In JoVE (1)

Other Publications (31)

Articles by Thomas F. Krauss in JoVE

Other articles by Thomas F. Krauss on PubMed

Photonic Crystals: Cavities Without Leaks

Nature Materials. Dec, 2003  |  Pubmed ID: 14647490

Physics. Control at the Quantum Level

Science (New York, N.Y.). May, 2005  |  Pubmed ID: 15905388

All-optical Control of Microfluidic Components Using Form Birefringence

Nature Materials. Jul, 2005  |  Pubmed ID: 15965480

The reflection and refraction of light at a dielectric interface gives rise to forces due to changes in the photon momentum. At the microscopic level, these forces are sufficient to trap and rotate microscopic objects. Such forces may have a profound impact in the emergent area of microfluidics, where there is the desire to process minimal amounts of analyte. This places stringent criteria on the ability to pump, move and mix small volumes of fluid, which will require the use of micro-components and their controlled actuation. We demonstrate the modelling, fabrication and rotation of microgears based on the principle of form birefringence. Using a geometric anisotropy (a one-dimensional photonic crystal etched into the microgear), we can fabricate microgears of known birefringence, which may be readily rotated by manipulating the input polarization in a standard optical trap. This methodology offers a new and powerful mechanism for generating a wide range of microfabricated machines, such as micropumps, that may be driven by purely optical control.

Integrated Monolithic Optical Manipulation

Lab on a Chip. Sep, 2006  |  Pubmed ID: 16929390

We present a new approach to optical manipulation that integrates microfluidic channels directly onto semiconductor laser material creating a compact integrated optical trap that requires no alignment and is wholly portable.

Low Loss Silicon on Insulator Photonic Crystal Waveguides Made by 193nm Optical Lithography

Optics Express. Mar, 2006  |  Pubmed ID: 19503583

We show the successful fabrication and operation of photonic crystal waveguides on SOI, with lower silicon dioxide cladding remaining, using 193 nm DUV lithography. We demonstrate that 193 nm lithography gives more process latitude, allowing a wider range of periods and hole diameters to be printed, as well as reducing the optical proximity effect to a minimum. The smallest period /hole size variation printed successfully was 280 nm and 150 nm, which is very promising for ambitious future designs. Lowest losses obtained were 14.2 +/- 2.0 dB/cm for a W1 waveguide in a 400 nm lattice with an r/a of 0.25 at a frequency of 0.257 a/lambda, which approaches the best losses reported for air-bridge type W1s.

Monolithic Integration of Microfluidic Channels and Semiconductor Lasers

Optics Express. Aug, 2006  |  Pubmed ID: 19529141

We present a fabrication method for the monolithic integration of microfluidic channels into semiconductor laser material. Lasers are designed to couple directly into the microfluidic channel, allowing submerged particles pass through the output beams of the lasers. The interaction between particles in the channel and the lasers, operated in either forward or reverse bias, allows for particle detection, and the optical forces can be used to trap and move particles. Both interrogation and manipulation are made more amenable for lab-on-a-chip applications through monolithic integration. The devices are very small, they require no external optical components, have perfect intrinsic alignment, and can be created with virtually any planar configuration of lasers in order to perform a variety of tasks. Their operation requires no optical expertise and only low electrical power, thus making them suitable for computer interfacing and automation. Insulating the pn junctions from the fluid is the key challenge, which is overcome by using photo-definable SU8-2000 polymer.

Compact Polarization Rotators for Integrated Polarization Diversity in InP-based Waveguides

Optics Letters. Aug, 2007  |  Pubmed ID: 17671575

We present the design, fabrication, and operation of a polarization converter based on angled waveguides in the InP/InGaAsP material system. By combining design elements from mode evolution and birefringent devices, the total device length is kept short (less than 50 microm) and the insertion efficiency high at 81%+/-19%, which corresponds to an insertion loss of 1 dB. Devices operate broadband, i.e., the polarization conversion exceeds 15 dB over a 100 nm wavelength range. A polarization rotator with these specifications is a prime candidate for use in an integrated polarization diversity scheme.

Dependence of Extrinsic Loss on Group Velocity in Photonic Crystal Waveguides

Optics Express. Oct, 2007  |  Pubmed ID: 19550581

We examine the effects of disorder on propagation loss as a function of group velocity for W1 photonic crystal (PhC) waveguides. Disorder is deliberately and controllably introduced into the photonic crystal by pseudo-randomly displacing the holes of the photonic lattice. This allows us to clearly distinguish two types of loss. Away from the band-edge and for moderately slow light (group velocity c/20-c/30) loss scales sub-linearly with group velocity, whereas near the band-edge, reflection loss increases dramatically due to the random and local shift of the band-edge. The optical analysis also shows that the random fabrication errors of our structures, made on a standard e-beam lithography system, are below 1 nm root mean square.

Ultracompact and Low-power Optical Switch Based on Silicon Photonic Crystals

Optics Letters. Jan, 2008  |  Pubmed ID: 18197221

Switching light is one of the most fundamental functions of an optical circuit. As such, optical switches are a major research topic in photonics, and many types of switches have been realized. Most optical switches operate by imposing a phase shift between two sections of the device to direct light from one port to another, or to switch it on and off, the major constraint being that typical refractive index changes are very small. Conventional solutions address this issue by making long devices, thus increasing the footprint, or by using resonant enhancement, thus reducing the bandwidth. We present a slow-light-enhanced optical switch that is 36 times shorter than a conventional device for the same refractive index change and has a switching length of 5.2 microm.

Integration of Grating Couplers with a Compact Photonic Crystal Demultiplexer on an InP Membrane

Optics Letters. Apr, 2008  |  Pubmed ID: 18414565

We demonstrate the integration of a 30% efficient grating coupler with a compact photonic crystal wavelength demultiplexer (DeMUX). The DeMUX has seven output channels that are spaced 10 nm apart and is aimed at coarse WDM applications. The integrated devices are realized on a high-index-contrast InP membrane using a simple benzocyclobutene wafer bonding technique. Cross talks of -10 to -12 dB for four channels 20 nm apart are obtained without optimization.

Optical Vortex Trap for Resonant Confinement of Metal Nanoparticles

Optics Express. Mar, 2008  |  Pubmed ID: 18542599

The confinement and controlled movement of metal nanoparticles and nanorods is an emergent area within optical micromanipulation. In this letter we experimentally realise a novel trapping geometry near the plasmon resonance using an annular light field possessing a helical phasefront that confines the nanoparticle to the vortex core (dark) region. We interpret our data with a theoretical framework based upon the Maxwell stress tensor formulation to elucidate the total forces upon nanometric particles near the particle plasmon resonance. Rotation of the particle due to orbital angular momentum transfer is observed. This geometry may have several advantages for advanced manipulation of metal nanoparticles.

Systematic Design of Flat Band Slow Light in Photonic Crystal Waveguides

Optics Express. Apr, 2008  |  Pubmed ID: 18545325

We present a systematic procedure for designing "flat bands" of photonic crystal waveguides for slow light propagation. The procedure aims to maximize the group index - bandwidth product by changing the position of the first two rows of holes of W1 line defect photonic crystal waveguides. A nearly constant group index - bandwidth product is achieved for group indices of 30-90 and as an example, we experimentally demonstrate flat band slow light with nearly constant group indices of 32.5, 44 and 49 over 14 nm, 11 nm and 9.5 nm bandwidth around 1550 nm, respectively.

Reconfigurable Microfluidic Photonic Crystal Slab Cavities

Optics Express. Sep, 2008  |  Pubmed ID: 18825225

We demonstrate the spectral and spatial reconfigurability of photonic crystal double-heterostructure cavities in silicon by microfluidic infiltration of selected air holes. The lengths of the microfluidic cavities are changed by adjusting the region of infiltrated holes in steps of several microns. We systematically investigate the spectral signature of these cavities, showing high Q-factor resonances for a broad range of cavity lengths. The fluid can be removed by immersing the device in toluene, offering complete reconfigurability. Our cavity writing technique allows for tolerances in the infiltration process and provides flexibility as it can be employed at any time after photonic crystal fabrication.

High-Q Microfluidic Cavities in Silicon-based Two-dimensional Photonic Crystal Structures

Optics Letters. Oct, 2008  |  Pubmed ID: 18830353

We demonstrate postprocessed microfluidic double-heterostructure cavities in silicon-based photonic crystal slab waveguides. The cavity structure is realized by selective fluid infiltration of air holes using a glass microtip, resulting in a local change of the average refractive index of the photonic crystal. The microcavities are probed by evanescent coupling from a silica nanowire. An intrinsic quality factor of 57,000 has been derived from our measurements, representing what we believe to be the largest value observed in microfluidic photonic crystal cavities to date.

Slow Light Enhancement of Nonlinear Effects in Silicon Engineered Photonic Crystal Waveguides

Optics Express. Feb, 2009  |  Pubmed ID: 19219198

We report nonlinear measurements on 80microm silicon photonic crystal waveguides that are designed to support dispersionless slow light with group velocities between c/20 and c/50. By launching picoseconds pulses into the waveguides and comparing their output spectral signatures, we show how self phase modulation induced spectral broadening is enhanced due to slow light. Comparison of the measurements and numerical simulations of the pulse propagation elucidates the contribution of the various effects that determine the output pulse shape and the waveguide transfer function. In particular, both experimental and simulated results highlight the significant role of two photon absorption and free carriers in the silicon waveguides and their reinforcement in the slow light regime.

Complete Response Characterization of Ultrafast Linear Photonic Devices

Optics Letters. Nov, 2009  |  Pubmed ID: 19881613

We present a method to fully characterize linear photonic devices that change their properties on ultrashort time scales. When we feed the device with a broadband input pulse and detect the resulting output field for a sufficient number of arrival times of the input, the device response to any other input with smaller bandwidth can be extracted numerically, without the need for additional measurements. Our approach is based on the formalism of linear time-varying systems, and we experimentally demonstrate its feasibility for the example of an ultrafast nanophotonic switch.

Nonlinear Loss Dynamics in a Silicon Slow-light Photonic Crystal Waveguide

Optics Letters. Apr, 2010  |  Pubmed ID: 20364221

We directly investigate both experimentally and numerically the influence of optical nonlinear loss dynamics on a silicon waveguide based all-optical device. The dynamics of these nonlinear losses are explored through the analysis of optical limiting of an amplitude distorted 10 Gbit/s signal in a slow-light silicon photonic crystal waveguide. As the frequency of the distortion approaches the free-carrier recombination rate, free-carrier absorption reaches a steady state, leaving two-photon absorption the dominant dynamic nonlinear loss. Our results highlight the importance of engineering the free-carrier lifetime in silicon waveguides for high speed all-optical processing applications.

Investigation of Phase Matching for Third-harmonic Generation in Silicon Slow Light Photonic Crystal Waveguides Using Fourier Optics

Optics Express. Mar, 2010  |  Pubmed ID: 20389702

Using Fourier optics, we retrieve the wavevector dependence of the third-harmonic (green) light generated in a slow light silicon photonic crystal waveguide. We show that quasi-phase matching between the third-harmonic signal and the fundamental mode is provided in this geometry by coupling to the continuum of radiation modes above the light line. This process sustains third-harmonic generation with a relatively high efficiency and a substantial bandwidth limited only by the slow light window of the fundamental mode. The results give us insights into the physics of this nonlinear process in the presence of strong absorption and dispersion at visible wavelengths where bandstructure calculations are problematic. Since the characteristics (e.g. angular pattern) of the third-harmonic light primarily depend on the fundamental mode dispersion, they could be readily engineered.

Photonic Crystal Formed by the Imaginary Part of the Refractive Index

Advanced Materials (Deerfield Beach, Fla.). Jun, 2010  |  Pubmed ID: 20446300

Experimental Evidence of Guided-resonances in Photonic Crystals with Aperiodically Ordered Supercells

Optics Letters. Dec, 2010  |  Pubmed ID: 21124574

We report on the first experimental evidence of guided resonances (GRs) in photonic crystal slabs based on aperiodically ordered supercells. Using Ammann-Beenker (quasiperiodic, eightfold symmetric) tiling geometry, we present our study on the fabrication, experimental characterization, and full-wave numerical simulation of two representative structures (with different filling parameters) operating at near-IR wavelengths (1300-1600 nm). Our results show a fairly good agreement between measurements and numerical predictions and pave the way for the development of new strategies (based on, e.g., the lattice symmetry breaking) for GR engineering.

Low-power Continuous-wave Generation of Visible Harmonics in Silicon Photonic Crystal Nanocavities

Optics Express. Dec, 2010  |  Pubmed ID: 21165010

We present the first demonstration of frequency conversion by simultaneous second- and third-harmonic generation in a silicon photonic crystal nanocavity using continuous-wave optical excitation. We observe a bright dual wavelength emission in the blue/green (450-525 nm) and red (675-790 nm) visible windows with pump powers as low as few microwatts in the telecom bands, with conversion efficiencies of ∼ 10 (-5) /W and ∼ 10/ W(2) for the second- and third-harmonic, respectively. Scaling behaviors as a function of pump power and cavity quality-factor are demonstrated for both second- and third order processes. Successful comparison of measured and calculated emission patterns indicates that third-harmonic is a bulk effect while second-harmonic is a surface-related effect at the sidewall holes boundaries. Our results are promising for obtaining practical low-power, continuous-wave and widely tunable multiple harmonic generation on a silicon chip.

Waveguide Confined Raman Spectroscopy for Microfluidic Interrogation

Lab on a Chip. Apr, 2011  |  Pubmed ID: 21225053

We report the first implementation of the fiber based microfluidic Raman spectroscopic detection scheme, which can be scaled down to micrometre dimensions, allowing it to be combined with other microfluidic functional devices. This novel Raman spectroscopic detection scheme, which we termed as Waveguide Confined Raman Spectroscopy (WCRS), is achieved through embedding fibers on-chip in a geometry that confines the Raman excitation and collection region which ensures maximum Raman signal collection. This results in a microfluidic chip with completely alignment-free Raman spectroscopic detection scheme, which does not give any background from the substrate of the chip. These features allow a WCRS based microfluidic chip to be fabricated in polydimethylsiloxane (PDMS) which is a relatively cheap material but has inherent Raman signatures in fingerprint region. The effects of length, collection angle, and fiber core size on the collection efficiency and fluorescence background of WCRS were investigated. The ability of the device to predict the concentration was studied using urea as a model analyte. A major advantage of WCRS is its scalability that allows it to be combined with many existing microfluidic functional devices. The applicability of WCRS is demonstrated through two microfluidic applications: reaction monitoring in a microreactor and detection of analyte in a microdroplet based microfluidic system. The WCRS approach may lead to wider use of Raman spectroscopy based detection in microfluidics, and the development of portable, alignment-free microfluidic devices.

Fabrication of Low Loss Dispersion Engineered Chalcogenide Photonic Crystals

Optics Express. Jan, 2011  |  Pubmed ID: 21369015

We demonstrate low loss photonic crystal waveguides in chalcogenide (Ge(33)As(12)Se(55)) glasses. The measured losses are as low as 21 dB/cm. We experimentally determine the refractive index of the thin film chalcogenide glass to be n = 2.6 and demonstrate that dispersion engineering can be performed up to a group index of ng = 40 in this relatively low refractive index contrast system.

Four-wave Mixing in Photonic Crystal Waveguides: Slow Light Enhancement and Limitations

Optics Express. Feb, 2011  |  Pubmed ID: 21369277

We demonstrate continuous wave four-wave mixing in silicon photonic crystal waveguides of 396 μm length with a group index of ng=30. The highest observed conversion efficiency is -24 dB for 90 mW coupled input pump power. The key question we address is whether the predicted fourth power dependence of the conversion efficiency on the slowdown factor (η≈S4) can indeed be observed in this system, and how the conversion efficiency depends on device length in the presence of propagation losses. We find that the expected dependencies hold as long as both realistic losses and the variation of mode shape with slowdown factor are taken into account. Having achieved a good agreement between a simple analytical model and the experiment, we also predict structures that can achieve the same conversion efficiency as already observed in nanowires for the same input power, yet for a device length that is 50 times shorter.

Experimental Observation of Evanescent Modes at the Interface to Slow-light Photonic Crystal Waveguides

Optics Letters. Apr, 2011  |  Pubmed ID: 21479019

We experimentally study the fields close to an interface between two photonic crystal waveguides that have different dispersion properties. After the transition from a waveguide in which the group velocity of light is v(g) ~ c/10 to a waveguide in which it is v(g) ~ c/100, we observe a gradual increase in the field intensity and the lateral spreading of the mode. We attribute this evolution to the existence of a weakly evanescent mode that exponentially decays away from the interface. We compare this to the situation where the transition between the waveguides only leads to a minor change in group velocity and show that, in that case, the evolution is absent. Furthermore, we apply novel numerical mode extraction techniques to confirm experimental results.

Ultracompact 160 Gbaud All-optical Demultiplexing Exploiting Slow Light in an Engineered Silicon Photonic Crystal Waveguide

Optics Letters. May, 2011  |  Pubmed ID: 21540983

We demonstrate all-optical demultiplexing of a high-bandwidth, time-division multiplexed 160 Gbit/s signal to 10 Gbit/s channels, exploiting slow light enhanced four-wave mixing in a dispersion engineered, 96 μm long planar photonic crystal waveguide. We report error-free (bit error rate<10⁻⁹) operation of all 16 demultiplexed channels, with a power penalty of 2.2-2.4 dB, highlighting the potential of these structures as a platform for ultracompact all-optical nonlinear processes.

Third-harmonic Generation in Slow-light Chalcogenide Glass Photonic Crystal Waveguides

Optics Letters. Aug, 2011  |  Pubmed ID: 21808323

We demonstrate third-harmonic generation (THG) in a dispersion-engineered slow-light photonic crystal waveguide fabricated in AMTIR-1 chalcogenide glass. Owing to the relatively low loss and low dispersion in the slow-light (c/30) regime, combined with the high nonlinear figure of merit of the material (∼2), we obtain a relatively large conversion efficiency (1.4×10(-8)/W(2)), which is 30× higher than in comparable silicon waveguides, and observe a uniform visible light pattern along the waveguide. These results widen the number of applications underpinned by THG in slow-light platforms, such as the direct observation of the spatial evolution of the propagating mode.

Slow-light Enhanced Correlated Photon Pair Generation in a Silicon Photonic Crystal Waveguide

Optics Letters. Sep, 2011  |  Pubmed ID: 21886228

We report the generation of correlated photon pairs in the telecom C-band at room temperature from a dispersion-engineered silicon photonic crystal waveguide. The spontaneous four-wave mixing process producing the photon pairs is enhanced by slow-light propagation enabling an active device length of less than 100 μm. With a coincidence to accidental ratio of 12.8 at a pair generation rate of 0.006 per pulse, this ultracompact photon pair source paves the way toward scalable quantum information processing realized on-chip.

Ultrafast Tilting of the Dispersion of a Photonic Crystal and Adiabatic Spectral Compression of Light Pulses

Physical Review Letters. Jan, 2012  |  Pubmed ID: 22400742

We demonstrate, by theory and experiment, the ultrafast tilting of the dispersion curve of a photonic-crystal waveguide following the absorption of a femtosecond pump pulse. By shaping the pump-beam cross section with a nanometric shadow mask, different waveguide eigenmodes acquire different spatial overlap with the perturbing pump, leading to a local flattening of the dispersion by up to 11%. We find that such partial mode perturbation can be used to adiabatically compress the spectrum of a light pulse traveling through the waveguide.

Ultrafast Tunable Optical Delay Line Based on Indirect Photonic Transitions

Physical Review Letters. May, 2012  |  Pubmed ID: 23003252

We introduce the concept of an indirect photonic transition and demonstrate its use in a dynamic delay line to alter the group velocity of an optical pulse. Operating on an ultrafast time scale, we show continuously tunable delays of up to 20 ps, using a slow light photonic crystal waveguide only 300 μm in length. Our approach is flexible, in that individual pulses in a pulse stream can be controlled independently, which we demonstrate by operating on pulses separated by just 30 ps. The two-step indirect transition is demonstrated here with a 30% conversion efficiency.

Four-wave Mixing in Slow Light Photonic Crystal Waveguides with Very High Group Index

Optics Express. Jul, 2012  |  Pubmed ID: 23038300

We report efficient four-wave mixing in dispersion engineered slow light silicon photonic crystal waveguides with a flat band group index of ng = 60. Using only 15 mW continuous wave coupled input power, we observe a conversion efficiency of -28 dB. This efficiency represents a 30 dB enhancement compared to a silicon nanowire of the same length. At higher powers, thermal redshifting due to linear absorption was found to detune the slow light regime preventing the expected improvement in efficiency. We then overcome this thermal limitation by using oxide-clad waveguides, which we demonstrate for group indices of ng = 30. Higher group indices may be achieved with oxide clad-waveguides, and we predict conversion efficiencies approaching -10 dB, which is equivalent to that already achieved in silicon nanowires but for a 50x shorter length.

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