Optical tags have been proposed in the past for optical communication between far objects. The two-way optical link is established when the laser beam from the source reaches the optical tag, gets modulated by it and then reflected back to the source. Each optical tag should ideally have a phase conjugator for reversing the direction of the optical rays along the same path. This property dramatically enhances the coupling and the signal to noise ratio of the system in situations where the source and the optical tag are not on the line of sight of each other. In a turbid medium, the effect of scattering and phase change requires more in-depth studies. Here, we investigate this topic both experimentally and theoretically. As for the latter, we use transmission matrix approach (TMA) for full-wave solution of electromagnetic wave propagation and retroreflection in the turbid medium. In particular, we consider homodyne detection systems which rely on interferometric effects to eliminate the background stray rays and boost the signal to noise ratio. In this talk, we present our results to demonstrate that unlike flat reflectors, the retroreflectors eliminate the angular sensitivity up to 80 degrees of rotation. Whenever possible, we refer the one by one relation between the numerical simulations, theoretical analysis, and the experiments to pinpoint the origin of this enhancement. We also discuss the effect of the retroreflector size on the observed enhancement. Our results reveal the importance of retroreflectors for unprecedented signal enhancement for the emerging biomedical and atmospheric applications.
Excitons, bound electron-hole pairs, possess distinct physical properties from free electrons and holes that can
be employed to improve the performance of optoelectronic devices. In particular, the signatures of excitons are
enhanced optical absorption and radiative emission. These characteristics could be of major benefit for the laser
cooling of semiconductors, a process which has stringent requirements on the parasitic absorption of incident
radiation and the internal quantum efficiency. Here we experimentally demonstrate the dominant ultrafast excitonic
super-radiance of our quantum well structure from 78 K up to room temperature. The experimental results are
followed by our detailed discussions about the advantages and limitations of this method.
Near-infrared optical coherence tomography (OCT) has gained a lot of attention due to the fact that it is relatively cheap, non-invasive and provides high resolution and fast method of imaging. However the main challenge of this technique is the poor signal to noise ratio of the images of the tissue at large depths due to optical scattering. The signal to noise ratio can be improved by increasing the source power, however the laser safety standards (ANSI Z136.1) restricts the maximum amount of power that can be used safely to characterize the biological tissue. In this talk, we discuss the advantage of implanting a micro-lens inside the tissue to have a higher signal to noise ratio for confocal and OCT measurements. We explain the theoretical background, experimental setup and the method of implanting the micro lens at arbitrary depths within a live mouse brain. The in-vivo 3D OCT and two-photon microscopy images of live mouse with implanted micro-lens are presented and significant enhancement of signal to noise ratio is observed. The confocal and OCT measurements have been performed with super-luminescent LEDs emitting at 1300 nm. We believe that the high resolution and high sensitivity of this technique is of fundamental importance for characterization of neural activity, monitoring the hemodynamic responses, tumors and for performing image guided surgeries.
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.
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.
Suspended semiconductor structures with high thermal isolation provide high temperature sensitivity of the dissipated thermal power. Therefore they can be used to obtain essential information about the underlying mechanisms of anti-Stokes laser cooling. Here, we experimentally investigate the electron-hole pair recombination processes in suspended and non-suspended MQW structures from 77 K up to room temperature. Excitation dependent and time-resolved micro-photoluminescence measurements have been used to conduct this study. To include the effects of lateral carrier diffusion, we preformed finite-element time-resolved analysis of carrier recombination and diffusion and thermal transients. The potential of these structures for laser cooling purposes is discussed.
Our group has designed and developed a novel telecom band photon detector called the electron-injection detector. The detector provides a high avalanche-free internal-amplification and a stable excess noise factor of near unity while operating at linear-mode with low bias voltages. In our previous reports on un-isolated detectors, the large dark current of the detectors prevented long integration times in the camera. Furthermore, the bandwidth of the un-isolated detectors was in the KHz range. Recently, by changing the 3D geometry and isolating the detectors from each other, we have achieved 3 orders of magnitude reduction in dark current at same bias voltage and temperature compared to our previous results. Isolated detectors have internal dark current densities of 0.1nA/cm2 at 160 K. Furthermore, they have a bandwidth that is 4 orders of magnitude higher than the un-isolated devices. In this paper we report room temperature and low temperature characteristics of the isolated electron-injection detectors. We show that the measured optical gain displays a small dependence on temperature over our measured range down to 220 K.
Plasmonic structures produce well-known enhancement of the near-field optical intensity due to sub-wavelength optical confinement. These properties can produce a significant change of transmission and reflection upon small mechanical change of the antenna configuration. We have developed a method based on this enhanced sensitivity for cooling and amplification of a moving mirror. Using finite difference time domain method and standard optomechanical coupled-equation, different regimes of operation such as laser detuning and cavity length were studied to compare the effect of the near-field enhancement with the conventional radiation pressure. Using practical microcavity parameters, we demonstrate significantly higher cooling - or amplification- efficiency for the near-field plasmonic effect. Moreover, the volume of the system is very small. We believe that the significant efficiency improvement and reduced volume due to the proposed near-field effect can make this approach practical for many applications ranging from gravitational wave detection to photonic clocks, high precision accelerometers, atomic force microscopy, laser cooling and parametric amplification.
Realization of anti-Stokes cooling requires high enough photon extraction efficiency as well as quantum efficiency, making the implementation of this technique extremely difficult for semiconductors. Here, for the first time, we demonstrate that the Coulomb interaction between photogenerated electron-hole pairs in strong piezoelectric materials such as GaN/InGaN quantum wells could assist laser cooling. By comparing to the cavity back-action mechanism, we also explain how this process depends upon laser detuning with respect to bandgap. To demonstrate the advantage of this method even further, we present simulations by using experimentally reported parameters of GaN and In0.15Ga0.85N, in order to conclude that the net cooling is indeed possible even with current III-nitride growth technology.
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.
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.
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