J. R. Koehler

CW-pumped single-pass frequency comb generation by resonant optomechanical nonlinearity in dual-nanoweb fiber

Recent experiments in the field of strong optomechanical interactions have focused on either structures that are simultaneously optically and mechanically resonant, or photonic crystal fibers pumped by a laser intensity modulated at a mechanical resonant frequency of the glass core. Here, we report continuous-wave (CW) pumped self-oscillations of a fiber nanostructure that is only mechanically resonant. Since the mechanism has close similarities to stimulated Raman scattering by molecules, it has been named stimulated Raman-like scattering. The structure consists of two submicrometer thick glass membranes (nanowebs), spaced by a few hundred nanometers and supported inside a 12-cm-long capillary fiber. It is driven into oscillation by a CW pump laser at powers as low as a few milliwatts. As the pump power is increased above threshold, a comb of Stokes and anti-Stokes lines is generated, spaced by the oscillator frequency of ∼6  MHz. An unprecedentedly high Raman-like gain of ∼4×106  m−1 W−1 is inferred after analysis of the experimental data. Resonant frequencies as high as a few hundred megahertz are possible through the use of thicker and less-wide webs, suggesting that the structure can find application in passive mode-locking of fiber lasers, optical frequency metrology, and spectroscopy.

Resolving the mystery of milliwatt-threshold opto-mechanical self-oscillation in dual-nanoweb fiber

It is interesting to pose the question: How best to design an optomechanical device, with no electronics, optical cavity, or laser gain, that will self-oscillate when pumped in a single pass with only a few mW of single-frequency laser power? One might begin with a mechanically resonant and highly compliant system offering very high optomechanical gain. Such a system, when pumped by single-frequency light, might self-oscillate at its resonant frequency. It is well-known, however, that this will occur only if the group velocity dispersion of the light is high enough so that phonons causing pump-to-Stokes conversion are sufficiently dissimilar to those causing pump-to-anti-Stokes conversion. Recently it was reported that two light-guiding membranes 20 μm wide, ∼500 nm thick and spaced by ∼500 nm, suspended inside a glass fiber capillary, oscillated spontaneously at its mechanical resonant frequency (∼6 MHz) when pumped with only a few mW of single-frequency light. This was surprising, since perfect Raman gain suppression would be expected. In detailed measurements, using an interferometric side-probing technique capable of resolving nanoweb movements as small as 10 pm, we map out the vibrations along the fiber and show that stimulated intermodal scattering to a higher-order optical mode frustrates gain suppression, permitting the structure to self-oscillate. A detailed theoretical analysis confirms this picture. This novel mechanism makes possible the design of single-pass optomechanical oscillators that require only a few mW of optical power, no electronics nor any optical resonator. The design could also be implemented in silicon or any other suitable material.

Effects of squeezed-film damping on the optomechanical nonlinearity in dual-nanoweb fiber

The freely-suspended glass membranes in a dual-nanoweb fiber, driven at resonance by intensity-modulated light, exhibit a giant optomechanical nonlinearity. We experimentally investigate the effect of squeezed-film damping by exploring the pressure dependence of resonant frequency and mechanical quality factor. As a consequence of the unusually narrow slot between the nanowebs (22 μm by 550 nm), the gas-spring effect causes a pressure-dependent frequency shift that is ∼15 times greater than typically measured in micro-electro-mechanical devices. When evacuated, the dual-nanoweb fiber yields a quality factor of ∼3 600 and a resonant optomechanical nonlinear coefficient that is ∼60 000 times larger than the Kerr effect.

Coherent control of flexural vibrations in dual-nanoweb fibers using phase-modulated two-frequency light

Coherent control of the resonant response in spatially extended optomechanical structures is complicated by the fact that the optical drive is affected by the back-action from the generated phonons. Here we report a new approach to coherent control based on stimulated Raman-like scattering, in which the optical pressure can remain unaffected by the induced vibrations even in the regime of strong optomechanical interactions. We demonstrate experimentally coherent control of flexural vibrations simultaneously along the whole length of a dual-nanoweb fiber, by imprinting steps in the relative phase between the components of a two-frequency pump signal,the beat frequency being chosen to match a flexural resonance. Furthermore, sequential switching of the relative phase at time intervals shorter than the lifetime of the vibrations reduces their amplitude to a constant value that is fully adjustable by tuning the phase-modulation depth and switching rate. The results may trigger new developments in silicon photonics, since such coherent control uniquely decouples the amplitude of optomechanical oscillations from power-dependent thermal effects and nonlinear optical loss.

Coherent control of flexural vibrations in dual-nanoweb fibre using phase-modulated two-colour CW laser light

Optical fields that are spatio-temporally shaped in amplitude and/or phase can be used to coherently control light-matter interactions. An example is the use of femtosecond pulse trains to selectively drive GHz phonons in the core of a photonic crystal fibre (PCF) [1]. These phonons generate, by stimulated Raman-like scattering (SRLS), Stokes and anti-Stokes side-bands within the same optical mode, all spaced by a pump-frequency-independent SRLS shift. Giant SRLS gain was recently measured in a system of two closely spaced and optically-coupled silica “nanowebs” suspended inside an evacuated capillary fibre [2] (Fig. 1(a)). Optical gradient forces give rise to a strong optomechanical nonlinearity, as a result of tight coupling between the optical mode and the lowest-order flexural resonance in each nanoweb (∼5.6 MHz). When tuned to this frequency, the beat-note (“optical force”) of two-colour light at ∼1550 nm (95% pump and 5% Stokes power) excites a large population of long-lived phonons (lifetime τ). Since the effect of group velocity dispersion is negligible over this small frequency shift, these phonons mediate phase-matched SRLS transitions between many pairs of adjacent side-bands. As a result the beat-note force evolves smoothly along the whole fibre, despite the presence of strong structural non-uniformities [2]. Moreover, since the group velocity of the SRLS phonons is nearly zero, the nanoweb deflection responds synchronously at every position to variations in the launched optical fields.

Interplay of Cascaded Raman- and Brillouin-like Scattering in Nanostructured Optical Waveguides

We formulate a generic concept of engineering optical modes and mechanical resonances in a pair of optically coupled light-guiding membranes for achieving cascaded light scattering to multiple Stokes and anti-Stokes orders. Light pressure exerted on the webs swings standing- and propagating-wave flexural vibrations associated with Raman-like intramodal and Brillouin-like intermodal transitions. Due to negligible optical group velocity dispersion, the Raman-like light scattering generates a frequency comb for a single optical mode while the Brillouin-like light scattering creates inserted frequency combs with even- and odd-order side-bands appearing alternatingly in the fundamental and higher-order optical modes via exciting an effective optoacoustic grating of backward- and forward-propagating flexural phonons. Adjustment of nanoweb widths makes both these processes to occur with the same Stokes shift. As a result, the system drives intricate optomechanical patterns permitting periodic reversal of the ene...

Pressure-tuning of the optomechanical nonlinearity in dual-nanoweb fibre

Optical driving of evacuated dual-nanoweb fibre at an acoustic resonance yields an optomechanical nonlinear coefficient ~90,000× greater than the Kerr-effect. Squeezed-film damping caused the resonant frequency to fall, and Q-factor to rise, with decreasing pressure.

Enhanced optomechanical nonlinearity in evacuated dual-nanoweb fiber

The non-resonant optomechanical nonlinearity is enhanced in dual-nanoweb fibers by improved design. A 10-fold increase in Q-factor, resulting in a resonant nonlinearity 200,000 times larger than the Kerr effect, is observed after evacuating the fiber.