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.
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).
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.
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.
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.
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.
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.
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.
We present our latest results from a novel nano-injector-base photon detector. Previously, we have demonstrated the
excellent noise performance and large linear gain of single-element devices at room temperature. Here we demonstrate
the first focal plane array (FPA) made from this unique device, and show that they hold their high gain and low noise
performance at a good array uniformity. The high internal gain produces significantly better images at high frame rates,
or at low light level conditions.
The terahertz region (1-10 THz) has potential applications in many areas, such as chemical sensing, medical
imaging and free-space optical communications. With the demonstration of terahertz sources, it is quite necessary to
develop the detection technology in terahertz. Here we propose an electrically tunable quantum dot infrared
photodetector to detect the terahertz region. The proposed detector applies a lateral electrical confinement on the
quantum wells and forms a quantum disk in the quantum well area. The two-dimensional quantum confinement of
quantum disk combining the vertical confinement from the quantum barrier forms a quantum dot structure. Using the
energy states and intersublevel energy spacing in the quantum dot, the detector can be used to detect the terahertz region.
Changing the lateral electrical confinement, the intersublevel energy spacing can also be tuned and in hence different
wavelengths can be detected. Our modeling and simulation results show the tunability of peak detection wavelength of
the photodetector from ~3.3 to ~6.0 THz with a gate voltage applied on the detector from -2 to -5 V. The peak
absorption coefficients of the detection are shown in the range of 103 cm-1. Compared with current quantum dot
photodetectors produced by self-assembled growth method the detector proposed here is easier to be tuned and the
effective sizes have a much higher uniformity, because of using electrical confinement.
We present here a novel design to form an artificial quantum dot with electrical confinement and apply it to a Quantum
Cascade Laser structure to realize a Quantum Dot Cascade Laser. A two-dimensional finite element method has been
used to numerically simulate the novel design of electrical formation of an artificial quantum dot. The size of the
quantum dot is electrically tunable and can be applied to quantum cascade laser structure to reduce the non-radiative LO-phonon
relaxation. Numerical modeling with cylindrical symmetry is custom developed using Comsol multiphysics to
evaluate the electrical performance of the device and optimize it by varying design parameters, namely, the doping
density of different layers and thickness of the cladding and active regions. The typical s-, p-, d- and f- wave functions
have been calculated. Numerical simulations show that the energy level separation could be as large as 50 meV by
electrical confinement. We also demonstrate the road map for the fabrication of such a device using a maskless super
lens photolithography technique. We have achieved a uniform array of nano-contacts of size ~ 200nm, required for the
device, using photolithographic technique with a UV source of λ ~ 400nm. The entire processing involves 7
photolithographic steps. This new device - "Quantum dot cascade laser", promises low threshold current density and
high wall-plug efficiency.
We present theoretical and experimental results for a novel photon detector based on a
strong opto-electro-mechanical effect. In this new architecture, photo-generated carriers
are compresses under a suspended nano-injector to produce significant electrostatic
pressure. The pressure results in a reduced gap between the nano-injector and the
semiconductor, which in turn increases the tunnel based electron injection dramatically.
Our experimental results show very good sensitivity at 1.55 μm at room temperature. We
also show that Casimir force has a considerable effect on the device performance, due to
the small gap between nano-injector and semiconductor.
With nanotechnology becoming widely used, many applications such as plasmonics, sensors, storage devices, solar
cells, nano-filtration and artificial kidneys require the structures with large areas of uniform periodic nanopatterns. Most
of the current nano-manufacturing techniques, including photolithography, electron-beam lithography, and focal ion
beam milling, are either slow or expensive to be applied into the areas. Here, we demonstrate an alternative and novel
lithography technique - Nanosphere Photolithography (NSP) - that generates a large area of highly uniform periodic
nanoholes or nanoposts by utilizing the monolayer of hexagonally close packed (HCP) silica microspheres as super-lenses
on top of photoresist. The size of the nanopatterns generated is almost independent of the sphere sizes and hence
extremely uniform patterns can be obtained. We demonstrate that the method can produce hexagonally packed arrays of
hole of sub-250 nm size in positive photoresist using a conventional exposure system with a broadband UV source
centered at 400 nm. We also show a large area of highly uniform gold nanoholes (~180 nm) and nanoposts (~300nm)
array with the period of 1 μm fabricated by the combination of lift-off and NSP. The process is not limited to gold.
Similar structures have been shown with aluminum and silicon dioxide layer. The period and size of the structures can
also be tuned by changing proper parameters. The technique applying self-assembled and focusing properties of micro-/nano-spheres into photolithography establishes a new paradigm for mask-less photolithography technique, allowing
rapid and economical creation of large areas of periodic nanostructures with a high throughput.
KEYWORDS: Signal to noise ratio, Infrared imaging, Short wave infrared radiation, Photodetectors, Sensors, Infrared radiation, Picosecond phenomena, Single photon detectors, Temperature metrology, Absorption
A novel short wave infrared single photon detector was conceived for wavelengths beyond 1 μm. The detector, called the nano-injection photon detector, is conceptually designed with biological inspirations taken from the eye. Based on a detection process similar to the human visual system, the detector couples a nano-scale sensory region with a large absorption volume to provide a low-noise internal amplification mechanism, high signal-to-noise ratio and quantum efficiency. Tens of thousands of devices were fabricated in different configurations with conventional processing methods in more than 20 iterations. For low speed imaging applications, the detectors have shown gain values reaching 10,000 with bias voltages around 1 V. Ultra-low noise levels were measured at gain values exceeding 4,000 at room temperature: Fano factors as low as 0.55 has been achieved, which indicated a statistically stable amplification mechanism and resulting sub-Poissionian shot noise. An alternate version of the detector, which is specialized towards high-speed applications, has also been developed with slight changes in processing steps. The fast detectors with bandwidth beyond 3 Ghz were demonstrated which provide gain values around 20. The measured risetime was less than 200 ps. Femtosecond pulsed illumination measurements exhibited ultra-low jitter around 15 ps. Transient delay experiments revealed that the measured jitter is due to the transit time in the large absorption region. Hence the amplification process has insignificant time-uncertainty in addition to low amplitude-variance (noise), which is consistent with statistically stable nature of amplification.
Short wave infrared (SWIR) imaging systems have several advantages due to the
spectral content of the nightglow and better discrimination against camouflage.
Achieving single photon detection sensitivity can significantly improve the image quality
of these systems. However, the internal noise of the detector and readout circuits are
significant barriers to achieve this goal. One can prove that the noise limitations of the
readout can be alleviated, if the detector exhibits sufficiently high internal gain.
Unfortunately, the existing detectors with internal gain have a very high noise as well.
Here we present the recent results from our novel FOcalized Carrier aUgmented Sensor
(FOCUS). It utilizes very high charge compression into a nano-injector, and subsequent
carrier injection to achieve high quantum efficiency and high sensitivity at short infrared
at room temperature. We obtain internal gain values exceeding several thousand at bias
values of less than 1 volt. The current responsivity at 1.55 μm is more than 1500 A/W,
and the noise equivalent power (NEP) is less that 0.5 x10-15 W/Hz1/2 at room temperature.
These are significantly better than the performance of the existing room temperature
devices with internal gain. Also, unlike avalanche-based photodiodes, the measured
excess noise factor for our device is near unity, even at very high gain values. The stable
gain of the device combined with the low operating voltage are unique advantages of this
technology for high-performance SWIR imaging arrays.