Heedeuk Shin

Control of coherent information via on-chip photonic–phononic emitter–receivers

Rapid progress in integrated photonics has fostered numerous chip-scale sensing, computing and signal processing technologies. However, many crucial filtering and signal delay operations are difficult to perform with all-optical devices. Unlike photons propagating at luminal speeds, GHz-acoustic phonons moving at slower velocities allow information to be stored, filtered and delayed over comparatively smaller length-scales with remarkable fidelity. Hence, controllable and efficient coupling between coherent photons and phonons enables new signal processing technologies that greatly enhance the performance and potential impact of integrated photonics. Here we demonstrate a mechanism for coherent information processing based on travelling-wave photon–phonon transduction, which achieves a phonon emit-and-receive process between distinct nanophotonic waveguides. Using this device, physics—which supports GHz frequencies—we create wavelength-insensitive radiofrequency photonic filters with frequency selectivity, narrow-linewidth and high power-handling in silicon. More generally, this emit-receive concept is the impetus for enabling new signal processing schemes.

Stimulated brillouin scattering in nanoscale silicon step-index waveguides: A general framework of selection rules and calculating SBS gain

We develop a general framework of evaluating the Stimulated Brillouin Scattering (SBS) gain coefficient in optical waveguides via the overlap integral between optical and elastic eigen-modes. This full-vectorial formulation of SBS coupling rigorously accounts for the effects of both radiation pressure and electrostriction within micro- and nano-scale waveguides. We show that both contributions play a critical role in SBS coupling as modal confinement approaches the sub-wavelength scale. Through analysis of each contribution to the optical force, we show that spatial symmetry of the optical force dictates the selection rules of the excitable elastic modes. By applying this method to a rectangular silicon waveguide, we demonstrate how the optical force distribution and elastic modal profiles jointly determine the magnitude and scaling of SBS gains in both forward and backward SBS processes. We further apply this method to the study of intra- and inter-modal SBS processes, and demonstrate that the coupling between distinct optical modes are necessary to excite elastic modes with all possible symmetries. For example, we show that strong inter-polarization coupling can be achieved between the fundamental TE- and TM-like modes of a suspended silicon waveguide.

Large Brillouin amplification in silicon

Net Brillouin amplification is demonstrated in silicon with just 5 mW pumping. Greater than 5 dB amplification is achieved.

Reducing pulse distortion in fast-light pulse propagation through an erbium-doped fiber amplifier

When a pulse superposed on a cw background propagates through an erbium-doped fiber amplifier with a negative group velocity, either pulse broadening or pulse compression can be observed. These effects can be explained in terms of two competing mechanisms: gain recovery and pulse spectrum broadening. The distortion of the pulse shape caused by these effects depends on input pulse width, pump power, and background-to-pulse power ratio. With the proper choice of these three parameters, we can obtain significant pulse advancement with minimal pulse distortion.

Quantum spatial superresolution by optical centroid measurements.

Quantum lithography (QL) has been suggested as a means of achieving enhanced spatial resolution for optical imaging, but its realization has been held back by the low multiphoton detection rates of recording materials. Recently, an optical centroid measurement (OCM) procedure was proposed as a way to obtain spatial resolution enhancement identical to that of QL but with higher detection efficiency (M. Tsang, Phys. Rev. Lett. 102, 253601 (2009)). Here we describe a variation of the OCM method with still higher detection efficiency based on the use of photon-number-resolving detection. We also report laboratory results for two-photon interference. We compare these results with those of the standard QL method based on multiphoton detection and show that the new method leads to superresolution but with higher detection efficiency.

Propagation of smooth and discontinuous pulses through materials with very large or very small group velocities

We investigate the propagation of optical pulses through two different solid- state optical materials, ruby and alexandrite, for which the group velocity can be very small (vgc )o r superluminal (vgc or negative). We find that for smooth pulses the fractional delay or advancement is maximized through the use of pulses with durations comparable to the response time of the physical process—coherent population oscillations—that leads to these extreme group velocities. However, we find that the transmitted pulse shape becomes distorted unless the pulse is much longer or much shorter than this response time. We also investigate the transmission of pulses that possess an abrupt change in pulse amplitude. We find that, to within experimental accuracy, this nearly discontinuous jump propagates at the usual phase velocity of light c/n ,e ven though the smoothly varying portions of the pulse propagate at the group velocity.

Large nonlinear optical response of polycrystalline Bi 3.25 La 0.75 Ti 3 O 12 ferroelectric thin films on quartz substrates

We measure the nonlinear susceptibility of Bi(3.25)La(0.75)Ti(3)O(12) (BLT) thin films grown on quartz substrates using the Z-scan technique with picosecond laser pulses at a wavelength of 532 nm. The third-order nonlinear refractive index coefficient gamma and absorption coefficient beta of the BLT thin film are 3.1 x 10(-10) cm(2)/W and 3 x 10(-5) cm/W, respectively, which are much larger than those of most ferroelectric films. The results show that the BLT thin films on quartz substrates are good candidate materials for applications in nonlinear optical devices.

Implementation of sub-Rayleigh-resolution lithography using an N-photon absorber

A nonlinear optical, phase-shifted-grating method for improving the resolution of feature sizes is implemented experimentally using an N-photon lithographic material. For the recording medium, we used poly(methyl-methacrylate) (PMMA), which is a UV lithographic material that can be excited by multi-photon absorption in the visible region. We achieved a two-fold enhancement of the resolution over the standard Rayleigh limit of half of the wavelength.

Forward Brillouin scattering in hollow-core photonic bandgap fibers

We quantify the strength of stimulated forward Brillouin scattering in hollow-core photonic bandgap fiber through a combination of experiments and multi-physics simulations. Brillouin spectroscopy methods reveal a family of densely spaced Brillouin-active phonon modes below 100 MHz with coupling strengths that approach those of conventional silica fiber. The experimental results are corroborated by multi-physics simulations, revealing that relatively strong optomechanical coupling is mediated by a combination of electrostriction and radiation pressure within the nano-scale silica-air matrix; the nontrivial mechanical properties of this silica-air matrix facilitate the large optomechanical response produced by this system. Simulations also reveal an incredible sensitivity of the Brillouin spectrum to fiber critical dimensions, suggesting opportunity for enhancement or suppression of these interactions. Finally, we relate the measured and calculated couplings to the noise properties of the fiber as the foundation for phase- and polarization-noise estimates in hollow-core fiber. More generally, such Brillouin interactions are an important consideration in both the high and low optical intensity limits.

Novel mechanism of fast relaxation of photo-induced anisotropy in a poly(malonic esters) containing p-cyanoazobenzene

The relaxation behavior of photo-induced anisotropy has been investigated in a side-chain liquid crystalline(LC) poly(malonic ester) (PCN). Abnormal phenomenon, increment of the induced birefringence, in decaying process was observed just after the pumping beam polarized perpendicularly to the recording beam was illuminated in the relaxation process. This is difficult to explain using the present models, the thermal cis-trans isomerization or the free volume effect for the relaxation process. We suggest a new model considering the elastic force between the side-chain and the backbone in the polymeric film. According to our model, the backbone can be reoriented together with the side-chain, and fast relaxation is associated with the elastic force.