604-nm high-order vortex beams directly generated from a Pr:YLF laser with a cavity-loss-induced gain switching mechanism

https://doi.org/10.1016/j.optlastec.2020.106185Get rights and content

Highlights

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    First demonstration of 604-nm vortices directly generated from a Pr:YLF laser.

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    Topological charge up to three was achieved through designated cavity design.

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    Cavity-loss-induced gain switching mechanism between 604 and 607 nm emission.

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    Output power reached 470, 230, and 60 mW for the 1st-, 2nd-, and 3rd-order vortex.

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    Slop efficiencies were 24.5%, 17.5%, and 11.4%, respectively.

Abstract

In this work, we demonstrated, for the first time to the best of our knowledge, high-order vortex beams at 604 nm that were directly emitted from a Pr:YLF crystal laser. By combining a blue-diode-pumped v-type cavity for controllable Gouy phase introduction and off-axis crystal rotation for high-order HG modes generation, topological charged 604-nm vortex lasers were attained up to the third order. Furthermore, the vortex beams were found to be lasing at 604 nm that was often deemed less prone to lase compared with 607-nm in the same energy manifold. By a detailed spectroscopic study and rate equation analysis, we proposed and confirmed that the 604-nm transition of Pr:YLF crystal could be favorable in the vortex beam generation by a cavity-loss-induced gain switching process, in which the net gain at 604-nm could surpass that of 607-nm as the cavity loss was amassed beyond a threshold value.

Introduction

High-order optical vortices, carrying more than one topological charge for each photon, are ideal light sources for macroscopic quantum entanglement measurement [1], angular remote sensing [2], precise spatial measurement [3], and laser printing of chiral plasmonic nanojets [4]. Optical vortices directly generated from lasers, which constitute cavity eigenstates and possess inherently superior beam quality, are preferable choices [5]. Higher-order vortex beams in the orange that is at the absorption band of certain metal phthalocyanines, uranium-doped CaF2 crystals, and hybrid liposome/Au nanoparticles, may offer new perspectives for applications in nonlinear absorption spectroscopy [6], crystallographic characterization [7], and biomedical photonics [8].

One of the promising and well-studied gain media for orange lasers is the praseodymium-doped yttrium lithium fluoride (Pr:YLF) [9], [10], [11], [12], [13], [14]. To realize optical vortex beams, off-axis pumping scheme has been widely used in generating high-order Hermite-Gauss (HG) modes that can be transformed to corresponding high-order Laguerre-Gauss (LG) modes by external mode convertors [15], [16], [17]. Such approach was recently exploited to produce red and orange vortex beams directly from a Pr:YLF laser cavity, however, the vortices achieved were limited to first-order vortex beams [18], [19].

In this paper, we demonstrated direct emission of high-order LG vortex beams in the orange spectra from a Pr:YLF laser utilizing an equivalent off-axis pumping scheme combined with a robust π/2 Gouy phase introduction. Furthermore, upon studying the emitted spectra of LG vortex beams up to the 3rd order, we discovered that the wavelength was centered at 604 nm, instead of the commonplace 607 nm in the orange. Based on a thorough spectroscopic study and rate-equation analysis, we confirmed that, to the contrary of common notions, the net gain of 604-nm transition at room temperature could be higher than that of 607-nm by a simple cavity loss management when the total loss was beyond a threshold value, resulting in a swift gain switching from 607 to 604 nm with the proper setup.

Section snippets

Experimental setup

To directly generate orange LG vortex beams, as illustrated in Fig. 1, we utilized a three-mirror v-type Pr:YLF cavity. The gain medium was a cubic a-cut crystal with a relatively low doping concentration of 0.3 at.%. The low doping level was to not only reduce the thermal load and avoid the risk of crystal fracturing [14], but also weaken the ground-state re-absorption [10]. A relatively large crystal thickness of 10 mm was consequently utilized to increase the pump absorption. The Pr:YLF

Results and discussion

For vortex beam generation, LG modes can be converted from HG modes by superimposing a π/2 Gouy phase, while HG modes can be acquired by lowering the threshold below that of Gaussian mode with proper crystal rotation [23]. The set of cavity parameters, l1, l2, and θ, required to bring the π/2 Gouy phase into the laser, can be calculated by the ABCD matrix law [21] with results plotted in Fig. 2(a). In general, l1 increases with the decrease of θ, and l2 increases with both l1 and θ decreasing.

Validation of the cavity-loss-induced gain switching and emission spectrum change

The switching of the emitted laser spectrum from 607 to 604 nm along with the vortex beam occurrence has led us to investigate on the spectroscopic characteristics in Pr:YLF lasing. It is well known that Pr:YLF crystal has two close emission peaks in the same energy manifold at 604 and 607 nm, respectively [11]. Traditionally, the perception is that the 607-nm transition is prone to lasing because it has higher gain and lower loss comparing with those of 604-nm [12]. This preconceived notion

Conclusion

To conclude, in this paper we presented a diode-pumped Pr:YLF laser that directly generated high-order vortex laser beams at 604 nm. Utilizing an elaborate design to superpose an intra-cavity π/2 Gouy phase shift and a delicate cavity loss management that preferred the 604-nm transition line, LG vortex laser emissions of both chiralities were achieved from HG modes conversion. Vortex beams up to the 3rd-order were attained directly out of the laser cavity with appreciable maximal output powers

CRediT authorship contribution statement

Saiyu Luo: Conceptualization, Methodology, Software, Formal analysis, Investigation, Writing - original draft, Visualization. Zhiping Cai: Validation. Chuanxiang Sheng: Resources, Funding acquisition. Li Li: Writing - review & editing, Supervision, Funding acquisition, Project administration. Qian Chen: Resources, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work is supported by NSAF (No. U1830123), the National Natural Science Foundation of China (No. 61627802), the Natural Science Foundation of Jiangsu Province (No. BK20180460), and the High-Level Educational Innovation Team Introduction Plan of Jiangsu, China.

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