An ultra-small telecentric lens with sub-millimeter thickness is proposed. This lens with 0.2 numerical aperture and high field of view is a good candidate to be used in multi-aperture super resolution imagers. Point spread function and the telecentricity of the lens is extracted numerically and measured experimentally. The ray-optics simulation results show nearly diffraction limited performance for the lens.
A thermal photovoltaic cell (TPV) is an optical heat engine that can extract energy from an emitter with elevated temperature. In theory, the efficiency of a TPV can reach to 80% by wavelength conversion, yet in practice, only 3.2% efficiency has been achieved. The main physical drawback is to maintain the device operation at very high temperature while managing total solar spectrum absorption and efficient coupling of the narrow-band thermal radiation into the photovoltaic cell. In this vein, utilizing of a nanophotonic structure to undergo the wavelength conversion of solar energy is inevitable. Furthermore, low cost, large area and high throughput realization of such a structure brings TPV beyond the research lab.
Simultaneous tailoring of UV/visible and mid-infrared spectrums requires sub-100-nm feature size, which is challenging with conventional photolithography if it is not impossible. We have developed a microsphere deep-UV lithography that can produce minimum feature size of ~ 50 nm at extremely low cost and high throughput. In this work, we demonstrate a metasurface platform fabricated with this lithography technique which has omni-polarization and -angle absorption in visible spectrum and efficient emission at mid-infrared as confirmed both by FDTD simulation and Fourier transform infrared spectroscopy (FTIR) measurement. The developed technique is promising technology to expedite TPV in real-life energy harvesting applications.
KEYWORDS: Antennas, Near field optics, Near field, Electron beam lithography, Nonlinear optics, Reactive ion etching, Switching, Scattering, Plasmonics, Nanoantennas
We have introduce optomechanical nanoantennae, which showed dramatic changes in scattering
properties by minuscule changes in geometry. These structures are very compact, with a volume 500
times smaller than free space optical wavelength volume. Through these optical elements, far-field can
directly control the near-field of antenna by mechanical reconfiguration. Here we present the functionality
of the optomechanical nanoantenna and challenges in fabricating and measuring these devices.
KEYWORDS: Sensors, Antennas, Quantum efficiency, Optical antennas, Photodetectors, Near field optics, Near field, Absorption, Semiconductors, Signal to noise ratio
Fast photodetectors, with high sensitivity and quantum efficiency are highly demanded elements for many applications, such as optical communication, optical interconnects, quantum key distribution, and infrared imaging. Therefore, it is crucial to optimize the quantum efficiency-bandwidth product, while maintaining a low noise. Integration of optical antennas with photodetectors leads to high quantum efficiency due to enhancing light – matter interaction as well as reducing the volume of the detector beyond the diffraction limit. Although, a direct consequence of shrinking the detection size is the noise reduction, the local density of state (LDOS) will be enhanced in the near-field of optical antenna. LDOS enhancement can lead to an increased noise current, by reducing the carrier lifetime due to the enhancement of the radiative recombination rate. This effect could become significant, especially for interband detectors, where the carrier lifetime is quite long compared with intersubband detectors. Here, we simulate and study this effect on the interband and intersubband infrared photodetectros using FDTD simulation.
KEYWORDS: Quantum well infrared photodetectors, Plasmonics, Signal to noise ratio, Signal processing, Metals, Quantum efficiency, Sensors, Absorption, Photoresist materials, Etching
Quantum Well Infrared Photodetector (QWIP) is an attractive candidate for long-wave infrared detection but is limited due its low quantum efficiency and its polarization sensitivity. Here we propose a detector with an embedded plasmonic structure surrounding the detector that is protected. Our detector uses an array of pillars surrounded by a plasmonic metal and contacted from the top making one “super pixel”. This structure is within close proximity of the active medium and is protected by the top contact. This configuration also eliminates non-absorbing semiconductor eliminating significant dark current.
Nano-fabrication technologies are usually associated with complication, high cost, and limited area of coverage.
However, advances in optics and nanophotonics constantly demand novel fabrications for nano-manufacturing
systems with extraordinary optical, electrical, mechanical, or thermal responses. While, these properties are vital for
health, energy, and information technology applications, proposing new methods of fabricating nanostructures that
can be compatible with high throughput and large scale manufacturing is quite desirable. Here, we propose a deep
ultra-violet (DUV) photolithography technique that can produce a variety of periodic nanostructure clusters with
sub-100 nm feature sizes. The method is based on microsphere nanolithography, which focuses DUV field into a socalled
photonic nano-jet – a propagative intensive field underneath the sphere. The position of a photonic nano-jet
can be moved by changing the angle of exposure. The DUV microsphere nanolithography is inherently self-aligned,
mask-less and optics-less (the bulky optical element such as lens is not required), which makes this method
attractive for low-cost and high-throughput nano-manufacturing schemes, such as roll-to-roll production. Here, we
present fabricated arrays of nanoscale complex structures to demonstrate the capabilities of this nanolithography
method.
Various metallic nano-structured thin films were fabricated by oblique angle deposition. Their optical, electrical and
structural properties were investigated to explore potential applications in optoelectronic field. The shape, size and
density of metal films were discussed based on SEM images and their thermal characteristics. The optical reflectance,
transmittance, and absorptance measurements showed unique optical properties of each metallic nano-structured films.
Indeed, ellipsometry measurement and resistance measurement were performed to investigate directivity of nanocolumnar
films depending on polarization properties, and conductivity, respectively.
Fabrication of nanostructures for applications such as plasmonics and metamaterials are typically accompanied by a slow production and limited area due to the required sub-micron feature sizes. In these applications, periodic array of metal/dielectric features can produce optical resonance responses such as optical field enhancement response, Fano response, chiral response, and negative refractive index. Here, we propose a mask-less photolithography technique that can produce a variety of periodic nanostructure clusters. The method is based on microsphere nanolithography, which focuses UV field into the so-called photonic jet which is a propagative intensive field underneath the sphere. The position of photonic jet can be moved by changing the angle of exposure. The method introduces a controllable scheme to realize nano-gap size by controlling the angle of exposure. The feature sizes generated by this method are about one third of exposure wavelength. The method is compatible with highthroughput nano-manufacturing schemes, such as roll-to-roll production. Here we present some examples to demonstrate the capabilities of this method in producing an array of complex plasmonic molecules over a large area. The periodicity of array and element’s diameter can be tuned by microsphere size and exposure/developing time, respectively. Tilted exposure lithography inherently is self-aligned and readily extendible to deep UV lithography due to absent of mask and optical elements. FDTD simulation agrees well with our experimental results, and suggests that much smaller feature sizes can be achieved at shorter wavelengths.
KEYWORDS: Near field optics, Atomic force microscopy, Near field scanning optical microscopy, Antennas, Plasmonics, Optical amplifiers, Near field, Microscopes, Modulation, Molecules
We have developed a NSOM technique that can map both the near optical field and the optical force using an atomic force microscope. This technique could be very useful for characterizing MEMs/NEMs devices, plasmonic nanoantennas, nano-photonic devices and biologically active substrates. Unlike conventional NSOM techniques that rely on an aperture fabricated on the end of an AFM tip to collect the optical signal this apertureless technique uses a lockin amplifier locked to the AFM tip vibrational frequency, to correlate the amplitude modulation of the back reflected optical signal to the strength of the optical field. And since we are not limited by the fabrication of an aperture the spatial resolution of the map is limited only by the size of a sharp AFM tip which for metallic coated tips can have a radius of curvature of 10 to 20 nm. For optical force mapping the incident laser is modulated and the lock-in amplifier is used to correlate the amplitude modulation of the vibrating AFM tip to strength of the optical gradient force. And in this way one can get a very accurate mapping of both the optical force and the optical field for any substrate of interest as long as it can be back illuminated. Lastly with an electrically monolithic substrate it is possible to correlate the amplitude modulation of the tunneling current to the optical field and obtain a spatial mapping that has a resolution of an STM, about 1 nm or maybe less.
In order to lessen the strain of cooling requirements on mid-infrared detectors, reducing the volume of the detecting medium is one promising solution. It is necessary to augment the absorption (quantum efficiency) lost when shrinking the detector volume. We present a Quantum Well Infrared Photodetector with a plasmonic structure embedded within and around the detection media. This device has a self-aligned plasmonic-hole array designed for 8μm wavelength and a planar top contact to the array of detector material. This arrangement has an expected field enhancement of an order of magnitude and lends itself to making a Focal Plane Array.
KEYWORDS: Antennas, Quantum efficiency, Dielectrics, Absorption, Optical antennas, Near field optics, Diffraction, Quantum information, Near field, Chemical elements
Coupling between electronic state and far field light, including absorption and spontaneous emission, is a central
issue for applications such as quantum metrology, optical quantum information, single molecule fluorescence
spectroscopy, and ultra sensitive detection which demand on high quantum efficiency. In such applications,
propagating far field light with diffraction limited spatial distribution has to be coupled to the electronic state of a
quantum absorber/emitter with a size far below the diffraction limit. Such a significant contrast between the
wavelengths of photon and electron sets limitations on the light-matter interaction strength. The most straight
forward solution is to convert far-field modes to near-field modes with dimensional scale closer to the electronic
state. The process of converting far field to near field and vice versa can be conducted by an antenna as an
intermediate element between far field mode and electronic state in a quantum element (absorber/emitter). Here,
we classify optical antenna based on their performance into three categories. Considering each category advantage,
we propose a hybrid antenna with superior performance. A quantum efficiency of about 50% is predicted for a
semiconductor with volume of ~λ3/170. Despite the weak optical absorption coefficient of 2000 cm-1 in the long
infrared wavelength of ~8 μm, very strong far-filed coupling has been achieved, as evidenced by an axial directivity
gain of 16 dB, which is only 3 dB bellow of theoretical limit.
Controlling the far field pattern of the electromagnetic (EM) waves has many applications including wireless
communications, radar detection, and industrial applications. The dynamic control of EM patterns is called beam
steering. Despite advantages in each technique, the speed, angular range, and spectral range of beam steering is limited
due to mechanical and optical properties of such systems. Here we present a beam steering method by means of an array
of optomechanical nanoantennas in which the generated optical force of each antenna results in changes to the antenna
response due to mechanical reconfiguration. As a result, the antenna far field phase is changed due to the mechanical
movement generated by the optical force. Depending on the mechanical properties of the movable component of the
antenna, the phase of the antenna can be tailored for a given optical source power. FDTD simulations are used to
calculate the optical response of antenna. A phase array of optomechanical nanoantennas is used to do beam steering.
The main far field lobe is steered by 0.5 degrees as a result of the mechanical reconfiguration of the phased array.
Here we present an antenna-integrated QCL which can be actively and optically modulated using light in the near infrared, creating an optical nanocircuit – coupling two different frequency antennas with a nonlinear active switching element. For our design, we chose two cross-polarized bow-tie antennas with an aligned central spot. We have used detailed FDTD simulations to choose the length of each bow-tie. The larger bow-tie antenna is resonant with the QCL at 6.1 μm wavelength and is aligned perpendicular to the active region of the device because QCL emits TM polarized light. The smaller bow-tie is resonant with the incoming modulating light at 1550 nm and is aligned perpendicularly to the first bow-tie. There is a rectangular region of amorphous germanium below the smaller bow-tie which acts as an absorber at 1550 nm. When light at 1550 nm is incident upon the device, it is focused and enhanced by the smaller bowtie, creating a region of large absorption in the germanium rectangle below. Free carriers are generated, shorting the larger bow-tie which is already focusing and enhancing light from the QCL mode. When the bow-tie arms of the larger bow-tie are shorted by these free carriers, the focusing and enhancement of the light by the larger bow-tie of the QCL mode is severely diminished, affecting the entire laser output, even the far field. Simulation results, fabrication details, and finally experimental results are discussed. Such an all-optical switch could be useful for telecommunications, free space communications, or rangefinding applications.
KEYWORDS: Quantum cascade lasers, Modulation, Antennas, Plasmonics, Atomic force microscopy, Sensors, Amplitude modulation, Near field optics, Reflectivity, Near field
We report mechanical frequency and amplitude modulation of a quantum cascade laser (QCL) integrated with a
plasmonic antenna operating at ~6.1 μm. We have observed a shift in the lasing frequency by over 30 GHz and an
intensity modulation of ~74% when an atomic force microscope (AFM) tip approaches the hot spot of a metal-dielectricmetal
(MDM) bow-tie antenna integrated onto the facet of the laser. The tip diameter is ~λ/60 and in non-contact mode
its amplitude of motion is ~λ/120. We have presented a theoretical model based on the rate equations for a QCL which
affirms our experimental observations. Our experiment demonstrates the strong influence of the hot spot on the laser
cavity modes, despite the fact that the former is many orders of magnitude smaller than the latter. We have compared
our device to a previous mechanically frequency modulated QCL and calculated a figure of merit, change in frequency
divided by change in distance of the mechanical component (Δf/Δd), which is an order of magnitude higher, while our
design uses a volumetric change per λ3 that is five orders of magnitude smaller. Our device differs from optical gradient
force actuated devices in that our device is externally mechanically actuated while those devices are self actuated
through the optical force. This sensitivity of the laser cavity mode to the position of a nanometer-scale metallic absorber
opens up the opportunity for modulating large amount of optical power by changing the optical properties of a miniscule
volume in an integrated, chip-scale device.
Mid-infrared photodetectors are the subject of many research efforts within the last two decades for enhancing their
operating parameters such as temperature stability, detectivity and quantum efficiency. This is due to their wide range of applications like biosensing, night vision, and short range communication. However, mid-infrared photons have much smaller energy compared with the band gap energy of well known semiconductors including III-V and II-VI families. One way to overcome this problem is to utilizing quantum confinement effects by absorbing a photon through the intersubband transition of a conduction electron or valance hole. Fabricating devices at the nanoscale size to achieve quantum confinement is costly and imposes limitations for further device preparation. In addition, the optical properties of quantum confined devices are sensitive to nanoscale geometrical parameters which make them vulnerable to fabrication imperfections. The other approach of detecting mid-infrared light is by exploiting the non-degenerate two photon absorption process (TPA). Two photons with different energies can be absorbed simultaneously by a semiconductor with the band gap energy less than the overall energy of two photons. Thus, a mid-infrared photon as the signal can be detected by a bulk semiconductor with much larger band gap energy when a near-infrared photon as the gate assists the absorption process through TPA.
KEYWORDS: Near field optics, Atomic force microscopy, Antennas, Near field, Plasmonics, Optical amplifiers, Near field scanning optical microscopy, Sensors, Optical fibers, Modulation
Spatial mapping of optical force near the hot-spot of a metal-dielectric-metal bow-tie nanoantenna at a wavelength of 1550 nm is presented. Non contact mode atomic force microscopy is used with a lock-in method to produce the map. Maxwell's stress tensor method has also been used to simulate the force produced by the bow-tie and it agrees with the experimental data. If dual lock-in amplifiers are used, this method could potentially produce the near field intensity and optical force map simultaneously, both with high spatial resolution. Detailed mapping of the optical force is critical for many emerging applications such as plasmonic biosensing and optomechanical switching.
KEYWORDS: Antennas, Quantum cascade lasers, Near field optics, Optical antennas, Near field scanning optical microscopy, Metals, Near field, Plasmonics, Mid-IR, Nanorods
Many important bio and chemical molecules have their signature frequency (vibrational resonance) matching the
mid infrared region (2-10 μm) of the optical spectrum. But building a bio-sensor, sensitive in this spectral regime, is
extremely challenging task. It is because of the weak light-particle interaction strength due to huge dimensional
mismatch between the probed molecules (typically ~ 10's of nm) and the probing wavelength (order of micron). We
exploit the optical antenna to overcome this problem by squeezing the optical modes. This modal confinement
happens only in the near-field region of the antenna and thus we have built an apertureless near-field scanning
optical microscope (a-NSOM) to demonstrate it experimentally. Further, we have integrated these plasmonic
antennas with mid-infrared sources known as Quantum Cascade Lasers (QCL). Our antenna structure is based on
metal-dielectric-metal (MDM) and we have shown how they can generate higher electrical field enhancement
compared to single metal design. Antenna integrated QCL operated at room temperature and its wavelength of
operation was measured to be ~ 6μm. We have used 3D finite-difference-time-domain (FDTD) simulations to
optimize the different component of the MDM antenna. After optimizing, we fabricated the antenna on the facet of
QCL using focused ion beam (FIB) and measured using a-NSOM. We have shown that the optical mode can be
squeezed down to a few 100's of nm which is much smaller than the incident light wavelength (λ~6μm). We also
propose a microfluidic approach to build a typical mid-infrared bio-sensor where the probed molecules can be
transferred to the near field region of the antenna through fluidic channels. Such scheme of building bio-sensor can
overcome the barrier of weak light-particle interaction and eventually could lead to building very efficient, compact,
mid-infrared bio-sensors.
KEYWORDS: Photonic crystals, Quantum cascade lasers, Near field scanning optical microscopy, Near field optics, Near field, Mid-IR, Silica, Gold, Plasmonics, Molecules
Planar photonic crystal (PPC) has recently attracted much attention as a promising platform for the realization of
compact nanocavity devices. Our proposed photonic crystal (PC) structure consists of a periodic hole array with a
point defect at the center. The device has been integrated on the facet of a quantum cascade laser working in the
mid-infrared region of optical spectrum. Finite-difference time domain (FDTD) simulations have been performed to
optimize the design structure. Simulations showed that with a periodicity of the holes (Λ) between 1.3um and
1.4um, the near field enhancement at the center of the cavity on the same level as the top metal surface can be as
high as 10 times the incident electric field. The radius of the hole and center cavity radius are 0.45 and 0.2 times Λ.
The structure was simulated at experimentally measured operating wavelength (λ=5.98um) of our device. During
fabrication, we used a buffer SiO2 layer thickness of 100nm followed by metal-dielectric-metal structure with layer
thicknesses of Au - SiO2 -Au (100/20/ 100 nm). Next, the MDM photonic crystal design was fabricated on the
MDM coated facet of the QCL using focused ion beam (FIB) milling. The integrated device has been tested using an
apertureless mid-infrared near field scanning optical microscopy (a-NSOM). The measurement set-up is based on an
inverted microscope coupled with a commercially available Atomic Forced Microscopy (AFM). Using this
technique, we could simultaneously measure the topography and NSOM image of the photonic crystal integrated
QCL. It showed that the combination of high quality factor and extremely low mode volume of the PC design can
squeeze the optical mode within a nanometric spot size ~ 450nm. The experimental results is a proof of concept,
although we believe, further optimization and improvisation with different PC designs can lead squeezing the optical
mode into a much smaller volume. Such integrated device are capable of focusing radiant infrared light down to
nanometer length scale and strongly enhance the near field intensity which can be extremely useful in molecular
sensing.
An apertureless near-field scanning optical microscope (a-NSOM) setup is described. Special
consideration is given to important system components. Surface plasmons are defined, as is their relationship to a-
NSOM and their interaction with the scanning probe tip. We used this set-up to measure a metal-dielectric-metal
(MDM) antenna integrated with a quantum cascade laser (QCL). The former is introduced and described. The role
of the atomic force microscope (AFM) in the experiment is laid out and explained. Finally, the lock-in amplifier is
explained. Next, the system setup is introduced and explained from the point of view of the light path taken by
light generated in the laser. Finally, results are given for the MDM single nanorod antenna and the coupled MDM
nanorod antenna. Simulation, topography, and NSOM images are shown. Lastly, several experimental issues are
discussed as well as other types of NSOM.
Quantum well infrared photodetectors (QWIPs) have demonstrated applications in many different areas, such as
medical and biological imaging, environmental and chemical monitoring, and infrared imaging for space and night
vision. However, QWIPs still suffer from low quantum efficiency and detectivity compared with mercury cadmium
telluride (MCT) based interband photodetectors, which dominate current infrared detector market. Besides, n-type
QWIPs cannot detect the normal incident infrared radiations because of the polarization selection rules of intersubband
transitions. Here, we used periodic holes array perforated in gold film to convert normal-incident infrared light to
surface plasmon waves, which can excite the intersubband transitions and be absorbed by quantum wells (QWs). Our
3D FDTD simulation results show that electric field component in the QWs growth direction can be enhanced by more
than 5 times compared with the total electric field intensity without any plasmonic arrays. The experimental results
show that the photodetector has a peak detection wavelength at ~8 μm with a high detectivity of ~7.4×1010 Jones, and the
photocurrent spectrum was very close to the simulation result of the electric field enhancement spectrum.
In the near field region, optical antennas can generate local hot spots with high energy density. It can be
very useful in increasing the photon-matter interactions for bio-sensing applications. There are several important
bio-molecules having signature frequency (vibrational resonance) matching the mid infrared region of the optical
spectrum. Thus mid-infrared antenna integrated with Quantum cascade laser (QCL) is highly desirable as it is
currently considered to be one of the most efficient mid-infrared laser sources with a huge gamut of commercial
applications. Here, we present a novel metal-dielectric-metal (MDM) based plasmonic nanorod antenna integrated
on the facet of a room temperature working Quantum Cascade Laser. Simulations showed that at an optimized SiO2
thickness of 20nm, the antenna can generate a local electric field with intensity 500 times higher than the incident
field intensity. Further, it can increase the number of regions with local hot spots due to a higher number of
geometrical singularities or sharp edges present in the MDM structure. This feature can be extremely useful,
especially for bio-sensing applications. All device structures have been optimized based on 3d finite-difference timedomain
(FDTD) numerical simulations. The antenna was fabricated on the facet of QCL using focused ion beam
(FIB). The integrated plasmonic QCL has been measured using an apertureless mid-infrared near field scanning
optical microscopy (a-NSOM). The measurement set-up is based on an inverted microscope coupled with a
commercially available Atomic Forced Microscopy (AFM). We have experimentally found that such integrated
nano antenna can generate a very narrow optical spot size, much below the diffraction limit, with high power density
that matches well with the simulation results.
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