligned with the peak of its CQD gain. The cavity mode wavelength could be tuned by adjusting the thicknesses of the CQD films or the surrounding SiO2 layers in the cavity. In this work, we fabricated three VCSELs with different CQD film thickness to tune the cavity mode wavelength. Figure 2c shows the spectra for the three VCSELs obtained using a low pump power (1.36 mJ/cm2), where the black (VCSEL1), red (VCSEL2), and blue (VCSEL3) curves correspond to the devices with 35-, 40-, and 50-nm-thick CQD films, respectively, in their cavities. With increasing CQD film thickness, the cavity mode red-shifts from 616 nm (VCSEL1) to 640 nm (VCSEL3). Here, the cavity spectra were measured by employing a method similar to that used for the ASE measurement, except that a convex lens, instead of the cylindrical lens, was used to focus the pump laser into a circular 75-μm-diameter spot. The cavity wavelengths of VCSEL1 and VCSEL2 are relatively well matched with the ASE peaks (Figure 1b), while that of VCSEL3 shows a large offset. Below the lasing threshold, the cavity Q-factors, defined as Q = λ/Δλ, were measured to be 340, 360, and 370 for VCSEL1, VCSEL2, and VCSEL3, respectively, which were sufficiently large to enable lasing.32

ASE threshold for this sample is about 6 mJ/cm2, as can be seen in this figure. This threshold energy density is relatively higher than those previously reported,7,11,31 necessitating further improvement of our CQD gain. The ASE threshold is also related to the pulse width of the pump laser because of the Auger effect and thermal degradation of the CQD gain. Using femtosecond laser pulses, the ASE threshold is reduced to about 0.12 mJ/cm2, a 50 times reduction compared to nanosecond OPO pump pulses (Supporting Information). A minimum CQD film thickness is necessary to provide sufficient gain to achieve ASE. Figure 1c shows the measured PL intensity as a function of pumping energy density for various CQD films. For film thicknesses of 95 and 110 nm, only sublinear PL intensity increases with increasing pump power are observable, with no indication of ASE. However, the PL intensities of CQD films thicker than 120 nm exhibit kinks indicating the onset of ASE. Using a variable stripe length (VSL) method,2 we estimated the modal gain of the CQD films. The inset in Figure 1c shows the measured PL intensity (black points) as a function of a stripe length (slit width) for a 135-nm-thick CQD film at a pump energy density of 28.9 mJ/ cm2. The red curve represents a fit of the measured data to the function I = A + B(egx − 1)/g, where g is the modal gain, x is the slit width, I is the measured emission intensity, and A and B are fitting parameters. The modal gain was calculated to be 28 ± 3 cm−1 at this pump energy density, which is relatively lower than those previous reported.7,31 Next, we fabricated CQD VCSELs with thin CQD films and investigated their lasing characteristics. Figure 2a shows a transmission electron microscope (TEM) image of one such device (top panel) and a magnified image of its cavity (bottom panel). Each device consisted of a bottom DBR with nine pairs of SiO2 and TiO2 layers, a cavity, and a top DBR with 10 pairs of TiO2 and SiO2 layers. The bottom and top DBRs consisted of alternating layers of λ/(4n)-thick SiO2 and TiO2 films, where λ = 620 nm and n is the refractive index of either SiO2 or TiO2. The transmittance of bottom and top DBRs were measured to be below 0.5% at the band gap region, indicating high reflection C

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Figure 3. (a) Measured spectra of VCSEL1 at various pumping energy densities. Top graph (red) has been compressed by 500 times, and each graph has been shifted vertically for clarity. (b) Image of the VCSEL output beam after the long-pass filter. (c) Line widths (top panel) and measured intensities (bottom panel) of VCSEL1 (black), VCSEL2 (red), and VCSEL3 (blue) versus pumping energy density. Inset: Linear plot of intensity versus pumping energy density. Data points: measured data; solid lines: linear fits.

higher, 54 mJ/cm2. Despite the thicker gain medium in the VCSEL3 cavity, the larger spectral mismatch between the cavity mode and the CQD gain (mostly ASE) is responsible for this higher lasing threshold. All three CQD VCSELs exhibited lasing thresholds on the order of tens of mJ/cm2, similar to their ASE thresholds. The relatively large lasing thresholds we obtained are attributed to the long pulse width (Supporting Information) and insufficient gain from CQD layers. As our cavities possess a relatively large mode volume, using a smaller mode volume cavity could be important to overcome the fast Auger decay by enhancing the Purcell effect of CQD emission. Besides the long pump pulse width used in this study, insufficient gain from the employed CQD films could be responsible for the high lasing thresholds of these devices, requiring improvement of gain properties of incorporated CQD materials. Furthermore, in each of our VCSELs, most of the pump beam (more than 90%) was not absorbed by the CQD film, but rather was transmitted through or reflected from the device, which would also require higher pumping energy. By simply increasing the CQD film thickness, the pump beam absorption volume of the gain medium could be increased, thereby increasing the modal gain at the same pump power. For this purpose, we fabricated a VCSEL with three layers of CQD films in its cavity. The bottom right inset in Figure 4 shows a TEM image of the fabricated device. Three CQD layers with thicknesses of 120, 120, and 55 nm, in sequence, were deposited near the three antinodes of the optical field in the 3λ/2-thick cavity, and SiO2 layers were alternated with the CQD layers. The cavity mode occurred at 623 nm (top left inset), which corresponds closely to the CQD gain peaks similar to VCSEL1 and VCSEL2. Figure 4 shows the measured emission intensity as a function of the pump energy density. By performing a linear fit of the measured data, we obtained a lasing threshold of 9 mJ/cm2 for this device. Compared with the thresholds of the single-layer CQD VCSELs, that of the three-layer CQD VCSEL is lower by about

To perform the lasing experiment, we measured the cavity spectra with increasing pump power. Figure 3a shows the measured spectra of VCSEL1 at various pumping energy densities. Because the cavity thickness was only λ/2, a single longitudinal mode is observable for this device regardless of the pump power. Figure 3b shows the image of the VCSEL output beam, where the bright and spatially well-defined red spot is observable. The line widths and measured intensities of VCSEL1 (black) as well as VCSEL2 (red) and VCSEL3 (blue) are plotted versus pump energy density in the top and bottom panels in Figure 3c, respectively. The cavity line width of VCSEL1 is 1.84 nm (Q = 340) at a low pump power of 0.23 mJ/cm2, does not exhibit rapid changes up to a certain pump energy density, and then suddenly drops near the onset of lasing. The cavity line width is measured to be about 1.25 nm at the lasing threshold. Considering the precision of this measurement was limited by the spectrometer resolution, the actual line width is thought to be below 1 nm at the lasing threshold (Supporting Information). The line width shows a gradual increase from below 1.25 nm (Q > 500) at the lasing onset to about 1.3 nm (Q = 470) at the pump power of 280 mJ/cm2. This line width broadening could be due to the temperature fluctuation at high pump power, which results in cavity mode drift near the center cavity wavelength. For the same device, the PL intensity slowly increases at low pump powers but starts to increase very rapidly near the lasing threshold. The inset in Figure 3c shows a linear plot of the measured PL intensity as a function of pump power density. By fitting the data with a linear function, we obtained a threshold pump energy density of ∼20 mJ/cm2. For VCSEL2, we observed a similar behavior to VCSEL1. The red curve in Figure 3c shows a VCSEL2 line width of 1.71 nm (Q = 360) at 0.23 mJ/cm2 that then decreases to 1.25 nm near the lasing threshold. The threshold pump density for VCSEL2 was measured to be 21 mJ/cm2, similar to that of VCSEL1 (inset in the bottom panel of Figure 3c). However, for VCSEL3 (blue curve), the lasing threshold was significantly D

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Optical properties of CQDs; ASE and lasing measurement with femtosecond pump pulses; high-resolution measurement of a single-mode VCSEL (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 4. Measured intensity of a three-layer CQD VCSEL versus pumping energy density. Top left inset shows cavity spectra at pumping energy densities of 0.56 mJ/cm2 (below lasing threshold) and 21.0 mJ/cm2 (above threshold). Bottom right inset shows a TEM image of the fabricated device. Bar indicates 0.5 μm length.

a factor of 2. The CQD layer was much thicker in the threelayer CQD VCSEL than in the single-layer CQD VCSELs, but this threshold reduction is rather moderate. We suspect that surface roughness that accumulated due to the thick CQD films and sequentially deposited top DBR layers could have caused additional losses in this device. CQD films can also be deposited with solvent-free transfer printing,29 which could be used to fabricate more uniform CQD films in cavities. For the three-layer CQD VCSEL, the cavity mode shows slight blue shift above the threshold (Figure 4 inset), which also occurs at single-layer CQD VCSELs but with lower amount (Figure 3a). The blue shift of the cavity mode could be due to the fact that the maximum of CQD gain is located at the wavelength shorter than the cavity mode, where higher gain can be obtained at the shorter wavelength part of the cavity mode. In conclusion, we fabricated monolithic CQD VCSELs and demonstrated single-mode lasing using densely packed CQD films as thin as 35 nm. Multiple layers of CQD films with various thicknesses were also monolithically integrated into a VCSEL structure. By applying nanosecond pump pulses, lasing thresholds of 20 and 9 mJ/cm2 were obtained for single-layer and three-layer CQD VCSELs, respectively. The CQD films used in this work are sufficiently thin to enable electrical carrier injection if combined with proper carrier-transporting layers. The relatively large threshold is mainly caused by the low CQD gain. Careful adjustment of the quantum dot core/shell interface18,19 and replacement of organic ligands with inorganic ones31 could be promising routes to enhance the CQD gain by lowering the Auger decay and enhancing the CQD filling factor. Furthermore, employing microcavities with higher Q-factors and smaller mode volumes25−27 as well as better spectral match with gain could also lower the lasing threshold, thus enabling the fabrication of CQD lasers that could be integrated into practical photonic systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.6b00327. E

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