Nonsuspended optomechanical crystal cavities using As2S3 chalcogenide glass

Renduo Qi; Qiancheng Xu; Ning Wu; Kaiyu Cui; Wei Zhang; Yidong Huang

doi:10.1364/PRJ.417933

Abstract

An optomechanical crystal cavity with nonsuspended structure using

{As}_{2} S_{3}

material is proposed. The principle of mode confinement in the nonsuspended cavity is analyzed, and two different types of optical and acoustic defect modes are calculated through appropriate design of the cavity structure. An optomechanical coupling rate of 82.3 kHz is obtained in the proposed cavity, and the designed acoustic frequency is 3.44 GHz. The acoustic mode coupling between two nonsuspended optomechanical crystal cavities is also demonstrated, showing that the proposed cavity structure has great potential for realizing further optomechanical applications in multicavity systems.

Cavity optomechanics explores the interaction between optical and mechanical modes in optical/mechanical resonance systems, and it is promising for a wide range of applications such as high-precision sensing [1 - 3], laser cooling [4], and nonreciprocal devices [5, 6]. Various microcavity systems have been investigated for cavity optomechanics, including microtoroids [7], microdisks [8], and optomechanical crystal cavities [9 - 13]. Among these systems, optomechanical crystal cavities have attracted much attention because of their smaller mode volumes and higher optomechanical coupling rates [14, 15]. So far, silicon is the dominant material for optomechanical crystal cavities, which are usually fabricated on silicon-on-insulator (SOI) substrates. However, since the sound velocity in silicon is larger than that in silica, it is hard to confine acoustic modes in the silicon layers [16]. Therefore, suspended cavity structures are required in these systems to confine the acoustic modes and enhance the overlap between optical and acoustic modes. However, the suspended structures would increase the difficulty of fabrication and limit the structural complexity of the devices. In addition, extra structure designs are required in the suspended cavity systems to realize further functions such as acoustic mode coupling [17, 18].

Recently, chalcogenide glasses (ChGs) have drawn a lot of interest in nonlinear optics because of their large Kerr nonlinearity as well as low two-photon absorption [19,20]. More importantly, these materials have relatively high refractive index and low sound velocity, so that both optical modes and acoustic modes can be confined in the ChG core without using suspended structures [21]. Based on this feature, different integrated ChG devices have been fabricated for applications such as stimulated Brillouin scattering [22–24], which also results from the interaction between photons and phonons in traveling acoustic modes. Therefore, it could be expected to achieve effective optomechanical coupling based on nonsuspended ChG devices. The nonsuspended structures have the advantage of more flexible designs, and they can directly realize functions such as acoustic mode coupling among cavity arrays and external modulations without extra structures.

{As}_{2} S_{3}

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Figure 1.(a) The top view schematic of the nonsuspended

{As}_{2} S_{3}

optomechanical crystal cavity. (b) A unit cell of the optomechanical crystal in the mirror region.

Figure 2.(a) Optical band structure of the unit cell in the mirror region. (b) Acoustic band structure of the unit cell in the mirror region. (c) Electric field component

E_{y}

of the optical defect mode at the X point. Left: distribution of

E_{y}

at plane

x = d_{1} / 2

. Right: distribution of

E_{y}

at plane

z = 0

. [The coordinate of the cavity center is set as (0,0,0).] (d) Displacement field component

u_{x}

of the acoustic defect mode at the X point. Left: distribution of

u_{x}

at plane

x = d_{1} / 2

. Right: distribution of

u_{x}

at plane

z = 0

g = \frac{\partial ω_{0}}{\partial α} x_{zpf},

r_{1}

Figure 3.(a) Displacement field profile (

u_{x}

) of a confined acoustic Bloch mode at the

Γ

point. (b) 2D FT spectrum of

u_{x}

of the acoustic Bloch mode at the cavity interface perpendicular to the

z

direction (left) and interface perpendicular to the

y

direction (right). (c) Electric field profile (

E_{y}

) of a confined optical Bloch mode at the

Γ

point. (d) 2D FT spectrum of

E_{y}

of the optical Bloch mode at the cavity interface perpendicular to the

z

direction (left) and interface perpendicular to the

y

direction (right). (e) Electric field component

E_{y}

of the optical defect mode at the

Γ

point. Left: distribution of

E_{y}

at plane

x = d_{1} / 4

. Right: distribution of

E_{y}

at plane

z = 0

. (f) Displacement field component

u_{x}

of the acoustic defect mode at the

Γ

point. Left: distribution of

u_{x}

at plane

x = d_{1} / 4

. Right: distribution of

u_{x}

at plane

z = 0

. (g) 2D FT spectrum of

E_{y}

of the optical defect mode at the cavity interface perpendicular to the

y

direction (left). 2D FT spectrum of

u_{x}

of the acoustic defect mode at the cavity interface perpendicular to the

y

direction (right).

Figure 4.(a) A dual-cavity system of nonsuspended

{As}_{2} S_{3}

optomechanical crystal cavities with a separation distance

L

. (b) Profiles of the calculated supermodes in the dual-cavity system. Top: the odd mode. Bottom: the even mode. (c) Left ordinate: calculated acoustic coupling rates between two identical cavities (black solid line) and between two different cavities with the same acoustic frequency (black dashed line). Right ordinate: optical coupling rates between two identical cavities (red solid line) and between two different cavities with the same acoustic frequency (red dashed line).

In some applications, the optical coupling between cavities is not expected while acoustic coupling is required. It can be realized by tuning the optical frequency of one cavity while keeping the acoustic frequency unchanged, so that optical coupling could not occur due to the inconsistent optical frequencies of the two cavities. A possible way to realize this is to adjust the radii of silica holes and lattice constants in the defect region of one cavity. The black dashed line in Fig. 4 (c) shows the calculated acoustic coupling rates between two nonsuspended optomechanical crystal cavities with different structural parameters. These two cavities have the same acoustic frequency, while the optical frequencies are 193 THz and 197 THz, respectively. The result shows that acoustic mode coupling also occurs on this condition. The optical coupling rates between two cavities are also calculated and plotted in Fig. 4 (c). The red solid line indicates the optical coupling rate between two identical cavities, and the red dashed line is the coupling rate between cavities with different optical frequencies. It can be seen that optical mode coupling could be suppressed by this way, while the acoustic mode coupling condition remains almost unchanged.

{As}_{2} S_{3}