通過破壞聚合物微腔表面直接獲得單模激光

回音壁模式(WGM)微腔在超小模式體積中對光有強烈的束縛能力,吸引了大家極大的研究興趣。在過去數年中,出現了多種不同形狀的WGM,如微環、微盤、圓柱形、球形、螺旋形、多邊形和微瓶腔等,但是由于缺少模式抑制技術,通常得到的是多模激光信號。在傳感應用方面,多模激光在區分所選的模式信號時非常棘手,因此,單模激光的出現在傳感領域和光譜學領域就顯得尤為關鍵。WGM單模激光信號可由以下四種方式獲得:減小微腔尺寸、游標效應、宇稱-時間對稱效應和壓電效應。

在上述提及的不同形狀的WGM微腔中,沿其長軸方向獨立分布的各個模式使得微瓶諧振腔成為一種全新且極具吸引力的微腔激光器。激發光可以有效激發微腔表面的WGM并選擇想要的激光模式,通過調節干涉激發光的增益機制及拉錐光纖破壞微球腔表面的方式可以獲得想要的單模激光信號,并將微腔結構放置在柔性襯底上,可以實現應力傳感的目的。而且,微瓶諧振腔的制備通常需要復雜且昂貴的光學設備,一種便宜且容易的制備方式更能獲得大家的青睞。與此同時,通過改變微腔的表面和形狀,實現模式選擇的方式大大吸引了研究者的興趣,如在微球腔表面劃出一個小洞或一個小槽,都可以增加微腔表面的散射光,達到模式抑制的作用。

上海理工大學谷付星教授課題組利用鎢探針破壞微球腔表面的形狀后獲得單模激光。相關成果發表在Chinese Optics Letters 2019年第17卷第12期上(Saima Ubaid, Feng Liao, Tao Guo, Zhaoqi Gu, Shuangyi Linghu, Yanna Ma, Jiaxin Yu, Fuxing Gu. Direct single-mode lasing in polymer microbottle resonators through surface destruction[J]. Chinese Optics Letters, 2019, 17(12): 121401),該課題組博士生Ubaid Saima為論文第一作者。

首先將增益介質R6G摻雜在高分子溶液中,并利用自組裝的制備方式獲得微瓶腔結構,將鎢探針安裝在三維調節架上,精確控制鎢探針接觸微腔的位置,小范圍地破壞微腔表面的形狀,以實現激光模式減少的目的。當均勻地激發光照射微腔表面時,由于微腔表面被破壞,遭受更高損耗的高階模式將會被抑制,最終得到單模激光信號。值得注意的是,模式的抑制程度與鎢探針破壞的位置數量和微腔尺寸緊密相關,如果微腔尺寸較小時,鎢探針只需輕輕觸摸微腔表面就可以得到單模激光;反言之,一旦增加尺寸,就很難得到純凈的單模信號。總而言之,該方法是抑制激光模式的有效方法。另外,該方法獲得的單模激光具有較高的單模抑制比和較低的激光閾值。

谷付星教授認為,將來可以通過使用具有納米級定位分辨率的壓電位移平臺來驅動鎢探針,這會進一步改進該系統,以便更精確地選擇單個激光模式。該方法非常靈活、方便且價格低廉,在激光和傳感領域有著廣泛的應用前景。

通過簡單地使用鎢探針破壞聚合物微瓶腔的外頸表面來產生單模激光。軸向階數較大的高階微瓶腔瓶模將遭受較大的損失,使得用自由空間光束輸運就可以直接產生單模激光。

Direct single-mode lasing in polymer microbottle resonators through surface destruction

Owing to strong confinement of light in ultra-small mode volume, whispering gallery mode (WGM) microresonators have attracted great interest. In previous years, different whispering gallery mode geometries have been investigated such as rings, disks, cylinders, spheres, toroids, polygons and microbottles but due to absence of mode suppression techniques, microlaser produced through these geometries is multimode. Multimode occurrence can be troublesome in distinguishing selective modes for sensing application. So, realization of single mode laser is an essential requirement for sensing and spectroscopy applications. Single WGM lasing can be obtained by decreasing the size of microresonators, coupled cavities in which one cavity serves as a spectral filter to the other one by using the Vernier effect, parity-time symmetry effect and piezoelectric effect.

Among all aforementioned geometries of WGM microresonators, microbottle resonators are new and interesting candidates owing to the highly non-degenerated WGMs with spatially well-separated intensity along the long-axis direction. These modes can be efficiently activated depending on the excitation set-up and make it possible to select the desired modes. Gain engineering via spatial interference excitation and loss engineering via fiber-taper have been investigated to fabricate single mode laser which is then used in a stretchable device for strain sensing. Nevertheless, the fabrication of microbottle resonators usually requires complex and expensive optical systems. It is preferred to have an inexpensive and simple way to fabricate the microbottle resonators. Alternatively, controlling of modes selection via engineering the surfaces and shapes of the WGM microresonators is of great interest for the researchers. Scattering of the light out of the microresonators by adding micro-droplets and micro-grooves on the surface of microbottles can suppress and attenuate the bottle modes.

The microlaser introduced by Dr. Saima Ubaid from a research group which is led by Prof. Fuxing Gu from the University of Shanghai for Science and Technology is single mode which is achieved by surface destruction of polymer microbottle resonators through tungsten probe. The relevant research results are published in Chinese Optics Letters, Vol. 17, Issue 12, 2019 (Saima Ubaid, Feng Liao, Tao Guo, Zhaoqi Gu, Shuangyi Linghu, Yanna Ma, Jiaxin Yu, Fuxing Gu. Direct single-mode lasing in polymer microbottle resonators through surface destruction[J]. Chinese Optics Letters, 2019, 17(12): 121401).

To obtain single mode laser, a bottle shaped polymer resonator is used. Self-assembly fabrication technique is used to fabricate laser dye doped polymer microbottle resonators. The external surface of the microbottle resonators is destroyed by a tungsten probe to reduce the numbers of lasing modes. The tungsten probe is mounted on a triple-axis micromanipulator and is precisely struck to the neck of a microbottle resonator, and in this way small part of resonator’s surface is destroyed. When this resonator is pumped by an ordinary free-space light irradiation, the higher-order bottle modes with large axial orders spatially located around the neck surface of the microresonator will suffer large optical losses while rest of them are excited and we can directly achieve single mode lasing from the fundamental bottle modes by destroying again and again. The modes suppression depends on the amount of surface destruction by tungsten probe and the sizes of the microbottle resonators. When this surface destruction method is applied to small microbottle resonators, achievement of single mode lasing is very easy by just slightly touching tungsten probe. While increasing further the outer diameter of microbottle resonators, it is difficult to achieve high purity single mode lasing yet it is an effective way to get mode reduction in these resonators. Additionally, the obtained single mode lasing has attractive advantages of high side mode suppression ratios and low thresholds.

Prof. Fuxing Gu believes that this method can be improved by using piezo-stages with nanoscale positioning resolutions to drive the tungsten probes in the future, so that we can select the single lasing modes more precisely. This method is very flexible, inexpensive and convenient, and may find wide promising applications such as lasing and sensing devices.

Single-mode lasing by simply using a tungsten probe to destroy the external neck surface of polymer microbottle resonators. The higher-order bottle modes with large axial orders will suffer large losses. Using free-space light beam will directly generate single-mode lasers.