*Nat Commun ; 10(1): 3122, 2019 Jul 16.*

**| MEDLINE**| ID: mdl-31311928

##### RESUMO

There has been significant recent interest in synthetic dimensions, where internal degrees of freedom of a particle are coupled to form higher-dimensional lattices in lower-dimensional physical structures. For these systems, the concept of band structure along the synthetic dimension plays a central role in their theoretical description. Here we provide a direct experimental measurement of the band structure along the synthetic dimension. By dynamically modulating a resonator at frequencies commensurate with its mode spacing, we create a periodically driven lattice of coupled modes in the frequency dimension. The strength and range of couplings can be dynamically reconfigured by changing the modulation amplitude and frequency. We show theoretically and demonstrate experimentally that time-resolved transmission measurements of this system provide a direct readout of its band structure. We also realize long-range coupling, gauge potentials and nonreciprocal bands by simply incorporating additional frequency drives, enabling great flexibility in band structure engineering.

*Proc Natl Acad Sci U S A ; 114(46): E9755-E9756, 2017 11 14.*

**| MEDLINE**| ID: mdl-29093167

*Proc Natl Acad Sci U S A ; 114(17): 4336-4341, 2017 04 25.*

**| MEDLINE**| ID: mdl-28396436

##### RESUMO

We derive four laws relating the absorptivity and emissivity of thermal emitters. Unlike the original Kirchhoff radiation law derivations, these derivations include diffraction, and so are valid also for small objects, and can also cover nonreciprocal objects. The proofs exploit two recent approaches. First, we express all fields in terms of the mode-converter basis sets of beams; these sets, which can be uniquely established for any linear optical object, give orthogonal input beams that are coupled one-by-one to orthogonal output beams. Second, we consider thought experiments using universal linear optical machines, which allow us to couple appropriate beams and black bodies. Two of these laws can be regarded as rigorous extensions of previously known laws: One gives a modal version of a radiation law for reciprocal objects-the absorptivity of any input beam equals the emissivity into the "backward" (i.e., phase-conjugated) version of that beam; another gives the overall equality of the sums of the emissivities and the absorptivities for any object, including nonreciprocal ones. The other two laws, valid for reciprocal and nonreciprocal objects, are quite different from previous relations. One shows universal equivalence of the absorptivity of each mode-converter input beam and the emissivity into its corresponding scattered output beam. The other gives unexpected equivalences of absorptivity and emissivity for broad classes of beams. Additionally, we prove these orthogonal mode-converter sets of input and output beams are the ones that maximize absorptivities and emissivities, respectively, giving these beams surprising additional physical meaning.

*Opt Express ; 22(1): 646-60, 2014 Jan 13.*

**| MEDLINE**| ID: mdl-24515024

##### RESUMO

We present an iterative design method for the coupling and the mode conversion of arbitrary modes to focused surface plasmons using a large array of aperiodically randomly located slits in a thin metal film. As the distance between the slits is small and the number of slits is large, significant mutual coupling occurs between the slits which makes an accurate computation of the field scattered by the slits difficult. We use an accurate modal source radiator model to efficiently compute the fields in a significantly shorter time compared with three-dimensional (3D) full-field rigorous simulations, so that iterative optimization is efficiently achieved. Since our model accounts for mutual coupling between the slits, the scattering by the slits of both the source wave and the focused surface plasmon can be incorporated in the optimization scheme. We apply this method to the design of various types of couplers for arbitrary fiber modes and a mode demultiplexer that focuses three orthogonal fiber modes to three different foci. Finally, we validate our design results using fully vectorial 3D finite-difference time-domain (FDTD) simulations.

*Opt Express ; 21(17): 20220-9, 2013 Aug 26.*

**| MEDLINE**| ID: mdl-24105567

##### RESUMO

We show how a spatial mode can be extracted from a light beam, leaving the other orthogonal modes undisturbed, and allowing a new signal to be retransmitted on that mode. The method is self-aligning, avoids fundamental splitting losses, and uses only local feedback loops on controllable beam splitters and phase shifters. It could be implemented with Mach-Zehnder interferometers in planar optics. The method can be extended to multiple simultaneous mode extractions. As a spatial reconfigurable optical add-drop multiplexer, it is hitless, allowing reconfiguration without interrupting the transmission of any channel.

*Opt Express ; 21(8): 10228-33, 2013 Apr 22.*

**| MEDLINE**| ID: mdl-23609731

##### RESUMO

We demonstrate the use of a subwavelength planar metal-dielectric resonant cavity to enhance the absorption of germanium photodetectors at wavelengths beyond the material's direct absorption edge, enabling high responsivity across the entire telecommunications C and L bands. The resonant wavelength of the detectors can be tuned linearly by varying the width of the Ge fin, allowing multiple detectors, each resonant at a different wavelength, to be fabricated in a single-step process. This approach is promising for the development of CMOS-compatible devices suitable for integrated, high-speed, and energy-efficient photodetection at telecommunications wavelengths.

##### Assuntos

Germânio/química , Nanotecnologia/instrumentação , Fotometria/instrumentação , Ressonância de Plasmônio de Superfície/instrumentação , Telecomunicações/instrumentação , Desenho de Equipamento , Análise de Falha de Equipamento , Germânio/efeitos da radiação , Fotografação/métodos*J Opt Soc Am A Opt Image Sci Vis ; 30(2): 238-51, 2013 Feb 01.*

**| MEDLINE**| ID: mdl-23456059

##### RESUMO

We analyze how complicated a linear optical component has to be if it is to perform one of a range of functions. Specifically, we devise an approach to evaluating the number of real parameters that must be specified in the device design or fabrication, based on the singular value decomposition of the linear operator that describes the device. This approach can be used for essentially any linear device, including space-, frequency-, or time-dependent systems, in optics, or in other linear wave problems. We analyze examples including spatial mode converters and various classes of wavelength demultiplexers. We consider limits on the functions that can be performed by simple optical devices, such as thin lenses, mirrors, gratings, modulators, and fixed optical filters, and discuss the potential for greater functionalities using modern nanophotonics.

*Opt Express ; 21(5): 6360-70, 2013 Mar 11.*

**| MEDLINE**| ID: mdl-23482206

##### RESUMO

We propose a device that can take an arbitrary monochromatic input beam and, automatically and without any calculations, couple it into a single-mode guide or beam. Simple feedback loops from detectors to modulator elements allow the device to adapt to any specific input beam form. Potential applications include automatic compensation for misalignment and defocusing of an input beam, coupling of complex modes or multiple beams from fibers or free space to single-mode guides, and retaining coupling to a moving source. Straightforward extensions allow multiple different overlapping orthogonal input beams to be separated simultaneously to different single-mode guides with no splitting loss in principle. The approach is suitable for implementation in integrated optics platforms that offer elements such as phase shifters, Mach-Zehnder interferometers, grating couplers, and integrated monitoring detectors, and the basic approach is applicable in principle to other types of waves, such as microwaves or acoustics.

*Opt Express ; 21(1): 867-76, 2013 Jan 14.*

**| MEDLINE**| ID: mdl-23388980

##### RESUMO

We demonstrate electroabsorption contrast greater than 5 dB over the entire telecommunication S- and C-bands with only 1V drive using a new Ge/SiGe QW epitaxy design approach; further, this is demonstrated with the thinnest Ge/SiGe epitaxy to date, using a virtual substrate only 320-nm-thick. We use an eigenmode expansion method to model the optical coupling between SOI waveguides and both vertically and butt-coupled Ge/SiGe devices, and show that this reduction in thickness is expected to lead to a significant improvement in the insertion loss of waveguide-integrated devices.

*Opt Express ; 20(27): 28388-97, 2012 Dec 17.*

**| MEDLINE**| ID: mdl-23263074

##### RESUMO

We design an extremely compact photonic crystal waveguide spatial mode converter which converts the fundamental even mode to the higher order odd mode with nearly 100% efficiency. We adapt a previously developed design and optimization process that allows these types of devices to be designed in a matter of minutes. We also present an extremely compact optical diode device and clarify its general properties and its relation to spatial mode converters. Finally, we connect the results here to a general theory on the complexity of optical designs.

##### Assuntos

Lasers Semicondutores , Refratometria/instrumentação , Ressonância de Plasmônio de Superfície/instrumentação , Desenho de Equipamento , Análise de Falha de Equipamento , Miniaturização*Opt Express ; 20(21): 23985-93, 2012 Oct 08.*

**| MEDLINE**| ID: mdl-23188365

##### RESUMO

We show that every linear optical component can be completely described as a device that converts one set of orthogonal input modes, one by one, to a matching set of orthogonal output modes. This result holds for any linear optical structure with any specific variation in space and/or time of its structure. There are therefore preferred orthogonal "mode converter" basis sets of input and output functions for describing any linear optical device, in terms of which the device can be described by a simple diagonal operator. This result should help us understand what linear optical devices we can and cannot make. As illustrations, we use this approach to derive a general expression for the alignment tolerance of an efficient mode coupler and to prove that loss-less combining of orthogonal modes is impossible.

##### Assuntos

Modelos Lineares , Modelos Teóricos , Dispositivos Ópticos , Simulação por Computador , Projeto Auxiliado por Computador , Desenho de Equipamento , Análise de Falha de Equipamento , Luz , Espalhamento de Radiação*Opt Express ; 20(20): 22735-42, 2012 Sep 24.*

**| MEDLINE**| ID: mdl-23037424

##### RESUMO

We propose and demonstrate a novel nanoscale resonant metal-semiconductor-metal (MSM) photodetector structure based on silicon fins self-aligned to metallic slits. This geometry allows the center wavelength of the photodetector's spectral response to be controlled by the silicon fin width, allowing multiple detectors, each sensitive to a different wavelength, to be fabricated in a single-step process. In addition, the detectors are highly efficient with simulations showing ~67% of the light (λ = 800 nm) incident on the silicon fin being absorbed in a region of thickness ~170 nm whereas the absorption length at the same wavelength is ~10 µm. This approach is promising for the development of multispectral imaging sensors and low-capacitance photodetectors for short-range optical interconnects.

##### Assuntos

Nanopartículas Metálicas/química , Nanopartículas Metálicas/efeitos da radiação , Nanotecnologia/instrumentação , Fotometria/instrumentação , Semicondutores , Silício/química , Ressonância de Plasmônio de Superfície/instrumentação , Desenho de Equipamento , Análise de Falha de Equipamento , Luz*Opt Express ; 20 Suppl 2: A293-308, 2012 Mar 12.*

**| MEDLINE**| ID: mdl-22418679

##### RESUMO

We analyze energy consumption in optical modulators operated in depletion and intended for low-power interconnect applications. We include dynamic dissipation from charging modulator capacitance and net energy consumption from absorption and photocurrent, both in reverse and small forward bias. We show that dynamic dissipation can be independent of static bias, though only with specific kinds of bias circuits. We derive simple expressions for the effects of photocurrent on energy consumption, valid in both reverse and small forward bias. Though electroabsorption modulators with large reverse bias have substantial energy penalties from photocurrent dissipation, we argue that modulator diodes with thin depletion regions and operating in small reverse and/or forward bias could have little or no such photocurrent energy penalty, even conceivably being more energy-efficient than an ideal loss-less modulator.

*Opt Express ; 20(28): 29164-73, 2012 Dec 31.*

**| MEDLINE**| ID: mdl-23388742

##### RESUMO

We demonstrate vertical-incidence electroabsorption modulators for free-space optical interconnects. The devices operate via the quantum-confined Stark effect in Ge/SiGe quantum wells grown on silicon substrates by reduced pressure chemical vapor deposition. The strong electroabsorption contrast enables use of a moderate-Q asymmetric Fabry-Perot resonant cavity, formed using a film transfer process, which allows for operation over a wide optical bandwidth without thermal tuning. Extinction ratios of 3.4 dB and 2.5 dB are obtained for 3 V and 1.5 V drive swings, respectively, with insertion loss less than 4.5 dB. For 60 ?m diameter devices, large signal modulation is demonstrated at 2 Gbps, and a 3 dB modulation bandwidth of 3.5 GHz is observed. These devices show promise for high-speed, low-energy operation given further miniaturization.

*Nano Lett ; 11(7): 2693-8, 2011 Jul 13.*

**| MEDLINE**| ID: mdl-21627101

##### RESUMO

A novel type of multiple-wavelength focusing plasmonic coupler based on a nonperiodic nanoslit array is designed and experimentally demonstrated. An array of nanoslits patterned on a thin metal film is used to couple free-space light into surface plasmon polaritons (SPPs) and simultaneously focus different-wavelength SPPs into arbitrary predefined locations in the two-dimensional plane. We design and fabricate a compact triplexer on a glass substrate with an integrated silicon photodetector. The photocurrent spectra demonstrate that the incident light is effectively coupled to SPPs and routed into three different focal spots depending on the wavelength. The proposed scheme provides a simple method of building wavelength-division multiplexing and spectral filtering elements, integrated with other plasmonic and optoelectronic devices.

##### Assuntos

Nanotecnologia/instrumentação , Ressonância de Plasmônio de Superfície/instrumentação , Algoritmos , Desenho de Equipamento , Ouro/química , Luz , Membranas Artificiais , Tamanho da Partícula , Silício/química , Propriedades de Superfície*Opt Lett ; 36(4): 591-3, 2011 Feb 15.*

**| MEDLINE**| ID: mdl-21326466

##### RESUMO

We present an extremely compact wavelength division multiplexer design, as well as a general framework for designing and optimizing frequency selective devices embedded in photonic crystals satisfying arbitrary design constraints. Our method is based on the Dirichlet-to-Neumman simulation method and uses low rank updates to the system to efficiently scan through many device designs.

*Opt Express ; 18(25): 25596-607, 2010 Dec 06.*

**| MEDLINE**| ID: mdl-21164905

##### RESUMO

We measure the intervalley scattering time of electrons in the conduction band of Ge quantum wells from the direct Γ valley to the indirect L valley to be ~185 fs using a pump-probe setup at 1570 nm. We relate this to the width of the exciton peak seen in the absorption spectra of this material, and show that these quantum wells could be used as a fast saturable absorber with a saturation fluence between 0.11 and 0.27 pJ/µm. We also observe field screening by photogenerated carriers in the material on longer timescales. We model this field screening by incorporating carrier escape from the quantum wells, drift across the intrinsic region, and recovery of the applied voltage through diffusive conduction.

##### Assuntos

Germânio/química , Pontos Quânticos , Silício/química , Projeto Auxiliado por Computador , Transporte de Elétrons , Desenho de Equipamento , Análise de Falha de Equipamento , Germânio/efeitos da radiação , Luz , Modelos Químicos , Teoria Quântica , Semicondutores , Silício/efeitos da radiação*Appl Opt ; 49(25): F59-70, 2010 Sep 01.*

**| MEDLINE**| ID: mdl-20820203

##### RESUMO

Optical interconnects are progressively replacing wires at shorter and shorter distances in information processing machines. This paper summarizes the progress toward and prospects for the penetration of optics all the way to the silicon chip.

*Opt Express ; 17(18): 15409-19, 2009 Aug 31.*

**| MEDLINE**| ID: mdl-19724539

##### RESUMO

We describe a compact modulator based on a photonic crystal nanocavity whose resonance is electrically controlled through an integrated p-i-n junction. The sub-micron size of the nanocavity promises very low capacitance, high bandwidth, and efficient on-chip integration in optical interconnects.