Two-dimensional semiconductor materials have shown strong non-linear optical properties at the monolayer limit due to the lack of inversion symmetry. These non-linear materials can ideally be combined with top-down planar fabrication methods to offer new optical devices that can be integrated on-chip as compared to large bulk non-linear crystals. Here, we show enhanced second harmonic generation (SHG) from monolayer MoS2 embedded within an all-dielectric Fabry-Perót microcavity that is resonant at the pump wavelength. Power dependency shows a 10-fold increase in second harmonic generation matching theoretical estimate.
© 2016 Optical Society of America
Novel optical properties recently found in two dimensional atomic crystals (2DAC) such as strong photoluminescence [1,2], valley polarization , coherence , and giant non-linear response , enable these two dimensional materials to be used as building blocks for optical components small enough to be integrated with modern semiconductor technologies. Many 2DACs lack inversion symmetry granting them strong non-linear optical response because higher order terms in the power series expansion of the polarization field within individual monolayers fail to get cancelled out by adjacent crystal layers found in their bulk counterpart . This allows odd number of layers of 2DACs to act as photon exchange hubs wherein photons can be exchanged between the various frequency components of the electromagnetic field in a single quantum-mechanical process . Second harmonic generation (SHG) is a fundamental non-linear quantum mechanical photon exchange process converting two photons of frequency ω into a single photon of frequency 2ω. This frequency doubling process is technologically important for the generation of lasers in the blue or UV regions  and general tunable laser sources which at present are only made with bulky non-linear crystals.
2DAC made of monolayer transition metal dichalcogenides (TMD) materials (i.e. MoS2, MoSe2, WS2) have shown comparable and even stronger SHG efficiencies than common non-linear crystals due to their lack of inversion symmetry [5,9,10]. Recent advances in SHG of monolayer materials have shown giant second harmonic (SH) response , crystal orientation dependence [11,12], and electrical control of SHG . The non-linear optical response of nanomaterials can be increased further by placing them inside an optical cavity which provides enhanced electromagnetic field density and an increased number of cavity photon passes, increasing the likelihood of SH conversion [14–17]. In this Letter, we use a monolithic microcavity with a single resonance at the pump wavelength to enhance SH emission at room temperature from a monolayer of MoS2 embedded within the resonator structure and characterize the SH enhancement in the presence of the microcavity.
We used a planar Fabry-Perót microcavity such as the one shown schematically in Fig. 1(a) consisting of distributed Bragg reflectors (DBR) for both the bottom and top mirrors to enhance interaction between the two-dimensional material and the fundamental beam inside the cavity. Each Bragg reflector consist of alternating layers of SiNx and SiO2 and on a glass substrate. The pairs of SiNx and SiO2 layers had thickness of , with being the cavity wavelength near 800 nm and n is the refractive index of the material. The top and bottom DBRs were designed to be highly reflective between 700 – 1000 nm where the pump wavelength lies while being transparent at the second harmonic wavelength,. Plasma enhanced chemical vapor deposition (PECVD) was used to grow the DBR layers with the measured refractive index for SiNx and SiO2, being n = 2.23 and n = 1.5, respectively. The bottom DBR mirror was grown on a glass substrate with 5 SiNx and SiO2 pairs, and with the top DBR mirror grown with 4 pairs so the overall structure is more transparent from above. The MoS2 layer was sandwiched between two quarter wavelength thick SiO2 layers forming the cavity and was grown between the DBR mirrors. The first SiO2 cavity layer was grown on the bottom DBR mirror with PECVD. The MoS2 was then transferred to this bottom cavity layer while the top SiO2 cavity layer and the top DBR were sequentially grown above the transferred MoS2 layer to complete the microcavity structure.
Monolayer MoS2 was grown on a 300 nm SiO2 on Si substrate at 650°C by ambient pressure chemical vapor deposition (APCVD). High purity solid precursors of MoO3 and S powders were adopted for the synthesis. Prior to the growth, Perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) solution were uniformly coated on the substrate surface as seeding promoters to enhance the nucleation and growth of the monolayers. The CVD growth of monolayer MoS2 is carried out in 1 inch furnace with a growth time of 40 minutes and a heating rate of 15°C/min. After the growth, the residual reactants were removed with an Ar gas flow of 100 sccm and the quartz tube was took out of the furnace to rapidly cool down the entire system. More details on the synthesis were presented in previous papers [18,19]. The CVD-grown monolayers were removed from SiO2/Si substrate by directly immersing the samples into the solution of DI water at room temperature. Immersed monolayers detached from the substrate and float on the DI water surface which were then fished out with the bottom DBR sample. PECVD growth of the top SiO2 cavity and top DBR layers were carried out on the transferred sample to complete the microcavity. Figure 1(b) shows the optical microscope image of the MoS2 flakes after traferring to the microcavity structure.
3. SHG calculations and measurement
A completed microcavity with 5 and 4, DBR periods (5,4) for the bottom and top mirrors respectively, shows a cavity mode at nm represented as a dip in the reflection spectrum as seen in Fig. 1(c) The microcavity shows high reflectivity, (> 90%) between 700 – 1000 nm with the cavity resonance at . Although low reflectivity is desired at the second harmonic frequency, we observed slightly higher than predicted by simple transfer matrix calculations due to thickness and refractive index variations in the DBR.
The (5,4) microcavity has a measured unloaded q-factor = 74, where as obtained from reflectivity shown in Fig. 1(c). Using the relationship between q-factor and cavity finesse, (Eq. (1), the resonant field enhancement of the pump laser inside a cavity that is perfectly impedance matched (all incident radiation is coupled into cavity) is given by ,
Second harmonic generation emitted from monolayer MoS2 was measured at room temperature during different stages of the microcavity fabrication process: bare as grown MoS2, MoS2 transferred to bottom cavity and DBR layers, and MoS2 embedded within a completed microcavity. The observed optical microscope SH images are shown in Fig. 2(a), 2(b) and 2(c). Optical measurements were obtained with a femtosecond Ti:sapphire laser (Spectra Physics Mai Tai) tuned to the cavity mode wavelength, 818 nm, and focused onto the sample in a reflection setup with a 100 × objective (0.85 NA). Average power used 1 mW, (peak power, 39 W/cm2), at 80 Mhz repetition rate. Emission was collected after passing through a short pass filter and detected with an avalanche photodiode in a scanning confocal setup. The stage was scanned while exposed to the laser creating scanned confocal images. Power measurements were obtained by taking the average photon counts of the area of a single CVD triangular flake (~40x40 μm2) from each SHG confocal image.
Second harmonic power emission from MoS2 samples observed during each microcavity fabrication stage show the expected quadratic dependence on input pump power as shown in Fig. 2(d). Although the majority of the monolayer MoS2 was successfully transferred to the bottom DBR and cavity layers, this method did not preserve the crystallinity of the MoS2 over large areas. The MoS2 layers can be seen to be merged having with different crystal boundaries in direct contact as can be seen in Fig. 2(b) and 2(c).
The highest second harmonic emission for transferred samples are observed from the crystal boundaries as opposed to the seed of the MoS2 monolayer located at the center of the MoS2 flake for as grown CVD samples as has been previously reported [5,10]. Although the transferred MoS2 samples lose their triangular crystalline form as seen in Fig. 2(b) and 2(c) during the water transfer, the overall SH emission is comparable and slightly greater than the emission from as grown samples. This could be due to the increase of higher emitting edge or crystalline boundaries for transferred samples or the incrased reflectivity of the bottom DBR as compared to the as grown samples. Monolayer MoS2 samples fully embedded within a (5,4) cavity shows an enhancement of ~10 over as grown MoS2 samples as shown in Fig. 2(d), which roughly matches SHG enhancement predicted by Eq. (1).
There was no background SHG from the cavity as sites without MoS2 flakes showed no SHG near powers used for MoS2 measurements (~1 mW avg. power, ≤ 50 W/cm2). However at higher input powers, (8.5 mW avg. power, ≥ 100 W/cm2), the bare cavity showed measurable but considerably lower SHG (≤ 1%) as compare to active MoS2 sites. These isolated SHG emission sites could be associated with PECVD-grown Si-nanocrystal impurities in the dielectric layers or could originate from tiny MoS2 flakes that were scattered during the wet transfer process. Interestingly, SHG can also be observed away from the central cavity mode due to the losses in the DBR mirrors. Exciting the MoS2 regions when pumping off the cavity resonance, (876 nm), the MoS2 regions shows ~25% of the SHG compared with on resonant excitation, but the emission is only visible from the crystalline borders unlike the emission seen at resonance as can be seen from Fig. 3(a). Shown in Fig. 3(b) is the reflectivity of the microcavity and the off resonabnt pump (red arrow).
In summary, using a relatively low-Q cavity we showed enhanced SH emission of a 2D material semiconductor inside a DBR-based optical microcavity. Enhancing only the fundamental mode we produced an SH emission of over one order of magnitude. Further improvements to this design could be made using a polymer-based transfer method to keep the crystallinity of the MoS2 intact, reducing the mode volume of the cavity, exciting SHG near the resonance of an excitonic state of the 2D material, and investigating doubly resonant cavity designs [21,22].
We are grateful to Mircea Cotlet for help obtaining scanning confocal images of our samples. Work at CCNY was supported through the EFRI 2-DARE program (grant# EFMA-1542863). This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. Y.H. Lee thank the funding support from Ministry of Science and Technology (MOST 103-2112-M-007-001-MY3 and MOST 104-2633-M-007-001) and Academia Sinica Research Program on Nanoscience and Nanotechnology, Taiwan.
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