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Abstract
A single layer WSe2 crystal is transferred on the tip of a single mode fiber that is placed in the cavity of a Yb3+ doped picosecond fiber oscillator. This intracavity monolayer generates second harmonics without affecting the stable mode-locking and laser output of the fiber oscillator. This method utilizes the higher intracavity optical intensities to increase the SHG conversion efficiency, thus combining the high nonlinear susceptibility and the phase-matching independence of a monolayer with the versatility of a fiber laser in an all-fiber integrated system. We also demonstrated a procedure to verify the monolayer placement on the fiber core by hyperspectral mapping. This intracavity SHG has possible applications for self-referencing f-2f interferometry in fiber-laser frequency combs.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Intracavity Second Harmonics Generation (SHG) in fiber lasers has proven to be an efficient frequency conversion method utilizing the compact form factor, superior optical integration, and thermal stability of fiber lasers [1–4]. Recently, two-dimensional (2D) materials have garnered great interest due to their enhanced nonlinear response in areas such as sum frequency generation, four-wave mixing, high harmonic generation and saturable absorption [5–9]. Transition metal dichalcogenides (TMDs), a sub-group of 2D materials, have been extensively studied due to their promise for optoelectronic and nanophotonic applications such as photo-detectors, tunable light sources and single-photon emitters [10–14].
As opposed to centrosymmetric 2D monolayers like graphene, TMD monolayers have a crystalline asymmetry giving rise to a remarkably high second-order nonlinear optical susceptibility [15–17] which was estimated to be around 0.5 nm/V [18,19] for TMD monolayers on fused silica substrates. Single-layered (monolayer) TMDs such as Molybdenum Disulfide (MoS2) and Tungsten Diselenide (WSe2) possess a direct bandgap [20–22] and inversion symmetry breaking [17,23,24]. TMDs can be fabricated on a large scale by chemical vapor deposition (CVD) with a wide range of band-gap tunability via doping [25] and alloying [26]. Recent advances in direct growth on optical fibers promise scalable fiber integration of TMDs in the near future [27]. The tunability, ease of integration, and hybridization with other materials makes them promising candidates for integrated nonlinear optical devices such as on-chip frequency converters, nanoresonators, nonlinear phase modulators, refractive index sensors and nonlinear hologram generators [28–32]. Fiber integration makes 2D TMDs promising candidates for fiber-integrated optoelectronic and photonic applications such as optical attenuation, electro-refractive modulation and optical nonlinearity enhancement thanks to their nonlinear optical response and exciton-dominated photoluminescence [33–36].
One specific advantage of TMD monolayers for SHG is that they eliminate the need for phase-matching as their optical thickness is much smaller compared to the typical wavelength of the fundamental wave [37,38]. This extreme thinness also allows them to be placed inside the cavity of fiber lasers as they can be securely placed on fiber tips by van der Waals bonding [39]. It was shown that multilayer TMDs integrated into fiber laser cavities can be used as saturable absorbers for passive Q-switching and mode-locking [40–42].
In this study, we demonstrate an intracavity WSe2 monolayer in a passively mode-locked, Yb3+ doped, picosecond fiber laser that generates second harmonics without reducing the quality of the laser cavity (i.e., stable mode-locking and laser output). Placing a monolayer TMD inside the cavity utilizes the higher intracavity optical intensities and increases the SHG conversion efficiency [43]. Our method combines the high nonlinear susceptibility and the phase-matching independence of TMDs with the versatility of fiber lasers in an all-fiber integrated system.
2. Methods
A passively mode-locked, polarization-maintaining (PM) fiber laser system is set up using a linear cavity, which is externally pumped with a 975 nm pump diode laser via a fused PM wavelength division multiplexer (WDM). The WDM behaves like a mirror with 78% reflection for 980 nm, therefore reflecting the pump light back into the cavity, while it acts like a semi-transparent medium with 75% transmission at 1030 nm, therefore transmitting 75% of the pulsed light toward the output of the fiber laser. All the fibers in the system are PM, angle-polished (8$^{\circ }$), single-mode fibers with a core diameter of 5.5 $\mathrm{\mu}$m producing an effective mode field diameter of 6.5 $\mathrm{\mu}$m. The fiber ends are dome shaped to ensure a direct contact of the fiber cores when joining two fibers. The effective laser cavity is defined between the saturable absorbing mirror (SAM) and the midpoint of the Fiber Bragg Grating (FBG). The 30 cm long Yb doped fiber, the 1 m long passive fiber, the 15 cm long passive fiber connected to the SAM , and half of the FBG create a cavity length of approximately 2 m, which yields a repetition rate of 47 MHz. The SAM is made by Batop GmbH (SAM-1030-32-1ps), which consists of an InGaAs absorbing layer and has a relaxation time of 1.1 ps resulting in a pulse duration of 3.6 ps. The FBG consists of doped silica layers with alternating refractive indices, serving as an output coupler and a wavelength locker. The FBG has a maximum reflectance of 0.87 at 1030 nm, locking the wavelength to a spectral width of 0.8 nm (FWHM). When the 975 nm diode laser is driven with 100 mA of current, an average optical power of 35 mW is generated, which corresponds to the lower bound of the mode-locking regime. Mode-locking is maintained to an upper bound of 180 mA of pump current, corresponding to an average optical power of 65 mW. Using the described setup, it is possible to achieve pulse energies between 0.13 and 0.23 nJ in the mode-locking regime when the monolayer is inside the cavity.
For the sample preparation, a single layer of WSe2 is mechanically exfoliated using the "Scotch Tape" method [44]. Potential monolayers are identified by their optical contrast under a light microscope, and the layer thickness is verified by photoluminescence (PL) spectroscopy. Monolayer flakes large enough to cover a significant portion of the effective mode field area (6.5 um in diameter) are then transferred onto the tip of a dome-shaped, angle polished, 1 m long passive fiber as schematically shown in Fig. 1(a) [45]. A chemical-free transfer method [39] is employed to avoid any polymer residue on the fiber tip. The integrity and the position of the transferred flake on the fiber core are verified using PL mapping. Utilizing 532 nm laser light, perpendicularly incident on the fiber tip surface with a spot size of 2 $\mathrm{\mu}$m, the PL of the monolayer is mapped over an area covering the core and the cladding of the fiber by moving the fiber by a piezo stage with 0.5 $\mathrm{\mu}$m steps. After determining the flake was on the core, this 1 m long passive fiber is placed back in the original configuration, sandwiching the flake between the passive fiber and Yb doped fiber as shown in Fig. 1(d). This process minimizes the air gap between the fibers and avoids back-reflections. An objective lens with a 0.40 numerical aperture collimates the fiber laser output, and a band-pass filter eliminates the fundamental wave. The output spectrum is recorded for increasing pump currents, ranging from 0 to 178 mA with the flake in the cavity. The same is repeated without the flake in the cavity. The cavity settings are kept the same for both "with monolayer" and "without monolayer" cases. All measurements are performed at room temperature.
Fig. 1. a) Illustration of the monolayer WSe2 flake placed at the tip of the fiber. The fiber surface is dome shaped. The flake covers the core of the single-mode fiber. b) White light image of the fiber area as shown by the dashed lines. The potential monolayer is highlighted in yellow. The green circle shows the fiber core location. c) Output spectrum of the fiber oscillator. Peak wavelength is observed to be around 1029 nm. The best fit is provided as a guide for the eye. d) Schematic diagram of the fiber laser setup. The flake is placed between the Yb doped fiber and 1 m long passive fiber. Effective cavity length is approximately 2 m. SAM: Saturable Absorber Mirror. FBG: Fiber Bragg Grating. WDM: Wavelength Division Multiplexer. PD: Photo Diode. L: Lens. F: Band-pass Filter.
3. Results and discussion
Fig. 1(a) includes a sketch of a flake placed on the core of an optical fiber visualizing the flake placement on the fiber tip. In the sketch, the TMD flake is shown in blue while the fiber core is depicted by a circle. A microscope image of the actual TMD flakes transferred onto the fiber tip is shown in Fig. 1(b). The potential monolayer is highlighted with yellow while an adjacent bilayer, a few surrounding few-layer, and bulk flakes are visible. To verify the number of layers for the potential monolayer, PL spectroscopy is used [46,47]. As shown in Fig. 2(a), the PL spectrum of the flake at the center shows high PL intensities with a peak wavelength of about 750 nm, which is a characteristic feature of single-layer WSe2 due to its direct bandgap of 1.65 eV [46–48]. Also, through-fiber PL spectrum indicates that the PL emission of the monolayer couples into the fiber and guided in the core through the fiber. The similarity between the through-fiber and in-reflection PL spectra shows that transmission through the fiber does not change the spectral properties of the PL response.
Fig. 2. a) PL spectrum transmitted collected after the fiber and in reflection geometry. b) Schematic diagram for the PL setup showing the collection of PL emission transmitted through the fiber by an objective lens with 0.25 NA and the collection of the reflected PL emission by an objective lens with 0.40 NA. Note that the same excitation method is used for both through-fiber and in-reflection measurements. White light is used to image the fiber surface. CW: Continuous Wave. WL: White Light. BS: Beam Splitter. F: Filter. NA: Numerical Aperture.
To investigate the flake placement and integrity after the transfer process, hyperspectral PL mapping is done in reflection geometry as shown in Fig. 2(b). The fiber tip surface containing the monolayer is mapped by laterally moving the fiber tip using a piezo-stage.
Fig. 3(a) shows the scattered laser light (532 nm) off the top surface of the fiber tip, while Fig. 3(b) shows the spatial distribution of the emitted 750 nm PL. Therefore, the intensity map shown in Fig. 3(b) verifies that the center flake is a monolayer and overlaps with the fiber core. One caveat in the exfoliation process is the resulting multilayered TMDs. Fig. 3(c) shows the PL emission at 850 nm that highlights the bilayer flake adjacent to the monolayer. Contrary to the monolayer flakes, which have a direct bandgap, bilayer flakes have an indirect bandgap yielding much less quantum efficiency, and therefore, reduced PL emission [47]. Moreover, the bandgap of the bilayer WSe2 is 1.55 eV (800 nm) [47] making it easy to distinguish from a monolayer. The PL intensities in Fig. 3(c) are shown at 850 nm in order to eliminate the tail of the high PL peak originating from the monolayer (peaking at 750 nm, stretching up to the vicinity of 800 nm). The white light image shown in Fig. 3(d) resembles the scattered laser light image in Fig. 3(a) since the scattered intensities of both laser light and white light are proportional to the flake thickness [49]. The scattered laser light image shows the fiber core beneath the monolayer, while the white light image provides the optical contrast verifying that the monolayer is intact.
Fig. 3. a) Map of the scattered laser light (532 nm) during the PL mapping. Green circles show the fiber core location. Brighter colors represent higher intensities. The scattered laser intensity from the bulk flakes is larger than that of thinner flakes. b) PL intensity map at 750 nm. Only the monolayer flake is visible since it has a direct bandgap at around 750 nm. c) PL map at 850 nm. The bilayer flake has the most prominent intensity since the bilayer has an indirect bandgap at around 805 nm. e) White light microscopy image of the same surface area.
Monolayer WSe2 is composed of one tungsten (W) and two selenium (Se) atoms resulting in an atomic trilayer where the two principal coordination for the W atoms are trigonal prismatic and octahedral [50]. This non-centrosymmetric crystal structure gives rise to a high nonlinear susceptibility. In particular, second-order nonlinear susceptibility of WSe2 has been shown to be three orders of magnitude larger than that of frequently used bulk crystals such as lithium niobate (LiNbO3), and quartz [17]. High SHG response is observed only in odd-layered TMD crystals due to symmetry breaking [37]. However, mechanical exfoliation yields multilayered flakes with 2H stacking that are centrosymmetric in even-numbered layers. While TMD bilayers have been demonstrated to generate second harmonics by means of various symmetry breaking mechanisms such as charge accumulation due to applied electric field [51] or lattice modification due to applied mechanical strain [52], resulting nonlinear susceptibilities are a fraction of those in the monolayer case. Considering the lower SHG intensities and the fabrication challenges of even-layered TMDs, odd-layered TMDs are more practical to be used for SHG. Particularly, monolayer TMDs are good candidates for SHG since higher number of layers leads to the re-absorption of the second harmonics, consequently reducing the overall SHG intensity [18].
Fig. 4(a) is a plot of the background-corrected intensity at 515 nm (second harmonics of 1030 nm). The blue dots represent the intensity in counts per second (cts/s) when the monolayer is inside the laser cavity. The spectrum is collected for increasing peak powers to study the power dependence of the light with the wavelengths of 515 nm. The solid line is a modeled quadratic function with respect to peak powers. A quadratic model is chosen as the SHG intensity can be expressed as follows [53]
Fig. 4. a) Optical power dependence of the background-corrected SHG intensity at the wavelength of 515 nm. Solid lines represent the best fit which is a quadratic function in the monolayer case and a linear function otherwise. b) Peak power intensities of the fiber laser output as a function of the pump current of the laser diode which pumps the cavity. While similar peak powers are achieved in the two cases, there is no difference in the onset of mode-locking.
As seen in Eq. (1), SHG intensity has a quadratic dependence on the fundamental intensity $I_\omega$. This is a signature of second-order nonlinear phenomena, in this particular case, SHG. Therefore, the quadratic power dependence of the 515 nm light proves the existence of SHG. The red crosses in Fig. 4(a) represent the intensity at wavelengths of 515 nm without the monolayer in the cavity. The solid line is the best fit to the data, which is a linear function. This linear behaviour is likely resulting from the parasitic luminescence inside the gain medium [54]. In the absence of the monolayer, intermittent negative intensity values are observed resulting from the background correction. As the SHG signal sits on top of the parasitic luminescence in the fiber lasers output, a background correction was necessary. Please see the Supplement 1 for the raw data and the background correction. Based on the difference between these two cases shown in Fig. 4(a), we can conclude that the SHG response originates from the monolayer, not from an interface or a noncentrosymmetric feature that may be present in the laser cavity.
Material thickness is another parameter modulating the SHG intensity. Since the SHG intensity is proportional to the square of the crystal thickness, the SHG conversion efficiency is hindered by the extreme thinness (around 1 nm) of the TMD monolayers. Nonetheless, this extreme thinness also brings a unique feature of phase-matching independence. Since the typical thickness of TMD monolayers is much smaller than the wavelength and the coherence length of the fundamental light, the argument of the sinc function ($\Delta k\ l$) goes to zero, eliminating the need for phase matching. In this sense, TMD monolayers can be called "intrinsically phase-matched".
In Fig. 4(b), peak power of the fiber laser is plotted against the pump current of the 975 nm diode laser in the mode-locking regime. The plot shows linearly increasing peak powers between 5 W and 30 W with respect to the pump current which ranges from 100 mA and 180 mA. The results indicate that mode-locking is preserved, and similar peak powers are achieved in both absence and presence of the monolayer inside the cavity. This indicates that the dome shape of the fiber ends allow the fiber cores with the monolayer flake to touch even with multilayer flakes present on the cladding.
Given the FBG reflectance of 87% at 1030 nm, the optical intensities in the cavity can be roughly estimated to be eight times higher than what is measured outside the cavity. This alone can lead to a sixty-fold increase in SHG response due to the quadratic power dependence. In addition, the fundamental wave passes through the monolayer twice in each cycle, resulting in double-pass enhancement. While the double-pass conversion efficiency can theoretically be four times larger than the single-pass conversion efficiency [55], experimental studies show an enhancement of about two-fold due to phase-matching limitations [56,57]. Although the WSe2 monolayer does not require phase-matching, its absorption is roughly 10% at around the wavelengths of 515 nm [58]. Therefore, resulting enhancement due to the double-pass scheme is expected to be less than four-fold.
Another factor for the SHG conversion efficiency is the absorption of SHG light by the semiconductor SAM which consists of an InGaAs absorber layer. When the second harmonic wave is generated, it resonates in the cavity just like the fundamental wave and it is partially absorbed by the SAM in each cycle. Therefore, the resulting SHG intensity is despite the absorption of InGaAs layers at the wavelength of 515 nm which is approximately 20% for a 10 nm thick absorbing layer [59].
Since the resulting SHG intensity is affected by a combination of positive factors such as higher intra-cavity intensities and double-pass scheme as well as negative factors such as the SAM absorption, it is challenging to quantify the intracavity enhancement precisely. As a comparative study, we focused the output (1030 nm) of the fiber laser on a suspended monolayer outside the cavity and recorded the transmitted signal using the same collection method described in the methods section. However, we could not observe any counts at 515 nm after the background correction indicating that the SHG intensity is smaller than the noise level which is about 0.25 cts/s (the integration time was set to two minutes). On the contrary, a signal to noise ratio of 100-500 was observed in the SHG signal when the monolayer was present inside the cavity.
The fiber laser has a typical power density of approximately 100 MW/cm2 resulting in an intracavity SHG output of 50 cts/s. To have a comparison, we used a Ti:Sapphire oscillator with a regenerative amplifier which has a repetition rate of 250 kHz and a pulse duration of 200 fs at 800 nm. To achieve a similar SHG response with the Ti:Sapphire oscillator, we found that a peak power density of more than 10 GW/cm2 is needed when the laser beam is focused on a suspended monolayer outside the cavity.
While the second order nonlinear susceptibility of WSe2 for the fundamental wavelength of 800 nm is calculated to be roughly two times of 1030 nm case, the overall SHG response is reported to be similar for these fundamental wavelengths [19]. The SHG response for 1030 nm was observed to be enhanced so that it becomes comparable to that of 800 nm [19]. The proposed mechanism behind this SHG enhancement is the band-nesting effect [19,60]. Based on the similar SHG responses for 800 nm and 1030 nm, we can factor out the effect of fundamental frequency for this specific case. Therefore, we can conclude that two orders of magnitude less peak power density is needed when the monolayer is placed in the cavity of the fiber laser.
4. Conclusion
Conventionally, bulk crystals are used in free-space lasers for nonlinear processes [61]. While their thickness helps to increase the conversion efficiency, this comes with the challenge of phase matching. The common practice of optical path modulation by tuning the incidence angle introduces pulse distortions and incident power limitations [62]. TMD monolayers do not require phase matching. Contrary to bulk crystals, 2D TMDs can be easily placed in the cavity of fiber lasers. In this sense, TMD monolayers are ideal materials in applications such as detection of carrier envelope offset (CEO) [63]. By having the second harmonic signal generated by a TMD monolayer in the cavity of a fiber laser, it would be possible to make a robust fiber integrated system which is capable of self-referencing f-2f interferometry to be used as an accurate optical frequency standard.
In terms of repeatability of our results, the biggest challenge is the precise placement of a monolayer flake over the core of a single mode optical fiber. We used a dry viscoelastic transfer method that is relatively easier compared to wet transfer methods. Moreover, this dry method does not leave any chemical residue. Regarding sample longevity, monolayer TMDs have been reported to show no signs of oxidation for up to 10 months if they are kept in dark ambient conditions [64]. Sandwiching a monolayer between butt-coupled fiber tips creates an ideal environment where the monolayer is protected from ambient light and open-air oxygen exposure. The reported time scale is in line with our experience as our measurements are carried out in a time range of several months without observing any sample deterioration.
With the increased optical intensity in the cavity, high nonlinear susceptibility of the TMD monolayer, and efficiency of intracavity SHG, a fully integrated fiber laser system can provide second harmonics at relatively low peak powers (30-50 W) compared to extra-cavity SHG with free space lasers. SHG conversion efficiency can be further enhanced by means of mechanisms such as exciton resonance and band-nesting [19,60]. In this study, we used a picosecond fiber laser with 1030 nm wavelength which generates off-resonant second harmonics (515 nm) for the WSe2 monolayer, which has a band gap of 1.65 eV (750 nm) [48]. However, 515 nm (2.41 eV) is reported to be very close to the band-nesting energy of 2.42 eV [60] contributing to the enhancement of SHG. Additionally, there is approximately one order of magnitude difference in the SHG intensity between the fundamental wavelengths of 1550 nm (widely used in fiber optic communication) and 1030 nm [65] attributed to the 1s exciton resonance near the bandgap of monolayer WSe2 [66]. SHG intensities, therefore, can be further enhanced using a resonant fundamental wavelength such as 1550 nm due to the exciton-related resonance [48,66]. We believe that Er-doped fiber laser media (1550 nm) would be a candidate to study the resonant enhancement of SHG in WSe2 monolayers. In our study, we proposed a method that can be applied to a variety of fiber lasers and 2D materials such as Yb, Er, or Nd doped gain media and different TMD monolayers (WS2 , WSe2 , MoS2 , MoSe2, etc.).
Considering the recent application of TMDs such as MoS2 [42] in the form of SAMs, we can suggest that monolayer TMDs can be used as saturable absorbers that generate the second harmonics simultaneously. Another application would be to use TMDs as electrically tunable nonlinear emitters inside fiber laser cavities as the electrical control of SHG in WSe2 monolayers has been demonstrated recently [67]. A monolayer TMD placed between two fiber tips allows for electrical contact pads inserted between these two adjacent fibers. These contact pads can be connected to outside controllers opening up a possibility of an ultra-compact electrically tunable nonlinear element inside a fiber laser cavity.
Funding
Graduate Student Association, University at Buffalo (Mark Diamond Research Fund).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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