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Abstract
Layered WSe2 membrane prepared via chemical vapor deposition was used as the saturable absorber (SA) for seeking Q-switching in a direct 808 nm LD pumped Tm/Ho composite laser. For separating the Tm laser from modulated by the SA which leads to the failure in Q-switching, a spectral filter was inserted intra-cavity to form a hybrid cavity, where resonance enhancement in the Ho laser compared with using the conventional cavity was observed. Successful Q-switching with maximum average output power of 141 mW at 2090 nm was obtained under output coupling of 2%, where the shortest pulse width was 185 ns at repetition rate of 33 kHz, corresponding to a pulse energy of 4.36 µJ and peak power of 23.1 W. It is the first presentation of 2D material applied in the Tm/Ho composite laser at 2.1 µm, where the current SA could be replaced by other pulse modulation materials such as topological insulator, graphene, or golden nano-rod for seeking novel pulse Ho lasers.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Passively Q-switched (PQS) and mode-locked solid-state lasers with pulse width varying from microseconds to femtoseconds are attractive in material processing, medical treatment, and scientific researches [1–3]. Among them, the saturable absorber (SA) plays a key role for the compactness of the system comparing with the actively Q-switching manner using bulky acousto-optical or electro-optical modulators.
Since the first successful exfoliation of graphene from graphite in 2004 [4], researches on the properties and applications of 2D materials such as topological insulators [5], reduced Graphene oxide [6], and transition metal dichalcogenides (TMDs) have attracted much attentions for years. One of the famous properties of these material is the broad-band non-linear absorption band spanning from near IR to mid-IR, which matches the requirements of SA in controllable modulation depth, feasible absorption strength, and ultra-fast recovering time [7].
Belonging to family of MX2 (M denotes the transition metal elements, X the chalcogen elements), TMDs (WS2, MoS2, WSe2, and MoSe2) is one type of 2D materials with sandwich structure, where monolayer of metal atoms is inserted between two chalcogen layers [8]. Although the modulable wavelength range is limited by the direct bandgaps of TMDs (1.58eV in MoSe2 [9], 1.8eV in MoS2 [10]), TMDs were demonstrated to be novel SAs for mid-IR lasers due to the presence of edge and defect states inside the bandgaps [11]. Hence, compared with the mature semiconductor SA, TMDs are accessible and easy-prepared SAs for pulse lasers, especially at 2 µm [12–14], which get potential applications in fields of surgeries, radar system, and molecular spectroscopy.
Among 2 µm lasers, Ho lasers with large emission cross sections are efficient pump sources for non-linear frequency conversion toward the molecular fingerprint region of 3∼14 µm [15,16], as the emission wavelengths around 2.1µm are away from cut-off wavelengths of the famous mid-IR nonlinear crystals such as ZGP and OP:GaAs [17]. However, due to the lack of absorption band around 800 nm, Ho lasers are mainly pumped by the 1.9 µm lasers such as the InGaAs LDs [18] and Tm lasers [19,20] for the past two decades, which are bulky and expensive. Recently, via diffusion-bonding the Tm-doped and Ho-doped gain media into a single bulk structure, efficient Ho laser operation with broad pump wavelength band from 760 nm to 808 nm and wide cooling range from 9°C to 27°C was demonstrated in our previous work [21]. Hence, it is of significance to explore Q-switching in the above Tm/Ho composite laser with the popular SA fabricated with 2D materials such as TMDs. However, pulse operation for the above Tm/Ho composite laser was prevented when directly inserting SA into the laser resonator as common PQS lasers, due to the introduced modulation loss for the intra-cavity Tm laser and the lack of sufficient pump energy to the Ho-doped region.
In this paper, with a CVD-prepared WSe2 SA, we realized Q-switching in the Tm/Ho composite laser via filtering the intra-cavity Tm laser from Ho laser before modulated by the SA with a hybrid cavity structure, where significant resonance enhancement compared with the previous simple cavity was analyzed.
2. Preparation and characterization of the WSe2 SA
The highly crystalline WSe2 thin film was grew via selenizing WO3 powers in a hot-wall chemical vapor deposition (CVD) chamber, as depicted in Fig. 1(a). The WO3 powders were placed at center of the furnace with temperature of 990 °C at a ramping rate of 25 °C /min. The Se powders were placed at separate heat zone, where the temperature was maintained at 270 °C during the reaction. Vapors of Se and WO3 were drove to the sapphire substrate by an Ar/H2 flowing gas (Ar=100 sccm, H2=10sccm). Temperature of the substrate was at 890 °C when the center heat zone reached 990 °C. And then, the reaction was kept for 10 minutes to fabricate monolayer WSe2 and for 40 minutes to multi-layer WSe2, respectively.
Fig. 1. (a)Schematic of the growing process. (b,c) optical microscopies of the layered WSe2 prepared with 10 minutes and 40 minutes, respectively (size of the imaging areas is 877 µm× 660 µm, inset: detail image of WSe2 flakes at boundary of the monolayer WSe2).
Figure 1(b) and 1(c) are optical micrographs of the monolayer and multi-layer WSe2 by a 3D optical profiler (Sensofar INC.), where the domain boundaries are more obvious for the multi-layer WSe2 than that in the monolayer one, which indicates the overlap between each layered WSe2. Inset of Fig. 1(b) depicts single flake of WSe2 caught at boundary of the layered WSe2 prepared with 10 minutes. Atomic force microscopies (AFM) for the monolayer and multi-layer WSe2 are depicted in Fig. 2(a) and 2(b). According to the cross section profile in Fig. 2(c), thickness of the prepared WSe2 film is 0.72 nm, which in accord with thickness of monolayer WSe2 from mechanical exfoliation [22]. Thickness of 2.86 nm in Fig. 2(d) indicates 4 layers of WSe2 were grown.
Fig. 2. (a,b) AFM image for the monolayer and multi-layer WSe2 films respectively; (c,d) Cross section profiles form the cutlines in (a) and (b) respectively.
Raman spectra for the monolayer and multi-layer WSe2 excited by a 532 nm laser was shown in Fig. 3, where characteristic peak at 252 cm−1, corresponding to E12g mode was observed. Inset of Fig. 3(a) depicts the Raman signal at high energy bands, where peaks at 360 cm−1 and 376 cm−1, corresponding to the 2E1g and A1g+LA modes respectively are also identified [23]. Main difference in Raman spectrum between the monolayer and multi-layer WSe2 could be distinguished by the peak at 312 cm−1, which relates to the interlayer interaction [24]. Transmittance of the monolayer and multi-layer WSe2 are measured with a UV-VIS-NIR spectrophotometer (Lambda950, Perkin Elmer INC.) and shown in Fig. 3(b). The initial transmittances are 98.2% and 92.7% for the monolayer and multi-layer films respectively at 2100 nm, which accords with the increment in WSe2 layers.
Fig. 3. Raman spectra (a) and transmittance(b) of the monolayer and multi-layer WSe2. Inset: Raman signals in the high energy band.
3. Experimental setup
Schematic of the PQS Tm/Ho composite laser is depicted in Fig. 4. Both CVD multi-layer and monolayer WSe2 were wet transferred from sapphire to fused silica for preparing the SAs with the assistant of polymethyl methacrylate (PMMA). M1 was a plano-concave mirror with radius of curvature of 100 mm, which was coated high-reflection (HR) at 1.9∼2.1µm and anti-reflection (AR) at 760∼820 nm. M2 denotes the plano-plano output couplers with transmittances of 10% and 2% at Ho laser, which were coated HR at Tm laser(R>99.7% at 1.9∼2.02 µm). Fused silica spectral filter F1 AR coated at Ho laser (T>99.7% at 2.05∼2.12 µm) and HR at Tm laser (R>94% at 1.8∼2.02 µm) was inserted intra-cavity to form a hybrid cavity (HC) for separating the Tm laser from modulated by the SA and avoiding the cease in laser oscillation. Fiber coupled 808 nm diode laser (LD) with core diameter of 400 µm and NA of 0.22 was applied as the pump source, which was 1:1 imaged into the composite gain medium via two identical plan-convex lenses with focus length of 35 mm. The gain medium with diameter of 3 mm and length of 16 mm was integrated via diffusion bonding a 10 mm long 3.5 at.% Tm:YAG and a 6 mm long 0.8 at.% Ho:YAG crystal into a single bulk structure. Both surfaces of the composite gain medium were AR coated at 760∼820 nm and 1.9∼2.1µm. For removing extra heat from quantum deficit of the pumping process, the gain medium was wrapped with indium foil and mounted into cooper heat sink for water cooling at 16 °C. In the convex-plane resonator consisted with M1 and M2, tight focus pumping scheme for the PQS Ho laser was realized [25], where mode waists on the gain medium and the WSe2 SA were calculated to be 210 µm and 140 µm respectively without considering thermal lens of the gain medium.
Fig. 4. Experimental setup of the WSe2 PQS Tm/Ho composite laser (FS: pump focus system, PD: photo-electronic detector, PM: power meter).
4. Resonance enhancement in the hybrid cavity
Laser oscillation was obtained first from the short cavity (ShC) consisted of M1 and F1. As the filter F1 has transmittance of 4% at Tm laser, which also coated AR at Ho laser, single Tm laser operation was observed during the power scaling process, where maximum output powers of 930 mW at 2010.7 nm was obtained. Oscillation in Ho laser was obtained after M2 was added after F1 to from a hybrid cavity (HC), where maximum output powers of 806 mW and 543 mW were achieved with output couplings (OC) of 10% and 2% respectively. Removing F1 form the HC, in the simple cavity (SC) consisted of M1 and M2, maximum output powers decreased to be 467 W and 406 W with the 10% and 2% output couplers respectively, which indicates the resonance enhancement in the developed hybrid cavity, as depicted in Fig. 5(a) and 5(b). With the 10% OC, slope efficiency (SE) from the HC are 19.4%, which is higher than that of 11.1% from the SC. With the 2% OC, SE of 14.7% from the HC is 1.8 times higher than that of 8% from the SC. This resonance enhancement attributes to the fact that the Tm laser in the HC can be mainly confined inside the short cavity, which is helpful for an efficient Tm laser [26] and the higher pumping intensity to the Ho-doped region.
Fig. 5. (a,b) Power curves of the Tm/Ho composite laser with the short cavity (ShC), hybrid cavity (HC), and simple cavity (SC). (c) Evolution in lasing wavelength with the output power from the ShC and SCs. (d) Lasing spectra at the maximum output powers of the ShC (yellow dot line) and the SC with OC of 2% (red line) respectively, and at 93 mW of the SC with OC of 10% (blue dash line).
During the power scaling processes of the HC with different OCs, lasing wavelength was stabilized at 2122.2nm ±0.2 nm, which was the same as the SC with OC of 2% (Fig. 5(c)), where no spectral signal of the Tm laser was detected due to the well confinement of Tm laser. Figure 5(c) shows the evolution in lasing wavelength with the ShC and SCs respectively. Unlike the significant wavelength-drift in Tm:YAG lasers [27], the lasing wavelength stabilized at 2010.4 nm ± 0.3 nm from the ShC indicates the absorption loss from Ho-doped region of the composite gain medium. Dual wavelength oscillation in Tm laser and Ho laser was detected at output power between 72 mW and 192 mW from the HC with 10% OC. Typical lasing spectra from the ShC and SCs were shown in Fig. 5(d).
5. WSe2 passively Q-switched composite Ho laser
The prepared WSe2 SAs were inserted into the resonance-enhanced HC for exploring Q-switching of the Tm/Ho composite laser. Due to the high initial transmittance of the monolayer WSe2 SA, attempt in Q-switching with this type of SA was failure, where no Q-switching signal was detected from the HC with different OCs. Moreover, the damage in SA following with the significant increment in lasing efficiency occurred during the power scaling process at absorbed pump power around 4 W (Fig. 6(a)). Hence, the multi-layered WSe2 SA with higher damage threshold was applied.
Fig. 6. (a) Power curves of the Tm/Ho composite laser from HC with the monolayer and multi-layer WSe2 SAs respectively, where successful PQS only took place with the 2% output coupling. (b,c) Evolution in lasing wavelength with output power from the multi-layer SA modulated HC with 10% OC (b) and 2% OC (c), respectively. (d) Beam quality at the maximum PQS power of 141 mW (Insert: 2D beam profile at the beam waist).
Inserting the multi-layered WSe2 SA inside the HC, maximum output power of 351 mW and 141 mW were obtained with OC of 10% and 2% respectively (Fig. 6(a)). In HC with the 10% OC, the lasing wavelength switched from 2122 nm to 2011 nm and remained unchanged during the power scaling process (Fig. 6(b)) when replaced the mono-layer SA with the multi-layer SA, where there was still no successful Q-switching. We attributed this to the higher cavity loss for Ho laser when introducing the multi-layer SA, where additional loss from the uncoated fused silica substrate should be considered. With the cease in Ho laser, higher gain in the Tm-doped region supported the oscillation in Tm laser, which was confirmed by the fact that the lasing wavelengths maintained at 2011.3nm±0.2 nm was longer than that of 2010 nm from the ShC (Fig. 5(c)).
Decreasing the OC for compensating the SA loss further, successful PQS process was obtained with the 2% OC, where the lasing wavelength shifted from 2122nm to 2090 nm after inserting the multi-layer SA (Fig. 6(c)). During the power scaling process, pulse repetition rate of the Ho laser increased from 15 kHz at 18 mW to 33 KHz at 141 mW, where the corresponding pulse width decreased from 1058 ns to 185 ns (Fig. 7(a)). Basing on the output powers, repetition rates, and pulse widths, evolutions in pulse energy and peak power are shown in Fig. 7(b). Increasing the pump power, pulse energy increased from 1.2 uJ to 4.36 uJ with corresponding peak power from 1.13 W to 23.1 W. Unlike the linearly increased peak power, saturation in pulse energy stared at pump power about 6 W, which could be explained by the shortened time for energy accumulation in the Ho-doped region with the increased repetition rate. Using an InGaAs detector (DET05D/M, Thorlabs Inc) connected with a 2 GHz bandwidth oscilloscope (MSO 2034, Tektronix Inc.), typical PQS pulse trains at different repetition rates were recorded and depicted in Fig. 8. The time scales were kept at 20 µs/div to show the evolution in pulse trains, where profile of the shortest pulse was inset beneath. Beam quality at the maximum PQS power were measured to be with M2x=1.27 and M2y=1.17 in the horizontal direction and vertical direction respectively with a beam quality analyzer (Nanomode scan, Ophire Inc.). Comparing with the lasing wavelength of 2000.4 nm from MoS2 PQS Tm, Ho co-doped YAP laser [28], successful PQS at 2.1 µm with the TMD SA was realized here, owing to the resonance enhancement of the HC and the decrement in OC for compensating the cavity loss of Ho laser.
Fig. 7. Evolutions in pulse repetition rate, pulse width, pulse energy, and peak power with absorbed pump power.
Fig. 8. Typical pulse trains at repetition frequency of 15 kHz, 22 kHz, and 33 kHz respectively with profile of the shortest pulse inserted beneath
6. Summary
We demonstrated passively Q-switching in a Tm/Ho composite laser with the developed HC to separate the intra-cavity Tm laser from modulated by the CVD WSe2 SA. Resonance enhancement in the HC was observed where maximum output powers increased from 406 mW to 467 mW and from 540 mW to 800 mW with OCs of 2% and 10% respectively compared with using the SC at the same absorbed pump power. SA with around 4 layered WSe2 was applied for exploring the PQS Ho laser due to the low initial transmittance and damage threshold of monolayer WSe2 SA. Cease in Ho laser oscillation was observed when inserting the SA into the HC with 10% OC, where the lasing wavelength switched from 2122 nm to 2011 nm due to the increased cavity loss for Ho laser. Successful PQS was realized via using the 2% OC, where maximum average power of 141 mW at 2090 nm with the shortest pulse width of 185 ns at 33 kHz was obtained. This work facilitates the direct use of common 800 nm LDs for an accessible Q-switched Ho laser at room temperatures, which is compact and gets potential applications in radar, surgeries, spectroscopy, etc.
Funding
National Key Research and Development Program of China (2018YFB0407400); National Natural Science Foundation of China (61875200); China Postdoctoral Science Foundation (2018M642575).
Disclosures
The authors declare no conflicts of interest.
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