The nanoplasmonic properties of apertures in metal films have been studied extensively; however, we have recently
discovered surprising new features of this simple system with applications to super-focusing and super-scattering.
Furthermore, apertures allow for optical tweezers that can hold onto particles of the order of 1 nm; I will briefly
highlight our work using these apertures to study protein - small molecule interactions and protein - DNA binding.
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.
Our group has designed and developed a new SWIR single photon detector called the nano-injection detector that is conceptually designed with biological inspirations taken from the rod cells in human eye. The detector couples a nanoscale sensory region with a large absorption volume to provide avalanche free internal amplification while operating at linear regime with low bias voltages. The low voltage operation makes the detector to be fully compatible with available CMOS technologies. Because there is no photon reemission, detectors can be formed into high-density single-photon detector arrays. As such, the nano injection detectors are viable candidates for SPD and imaging at the short-wave infrared band. Our measurements in 2007 proved a high SNR and a stable excess noise factor of near unity. We are reporting on a high speed version of the detector with 4 orders of magnitude enhancement in speed as well as 2 orders of magnitude reduction in dark current (30nA vs. 10 uA at 1.5V).
The loss in optical antennas can affect their performance for their practical use in many branches of science
such as biological and solar cell applications. However the big question is that how much loss is due to the
joule heating in the metals. This would affect the efficiency of solar cells and is very important for single
photon detection and also for some applications where high heat generation in nanoantennas is desirable, for
example, payload release for cancer treatment. There are few groups who have done temperature
measurements by methods such as Raman spectroscopy or fluorescence polarization anisotropy. The latter
method, which is more reliable than Raman spectroscopy, requires the deposition of fluorescent molecules on
the antenna surface. The molecules and the polarization of radiation rotate depending upon the surface
temperature. The reported temperature measurement accuracy in this method is about 0.1° C. Here we present
a method based on thermo-reflectance that allows better temperature accuracy as well as spatial resolution of
500 nm. Moreover, this method does not require the addition of new materials to the nanoantenna. We present
the measured heat dissipation from bull’s-eye nanoantennas and compare them with 3D simulation results.
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.
Laser cooling of materials has been one of the important topics of photonic research during recent years. This is due to the compactness, lack of vibration, and integratibility of this method. Although laser refrigeration has been achieved in rare earth doped glass, no net cooling of semiconductors has been observed yet. The main challenge in this regard is the photon trapping inside the semiconductors, due to its high refractive index, which prevents the extraction of the energy from the material. Various methods have been proposed to overcome photon trapping but they are either not feasible or introduce surface defects. Surface defects increase the surface recombination which absorbs some portion of the photoluminescence and converts it to heat. We exploit the surface plasmons produced in silver nanoparticles to scatter the PL and make the extraction efficiency significantly higher without increasing the surface recombination. This is also important in the semiconductor lighting industry and also for enhancing the performance of solar cells by coupling the sunlight into the higher index absorbing region. Finite difference time domain simulations were used to find the total power extraction efficiency of the silver nanoparticles. It is also proposed for the first time to use the silver nanoparticles as mask for dry etching. The results for both etched and unetched cases were compared with each other. We also refer to a method of silver nanoparticle fabrication which is easy to apply to all kinds of cooling targets and is relatively cheaper than deposition of complex anti-reflective coatings.
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.
We present the design and initial characterization of a device geometry that is suitable for measuring the direct midinfrared
absorption of single molecules. The devices are based on a metal-dielectric-metal sandwich with optically thick
gold cladding layers which squeeze the gap mode, the optical mode in the dielectric layer, in the electric field direction;
with a photonic crystal cavity defect which can squeeze the gap mode in the planar direction. Simulations show a field
enhancement of 10000 times the incident field in the cavity defect at an excitation frequency of 87 THz. Experimentally,
devices with varying periods were made using a free standing Si3N4 film of 15 nm or 50 nm as the inner dielectric. These
devices show a red shift as the period is increased and more interesting there is also a red shift for the thinner dielectric
devices showing that the field is further squeezed in this layer. By placing a molecule into the cavity, there should be a
strong enough interaction between the light and the analyte so that its absorption spectra can be resolved.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.