Optomechanical crystal is a combination of both photonic and phononic crystal. It simultaneously confines light and mechanical motion and results in strong photon-phonon interaction, which provides a new approach to deplete phonons and realize on-chip quantum ground state. It is promising for both fundamental science and technological applications, such as mesoscopic quantum mechanics, sensing, transducing, and so on. Here high optomechanical coupling rate and efficiency are crucial, which dependents on the optical-mechanical mode-overlap and the mechanical frequency (phonon frequency), respectively. However, in the conventional optomechanical-crystal based on the same periodical structure, it is very difficult to obtain large optical-mechanical mode-overlap and high phonon frequency simultaneously. We proposed and demonstrated nanobeam cavities based on hetero optomechanical crystals with two types of periodic structure. The optical and mechanical modes can be separately confined by two types of periodic structures. Due to the design flexibility in the hetero structure, the optical field and the strain field can be designed to be concentrated inside the optomechanical cavities and resemble each other with an enhanced overlap, as well as high phonon frequency. A high optomechanical coupling rate of 1.3 MHz and a high phonon frequency of 5.9 GHz are predicted theoretically. The proposed cavities are fabricated as cantilevers on silicon-on-insulator chips. The measurement results indicate that a mechanical frequency as high as 5.66 GHz is obtained in ambient environment, which is the highest frequency demonstrated in one-dimensional optomechanical crystal structure.
Nanostructure is an effective solution for realizing optoelectronic devices with compact size and high performances simultaneously. This paper reports our research progress on integrated nanophotonic devices for optical interconnections. We proposed a parent-sub micro ring structure for optical add-drop multiplexer (OADM) with compact footprint, large free spectral range, and uniform channel spacing. All eight channels can be multiplexed and de-multiplexed with 2.6 dB drop loss, 0.36 nm bandwidth (>40 GHz), -20 dB channel crosstalk, and high thermal tuning efficiency of 0.15 nm/mW. A novel principle of optical switch was proposed and demonstrated based on the coupling of the defect modes in photonic crystal waveguide. Switching functionality with bandwidth up to 24 nm and extinction ratio in excess of 15 dB over the entire bandwidth was achieved, while the footprint was only 8 μm×17.6 μm. We proposed an optical orbital angular momentum (OAM) coding and decoding method to increase the data-carrying capacity of wireless optical interconnect. An integrated OAM emitter, where the topological charge can be continuously varied from -4 to 4 was realized. Also we studied ultrafast modulated nLED as the integrated light source for optical interconnections using a nanobeam cavity with stagger holes.
Broadband thermo-optic switch based on an ultra-compact W2 photonic crystal waveguide (PCW) is demonstrated with
an integrated titanium/aluminum microheater on its surface. The operating principle relies on shifting a transmission-dip
caused by the enhanced coupling between the defect modes in W2 PCW. As a result, broadband switching functionality
with larger extinction ratio can be attained. Moreover, microheaters with different width are evaluated by the power
consumptions and heating transfer efficiency, and an optimized slab microheater is utilized. Finally, switching
functionality with bandwidth up to 24 nm (1557~1581 nm) is measured by the PCW with footprint of only
8μm×17.6 μm, while the extinction ratio is in excess of 15 dB over the entire bandwidth. What’s more, the switching speed is obtained by the measurement of alternating current modulation. Response time for this thermo-optic switch is
11.0±3.0 μs for rise time and 40.3±5.3 μs for fall time, respectively.
In particular, the surface plasmon polariton (SPP) is attractive to enhance the spontaneous emission (SE) from active
materials due to the larger density of state (DOS) and smaller mode volume comparing with optical wave, namely
Purcell effect. Usually, the Purcell factor (PF) is calculated from the reduced form of Fermi’s golden rule, where only the
DOS and mode volume of photon (or SPP mode) are involved. Obviously, the PFs calculated with reduced form exclude
the influence of active material and only evaluate the effect of cavity or SPP waveguide. However, for a practical
emitter, the linewidth could not always be ignored. For example, the ensemble emission linewidth of mass Si- quantum
dots (QD) is about 220meV~400meV (90~160nm), which are much wider than the linewidth of the SPP DOS
In this work, the PF of SPP mode on Au-Si<sub>3</sub>N<sub>4</sub> grating is calculated with full integration formula of Fermi’s golden rule by taking account of the spontaneous emission linewidth from single Si-QD. The calculated PF is about 1.7~1.4 within the emission range of <i>†hω<sub>0</sub></i> =1.9~1.6eV. Comparing with the PF value of 266.9~30.1, which is calculated without including the emission linewidth of Si-QD, it could be easily concluded that the impact of rather wide emission linewidth is fatal for applying plasmonic enhancement. To obtain some useful guidelines, we also discuss the necessary linewidth for effective plasmonic enhancement on Si-QDs. It is found that if the emission linewidth could be decreased to several tens of μeV, plasmonic enhancement would be helpful.
Layer-thickness dependence of Si-nanocrystal (Si-NC) formation induced by furnace annealing in amorphous Si (a-Si)
/SiO<sub>2</sub> multilayers is experimentally demonstrated with a radio-frequency-sputtered sample that has a-Si layers with
different thicknesses. Further, a modified model is developed to explain the Si-NC formation based on the Gibbs free
energy variation and it takes into account the whole formation process including nucleation and following growth. The
theoretical results show that there is a lower limit of Si layer thickness below which the crystal formation cannot occur
for a-Si/SiO<sub>2</sub> multilayers, and the oxide interfaces cannot constrain the lateral growth of Si-NCs, which may lead to their
touches within the Si layers.
The photoluminescence spectra of amorphous silicon rich silicon nitride films with various compositions were
investigated. Two main luminescence peaks were identified for all samples and blueshift of photoluminescence were
observed after annealing treatment. With the help of X-ray photoelectron spectroscopy and Fourier transform infrared
measurement, the chemical composition and bonding environment of samples, which were grown with different reactant
gases flow rates of plasma enhanced chemical vapor deposition, were analyzed. According to all these measurement
results, it is confirmed that the main luminescence centers are radiative recombination defects, such as silicon and nitride
dangling bonds. With proper deposition conditions, all these radiative recombination defects could be activated at the
same time, so that ultra-wide photoluminescence spectra with full width at half maximum of about 250nm ~ 300nm were
obtained in visible region.
We propose an all fiber tunable band pass filter based on dual acousto-optic mode coupling scheme. By applying two
acoustic waves on a piece of etched single mode fiber, the feasibility of the filter is demonstrated experimentally. The
tunable characteristics of the device are discussed theoretically.
An optimized 1.66μm Q-switched fiber laser is demonstrated for Raman-based distributed temperature sensor. In a ring
type Q-switched erbium-doped fiber laser, 850m TrueWave fiber is introduced to serve as both delay line fiber and
Raman gain medium so that in addition to the wavelength shifted to 1.6μm, the pulse duration and the buildup time can
be relatively extended. By properly controlling the fall edge of the acousto-optic switch, the pulse duration of 30-74ns
with peak power of 1.3-3.3W is achieved. Based on this light source, a 2.4km Raman-based distributed temperature
sensor is obtained with temperature resolution of 1°C and spatial resolution of 8m.