1

Optical Microfluidic Waveguides and Solution Lasers of Colloidal Semiconductor

Quantum Wells

Joudi Maskoun, Negar Gheshlaghi, Furkan Isik, Savas Delikanli, Onur Erdem, E.Yegan

Erdem*, and Hilmi Volkan Demir*

J. M., N. G., F. I., Dr. S. D., Dr. O. E., Prof. E. Y. E. and Prof. H. V. D.

UNAM – Institute of Materials Science and Nanotechnology, Bilkent University, Ankara

06800, Turkey

Dr. S. D. and Prof. H. V. D.

Luminous! Center of Excellence for Semiconductor Lighting and Displays, School of

Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang

Avenue, Singapore 639798, Singapore

Dr. S. D. and Prof. H. V. D.

Division of Physics and Applied Physics, School of Physical and Mathematical Sciences,

Nanyang Technological University, 21 Nanyang Link, Singapore 639798, Singapore

Dr. O. E. and Prof. H. V. D.

Department of Electrical and Electronics Engineering, Department of Physics, Bilkent

University, Ankara 06800, Turkey

Prof. E. Y. E.

Department of Mechanical Engineering, Bilkent University, Ankara 06800, Turkey

E-mail: volkan@bilkent.edu.tr, hvdemir@ntu.edu.sg, yeganerdem@bilkent.edu.tr

Keywords: microfluidics, semiconductor quantum wells, solution optical gain, solution lasing

2

The realization of high-quality lasers in microfluidic devices is crucial for numerous

applications, including biological and chemical sensors, flow cytometry and the development

of advanced lab-on-chip (LOC) devices. In this paper, an ultralow-threshold microfluidic

single-mode laser is proposed and demonstrated using an on-chip Fabry-Pérot cavity.

CdSe/CdS@CdxZn1-xS core/crown@gradient-alloyed shell colloidal semiconductor quantum

wells (CQWs) dispersed in toluene are employed in the Fabry-Pérot cavity created inside a

poly(dimethylsiloxane) (PDMS) microfluidic device using SiO2 protected Ag mirrors to

achieve in-solution lasing. Lasing from such a microfluidic device having CQW solution as a

microfluidic gain medium is shown for the first time with a record-low optical gain threshold

of 17.1 μJ/cm² and lasing threshold of 68.4 μJ/cm² among all solution-based lasing

demonstrations. In addition, air-stable SiO2 protected Ag films were used and designed for the

first time in such a microfluidic laser to form highly tunable and reflective mirrors required to

attain a high quality Fabry-Pérot cavity. These realized record low thresholds emanate from our

high quality Fabry-Pérot cavity and core/crown@gradient-alloyed shell CQWs having giant

gain cross-section and slow Auger rates. This micro-fabricated CQW laser provides a compact

and inexpensive coherent light source for microfluidics and integrated optics covering the

visible spectral region.

3

1. Introduction

Microfluidics is an emerging technology for biological applications, including diagnostics and

drug delivery.[1] Owing to its ability to manipulate liquids at the microscale, microfluidics have

the potential to introduce new routes for chemical synthesis, biological analysis, sensing, and

cellular analysis, which require higher resolutions and lower detection limits than bulk

systems.[2,3] To date many improvements and components have been proposed and utilized to

broaden applications of these systems.[4-7] The implementation of lasers in microfluidic

networks to provide on-chip coherent/planar light sources is significantly important and desired

since such implementation of lasers in microfluidic chips opens up a wide range of possibilities

from optical detection to sensing which may lead to the development of fully automated chips.[8-

11] Microfluidic lasing and amplified spontaneous emission (ASE) using fluorescent dyes

embedded in liquid-liquid waveguides have been reported in literature.[12-16] Whitesides et al.

reported multimode lasing from a liquid-liquid waveguide laser using Rhodamine 640 at 1.1

mJ/cm2.[15] Similarly, Chen et al. observed white lasing from Rhodamine 610, Courmarin 540,

and Stilbene 420 at a threshold of 2.6 mJ/cm2.[16] The flow rates used in these devices had to be

fast enough to decrease the residence time of the gain material[15] to prevent photobleaching

due to the irreversible conversion of them into non-fluorescent molecules caused by photon

generation.[17] The necessity of continuous replenishment of dye solutions at high flow rates

requires sophisticated operation conditions such as external pumping and excess material

consumption.

CQWs, which make a prominent class of colloidal nanocrystals (NCs) with favorable properties,

such as large absorption cross-section,[18]giant modal gain,[19] large gain cross-section,[20] slow

Auger recombination[20-22] and narrow emission linewidth,[23] have been widely utilized for

optical gain and lasing applications.[24,25] These properties, especially large gain cross-section

and slow Auger recombination, make CQWs suitable for their utilization in lasers as optical

4

gain media[24] with lasing thresholds as low as 7.5 μJ/cm2.[20] Moreover, the high photo-stability

of CQWs[20,26] may allow for lasing from capillary-filled microfluidic devices, which is not

possible by using dyes due to their bleaching in static solutions. CQWs in solution have a lower

volume fraction than close-packed films of theirs, which makes the realization of in-solution

lasers from CQWs challenging.[27] Few studies have demonstrated solution-based lasing and

gain from NCs with lasing thresholds on the order of tens of mJ/cm2 using cylindrical and

spherical microcavities of optical fibers and microdrops, respectively, to create whispering

gallery mode (WGM) lasers by confining light at their concave boundaries.[28-30] However, the

reported thresholds are high due to the lower gain cross-section of the employed nanocrystals,

three orders of magnitude lower than those of CQWs.[27,30] Here, optical gain and single-mode

lasing from CdSe/CdS@CdxZn1-xS core/crown@gradient-alloyed shell CQWs in solution are

demonstrated in a microfluidic platform for the first time with a record low gain threshold in

solution-based lasing from nanocrystals. In this work, unlike the previous reports, a Fabry-Pérot

cavity inside microchannel provides optical feedback through highly reflective designed

mirrors to generate lasing action. CdSe/CdS@CdxZn1-xS core/crown@gradient-alloyed shell

CQWs were inserted into the channel from an inlet using capillary forces. Since this

microfluidic platform exhibiting lasing does not require external pumping, it has the advantage

of stability and ease of operation.

2. Results and Discussion

The microfluidic devices used in this study are made of PDMS to provide an easy integration

of the laser to existing microfluidic functionalities. The 4 mm long devices with 300 𝜇m width

and 100 𝜇m height were prepared by soft lithography technique, where patterns from a silicon

substrate were transferred to a PDMS mold (see Supporting Information). For lasing action, a

Fabry-Pérot cavity was created by depositing mirrors of different reflectance values on the walls

of the microfluidic channels using thermal evaporation. The devices were finally sealed with a

5

flat PDMS layer using O2 plasma bonding. Then, a pipette tip was inserted into the channel

inlet to fill the cavity with CQW solution. A laser was directed along the microchannel to induce

optical gain as shown in the schematic in Figure 1a and the optical output was collected from

the partially reflective mirror side as depicted in Figure 1b.

CdSe/CdS@Cd1-xZnxS core/crown@gradient-alloyed shell CQWs, which have been previously

reported by our group,[31] are used as the gain medium in this work. Photoluminescence and

absorption spectra of the synthesized core/crown@gradient-alloyed shell CQWs having three

monolayers of Cd1-xZnxS shell are presented in Figure 1c. Photoluminescence emission peaks

at 633 nm and exhibits a linewidth of ~30 nm. The inset in Figure 1c shows the transmission

electron microscopy (TEM) image of the CQWs having lateral dimensions of 26 nm by 14 nm.

Our CQWs were dispersed in toluene, which is normally incompatible with PDMS due to its

swelling capacities.[32] To address this issue and render our PDMS structure compatible with

toluene, the ratio of curing agent to PDMS was increased to 1:5 and devices were thermally

treated on a 200°C hot plate for 24 h.[33] Following the thermal treatment, mirrors were

deposited to create an FP cavity prior to the plasma bonding. The method of introduction of the

CQWs to the channel relies on capillary forces, which have the advantage of eliminating the

need for external pumping. To justify the capillary action, the contact angle of toluene on treated

PDMS was measured. According to the Young Laplace equation, the results show that capillary

forces are large enough to induce flow (see SI for details).

After the thermal treatment of the pattern holding PDMS substrates, the Fabry-Pérot cavity was

created by depositing mirrors on the walls of the microfluidic devices using a thermal

evaporator. The mirrors on the channel walls were deposited by tilting the PDMS replica in the

deposition chamber. For our experiments, the PDMS replicas were placed at a 75° angle using

holders made of PDMS, as shown in Figure S2. The back sides of the PDMS replica were

plasma bonded to glass in order to prevent excess bending of PDMS and consequently avoid

thin film damage. To obtain smooth mirror films on PDMS, the deposition of an adhesion layer

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of titanium or chromium is required[34] and increasing the adhesion layer thickness enhances

the smoothness of the thin films deposited on PDMS.[35] In the visible wavelength range, Ag,

Au, and Al are the most widely used as reflective materials with Ag having the best

performance.[36,37] However, Ag films oxidize in ambient conditions which results in significant

reduction in their reflectivity over time. To overcome this issue, a thin layer of SiO2 is deposited

to protect the Ag film and increase its durability. The mirrors were deposited in a thermal

evaporator chamber using 20 nm of Cr as the adhesion layer and 70 nm and 80 nm thick films

of Ag for the reflective mirrors. The reflectance values of these mirrors are measured to be

93% and 95% for 70 nm and 80 nm thick Ag films, respectively (see Methods). Finally, 15 nm

of SiO2 is sputtered to prevent oxidation of the Ag films. Moreover, thin films were deposited

on flat PDMS substrates for X-ray photoelectron spectroscopy (XPS) and reflectance

characterizations. XPS was performed to identify the elemental composition of the fabricated

mirrors and reflectance measurements were carried out to quantify the mirror performances.

The oxidation states of the mirror surfaces were investigated for bare Ag films and SiO2

protected Ag films using XPS. Both samples were exposed to oxygen plasma in order to mimic

experimental conditions.

The results for the mirror films with and without SiO2 protective coating are presented in

Figures 2a-c, and Figures 2d-e, respectively. As shown in Figure 2a, the Ag 3d5/2 and Ag 3d3/2

peaks were found at binding energies of 368.8 eV and 374.8 eV and satellite peaks at 372.6 and

378.6 eV, respectively. The combination of these peaks and splitting of 6.0 eV in Ag 3d doublet

indicates that the silver species exist as Ag0 on the surface and the peaks corresponding to Ag+

cations are not found.[38] The characteristic O 1s peak (Figure 2b) located at 532.5 eV is

ascribed to the lattice O2− of SiO2 and presence of other components such as OH, H2O, and

carbonate species adsorbed onto the surface. Figure 2c displays the XPS spectrum of Si 2p.

The peak located at 103.3 eV indicates that Si exists as SiO2. These results confirm that the

SiO2 layer deposition successfully prevents the formation of the silver oxide layer.

7

Figure 2d shows deconvolution of XPS silver peaks. Peak positions located at 368.8 and 368.2

eV correspond to Ag and Ag2O, respectively, along with disappearance of the satellite peak.

Since AgO is notoriously unstable in ultra-high vacuum, its presence cannot be confirmed nor

rejected. Detailed scans of the O 1s region is also recorded but not very insightful because AgO

contributions can overlap with carbonate groups from air contamination in the O 1s region

(Figure 2e). The only definitive assignment that can be made in the O 1s spectrum in Figure 2e

is the small peak at 529.9 eV, ascribed to Ag2O. The larger peak at 531.5 eV of the O 1s

spectrum could be ascribed to oxygen chemisorbed on a silver surface or Ag–O–Ag bonds.[39]

These results confirm the presence of an oxide layer on the surface of the unprotected Ag film.

The reflectance values of the mirrors are measured to be 93% and 95% for 70 nm and 80 nm

thick Ag films, respectively (see Methods). These results imply the smooth and uniform quality

of the deposited films on the PDMS substrate. A SEM image of the deposited mirror on the

channel wall is provided in Figure 2f, and the inset shows a magnification of the thin film

surface. The absence of cracks on mirror surface further confirms the smoothness and

uniformity of the deposited thin films.

To evaluate the optical gain of our CQWs in micro-channels, mirrorless devices were filled

with the core/crown@gradient-alloyed shell CQWs dispersed in toluene and then excited using

one-photon absorption. The micro-channels containing CQW solution were excited using

femtosecond pulses (pulse width≈120 fs) at 400 nm and 1 kHz repetition rate. A cylindrical

lens was used to obtain excitation with a stripe geometry. Emitted photons were collected by

using an optical fiber connected to a spectrometer. Toluene was used as a solvent in this works

instead of usually used hexane, becuase of high boiling point and refractive index of the toluene

compared to hexane and most of the other non-polar solvents. The high boiling of toluene

(110.6 °C) prevents the possibility of particle aggregation due to the solvent evaporation in the

channel. In addition, higher refractive index of toluene provides efficient confinement of the

electrical field within the gain medium which is desirable for optical gain applications.

8

Spontaneous emission centered at 637.5 nm with an emission linewidth of 29.2 nm is observed

from devices for pump fluences up to 17.1 μJ/cm2 as presented in Figure 3a. Increasing the

pump fluence further results in the emergence of a sharp peak with a narrow emission linewidth

of 6.8 nm centered around 648.1 nm, which is attributed to amplified spontaneous emission

(ASE). This attribution is corroborated by the increase in the slope of the total intensity versus

pump fluence curve beyond the threshold which is at 17.1 μJ/cm2 as can be seen in Figure 3b.

Previously optical gain from core/shell CQWs were demontrated using only two- and three

photon pumping.[27] This threshold of 17.1 μJ/cm2 from our designed CdSe/CdS@CdxZn1-xS

core/crown@gradient-alloyed shell CQW heterostructures exhibiting large modal gain[20] and

slow Auger recombination as a result of smooth confinement potential[20,40] is almost an order

of magnitude lower than the previously best threshold from solution based optical gain from

colloidal nanocrystals of all kinds. The previously demonstrated best ASE threshold in solution

is from perovskite nanocrystals with a threshold of 105 µJ/cm2. The redshift of ASE emission

(≈ 10 nm) is due to the multi-excitonic gain commonly observed in type-I structures and is

advantageous for lasing action since it reduces self-absorption and hence gain threshold.[20,29]

For the fabrication of the lasing device, two mirrors were deposited to the ends of the

microfluidic channel as described earlier. Similar to the operation conditions for the optical gain

measurements, the solution of CQWs was introduced to the channels via capillary action and

the channel was excited via one-photon absorption at 400 nm using stripe geometry by

employing a cylindirical lens. At low pump intensities, spontaneous emission dominates the

spectrum and has similar characteristics to the spontaneous emission observed before the onset

of ASE in the optical gain measurement experiment as can be seen in Figure 3c. At a pump

fluence of 68.4 μJ/cm² and beyond, the transition from spontaneous emission to lasing is evident

from the emergence of a narrow emission feature (FWHM of the lasing peak) along with

superlinear increase in the output intensity with the increasing input pump intensity as can be

seen in Figure 3c. The emission linewidth drops from 27.0 nm centered at 642.6 nm to 0.54

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nm centered at 645.6 nm. This redshifted lasing action signifies the multiexcitonic optical gain

in our device. Figure 3c presents the device emission at various pump fluences and Figure 3d

shows the drop in the linewidth with the emergence of lasing action . The quality factor (Q) of

our cavity exhibiting single mode lasing at 645.6 nm with FWHM of 0.54 nm is ~1195. This

high Q is due to the highly reflective and air stable Ag coated mirrors with SiO2 protection layer

designed for this work. At a pump fluence of 76.1 μJ/cm² and above, the laser emission saturates

and results in the expected “S”-shaped behavior as shown in Figure 3d. The output laser beam

focused onto a screen is shown in Figure 3e. The lasing threshold achieved in this work via

single-photon pumping is three orders of magnitude lower than the lasing thresholds observed

from other colloidal nanocrystals.[28,30] In addition, in all previous demonstrations of lasing

using solution of colloidal nanocrystals multimode lasing was demonstrated and single-mode

lasing has not been demonstrated.[28-30] This is also one of the reason for such high lasing

thresholds in the other demonstrations along with the lower modal gain of QDs and nanorods

compared to QWs[19] and slow Auger rates in our CQWs having smooth confinement

potential.[20,40] The ultralow threshold of single-mode lasing in our work also ensures the

stability of lasing since such high fluences used in previous works will likely promote

evoporation of the solvents as observed and detoriation of the nanocrystals[29] and as a result

diminish the lifetime and stability of these device. Irreversible blue shift of the of the gain

profile was observed in QD microdrop lasers as a result of such high fluences (three orders of

magnitude stronger than the fluences in this work) leading photo-oxidation.[29] This is likely

the reason of very few reports on solution based lasing. This makes our ultralow threshold

single-mode lasing is significantly important and our demonstration in here can pave the way

for realization of practical microfludic lasers based on colloidal CQWs. Also, it is worth

mentioning that few more modes emerge with lower intensity in our microfludic laser at higher

fluences as can be seen in Figure 3c as they overcome the losses in the cavity at higher fluences.

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3. Conclusion

In summary, we have demonstrated an ultralow threshold single-mode laser built in a PDMS

microfluidic structure using CdSe/CdS@CdxZn1-xS core/crown@gradient-alloyed shell CQWs

dispersed in toluene as the gain medium. Owing to the photo-stability of the

core/crown@gradient-alloyed shell CQWs solution and the strong capillary interaction with

PDMS, an easy to operate in-solution laser was achieved using a minimal amount of the gain

material. This is the first achievement of single-mode in-solution laser from colloidal

nanocrytals with record low threshold, which is three orders of magnitude better than previous

multimode in-solution lasers from colloidal nanocrytals. Such an ultralow threshold, which is

due to the giant gain cross-section and slow Auger rate in these core/crown@gradient-alloyed

shell CQWs, is significantly important for realization of practical microfludic lasers. The

fabricated microfluidic device may be extended to contain multiple channels, each filled with a

different ensemble of colloidal NCs emitting in different colors, thereby paving the way for the

development of easy-to-operate low-threshold white lasers in solution. Therefore, our capillary

microfluidic in solution laser provides a foundation to explore a rich variety of low cost,

broadband light sources, which can be easily integrated to existing microfluidic devices. The

remarkably low lasing threshold obtained with the in-solution CQWs can also help their

utilization in laminar-flow lasers, which enable tuning the output laser beam and cavity

parameters during the operation.

4. Experimental Section

Materials: The chemicals used in this study are provided in the SI along with their commercial

source.

Synthesis of cadmium myristate: This synthesis followed the recipe reported previously with

slight modifications.[31] 1.23 g of cadmium nitrate tetrahydrate was dissolved in 40 mL of

methanol while 3.13 g of sodium myristate was dissolved in 250 mL of methanol. Then these

solutions were mixed and stirred for couple of hours. The formed precipitate was filtered and

11

washed with methanol to remove unreacted species. Finally, the precipitate was dried under

vacuum overnight.

Synthesis of 4ML CdSe CQWs: The synthesis was performed according to a previously reported

recipe with slight modifications.[31] 170 mg of cadmium myristate and 12 mg selenium powder

was mixed with 15 mL ODE in a three-neck flask. The mixture was degassed at 90 °C and the

mixture was heated to 240 °C under argon atmosphere. When the color of the solution turned

to deep orange around 190-200 °C, 80 mg of cadmium acetate dihydrate was added to the

reaction mixture. The temperature of the reaction solution kept at 240 °C for 8 min for CQWs

to grow laterally. Then the reaction was quenched in water bath and 0.5 mL of OA was injected.

The product was precipitated with addition of acetone and redispersed in hexane. To remove

undesired particles, size selective precipitation with various amounts of ethanol was performed.

Preparation of growth solution for CdS crown: A previously reported recipe was followed with

slight modifications.[31] 480 mg of cadmium acetate dihydrate, 340 µL of OA and 2 ml ODE

were loaded into a bottle. After sonication for 30 minutes, the mixture was heated to 160 °C

with constant stirring under ambient atmosphere. The mixture was alternately sonicated in the

sonicater filled with boiling water and stirred at 160 °C on hot plate until white homogeneous

gel was obtained. Then it was mixed with 4 mL of 0.1 M S-ODE solution.

Synthesis of CdSe/CdS core/crown CQWs: CdSe/CdS core/crown CQWs were synthesized by

following previously reported recipe with slight modifications.[31] A desired amount of 4 ML

CdSe CQWs in hexane was loaded into a three-neck flask with 15 mL of ODE. The hexane was

removed under vacuum at 80 °C. Then the reaction mixture was heated to 240 °C under argon

atmosphere and a certain amount of growth solution for CdS crown was injected at the rate of

15 mL h−1. When a desired crown size was achieved, the reaction was quenched with water

bath. Then 10 mL of hexane was added. To precipitate CQWs, the mixture was centrifuged

after addition of ethanol. Subsequently, they were redispersed in hexane and washed with

12

ethanol and methanol to remove unreacted compounds, which plays a crucial role in growth of

shells with c-ALD.

Synthesis of CdSe/CdS@Cd1-xZnxS core/crown@shell CQWs: Core/crown@shell hetero-

CQWs were synthesized using a previously reported c-ALD recipe with some modifications.[31]

CdSe/CdS core/crown CQWs dispersed in 1 mL hexane were mixed with 4 mL of n-

methylformamide (NMF) and 40 mL of 40-48% aqueous solution of ammonium sulfide (as

sulfur shell growth precursor) and stirred for 2 min. Then, the reaction was quenched by

addition of 1 mL acetonitrile and excess toluene (till the mixture turned blurry) and the mixture

was precipitated via centrifugation. The precipitate was redispersed in NMF and the same

precipitation step was repeated to remove unreacted precursor. Finally, the CQWs were

dispersed in 4 mL of NMF. Cation precursor was prepared by mixing solutions of 0.4 M

Cd(NO3)2.4H2O and 0.4 M Zn(NO3)2.6H2O in NMF by volume fractions of 5% and 95%,

respectively. For the cation deposition step, 1 mL of cation precursor mixture was added to

NPL dispersion in NMF and stirred for 45 min. The same cleaning step was repeated to get rid

of excess precursors. To increase number of shells, the aforementioned steps were repeated

until three layers of shell was achieved. Lastly, CQWs were precipitated and 5 mL of hexane

and 100 mL of OLA was added on top of precipitate and the mixture was stirred overnight. To

remove excess ligands, the dispersion of CQWs was precipitated by addition of minimum

amount of ethanol and stored in toluene.

Device fabrication and mirror deposition: The microfluidic channels in this work were

fabricated by standard photolithography and soft lithography techniques. SU8 2050 photoresist

was spin coated on a 4-inch silicon wafer, and the patterns from a 5-inch dark filed glass mask

with 2.3 mm thickness were transferred to the wafer by aligning the mold and the mask and

exposing them to UV light using EVG-620 contact aligner using soft contact mode. The parts

of the photoresist, which were not exposed to light, were then washed with SU8 developer to

obtain a silicon mold with the desired channel geometries. The spin coater parameters were

13

adjusted to obtain a 100 μm thick SU8 2050 film on the mold to yield 100 µm channel heights,

and the contact aligner was set to 230 mJ cm-2 exposure intensity with 200 µm separation. Prior

to spin-coating SU8 2050, 2 µm thick SU8 2005 photoresist was coated on the wafer, with 120

mJ cm-2 exposure intensity and 100 µm separation to create a base layer to enhance the adhesion

of the structures on the mold.

The structures on the mold were then transferred to PDMS by mixing it with curing agent at a

ratio of 5:1 by weight, and heating the mixture in an oven at 80 °C for 40 min. The mold that

contains patterns for several microfluidic devices was then removed from the oven, the PDMS

was peeled off, and devices were cut and their holes were punched. The devices were

subsequently placed on a hot plate at 200 °C for 24 h, and mirrors were deposited in a thermal

evaporator. Prior to mirror deposition, the PDMS replica was covered with a thermal tape to

prevent the formation of thin films where undesired.

Subsequently, Cr, Ag, and SiO2 were deposited consecutively on the channel wall using

Leybold L560 Box Coater at a rate of 0.3 Å s-1, where Cr and Ag were laid down using thermal

evaporation and SiO2 was deposited using RF sputtering. Cr of 20 nm in thickness was used on

PDMS to enhance the adhesion of the silver mirrors, and silver mirror thicknesses of 70 and 80

nm were chosen for the partially reflective and totally reflective mirrors, respectively, to

provide the desired reflectance values. 15 nm thick SiO2 was deposited as a protective layer to

prevent the oxidation of silver mirrors. To create the microfluidic channels, the PDMS

substrates were finally sealed to flat PDMS substrates using NANOPLAS Plasma Asher DSB-

6000.

Characterization: The XPS measurements were carried out using Thermo Scientific X-ray

photoelectron spectrometer with a monochromatic AlKα source used to measure the amount of

surface oxidation. The sample surfaces were irradiated with a pass energy of 187.5 eV, and the

measurements were obtained from three different points on the surface with at least two sweeps

per data point. The used binding energy range for chemical identification was 0–1100 eV. Peak

14

positions recorded in the Handbook of XPS were used as the reference standard for spectra

identification and data interpretation. The absorption and PL spectra were obtained using

Agilent Varian Cary 5000 UV-Vis-NIR and Cary Eclipse spectrophotometer. TEM images of

the CQWs were taken using JEOL 2100F electron microscope. Scanning electron microscopy

of the cavity mirrors was performed using FEI NovaLab 600i SEM/FIB instrument operating

at 15 KV. AFM images of the mirror surface were obtained using Asylum Research MFP-3D

instrument. The reflectance of the mirror surfaces was measured by irradiating the mirror with

a Xenon lamp through an optical fiber and collecting the reflected photons through the same

fiber, which were detected by an optical spectrometer (Maya2000 Pro). The reflectance value

of the mirror was then calculated by dividing the intensity of the back reflected photons to that

of a 100% reflective mirror reference at the wavelength of interest (646 nm).

Optical gain and lasing experiments: These were performed using a Spectra Physics Spitfire

Pro XP 800 nm laser with a 120 fs pulse width and a 1 KHz repetition rate. A barium borate

(BBO) crystal was used to double the laser frequency and generate a 400 nm laser beam. After

the frequency upconversion, the remaining 800 nm beam was filtered out with a short-pass filter

and the 400 nm beam was focused onto the microfluidic channel on the device using a

cylindrical lens for a stripe excitation configuration. A neutral density filter was used to control

the intensity of the excitation laser. The sample luminescence was collected with an optical

fiber coupled to Maya2000 Pro spectrometer.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

The authors gratefully acknowledge the financial support received from TUBITAK 115E679,

115F297, 117E713.

Received: ((will be filled in by the editorial staff))

Revised: ((will be filled in by the editorial staff))

Published online: ((will be filled in by the editorial staff))

15

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Figure 1. (a) Design and operation of a microfluidic waveguide of CQWs, in which a FP

cavity is created by the deposition of two mirrors on the channel walls. An external pump

laser is used to induce excitation in the cavity. (b) Illustration of the Fabry-Pérot (FP) cavity

showing the mirrors and optical output. (c) Absorption (blue) and PL (red) spectra of the

CQWs used in this work, and the inset shows the TEM image of the CQWs. (d) Fluorescence

microscope image of a filled channel.

Figure 2. High resolution XPS spectra of (a) Ag 3d, (b) O 1s, (c) Si 2p, (d) Ag 3d, and (e) O

1s of silver mirrors with (a-c) and without (d,e) oxidation protection layer of SiO2 after

oxygen plasma. (f) SEM image of the microfluidic device showing the smooth mirror surface.

380 376 372 368 364

Inte

nsit

y (

a.

u.)

Binding Energy (eV)

O 1s Si 2d

Ag 3d3/2

Ag 3d Ag 3d5/2

Plasmon from

536 534 532 530 528 526

Binding Energy (eV)

O=CAg 3d3/2

Ag 3d5/2

Plasmon from

106 104 102 100

Binding Energy (eV)

380 376 372 368 364

Inte

nsit

y (

a.

u.)

Binding Energy (eV)536 534 532 530 528 526

Binding Energy (eV)

O 1SAg 3d Ag 3d5/2

Ag 3d3/2 O=C

O(Ag O)2

Ag(Ag O)2

a b c

d e f

19

Figure 3. (a) Photoluminescence spectra of CQWs in the microfluidic device under pulsed

excitation at various pump intensities. (b) Luminescence and linewidth vs pump fluence. (c)

Lasing of CQWs in microfluidic devices with mirrors. (d) The output intensity of the device

and linewidth values with increasing pump intensity. (e) Image of the output laser beam focused

onto a screen.

a b

c d

e

20

J. M., N. G., F. I., S. D., O. E., E. Y. E.*, and H. V. D.*

Optical Microfluidic Waveguides and Solution Lasers of Colloidal Semiconductor

Quantum Wells

Solution of semiconductor quantum wells (CQWs) colloidally synthesized and used as

the gain medium in a capillary microfluidic waveguide yields record low optical gain

threshold in solution. A Fabry-Pérot cavity was created between metallized sidewalls of the

ends of microfluidic channel and optically pumped along the side of the waveguide to

generate a single-mode output laser beam.

Keywords: microfluidics, semiconductor quantum wells, solution optical gain, solution lasing

21

Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2018.

Supporting Information

Title Optical Microfluidic Waveguides and Lasers of Colloidal Semiconductor Quantum Well

Solution

Joudi Maskoun, Negar Gheshlaghi, Furkan Isik, Savas Delikanli, Onur Erdem, E.Yegan

Erdem*, and Hilmi Volkan Demir*

1. Chemicals: Cadmium acetate dihydrate (Cd(OAc)2·2H2O) (>98%, Sigma-Aldrich),

cadmium nitrate tetrahydrate (Cd(NO3)2.4H2O) (>99.0%, Sigma-Aldrich), zinc nitrate

hexahydrate (Zn(NO3)2.6H2O) (98%, Sigma-Aldrich), trioctylamine (TOA) (98%, Sigma-

Aldrich), oleylamine (OLA) (70%, Sigma-Aldrich), N-methylformamide (NMF) (99%, Sigma

Aldrich), ammonium sulfide solution (40-48 wt. % in water, Sigma-Aldrich),

Trioctylphosphine (TOP) (~90%, technical grade, Sigma-Aldrich), oleic acid (OA) (90%,

technical grade, Sigma-Aldrich), selenium (powder, 100 mesh, 99.99%, trace metal basis,

Sigma-Aldrich), hexane (HPLC grade, ≥97.0%, Sigma Aldrich), acetonitrile (HPLC grade,

99.9%, Sigma-Aldrich), toluene (ACS reagent, ≥99.7%, Sigma-Aldrich), SU-8 2005 negative

photoresist (Microchem), SU-8 2050 negative photoresist (Microchem), SU-8 Developer

(Microchem), Sylgard 184 Polydimethylsiloxane (PDMS), and curing agent (Dow Corning)

were purchased and used without further purification.

2. Capillary flow: The introduction of a liquid droplet to the inlet of a microfluidic channel

may induce flow, depending on the capillary pressure induced by the channel on the droplet.

The pressure is expressed by the Young-Laplace equation:

∆𝑃 = 𝛾 (2cos𝜃

𝑅ℎ) (1)

where 𝛾 is the surface tension between toluene and air, 𝜃 is the contact angle of toluene on

PDMS, and 𝑅ℎ = 𝑤ℎ/(𝑤 + ℎ) is the hydraulic radius of the channel with w and h being the

height and width of the channel, respectively.[1] For the solution to fill the channel

22

spontaneously, the contact angle must be small enough to induce a pressure drop, as shown in

the equation. Static contact angle measurement technique was employed using a contact angle

goniometer, on which the PDMS sample was placed, and the toluene droplet was then released

from a syringe positioned above the PDMS surface. A high-resolution camera then captured

the profile of the droplet on the sample surface.

Introducing CQWs in toluene to the channel inlets resulted in observable capillary flow,

suggesting that the capillary forces are strong enough between the solution and PDMS. The

contact angle of toluene on treated PDMS was measured to be 30.1°, as shown in Figure S2.

Plugging 𝛾 = 0.028 N·m−1, 𝜃 = 30.1, w = 300 µm, and h = 100 µm in Equation S1 gives ∆𝑃 =

0.65 kPa, meaning that the capillary forces are large enough to induce flow in the channels.

Figure S1. Contact angle of toluene on treated PDMS surface.

23

3. Microfabrication and mirror deposition:

Figure S2. Fabrication steps for the microfluidic FP cavity. (a-c) Standard soft lithography

technique for pattern transfer from silicon mold to PDMS. (d) Deposition of the first mirror.

(e) Deposition of the second mirror after the device is flipped upside down in the chamber. (f)

Plasma bonding of PDMS device.

3. References

[1] H. Bruus, Theoretical microfluidics, Oxford University Press, 2008.