Random lasers
Optical scattering has long been considered detrimental to laser as it induces additional loss. However, strong scattering increases the path length or dwell time of light inside the gain medium, thus enhances optical amplification. We recently demonstrated that laser cavities could be formed through the process of light scattering and interference in random media. In contrast to the conventional lasers whose cavities are made of mirrors, cavities of random lasers are "self-formed" in disordered materials. Therefore, random laser is also called mirrorless laser. Optical scattering not only can provide feedback for lasing, but also may lead to spatial confinement of laser light on the micro-scale. We invented a new type of microlaser that is made of disordered medium. The fabrication of such micro random laser is much easier and cheaper than that for most microlasers. The random lasers have many potential applications, e.g., optical tagging and encoding, multi-color display, bio-chemical sensing, tumor detection, photodynamic therapy.
We are carrying out detailed experimental and theoretical studies of random lasers to understand their physical mechanism. Noise and fluctuations have been incorporated to the FDTD method to simulate lasing in complex systems. Spatially non-uniform pumping is employed to control lasing properties and create new lasing modes. We investigate lasing in deterministic aperiodic nanostructures which combines many of the advantages of random lasers with a well-defined repeatable fabrication process suitable for device technology.
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A micro random laser. TOP LEFT: Scanning electron microscope image of a micron-sized cluster of ~ 10,000 ZnO nanoparticles. Optical excitation of ZnO nanoparticles leads to lasing in the cluster. TOP RIGHT: Intensity of emission from the ZnO cluster as a function of the incident pump pulse energy, showing the threshold behavior. BOTTOM LEFT: Spatial distribution of laser emission in the cluster. BOTTOM RIGHT: Emission spectrum with two lasing peaks. |
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UV Photonic Crystal Light Sources
Ultraviolet (UV) lasers and light emitting diodes (LEDs) have potential applications in high-density optical storage, high-resolution laser printing, solid-state lighting and display. We are employing the photonic band structures to reduce the threshold of UV lasers and to increase the efficiency of UV LEDs. In collaboration with Professor R.P.H. Chang's group in the Materials Science and Engineering Department of Northwestern University, we have developed various techniques to fabricate two-dimensional and three-dimensional photonic crystals with a wide-band-gap semiconductor ZnO.
Recently we have realized the first UV photonic crystal (PhC) lasers in photonic crystal slabs under optical pumping. Due to the short wavelength, structural disorder that is introduced unintentionally during the fabrication process has a significant effect on the fundamental band gap. We propose a new approach, i.e. to utilize high-order band structures. This approach increases the feature size and reduces the amount of lattice defects. Our latest experiments demonstrate effective and robust lasing associated with high-order flat bands of ZnO inverse opals as compared to lasing in the fundamental gap.
We recently observed the frozen modes in 3D PhCs originating from the stationary inflection points of high-order photonic bands. The frozen modes strongly modify the intensity, directionality and polarization of photoluminescence from inverse opals. At the frequency of a stationary inflection point, the directional density of states diverges, greatly enhancing the spontaneous emission into the frozen mode. Perfect coupling of the frozen mode to the free photon mode outside the sample leads to efficient extraction of emission from the PhC. The enhanced emission has not only well-defined propagation direction but also specific polarization.
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The first UV photonic crystal laser. LEFT: Top-view scanning electron micrograph of a triangular lattice of air holes in a ZnO slab, fabricated by focused ion beam etching. The lattice constant is 130 nm and the air cylinder radius is 33 nm. Inset: Structural Fourier transform reveals the long-range periodicity in the six maxima at the positions corresponding to the perfect lattice. RIGHT: Optical image of a lasing mode, which is localized by structural disorder. The deviation of the fabricated pattern from the ideal honeycomb structure is shown with color. Unexpectedly, structural disorder leads to spontaneous optimization of optical confinement by automatically balancing the rates of in-plane and out-of-plane leakage of light from a photonic crystal slab. |
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Mesoscopic transport of photons
Anderson localization of electrons, which has become an important part of contemporary condensed matter physics, represents a disorder-induced phase transition in electron transport from classical diffusion regime where Ohm's law holds to a localized state in which the material behaves as an insulator. Its optical analog, Anderson localization of photons, has been a subject of intense studies over the past two decades. One obstacle to photon localization is optical absorption, which does not exist in an electronic system. The question we intend to address is whether optical amplification could enhance photon localization. We find that the coherent amplification enhances the long-range correlation of light transmitting through the random media and it also increases the fluctuations in transmission and reflection. The interplay of light localization and coherent amplification, which has no counter-part in the electronic system, adds a new dimension to the fundamental study of Anderson localization.
We also investigate the effects of optical nonlinearity on light localization and lasing in random media. The nonlinearity refers to the change of refractive index with light intensity. Nonlinear effect is strong in a random laser because the nonlinear coefficient is resonantly enhanced at the lasing frequency and the laser intensity is high due to spatial confinement in random media. Our recent experimental and theoretical studies show that the third-order nonlinearity not only changes the frequency and size of lasing modes, but also modifies the laser emission intensity and laser pulse width. When the nonlinear response time is longer than the lifetime of the lasing mode, the nonlinearity changes the laser output through modifying the size of the lasing mode. If the nonlinear response is faster than the buildup of the lasing mode, positive nonlinearity always extracts more laser emission from the random medium due to the enhancement of single particle scattering. Strong nonlinearity could leads to instability and chaos.
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A spectral-temporal image of laser emission from ZnO powder. The temporal shifts of lasing frequencies result from dynamic change of the refractive index of ZnO due to the third-order optical nonlinearity. |