Laser optomechanics

Abstract

Cavity optomechanics explores the interaction between optical field and mechanical motion. So far, this interaction has relied on the detuning between a passive optical resonator and an external pump laser. Here, we report a new scheme with mutual coupling between a mechanical oscillator supporting the mirror of a laser and the optical field generated by the laser itself. The optically active cavity greatly enhances the light-matter energy transfer. In this work, we use an electrically-pumped vertical-cavity surface-emitting laser (VCSEL) with an ultra-light-weight (130 pg) high-contrast-grating (HCG) mirror, whose reflectivity spectrum is designed to facilitate strong optomechanical coupling, to demonstrate optomechanically-induced regenerative oscillation of the laser optomechanical cavity. We observe >550 nm self-oscillation amplitude of the micromechanical oscillator, two to three orders of magnitude larger than typical, and correspondingly a 23 nm laser wavelength sweep. In addition to its immediate applications as a high-speed wavelength-swept source, this scheme also offers a new approach for integrated on-chip sensors.

Figure 1. Schematic of experimental apparatus of laser optomechanics.

The optical cavity houses the laser source as well as a mechanically moveable mirror with a reflection spectrum R(λ). The laser can be either optically pumped or electrically pumped. As the mirror oscillates with displacement x(t), the laser output wavelength λ(x, t) and power P(λ, t) changes with respect to time t. The spring constant k is a function of the optical wavelength and laser output power, which illustrate the optical spring effect. The various images visualize this wavelength changes: the lasing wavelength changes from red to blue as the effective cavity length shortens due to the mirror oscillation. Depending on the mirror reflectivity and gain, the mechanical oscillation can be sustained over a large displacement range.

Figure 2. HCG VCSEL and the mechanical oscillation of HCG.

(a) Schematic of the HCG VCSEL, which is composed of a capacitatively-actuated MEMS HCG top mirror, an active region with multiple quantum wells, and an immobile DBR bottom mirror. (b) Laser Doppler vibrometry measurement of quality factor of fundamental mechanical mode of MEMS oscillation in vacuum, Q_m = 3,640. (c–d) False color SEM image of the HCG viewed at 45^o from surface normal. c shows the steady HCG when the bias current I of the VCSEL is zero; (d) shows its oscillation at fundamental mode when I = 13 mA (laser threshold current is 5 mA at room temperature for this device). Due to the fast scanning rate of the SEM, the stroboscopic effect produces a periodic distortion of the HCG bars in the SEM image.

Figure 3. Regenerative oscillation of HCG VCSEL.

(a) The time-resolved lasing wavelength of the HCG VCSEL with the HCG oscillating at fundamental mode. (b) Optical spectrum obtained in an optical spectrum analyzer with high integration time, captured simultaneously with the time-resolved wavelength characterization of Fig. 3a, confirming the extent of the wavelength sweep. (c) The wavelength swept range versus the laser current and tuning voltage. The laser current and tuning voltage can be used to tune the optomechanical dynamics.

Figure 4. Regenerative oscillation of the HCG VCSEL in different mechanical oscillation modes.

(a–c) Oscillation at fundamental mode; (d–f), Oscillation at a high order mode. (a,d) shows the RF spectrum density of the laser optical output power. (b,e) shows the zoom-in view of the first RF tone in (a,d) (marked with the red dot); they are fitted with Voigt functions. The frequency offset in (b) and (e) is 345.6 kHz and 5.636 MHz respectively. (c,f) shows the laser optical output power in time-domain. The threshold current of the laser is 10.1 mA at room temperature. The laser current and tuning voltage in conditions (a-c) and (d-f) is 19.2 mA, 10 V, 12 mA, 0 V respectively.

Figure 5. Mechanical modes of the HCG, and the analysis of the dynamical back action.

(a–c), The mechanical modes of the HCG. a shows the fundamental mode, and (b–c) shows examples of the high order modes, where the HCG frame and bars are oscillating with the static bridges in (b) and only the HCG bars are oscillating with the static frame and bridges in (c). (d), Optical delay τ_c(x) and radiation pressure damping versus wavelength displacement for a radiation-pressure passive optomechanical cavity. (e) Optical delay τ_c(x) and radiation pressure damping versus wavelength displacement for an equivalent radiation-pressure laser optomechanical cavity.