Yikun Liu

Deterministic quasi-random nanostructures for photon control

Controlling the flux of photons is crucial in many areas of science and technology. Artificial materials with nano-scale modulation of the refractive index, such as photonic crystals, are able to exercise such control and have opened exciting new possibilities for light manipulation. An interesting alternative to such periodic structures is the class of materials known as quasi-crystals, which offer unique advantages such as richer Fourier spectra. Here we introduce a novel approach for designing such richer Fourier spectra, by using a periodic structure that allows us to control its Fourier components almost at will. Our approach is based on binary gratings, which makes the structures easy to replicate and to tailor towards specific applications. As an example, we show how these structures can be employed to achieve highly efficient broad-band light trapping in thin films that approach the theoretical (Lambertian) limit, a problem of crucial importance for photovoltaics.

Reverse-bias-induced bipolar resistance switching in Pt∕TiO2∕SrTi0.99Nb0.01O3∕Pt devices

Stoichiometric single-crystalline TiO2 thin films were grown on SrTi0.99Nb0.01O3 (Nb:STO) substrates by oxygen plasma-assisted molecular beam epitaxy. The Pt∕TiO2∕Nb:STO∕Pt devices showed extremely weak resistance switching hysteresis without applying reverse bias. However, when the reverse bias increased above −2V, the hysteresis became more and more prominent. Further, it was found that the low (high) resistance state can be set by applying sufficient reverse (forward) bias. The origin of the reverse-bias-induced bipolar switching behavior should be attributed to the modulation of Schottky-like barrier width by electrochemical migration of oxygen vacancies.

Fabrication of photonic crystals with functional defects by one-step holographic lithography

A one-step introduction of functional defects into a photonic crystal is demonstrated. By using a multi-beam phase-controlled holographic lithography, line-defects in a Bragg structure and embedded waveguides in a two-dimensional photonic crystal are fabricated. Intrinsic defect introduction into a 3-dimensional photonic crystal is also proposed. This technique gives rise to a substantial reduction of the fabrication complexity and a significant improvement on the accuracy of the functional defects in photonic crystals.

Direct femtosecond pulse compression with miniature-sized Bragg cholesteric liquid crystal.

Direct compression of femtosecond optical pulses from a Ti:sapphire laser oscillator was realized with a cholesteric liquid crystal acting as a nonlinear 1D periodic Bragg grating. With a 6 μm thick sample, the pulse duration could be compressed from 100 to 48 fs. Coupled-mode equations for forward and backward waves were employed to simulate the dynamics therein, and good agreement between theory and experiment was obtained.

Photonic Crystal Formed by the Imaginary Part of the Refractive Index

2010 WILEY-VCH Verlag Gmb Photonic crystals (PhCs) are widely studied photonic structures that provide unprecedented control over the propagation of light. The large majority of PhCs are formed by a periodic modulation of the refractive index, i.e., a modulation of the real part of the dielectric constant. Additional functionality can be created by including absorbing features into the structure, thus creating PhCs out of materials with complex dielectric index. This is particularly interesting when the photonic resonance created by the refractive index contrast overlaps with the absorption feature, hence creating a ‘‘resonantly absorbing PhC’’. An example of such a resonantly absorbing structure is that of a semiconductor saturable absorber mirror (SESAM) used in mode-locked lasers where quantum wells are incorporated into a multilayer Bragg stack. Similar structures referred to as Resonantly Absorbing Bragg Reflectors (RABR) have been used to demonstrate optical switching, optical storage, and nonlinear optical conversion. The examples above are based on Bragg mirrors or 1D photonic crystals. An extension to 2D and 3D structures has been proposed and demonstrated by backfilling the voids of a conventional photonic crystal with resonantly absorbing materials such as quantum dots and metal. Most of these structures are based on refractive index modulation with absorption providing additional features. In contrast, the structure we propose and demonstrate here, an extension of the 1D case approaches of Prineas et al. and Kozhekin et al., is formed exclusively by the absorbing feature, hence it is a true ‘‘imaginary refractive index’’ structure. A refractive index contrast naturally exists near the absorption feature, as required by the Kramers–Kronig relationship, but away from this feature, the refractive index contrast is practically zero. In order to demonstrate this effect experimentally, we created a template using holographic lithography with a diffractive optical element (DOE) (Fig. 1), which generates a 2D photonic lattice of SU-8 polymer disks (Fig. 2a). The disks are doped with a high concentration of the organic dye Rhodamine B (RhB) that has an absorption peak around 564 nm, so the absorption of the lattice is strongly dependent on wavelength. Subsequent filling of the voids with the same SU-8 polymer, but without dye doping, gives rise to an imaginary index photonic lattice (Fig. 2b). The diffraction pattern of the structure before back-filling is given by the dielectric modulation that exists for all wavelengths as in a conventional photonic lattice and is much like a rainbow (Fig. 3a). By contrast, only yellow/green light diffraction around 564 nm can be observed once the structure has been back-filled (Fig. 3b–d). The wavelength dependent diffraction clearly shows that the structure only acts as a PhC in the vicinity of the absorption window. Out of this window, the structure behaves as a uniform polymer layer. This new type of photonic crystal offers intriguing properties for further study in the field of saturable light absorption and emission control. The PhC may also be applied for optical switching and optical logic operation. The imaginary index structure can be treated as a 2D grating with wavelength-dependent phase and intensity modulation. According to diffraction theory, the intensity distribution I created by the grating is given by the Fourier transform of the transmission function of the grating t:

Light propagation in a resonantly absorbing waveguide array

Light propagation behavior in a resonantly absorbing waveguide array is analyzed. Both a Lorentzian line shape and an inhomogeneous broadened absorbing line shape are considered, with their imaginary and real part of the refractive index determined by a Kramers-Kronig relationship. The diffracted wave is shown to have the frequency spectra determined by the material absorption, dispersion as well as the waveguide structure. An interesting phenomenon is that a spectral hole is produced and becomes deeper in the diffraction spectrum as the thickness of the resonantly absorbing waveguide array increases. The experimental measurements conducted in a waveguide array are found to be in good agreement with the numerical results.

Ultrafast pulse compression, stretching-and-recompression using cholesteric liquid crystals

We have experimentally demonstrated the feasibility of direct compression, or stretching and recompression of laser pulses in a very wide temporal time scale spanning 10s fs to ~1 ps time with sub-mm thick cholesteric liquid crystal (CLC) cells. The mechanisms at work here are the strong dispersion at the photonic band-edges and nonlinear phase modulation associated with the non-resonant ultrafast molecular electronic optical nonlinearity. The observed pulse compression limit, spectral characteristics and intensity dependence of the compression are in good agreement with theoretical expectations and simulations based on a coupled-mode propagation model. Owing to the large degree of freedom to engineer the wavelength locations of CLC photonic bandgap and band-edges, these self-action all-optical processes can be realized with ultrafast lasers pulses in a very wide spectral region from the visible to near infrared, with potential applications in compact ultrafast photonic modulation devices/platforms.

Buffering and trapping ultrashort optical pulses in concatenated Bragg gratings

Strong retardation of ultrashort optical pulses, including their deceleration and stoppage in the form of Bragg solitons in a cascaded Bragg grating (BG) structure, is proposed. The manipulations of the pulses are carried out, using nonlinear effects, in a chirped BG segment which is linked, via a defect, to a uniform grating. The storage of the ultrashort pulses is shown to be very robust with respect to variations of the input field intensity, suggesting the feasibility of storing ultrafast optical pulses in such a structure. Physical estimates are produced for the BGs written in silicon.

Femtosecond soliton diode on heterojunction Bragg-grating structure

We numerically propose a scheme for realizing an all-optical femtosecond soliton diode based on a tailored heterojunction Bragg grating, which is designed by two spatially asymmetric chirped cholesteric liquid crystals. Our simulations demonstrate that with the consideration of optical nonlinearity, not only the femtosecond diode effect with nonreciprocal transmission ratio up to 120 can be achieved but also the optical pulse evolving into soliton which maintains its shape during propagation through the sample is observed. Further, the influence of pulse width and the carrier wavelength to the femtosecond diode effect is also discussed in detail. Our demonstrations might suggest a direction for experimentally realizing the femtosecond soliton diode based on the cholesteric liquid crystals.

Pseudo-random arranged color filter array for controlling moiré patterns in display

Optical display quality can be degraded by the appearance of moiré pattern occurring in a display system consisting of a basic matrix superimposed with a functional structured optical layer. We propose in this paper a novel pseudo-random arranged color filter array with the table number arranged with an optimal design scenario. We show that the moiré pattern can be significantly reduced with the introduction of the special color filter array. The idea is tested with an experiment that gives rise to a substantially reduced moiré pattern in a display system. It is believed that the novel functional optical structures have significant impact to complex structured display system in general and to the autostereoscopic and integrated display systems in particular.