Transport of nanoscale energy in the form of excitons is at the core of photosynthesis and the operation of a wide range of nanostructured optoelectronic devices such as solar cells, lightemitting diodes and excitonic transistors. Of particular importance is the relationship between exciton transport and nanoscale disorder, the defining characteristic of molecular and nanostructured materials. Here we report a spatial, temporal and spectral visualization of exciton transport in molecular crystals and disordered thin films. Using tetracene as an archetype molecular crystal, the imaging reveals that exciton transport occurs by random walk diffusion, with a transition to subdiffusion as excitons become trapped. By controlling the morphology of the thin film, we show that this transition to subdiffusive transport occurs at earlier times as disorder is increased. Our findings demonstrate that the mechanism of exciton transport depends strongly on the nanoscale morphology, which has wide implications for the design of excitonic materials and devices.

We investigate the design, fabrication and experimental characterization of high Quality factor photonic crystal nanobeam cavities in silicon. Using a five-hole tapered 1D photonic crystal mirror and precise control of the cavity length, we designed cavities with theoretical Quality factors as high as 1.4 × 10 7 . By detecting the cross-polarized resonantly scattered light from a normally incident laser beam, we measure a Quality factor of nearly 7.5 × 10 5 . The effect of cavity size on mode frequency and Quality factor was simulated and then verified experimentally.For the past decade, there has been a concerted research effort to develop ultra-high Quality (Q) factor electromagnetic cavities with dimensions comparable to the wavelength of light [1,2,3,4,5,6]. By shrinking the modal volume to near the fundamental limit of V = (λ/2n) 3 , these cavities have enabled new applications to emerge in ultrasmall lasers [7,8,9,10], strong light-matter coupling [11,12,13,14,15,16], optical switching [17], and chemical sensing [18,19], among others. Recently, there has been much interest in cavities realized in suspended nanobeams patterned with a onedimensional (1D) lattice of holes [20,21,22,23] due to their exceptional cavity figures of merit (Q and V ), relative ease of design and fabrication, and potential for novel optomechanical effects [24,25]. These apparently simple structures, which resemble very early microcavity prototypes [26], actually have Q/V factors which rival the best 2D planar photonic crystal cavities [3,4]. They also have many inherent advantages, including the possibility of realizing high Q/V cavities in moderate index materials such as SiN x [22] and facilitating coupling to ridge waveguides [21]. In addition, the near-field of the cavity is also highly "accessible", in the sense that there are two dimensions with total-internal-reflection (TIR) interfaces, which should facilitate bio-sensing applications as well as novel techniques for the dynamic control of cavity resonances [27].In this paper we describe the design, fabrication, and experimental characterization of silicon photonic crystal nanobeam (PhCnB) cavities operating near ∼ 1500 nm with measured Q factors of 7.5 ×10 5 . To our knowledge, this represents the highest Q factor ever measured in nanocavities based on photonic crystal nanobeams, and one of the highest Qs ever measured in any photonic crystal cavity. Electromagnetic field confinement in the structure [ Fig. 1(a)] is achieved by index guiding in two directions (y and z), and Bragg scattering from the 1D photonic crystal mirror in the third (x) direction. The mechanism of light confinement has been interpreted in terms of impedance matching [20,22,28] and the mode-gap effect [23]. Conceptually, the cavity can be viewed as a wavelength-scale Fabry-Perot cavity with photonic crystal mirrors which reflect and thus trap the nanobeam waveguide mode. Because the cavity mode penetrates some distance into the mirror, it is crucial that the fields do not abruptly terminate at the mirro...

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A deterministic design of an ultrahigh Q, wavelength scale mode volume photonic crystal nanobeam cavity is proposed and experimentally demonstrated. Using this approach, cavities with Q> 10 6 and on-resonance transmission T>90% are designed. The devices fabricated in Si and capped with low-index polymer, have Q=80,000 and T=73%. This is, to the best of our knowledge, the highest transmission measured in deterministically designed, wavelength scale high Q cavities.Photonic crystal (PhC) The large computational cost, in particular the computation time, needed to perform the simulation of high-Q cavities make this trial based method inefficient. Inverse engineering design, in which the physical structure is optimized by constructing specific target functions and constraints, was also proposed[14] [15]. A design recipe based on the desired field distribution is proposed in [16]. In this letter, we propose and experimentally demonstrate a deterministic method to design an ultrahigh Q, sub-wavelength scale mode volume, PhC nanobeam cavity( Figure.1) that is strongly coupled to the feeding waveguide(i.e. near unity on resonance transmission). The design approach is deterministic in the sense that it does not involve any trial-based hole shifting, re-sizing and overall cavity re-scaling to ensure ultra-high Q cavity. Moreover, the final cavity resonance has less than 2% deviation from a predetermined frequency. Our design method requires only computationally inexpensive, photonic band calculations (e.g. using plane wave expansion method), and is simple to implement.The Q factor of a PhC nanobeam cavity can be maximized by reducing the out-of plane scattering(Q sc ) due to the coupling to the radiation modes. As shown previously [3][16], scattered power (P sc ) can be expressed as an integral of spatial fourier frequencies within a light cone, calculated over the surface above the cavity:. The integral is minimized when major fourier components are tightly localized (in k-space) at the edge of the first Brillioun zone [4]. We start by considering the ideal field distribution on this surface which would minimize P sc . A general property of these nanobeam cavities is that it consists of the waveguide region of length L, that sup- * Electronic address: quan@fas.harvard.edu ports propagating modes, surrounded by infinitely long Bragg mirror on each side( Figure.1a). Without the loss of generality, we consider the TE-like cavity mode with Hz as a major field component. In the case of conventional periodic Bragg mirror, evanescent field inside the mirror can be expressed as sin(β Bragg x) exp(−κx), where κ is attenuation constant. The cavity field inside the waveguide region can be represented as sin(β wg x). As mentioned above, scattering loss decreases in mirror section when β Bragg = π/a, while phase matching between mirror and waveguide [7], β Bragg = β wg , minimizes the scattering loss at cavity-mirror interface. The spatial fourier transform of such cavity field is approximately a Lorentzian in the vicinity of π/a. As ...

Charge-transfer (CT) states, bound combinations of an electron and a hole on separate molecules, play a crucial role in organic optoelectronic devices. We report direct nanoscale imaging of the transport of long-lived CT states in molecular organic donor-acceptor blends, which demonstrates that the bound electron-hole pairs that form the CT states move geminately over distances of 5-10 nm, driven by energetic disorder and diffusion to lower energy sites. Magnetic field dependence reveals a fluctuating exchange splitting, indicative of a variation in electron-hole spacing during diffusion. The results suggest that the electron-hole pair of the CT state undergoes a stretching transport mechanism analogous to an 'inchworm' motion, in contrast to conventional transport of Frenkel excitons. Given the short exciton lifetimes characteristic of bulk heterojunction organic solar cells, this work confirms the potential importance of CT state transport, suggesting that CT states are likely to diffuse farther than Frenkel excitons in many donor-acceptor blends.

Reconfigurable optical filters are of great importance for applications in optical communication and information processing. of particular interest are tuning techniques that take advantage of mechanical deformation of the devices, as they offer wider tuning range. Here we demonstrate reconfiguration of coupled photonic crystal nanobeam cavities by using optical gradient force induced mechanical actuation. Propagating waveguide modes that exist over a wide wavelength range are used to actuate the structures and control the resonance of localized cavity modes. using this all-optical approach, more than 18 linewidths of tuning range is demonstrated. using an on-chip temperature self-referencing method, we determine that 20% of the total tuning was due to optomechanical reconfiguration and the rest due to thermo-optic effects. By operating the device at frequencies higher than the thermal cutoff, we show high-speed operation dominated by just optomechanical effects. Independent control of mechanical and optical resonances of our structures is also demonstrated.

We demonstrate amorphous and polycrystalline anatase TiO(2) thin films and submicrometer-wide waveguides with promising optical properties for microphotonic devices. We deposit both amorphous and polycrystalline anatase TiO(2) using reactive sputtering and define waveguides using electron-beam lithography and reactive ion etching. For the amorphous TiO(2), we obtain propagation losses of 0.12 ± 0.02 dB/mm at 633 nm and 0.04 ± 0.01 dB/mm at 1550 nm in thin films and 2.6 ± 0.5 dB/mm at 633 nm and 0.4 ± 0.2 dB/mm at 1550 nm in waveguides. Using single-mode amorphous TiO(2) waveguides, we characterize microphotonic features including microbends and optical couplers. We show transmission of 780-nm light through microbends having radii down to 2 μm and variable signal splitting in microphotonic couplers with coupling lengths of 10 μm.

We present dynamically reconfigurable photonic crystal nanobeam cavities, operating at ~1550 nm, that can be continuously and reversibly tuned over a 9.5 nm wavelength range. The devices are formed by two coupled nanobeam cavities, and the tuning is achieved by varying the lateral gap between the nanobeams. An electrostatic force, obtained by applying bias voltages directly to the nanobeams, is used to control the spacing between the nanobeams, which in turn results in tuning of the cavity resonance. The observed tuning trends were confirmed through simulations that modeled the electrostatic actuation as well as the optical resonances in our reconfigurable geometries.