Publications

2015
Deotare, P.B., et al. Nanoscale transport of charge-transfer states in organic donor-acceptor blends. Nat Mater 14, 1130-4 (2015).Abstract
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.
Deotare, P.B., et al. Nanoscale transport of charge transfer states in organic donor-acceptor blends. MRS Spring 2015 (2015).
Mukherjee, K., Deotare, P.B. & Fitzgerald, E.A. Improved photoluminescence characteristics of order-disorder AlGaInP quantum wells at room and elevated temperatures. Applied Physics Letters 106, 142109 (2015).Abstract
A set of nominally undoped CuPt-B type ordered (AlxGa1−x)0.5In0.5P quantum-wells with disordered (Al0.7Ga0.3)0.5In0.5P barriers were grown and characterized using transmission electron microscopy and photoluminescence spectroscopy. Such structures are potentially beneficial for light emitting devices due to the possibility of greater carrier confinement, reduced scattering into the indirect valleys, and band-offset adjustment beyond what is possible with strain and composition. Furthermore, the possibility of independently tuning the composition and the order-parameter of the quantum-well allows for the decoupling of the carrier confinement and the aluminum content and aids in the identification of carrier loss mechanisms. In this study, sharp order-disorder interfaces were achieved via the control of growth temperature between 650 °C and 750 °C using growth pauses. Improved high-temperature (400 K) photoluminescence intensity was obtained from quantum-wells with ordered Ga0.5In0.5P as compared to disordered Ga0.5In0.5P due to greater confinement. Additionally, in the ordered samples with a higher Al/Ga ratio to counter the band-gap reduction, the photoluminescence intensity at high temperature was as bright as that from conventional disordered heterostructures and had slightly improved wavelength stability. Room-temperature time-resolved luminescence measurements indicated a longer radiative lifetime in the ordered quantum-well with reduced scattering into the barrier. These results show that in samples of good material quality, the property controlling the luminescence intensity is the carrier confinement and not the presence of ordering or the aluminum content.
2014
Visualization of exciton transport in ordered and disordered molecular solids.
Akselrod, G.M., et al. Visualization of exciton transport in ordered and disordered molecular solids. Nat Commun 5, 3646 (2014).Abstract
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, light-emitting 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.
Deotare, P.B., Mahony, T. & Bulović, V. Organic Lasers for Biochemical Sensing. 35th Progress in Electromagnetics Research Symposium (PIERS) (2014).
Deotare, P.B., et al. Transient Mapping of Exciton Dynamics in MoS2 and WSe2 Monolayers. MRS Spring 2014 (2014).
Diamond nonlinear photonics
Hausmann, B.J.M., Bulu, I., Venkataraman, V., Deotare, P. & Lončar, M. Diamond nonlinear photonics. Nature Photonics 8, 369–374 (2014).Abstract
Despite progress towards integrated diamond photonics, studies of optical nonlinearities in diamond have been limited to Raman scattering in bulk samples. Diamond nonlinear photonics, however, could enable efficient, in situ frequency conversion of single photons emitted by diamond's colour centres, as well as stable and high-power frequency microcombs operating at new wavelengths. Both of these applications depend crucially on efficient four-wave mixing processes enabled by diamond's third-order nonlinearity. Here, we have realized a diamond nonlinear photonics platform by demonstrating optical parametric oscillation via four-wave mixing using single-crystal ultrahigh-quality-factor (1 × 106) diamond ring resonators operating at telecom wavelengths. Threshold powers as low as 20 mW are measured, and up to 20 new wavelengths are generated from a single-frequency pump laser. We also report the first measurement of the nonlinear refractive index due to the third-order nonlinearity in diamond at telecom wavelengths.
Non-linear mixing in coupled photonic crystal nanobeam cavities due to cross-coupling opto-mechanical mechanisms
Ramos, D., Frank, I.W., Deotare, P.B., Bulu, I. & Lončar, M. Non-linear mixing in coupled photonic crystal nanobeam cavities due to cross-coupling opto-mechanical mechanisms. Applied Physics Letters 105, (2014).
Ultracompact Low-Threshold Organic Laser
Deotare, P.B., Mahony, T.S. & Bulović, V. Ultracompact Low-Threshold Organic Laser. ACS Nano 8, 11080-11085 (2014). Publisher's Version
2013
Photonic Crystal Nanobeam Cavities for Tunable Filter and Router Applications
Deotare, P.B., Kogos, L.C., Bulu, I. & Loncar, M. Photonic Crystal Nanobeam Cavities for Tunable Filter and Router Applications. IEEE Journal of Selected Topics in Quantum Electronics 19, 3600210-3600210 (2013).Abstract
We investigate the suitability of photonic crystal nanobeam cavities for interconnect applications. Owing to their small footprint, exactly the same as that of an optical waveguide, as well as ultrahigh quality factor resonances that they support, nanobeam cavities are attractive candidates for realization of densely integrated on-chip optical networks. We discus tunability of these filters using thermo-optic, electromechanic, and optomechanic effects, and we compare different reconfiguration strategies in terms of tuning hold power, tuning efficiency, and maximum operating frequency.
Integrated high-quality factor optical resonators in diamond.
Hausmann, B.J.M., et al. Integrated high-quality factor optical resonators in diamond. Nano Lett 13, 1898-902 (2013).Abstract
The realization of an integrated diamond photonic platform, based on a thin single crystal diamond film on top of a silicon dioxide/silicon substrate, is reported. Using this approach, we demonstrate high-quality factor single crystal diamond race-track resonators, operating at near-infrared wavelengths (1550 nm). The devices are integrated with low-loss diamond waveguides terminated with polymer pads (spot size converters) to facilitate in- (out-) coupling of light from (to) an optical fiber. Optical characterization of these resonators reveal quality factors as high as ~250,000 and overall insertion losses as low as 1 dB/facet. Scattering induced mode splitting as well as signatures of nonlinear effects such as optical bistability are observed at an input pump power of ~100 mW in the waveguides.
Optomechanical and photothermal interactions in suspended photonic crystal membranes.
Woolf, D., et al. Optomechanical and photothermal interactions in suspended photonic crystal membranes. Opt Express 21, 7258-75 (2013).Abstract
We present here an optomechanical system fabricated with novel stress management techniques that allow us to suspend an ultrathin defect-free silicon photonic-crystal membrane above a Silicon-on-Insulator (SOI) substrate with a gap that is tunable to below 200 nm. Our devices are able to generate strong attractive and repulsive optical forces over a large surface area with simple in- and out- coupling and feature the strongest repulsive optomechanical coupling in any geometry to date (gOM/2π ≈65 GHz/nm). The interplay between the optomechanical and photo-thermal-mechanical dynamics is explored, and the latter is used to achieve cooling and amplification of the mechanical mode, demonstrating that our platform is well-suited for potential applications in low-power mass, force, and refractive-index sensing as well as optomechanical accelerometry.
Single particle detection in CMOS compatible photonic crystal nanobeam cavities.
Quan, Q., et al. Single particle detection in CMOS compatible photonic crystal nanobeam cavities. Opt Express 21, 32225-33 (2013).Abstract
We report the label-free detection of single particles using photonic crystal nanobeam cavities fabricated in silicon-on-insulator platform, and embedded inside microfluidic channels fabricated in poly-dimethylsiloxane (PDMS). Our system operates in the telecommunication wavelength band, thus leveraging the widely available, robust and tunable telecom laser sources. Using this approach, we demonstrated the detection of polystyrene nanoparticles with dimensions down to 12.5nm in radius. Furthermore, binding events of a single streptavidin molecule have been observed.
Deotare, P.B., G., A. & Bulović, V. Patterning J-aggregate thin films for controlled exciton dynamics. MRS Fall 2013 Poster, (2013).
Hausmann, B., Bulu, I., Venkataraman, V., Deotare, P. & Lončar, M. An on-chip optical parametric oscillator in diamond. Nonlinear Optics PTh3A.6 (2013). Publisher's VersionAbstract
We demonstrate optical parametric oscillation via four-wave mixing inwaveguide-integrated, single crystal diamond micro-ring resonators. Threshold powers as low as20 mW are enabled by high quality factor ( 1 million) resonators operating at telecomwavelengths.
2012
Submicrometer-wide amorphous and polycrystalline anatase TiO2 waveguides for microphotonic devices.
Bradley, J.D.B., et al. Submicrometer-wide amorphous and polycrystalline anatase TiO2 waveguides for microphotonic devices. Opt Express 20, 23821-31 (2012).Abstract
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.
Integrated TiO2 resonators for visible photonics.
Choy, J.T., et al. Integrated TiO2 resonators for visible photonics. Opt Lett 37, 539-41 (2012).Abstract
We demonstrate waveguide-coupled titanium dioxide (TiO(2) racetrack resonators with loaded quality factors of 2.2×10(4) for the visible wavelengths. The structures were fabricated in sputtered TiO(2) thin films on oxidized silicon substrates using standard top-down nanofabrication techniques, and passively probed in transmission measurements using a tunable red laser.
All optical reconfiguration of optomechanical filters.
Deotare, P.B., et al. All optical reconfiguration of optomechanical filters. Nat Commun 3, 846 (2012).Abstract
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.
Deotare, P.B., et al. All optical control of optomechanical filters. Conference on Lasers and Electro-Optics 2012 Poster, JW4A.87 (2012). Publisher's VersionAbstract
We demonstrate optical reconfiguration of coupled photonic crystal nanobeam cavities by using optical gradient force. The on-chip temperature sensing along with pulsed operation allows us to estimate and isolated thermal and optomechanical effects.
Deotare, P.B., Bulu, I., Quan, Q., Elic, R. & Lončar, M. Improving optomechanical contributions in silicon photonic devices. PECS-X Best Poster Award, (2012).

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