Mirror Surface Nanostructuring via Laser Direct Writing—Characterization and Physical Origins

Abstract

advances to this method, perform timeresolved measurements of the process, and analyze its physical origins.Thanks to this novel nanostructuring method, open-access optical microcavities have become a very flexible platform for experiments with two-dimensional photon gases, in particular, for the study of lasing phenomena and photon Bose-Einstein condensation (BEC). [8][9][10][11][12][13] A major future application of such systems may be spin glass simulation, allowing the system to be used as a computing platform. [14] Furthermore, the study of polaritonic quantum gases based on open cavities [15,16] may benefit from this method. Beyond studying optical condensation phenomena, nanostructuring of high-reflectivity mirrors via laser direct writing may also find novel applications in areas such as cavity ringdown spectroscopy, [17] wavefront shaping, [18] and precision interferometry [19,20] (and references therein).In addition to the permanent modification of mirror surfaces, it is also desirable to perform time-dependent control over the transverse flow of light in microcavities. Mirrors with integrated piezo arrays, [21] heater arrays, [22] and electrostatic membranes [23] may be used for this purpose. As in the previously discussed case of permanent nanostructuring, [7] absorptive layers in the mirror also offer a solution to the problem of time-dependent control. This includes refractive-index tuning via laser writing of the thermo-responsive polymer poly(N-isopropylacrylamide) (pNIPAM) [24] added to the cavity medium, and thermal expansion of the mirror surface, which will be demonstrated in this paper. The former introduces an attractive potential, while the latter creates a repulsive potential. Since both techniques work on different timescales, they may be used in a complementary manner. Additionally, a certain degree of spatial complexity can be introduced by wavefront shaping the heating laser.The dielectric mirrors used in this work are produced using ion beam sputtering (IBS), and have the following composition: a fused silica substrate, atop which lies an optically absorptive amorphous silicon layer of 30 nm thickness, followed by a quarter-wave dielectric stack with a reflectivity maximum at a wavelength of 600 nm. In most measurements presented in this work, this dielectric stack consists of 20 pairs of Ta 2 O 5 and SiO 2 . However, we also evaluate thinner dielectric stacks with 11 and 10 pairs. The essential point of this mirror design is the absorptive Si layer that provides a way to locally heat the dielectric stack and the optical medium using laser light-which is the basis of all the phenomena we show in this paper. In the first part of this work, we will demonstrate that such mirrorsThe addition of an optically absorptive layer to otherwise standard dielectric mirrors enables a set of laser direct writing nanostructuring methods that can add functionality to such mirrors while retaining their high reflectivity. A thorough characterization of this method is given in this paper, and its physic...