Developing Novel Quantum Cascade Laser(QCL)
Quantum cascade laser (QCL) is a unipolar semiconductor laser works based on intersubband transition.I have been working on development, fabrication and testing of such lasers.These lasers operate very efficiently in the mid-infrared region of the optical spectrum. This spectrum is especially important for sensing as there are several bio and chemical molecules which have signature vibrational frequency between 2-10um.
  • Modeled QCL band structure using 3D-Finite Element Method (Comsol Multiphysics)
  • Designed and developed multiple fabrication steps for the fabrication of standard QCL, Injectorless QCL and electrically tunable QCL (involves >50 processing steps)
  • Demonstrated injectorless QCL with record lowest voltage defect ~30meV
  • Demonstrated improved thermal performance by manipulating injector region of the QCL
    Plasmonic Optical Antenna Integrated QCL
    Optical antenna is a device that coupled the radiation energy to a confined region of subwavelength size.In contrast to RF antenna designs, where the focus is on optimizing far-field characteristics in order to obtain better long distance transmission and reception performance,optical antenna designs must first emphasize the near-field behavior. The near-field hotspot can enhance the light-particle interaction orders of magnitude and thus this technique can be especially useful for building an efficient bio-sensor
  • Performed 3D-FDTD simulations(Lumerical) to design a mid-IR optical antenna
  • Fabricated optical antenna integrated QCL using Focused Ion Beam milling
  • Demonstrated composite material-based(MDM) optical antenna (bow-tie, single and coupled nanorod, photonics crystal design)
  • Achieved a squeezed mid-IR optical mode (6um) with a spot size of ~100
  • Integrated optofluidics over the antenna integrated QCL as a means to deliver molecules near the antenna hotspot. This technique is helpful towards building compact, chip-scale mid-IR bio-sensor
    Near Field Scanning Optical Microscopy
    We have used apertureless near-field scanning optical microscopy (a-NSOM) to measure the near-field signal generated near the optical antenna.It works by light scattering off of a vibrating AFM tip and being modulated at that vibrational frequency.It then gets picked up by a detector which feeds into a lock-in amplifier, and the tip frequency is used as the reference. The output of the lock-in is fed to a computer which maps to the position of the AFM tip. The lock-in demodulates the signal and the output gives a map of the light which is scattered from the AFM tip with arbitrary gain.
  • Built an a-NSOM based on a commercially available Atomic Force Microscope (Agilent 5400) (AFM)
  • Performed near-field (within 50nm) study of the antenna integrated QCL
  • Demonstrated a transmission-based a-NSOM
    Mid-IR Bio-Sensing
    Mid-IR bio-sensor has huge commercial demand as several important bio and chemical molecules have significant absorption in the mid-IR spectrum. The key challenge for building such sensor is the weak light-particle interaction due to large dimensional mismatch between the probing light wavelength (order of microns) and the probed molecules (few nanometers). We have overcome this problem by using plasmonic optical nanoantennas, which can focus the radiant infrared light into nanometric length-scale and thus increasing the interaction by several orders of magnitude.
    Optical Force Measurement
    Photons have a momentum and tranfer of this momentum due to elastic scattering is the source of radiation pressure and optical force. We performed spatial mapping of optical force near the hot-spot of a metal-dielectric-metal bow-tie nanoantenna at a wavelength of 1550 nm. Non contact mode atomic force microscopy was used with a lock-in method to produce the map. Maxwell’s stress tensor method was also been used to simulate the force produced by the bow-tie and it agrees with the experimental data.