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Book:
1.
Z.
Lu and D. W. Prather, “Dispersion-Engineered Photonic Crystals,” ISBN:
3639063767, VDM Verlag Dr. Mueller e.K., 2008. |
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Journal Publications:
In 2012 2. R. Yang and Z. Lu, “Subwavelength Plasmonic Waveguides and Plasmonic Materials,” International Journal of Optics, vol. 2012, 258013 (12) (2012). 3. Z. Lu and W. Zhao, “Nanoscale
electro-optic modulators based on graphene-slot waveguides,” J. Opt. Soc. Am. B vol. 29, 1490-1496
(2012) (Top Downloaded paper). Abstract: Research on graphene has revealed
its remarkable electro-optic properties, which promise to satisfy the needs
of future electro-optic modulators. However, its ultrasmall thickness,
compared with operating light wavelength, downplays its role in an
optoelectronic device. The key to achieve efficient electro-optic modulation
based on graphene is to enhance its interaction with light. To this end, some
novel waveguides and platforms will be employed to enhance the interaction.
Herein, we present our recent exploration of graphene electro-optic
modulators based on graphene sandwiched in dielectric or plasmonic
waveguides. With a suitable gate voltage, the dielectric constant of graphene
can be tuned to be very small due to the effect of intraband electronic
transition, resulting in “graphene-slot waveguides” and greatly enhanced
absorption modes. Up to 3 dB modulation depth can be achieved within
800 nm long silicon waveguides, or 120 nm long plasmonic waveguides
based on three-dimensional numerical simulations. They have the advantages of
nanoscale footprints, small insertion loss, low power consumption, and
potentially ultrahigh speed, as well as being CMOS-compatible.
4. Z. Lu, W. Zhao, and K. Shi, “Ultracompact
electro-absorption modulators based on tunable epsilon-near-zero-slot
waveguides,” IEEE Photonics Journal,
vol. 4, 735-740(2012). Abstract: We present a promising application of
epsilon-near-zero (ENZ) materials in optical modulators. When a thin ENZ film
is sandwiched in a single-mode waveguide, an ENZ-slot waveguide is formed,
where the absorption can be greatly enhanced. We propose electroabsorption
modulators based on tunable ENZ materials and slot waveguides. Transparent
conducting oxides (TCOs) may be employed as the active slot, which can be
tuned between ENZ (high absorption) and epsilon-far-from-zero (low
absorption) by accumulation carriers. Numerical simulation shows that over
3-dB modulation depth can be achieved in a 250-nm-long TCO-slot waveguide.
The modulators have the advantages of nanoscale footprints, small insertion
loss, potentially ultrahigh speed, and easy fabrication.
5.
A. Kaur, S. Banerjee, W. Zhao, J.
Venkataraman, and Z. Lu, “Deep Subwavelength Power Concentration Based
Hyperbolic Metamaterials,” International
journal of optics, vol. 2012, 879392 (6) (2012). Abstract:
Hyperbolic metamaterials can manipulate electromagnetic waves by converting
evanescent waves into propagating waves and thus support light propagation
without diffraction limit. In this paper, deep subwavelength focusing (or
power concentration) is demonstrated both numerically and experimentally
using hyperbolic metamaterials. The results verify that hyperbolic
metamaterials can focus a broad collimated beam to spot size of ~λ0/6 using wired medium design for both normal and
oblique incidence. The nonmagnetic design, no-cut-off operation, and
preferred direction of propagation in these materials significantly reduce
the attenuation in electromagnetic waves.
6.
R. Yang, X. Huang, and Z. Lu, “Arbitrary
Super Surface Modes Bounded by Multilayered Metametal,” Micromachines
3(1), Special Issue on Nano-photonic Devices, 45-54 (2012). Abstract: The dispersion of the fundamental super mode confined along the boundary between a multilayer metal-insulator (MMI) stack and a dielectric coating is theoretically analyzed and compared to the dispersion of surface waves on a single metal-insulator (MI) boundary. Based on the classical Kretschmann setup, the MMI system is experimentally tested as an anisotropic material to exhibit plasmonic behavior and a candidate of “metametal” to engineer the preset surface plasmon frequency of conventional metals for optical sensing applications. The conditions to obtain artificial surface plasmon frequency are thoroughly studied, and the tuning of surface plasmon frequency is verified by electromagnetic modeling and experiments. The design rules drawn in this paper would bring important insights into applications such as optical lithography, nano-sensing and imaging.
In 2011 7. R. Yang and Z. Lu, “Silicon-on-Insulator Platform for
Integration of 3D Nanoplasmonic Devices,” IEEE Photon. Tech. Lett. 23,
1652(2011). Abstract: We experimentally demonstrate a compact platform to convert near-infrared signals between nanoplasmonic and nanophotonic domains. In particular, we utilize a pair of adiabatic nanoplasmonic tapers to assist direct-coupling and shrink the plasmonic guiding area to deep-subwavelength scale as small as 50-nm-by-80-nm with a propagation loss of 2.28 dB/µm. The integrated plasmonic devices developed on a silicon-on-insulator (SOI) substrate could build up a solid platform for plasmonic gauges of ultra-fast communications and optical sensing.
8. W. Zhao and Z. Lu, “Nanoplasmonic optical switch based
on Ga-Si3N4-Ga waveguide,” Opt. Eng. 50, 074002(2011). Abstract: In this paper, we propose an optical switch based on a metal-insulator-metal plasmonic waveguide with Si3N4 core sandwiched between two gallium (Ga) metal layers. Combining the unique structural phase transition property of gallium, within a total length of only 400 nm, an extinction ratio as high as 7.68 dB can be achieved in the proposed nanoplasmonic structure. The phase transition may be achieved by changing the temperature of the waveguide or by external light excitation.
9. W. Zhao, X. Huang, and Z. Lu, "Super Talbot effect
in indefinite metamaterial," Opt. Express 19, 15297-15303 (2011). Abstract: The Talbot effect (or the self-imaging effect) can be observed for a periodic object with a pitch larger than the diffraction limit of an imaging system, where the paraxial approximation is applied. In this paper, we show that the super Talbot effect can be achieved in an indefinite metamaterial even when the period is much smaller than the diffraction limit in both two-dimensional and three-dimensional numerical simulations, where the paraxial approximation is not applied. This is attributed to the evanescent waves, which carry the information about subwavelength features of the object, can be converted into propagating waves and then conveyed to far field by the metamaterial, where the permittivity in the propagation direction is negative while the transverse ones are positive. The indefinite metamaterial can be approximated by a system of thin, alternating multilayer metal and insulator (MMI) stack. As long as the loss of the metamaterial is small enough, deep subwavelength image size can be obtained in the super Talbot effect.
In 2010 10.
W.
Zhao, O. M. Eldaiki, R. Yang, and Z. Lu, "Deep subwavelength waveguiding
and focusing based on designer surface plasmons,"Optics Express,
vol. 18, 21498-21503 (2010). Abstract:
We experimentally demonstrate focusing and guiding electromagnetic (EM) waves
in a designer surface plasmonic waveguide with deep subwavelength mode cross
section. Our experiments show that a metal grating with suitable parameters,
functioning as a designer surface plasmonic waveguide, can support deep
subwavelength surface modes and the width of the modes can be squeezed also into
deep subwavelength by tapering the width of the waveguide. The results
provide a new insight into deep subwavelength waveguiding and focusing.
11.
W.
Zhao and Z. Lu, “Negative refraction imaging by three-dimensional
metamaterial at microwave frequencies,” Microwave and Optical
Technology Letters, vol. 52, 2253-2256 (2010). Abstract: We experimentally demonstrate negative refraction imaging at microwave frequencies by a three-dimensional (3D) metamaterial with simple cubic unit structure, which is fabricated based on the electronic textile technology.A four-layer metamaterial sample was tested in a series of experiments. The experimental result of the characteristic plasma frequency of the metamaterial was in good agreement with the theoretical result. The fabrication approach for 3D metamaterial may find many novel applications at microwave frequencies. 12.
R. Yang, R.
A. Wahsheh, Z. Lu, and M. A. Abushagur, “Efficient
light coupling between dielectric slot waveguide and plasmonic slot
waveguide,” Optics Letters, vol. 35, 649-651 (2010) Abstract: An efficient coupler between a dielectric waveguide and a plasmonic metal–insulator–metal (MIM) waveguide is proposed, modeled, fabricated, and characterized. Based on the platform of a silicon slot waveguide, a quasi-MIM plasmonic junction is formed via e-beam lithography and lift-off process. Coupling efficiency between the silicon slot waveguide and plasmonic waveguide up to 43% is obtained after normalizing to reference waveguides at 1550 nm. This coupling scheme can be potentially used for fast optical switching and small-footprint optical modulation.
In 2009 13.
R. A. Wahsheh, Z. Lu, and M. A. Abushagur, “Cross Talk Reduction in
Square Cavities,” IEEE Photonics Journal, vol. 1, 191-196
(2009). Abstract: Theoretically and experimentally, we demonstrate that low crosstalk between two crossed line-defect waveguides formed in a square lattice photonic crystal (PC) structure can be achieved using a resonant cavity at the intersection area. The PC resonator consists of cubic air-holes in silicon air-holes. The Q-factor of the cavity can be changed by increasing the number of holes that form the cavity. The theoretical and experimental crosstalk results are about -40 dB and -20 dB, respectively. 14.
R. A. Wahsheh, Z. Lu, and M. A. Abushagur, “Nanoplasmonic Directional
Couplers and Mach-Zehnder Inerferometers,” Optics Communications,
vol. 282, 4622-4626 (2009). Abstract: We present a novel design and analysis of two nano-scale plasmonic devices: a directional coupler and a Mach–Zehnder interferometer. The designs of the two devices are based on our recent work on the air-gap coupler that resulted in high coupling efficiency between a dielectric waveguide and a plasmonic waveguide. The two devices are embedded between two dielectric waveguides and operate at optical telecom wavelengths. The overall efficiency was 37% for a 2[1]2 directional coupler switch and above 50% for the proposed designs for a Mach–Zehnder Interferometer. The efficiency in the proposed devices can be increased using broader plasmonic waveguides. 15.
R. A. Wahsheh, Z. Lu, and M. A. Abushagur, “Nanoplasmonic Couplers and
Splitters,” Optics Express, vol. 17, 19033-19040
(2009) Abstract:
In this paper, we present novel designs and analysis of ultracompact couplers
and 1 × 2 splitters based on plasmonic waveguides. Numerical simulation shows
coupling efficiency up to 88% for the former one and 45% for each branch for
the latter one. The proposed coupler design has the advantages of improving
the alignment tolerance of the plasmonic waveguide with respect to the
dielectric waveguide and broadening the spectrum response of the splitter. In 2008 16.
R.
Yang, M. A. Abushagur, and Z. Lu, “Efficiently squeezing near infrared
light into a 21nm-by-24nm nanospot,” Optics Express,
vol. 16, 20142- 20148 (2008). Abstract: Recent work demonstrated light transmission through deep subwavelength slits or coupling light into waveguides with deep subwavelength dimension only in one direction. In this paper, we propose an approach to squeeze light (λ = 1550 nm) from a dielectric waveguide into a deep subwavelength spot. Vertical confinement is achieved by efficiently coupling light from a dielectric waveguide into a 20-nm metal-dielectricmetal plasmonic waveguide. The horizontal dimension of the plasmonic waveguide is then tapered into 20 nm. Numerical simulation shows that light fed from a dielectric waveguide can be squeezed into a 21nm-by-24nm spot with efficiency 62%.
In 2007 17.
Z. Lu,
“Efficient fiber-to-waveguide coupling through the vertical leakage from a
microring,” Optics Letters, vol. 32, 2861-2863 (2007). Abstract:
A fiber-to-waveguide coupler is proposed to efficiently couple light from a
single-mode fiber into a submicrometer semiconductor waveguide for integration
with optoelectronic circuits. A microring with a specific refractive index is
designed on the top of the semiconductor waveguide. The gradual vertical
leakage from the microring forms steady coupling into the semiconductor
waveguide. Coupling efficiency up to 93% is demonstrated using the
three-dimensional finite-difference time-domain method. A tapered-waveguide or
microring structure can be used to convert the lateral-mode size for coupling
light into a single-mode semiconductor waveguide 18.
Z.
Lu, B. Miao, T. R. Hodson, C. Lin, J. A. Murakowski, and D. W. Prather,
“ Negative refraction imaging in a hybrid photonic crystal device at
near-infrared frequencies,” Optics Express, vol. 15,
1286-1291 (2007). Abstract: We present the experimental demonstration of imaging of a point source by negative refraction at near-infrared frequencies using a hybrid photonic crystal device. The photonic crystal device, fabricated by patterning holes in 260nm silicon-on-insulator, integrates a triangular-lattice photonic crystal with a large photonic bandgap and square-lattice photonic crystal with negative refraction. Experimental results show that the output of a line-defect photonic bandgap waveguide provides a nearly ideal point source and then is imaged through the photonic crystal by negative refraction.
19.
Z.
Lu and D. W. Prather, “Calculation of effective permittivity,
permeability, and surface impedance of negative-refraction photonic
crystals,” Optics Express, vol. 15, 8340-8345 (2007). Abstract: We consider the eigen-fields of a two-dimensional negativerefraction photonic crystal and obtain negative effective permittivity and negative effective permeability. Effective permittivity, permeability, and surface impedance are calculated by averaging the eigen-fields. The value of the surface impedance is shown to be location-dependent and is validated by finite-difference time-domain simulations. The unique power propagation mechanism in the photonic crystal is demonstrated through time-evolution of eigen-fields. In 2006 20.
Z.
Lu, S. Shi, J. A. Murakowski, G. J. Schneider, C. A. Schuetz, and D. W.
Prather, “Experimental demonstration of self-collimation inside a
three-dimensional photonic crystal,” Physical Review Letters,vol.96,
No.17, 173902(4) (2006). Abstract:
We present our experimental demonstration of self-collimation inside a
three-dimensional (3D) simple cubic photonic crystal at microwave
frequencies. The photonic crystal was designed with unique dispersion
property and fabricated by a high precision computer-controlled machine. The
self-collimation modes were excited by a grounded waveguide feeding and
detected by a scanning monopole. Selfcollimation of electromagnetic waves in
the 3D photonic crystal was demonstrated by measuring the 3D field
distribution, which was shown as a narrow collimated beam inside the 3D
photonic crystal but a diverged beam in the absence of the photonic crystal. 21.
Z.
Lu, J. A. Murakowski, C. A. Schuetz, S. Shi, G. J. Schneider, J. P.
Samluk, and D.W. Prather “Perfect lens makes a perfect trap,” Optics
Express, vol.14, No.6, 2228-2235 (2006). Abstract: In this work, we present for the first time a new and realistic application of the “perfect lens”, namely, electromagnetic traps (or tweezers). We combined two recently developed techniques, 3D negative refraction flat lenses (3DNRFLs) and optical tweezers, and experimentally demonstrated the very unique advantages of using 3DNRFLs for electromagnetic traps. Super-resolution and short focal distance of the flat lens result in a highly focused and strongly convergent beam, which is a key requirement for a stable and accurate electromagnetic trap. The translation symmetry of 3DNRFL provides translation-invariance for imaging, which allows an electromagnetic trap to be translated without moving the lens, and permits a trap array by using multiple sources with a single lens. Electromagnetic trapping was demonstrated using polystyrene particles in suspension, and subsequent to being trapped to a single point, they were then accurately manipulated over a large distance by simple movement of a 3DNRFL-imaged microwave monopole source.
22.
I.
O. Mirza, S. Shi, Z. Lu, and D. W. Prather, “Calculation of the negative
refractive index of a LHM slab by the free-space mapping of the laterally
shifted refracted beam,” Microwave and Optical Technology Letters,
vol. 48, No. 3, 593 – 596 (2006). Abstract:
In this paper, we present a novel method to calculate the effective negative
refractive index of a left-handed material (LHM) - a composite of split-ring
resonators (SRRs) and thin wire structures. A LHM slab was first fabricated
and then the measured transmission and reflection results were used to
determine the LHM passband. To obtain the effective negative-refractive
index, a microwave beam was obliquely launched to illuminate the slab for
which a 2D spatial-field profile of the refracted beam outside the slab was
obtained by scanning the lateral shifting of the field with respect to normal
for a band of frequencies. A lateral shift of the refracted beam towards the
same side of the incident beam for the LHM passband confirmed negative
refraction in the LHM slab and the refractive index calculated for that
frequency region was conclusively negative according to Snells law. The
presented method provides an alternate and efficient way to measure the
effective refractive index of a LHM. 23.
S. K. Lohokare, Z.
Lu, C. A. Schuetz, and D. W. Prather, “Electrical Characterization of
Flip-Chip Interconnects Formed Using A Novel Conductive-Adhesive-Based
Process,” IEEE Transactions On Advanced Packaging, vol. 29,
No. 3, 542-547 (2006). Abstract:
Using conventional microfabrication techniques, we have developed a new,
low-cost wafer bumping process that enables a high degree of control over
patterning of conductive adhesive interconnects. This approach obviates the
need for development of dispensing and scraping head equipment that may
otherwise be required for mass fabrication of lithographically patterned
adhesive bumps. Flip-chip interconnects formed using this new process offer
better electrical performance as compared to those formed by squeegee-based
definition techniques. This is inferred in this paper by experimentally
demonstrating lower contact resistance with the polished bumps as compared to
the squeegeed bumps. Furthermore, in order to study the high-speed electrical
performance characteristics of these conductive adhesive bumps, a 10-GHz
1.55- m p-i-n photodetector fabricated in the antimonide material system was
used as case study. The results from the bandwidth characterization of the
polymer flip-chip-integrated detector showed minimum degradation in the
high-speed performance characteristics of the detector. In 2005 24.
Z.
Lu, J. A. Murakowski, C. A. Schuetz, S. Shi, G. J. Schneider, and D. W.
Prather, “Three-dimensional subwavelength imaging by a photonic-crystal flat
lens using negative refraction at microwave frequencies,” Physical
Review Letters, vol.95, No.15, 153901(4) (2005). Abstract: We experimentally demonstrate subwavelength resolution imaging at microwave frequencies by a three-dimensional (3D) photonic-crystal flat lens using full 3D negative refraction. The photonic crystal was fabricated in a layer-by-layer process. A subwavelength pinhole source and a dipole detector were employed for the measurement. By point-by-point scanning, we obtained the image of the pinhole source shown in both amplitude and phase, which demonstrated the imaging mechanism and subwavelength feature size in all three dimensions. An image of two pinhole sources with subwavelength spacing showed two resolved spots, which further verified subwavelength resolution.
25.
Z.
Lu, C. Chen, C. A. Schuetz, S. Shi, J. A. Murakowski, G. J. Schneider, and D.
W. Prather, “Sub-wavelength imaging by a flat cylindrical lens using
optimized negative refraction,” Applied Physics Letters, vol. 87,
No.9, 091907(3) (2005). 26.
Z.
Lu, C. A. Schuetz, S. Shi, C. Chen, G. P. Behrmann, and D. W. Prather,
“Experimental demonstration of self-collimation in low index contrast
photonic crystals in the millimeter wave regime,” IEEE Transactions
on Microwave Theory and Techniques, vol.53, No. 4, 1362-1368(2005). Abstract:
In this paper, we present the theoretical and experimental results for
self-collimation in low-index-contrast photonic crystals (PhCs) in the
millimeter-wave (MMW) region of the electromagnetic spectrum. The design of
the PhCs is based on their equifrequency contours and the two-dimensional
finite-difference time-domain simulation results. In the experiments, the MMW
PhCs are fabricated in Rexolite slabs by a CNC micro-milling system. A MMW
imaging system is built based on a vector network analyzer. The input source
is launched either through a waveguide or a monopole, while the field
distribution is acquired by scanning a monopole antenna over the surface of
the photonic crystal to detect the profile of the evanescent waves. In both
cases, we have observed and characterized the self-collimation effect for
both the amplitude and phase of the propagating electromagnetic wave in
low-index-contrast photonic crystals. 27.
Z.
Lu, S. Shi, C. A. Schuetz, J. A. Murakowski, and D. W. Prather,
“Three-dimensional photonic crystal flat lens by full 3D negative
refraction,” Optics Express, vol.13, No.15, 5592-5599 (2005). Abstract: We present the experimental demonstration of imaging by allangle negative refraction in a 3D photonic crystal flat lens at microwave frequencies. The flat lens is made of a body-centered cubic photonic crystal (PhC) whose dispersion at the third band results in group velocity opposite to phase velocity for electromagnetic waves. We fabricated the photonic crystal following a layer-by-layer process. A microwave imaging system was established based on a vector network analyzer, where two dipoles work as the source and the detector separately. By scanning the volume around the lens with the detector dipole, we captured the image of the dipole source in both amplitude and phase. The image of two incoherent sources separated by 0.44λ showed two resolvable spots, which served to verify subwavelength resolution.
28.
Z.
Lu, S. Shi, C. A. Schuetz, and D. W. Prather, “Experimental demonstration of
negative refraction imaging in both amplitude and phase,” Optics
Express, vol.13, No.6, 2007-2012(2005). Abstract: We studied a two-dimensional square-lattice photonic crystal with all-angle negative refraction at its first band. Using this photonic crystal, we designed and fabricated a flat lens functioning as a cylindrical lens by increasing the vertical dimension of the photonic crystal. Twodimensional finite-difference time-domain simulation validated negative refraction imaging. To perform the experiment, a microwave imaging system was built based on a vector network analyzer. Field distributions were acquired by scanning the imaging plane and object plane. The experiment demonstrated negative refraction imaging in both amplitude and phase, and obtained an image with feature size, 0.77λ. 29.
C.
Lin, Z. Lu, S. Shi, G. Jin, and D. W. Prather, “Experimentally
demonstrated filters based on guided resonance of photonic crystal
films,” Applied Physics Letters, vol. 87, 091102(3) (2005). Abstract:
We demonstrate a guided resonance filter based on photonic crystals PhCs[1], which are fabricated in a
high-permittivity material. The resulting spectra from a three-dimensional
analysis of the structure and experimental measurement results show sharp
dips and flattop transmissions. These provide promising properties in
constructing sensitive and compact wavelength-selective devices, such as
wavelength-division multiplexers. 30.
C.
Lin, Z. Lu, G. J. Schneider, and D. W. Prather, “Experimental
demonstration of prism coupling of infrared wave into planar photonic crystal
waveguide devices,” Optical Engineering, vol. 44, 114601
(2005). 31.
C.
Lin, C. Chen, S. Shi, Z. Lu, and D. W. Prather, “CO2 laser
light characterization of low loss planar-photonic crystals self-collimated
waveguides,” Optical Engineering, vol. 44, 114602 (2005). Abstract: We demonstrate nondefect mid-infrared waveguides based on the self-collimation effect in photonic crystals (PhCs). Due to the spatial dispersion properties serving to confine and route light, propagation loss as low as 2.56 dB/mm is obtained in a silicon-on-insulator (SOI)-based waveguide. The efficiency, together with their ability of arbitrary and structureless routing of electromagnetic beams, of these self-collimation devices provide a potential candidate for miniaturizing devices. In 2004 32.
Z.
Lu and D. W. Prather, “Total internal reflection-evanescent coupler for
fiber-to-waveguide integration of planar optoelectronic devices,” Optics
Letters, vol. 29, No.15, 1748-1750(2004). Abstract:
We present a method for parallel coupling from a single-mode fiber, or fiber
ribbon, into a silicon-on-insulator waveguide for integration with silicon
optoelectronic circuits. The coupler incorporates the advantages of the
vertically tapered waveguides and prism couplers, yet offers the flexibility
of planar integration. The coupler can be fabricated by use of either wafer
polishing technology or gray-scale photolithography. When optimal coupling is
achieved in our experimental setup, the coupler can be packaged by epoxy
bonding to form a fiber-waveguide parallel coupler or connector.
Two-dimensional electromagnetic calculation predicts a coupling efficiency of
77% 1.14-dB insertion loss) for a silicon-to-silicon coupler with a uniform
tunnel layer. The coupling efficiency is experimentally achieved to be 46%
(-3.4-dB insertion loss), excluding the loss in silicon and the reflections
from the input surface and the output facet. 33. S. K. Lohokare, C. A. Schuetz, Z. Lu, T. Dillon,
A. Sure, and D. W. Prather, “10GBps, 3-D, chip-level, parallel micro-optical
interconnects using a novel conductive polymer flip-chip process,” Optical
Engineering, vol. 43, No. 11, 2511-2517(2004). |
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Conference Publications and Presentations
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