Second-order Nonlinear Processes Research Articles

Polarization-induced phase shift of ultrafast photocurrents

Shift and injection currents are known to occur for linearly and circularly polarized optical excitations of semiconductors, respectively. Here, we show with room-temperature experiments that for excitation of discrete transitions the frequency dynamics of the coherent polarization changes this phase rule significantly. For above-bandgap optical excitation of semiconductors two main components of all-optically induced photocurrents have been recognized and are known as shift and injection currents (1). These currents can be linked to a second-order optical nonlinear process associated with difference- frequency mixing. Microscopically, shift currents and injection currents result from a polar distribution of carriers in real and momentum space, respectively. They are also known as linear and circular photogalvanic currents, since they occur for optical excitation with linearly and circularly polarized light, respectively (1-3). Here, we show that the frequency dynamics of the coherent polarization resulting from excitation of discrete transitions is capable of significantly altering this phase rule such that shift and injection currents occur for both, linear and circular polarizations. The measurements are performed on exciton transitions in a (110)-oriented, 5-nm wide GaAs quantum well (QW) sample, with the x, y, and z directions being parallel to the (001), (1 0), and (110) crystallographic directions, respectively.

The case for using gap plasmon-polaritons in second-order optical nonlinear processes

We show that using metal-insulator-metal (MIM) waveguides to carry out various second-order nonlinear optical processes not only provides highly desired tight optical confinement but also facilitates the phase-matching due to their inherently large anisotropy. This fact allows one to take advantage of otherwise inaccessible large nonlinear susceptibilities of the cubic zinc blende semiconductors. Our efficiency estimates show that since only the longer wavelength infra-red radiation propagates in the surface-plasmon-polariton (SPP) mode, the losses in the metal, while significant, do not preclude development of highly compact nonlinear optical devices on this integration-friendly semiconductor platform.

Metamaterials with Tailorable Nonlinear Optical Properties

Second-order nonlinear processes such as second-harmonic generation (SHG) require noncentrosymmetric structures. The development of second-order metamaterials, however, has been hampered by symmetry breaking due to sample defects and shape distortions. The resulting outcomes can be interpreted in terms of effective higher-multipole (magnetic and quadrupole) effects that strongly modify the radiative properties of the samples.

Phase conjugation based on single backward second-order nonlinear parametric process

We show that backward difference-frequency generation can be exploited to achieve phase conjugation in a second-order nonlinear medium. The backward configuration can be utilized to achieve broadband quasi-phase-matching, compared with the forward counterpart. Our calculation shows that a nonlinear reflectivity of close to 100% is achievable from a laser emitting an output power of ~1 mW. Such an efficient phase conjugator is made feasible by placing the nonlinear medium inside a pump laser cavity. In addition, a Fabry-Perot resonator at the input frequency is used to significantly improve the nonlinear reflectivity.

Terahertz emission from surface-immobilized gold nanospheres

Electromagnetic wave emission based on optical rectification at terahertz (THz) wavelengths was observed from surface-immobilized gold nanospheres (SIGNs) above a gold surface. Although the excitation wavelength is off-resonant with the localized surface plasmons, the THz emission field was observed to be approximately 4.8 times greater than that from a percolated gold thin film of 10 nm thickness. A theoretical calculation predicts that the light electric field is enhanced in the SIGN system, even at off-resonance wavelengths. The observed THz field amplitude was quadratic with the illumination light field, suggesting that the THz generation is due to a second-order nonlinear optical process.

Second-order nonlinear silicon-organic hybrid waveguides

We describe a concept for second-order nonlinear optical processes in silicon photonics. A silicon-organic hybrid (SOH) double slot waveguide is dispersion-engineered for mode phase-matching (MPM). The proposed waveguide enables highly efficient nonlinear processes in the mid-IR range. With a cladding nonlinearity of χ(2) = 230 pm/V and 20 dBm pump power at a CW wavelength of 1550 nm, we predict a gain of 14.7 dB/cm for a 3100 nm signal. The suggested structure enables for the first time efficient second-order nonlinear optical mixing in silicon photonics with standard technology.

Optical effects of spin currents in semiconductors

A spin current has novel linear and second-order nonlinear optical effects due to its symmetry properties. With the symmetry analysis and the eight-band microscopic calculation we have systematically investigated the interaction between a spin current and a polarized light beam (or the "photon spin current") in direct-gap semiconductors. This interaction is rooted in the intrinsic spin-orbit coupling in valence bands and does not rely on the Rashba or Dresselhaus effect. The light-spin current interaction results in an optical birefringence effect of the spin current. The symmetry analysis indicates that in a semiconductor with inversion symmetry, the linear birefringence effect vanishes and only the circular birefringence effect exists. The circular birefringence effect is similar to the Faraday rotation in magneto-optics but involves no net magnetization nor breaking the time-reversal symmetry. Moreover, a spin current can induce the second-order nonlinear optical processes due to the inversion-symmetry breaking. These findings form a basis of measuring a pure spin current where and when it flows with the standard optical spectroscopy, which may provide a toolbox to explore a wealth of physics connecting the spintronics and photonics.

Second-order nonlinear frequency conversion processes in plasmonic slot waveguides

We present a theoretical study on the possibilities to achieve nonlinear frequency conversion in plasmonic slot waveguides having a χ(2) nonlinear medium as its core. Second-harmonic generation is used as a referential process to discuss the possibilities in achieving strong nonlinear interactions. We show that geometrical dispersion allows for the possibility of modal phase matching without resorting to other mechanisms like birefringence phase matching or periodic poling of the nonlinear medium. We disclose that in strongly dissipative systems two effects, the damping of individual modes and the phase-matching condition, have to be carefully balanced to assure an efficient energy conversion. Besides second-harmonic generation, emphasis is put on exploring the application of potentially more importance: the parametric amplification in the waveguide with the purpose to enhance its propagation length.

Generation of polarization entangled photons using concurrent type-I and type-0 processes in AlGaAs ridge waveguides

A technique to generate polarization entangled photons using concurrent type-I and type-0 second-order nonlinear processes in monolithic Bragg reflection waveguides is presented and analyzed. Concurrent phase matching is achieved by lithographic tuning of the waveguide ridge width. Nearly perfect entanglement is achievable on-chip through appropriate epistructure design without the need of spectral filtering and group velocity compensation. Theoretical calculations predict that a high quantum interference visibility could be experimentally achieved with the pair generation rate of each process being approximately 3.0×10(6) pairs/s/mW/GHz.

Monolithic Photonics Using Second-Order Optical Nonlinearities in Multilayer-Core Bragg Reflection Waveguides

Recent advancements in phase-matching second-order nonlinear processes by using matching-layer-enhanced Bragg reflection waveguides (ML-BRWs) in Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> As material system are discussed. The limitations on the choice of the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> As layers for applications that require high pump power operation are highlighted. Multilayer-core ML-BRWs are proposed as a new waveguide design with relaxed constraints over the choice of the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> As layers composition. The tradeoffs associated with material bandgap on the efficiency of second-order nonlinear processes are examined by using this novel structure. The interplay among the various factors, including the nonlinear overlap factor, the effective second-order nonlinearity, and the third-order nonlinear effects result in the presence of an optimum detuning of the core bandgap from the operating wavelength for maximum conversion efficiency. Two different wafer structures are examined by using second-harmonic generation to elucidate these tradeoffs. The conversion efficiency is examined by using 30-ps, 2-ps, and 250-fs pulses at various pump average power levels.

Quantum cutting mechanism in NaYF_4:Tb^3+, Yb^3+

A quantum cutting mechanism for the sublinear near-IR power dependence property in Tb3+-Yb3+ codoped NaYF4 powders were investigated both experimentally and theoretically. The slopes of Yb3+ luminescence intensity versus excitation power were fitted to be between 0.5 and 1. We have developed a quantum cutting rate equation model to explain the anomalous sublinear phenomenon and an assessment factor was introduced to help understand the physical mechanism. Experimental results showed that the linear downconversion process combined with second-order nonlinear process induced the sublinear power dependence property with the latter to be the dominant process.

Stretching silicon's potential

The application of inhomogeneous strain to silicon photonic structures may lead to new optically active devices based on second-order nonlinear processes.

Asymptotic fields for a Hamiltonian treatment of nonlinear electromagnetic phenomena

We discuss the use of asymptotic-in and -out fields in a Hamiltonian treatment of nonlinear electromagnetic problems. Our approach allows a description of nonlinear phenomena in a number of geometries and is suitable to treat integrated photonic structures, particularly cavities. We present examples in which the nonlinear phenomena are weak enough for pump depletion to be neglected, and we report some results related to the description of second-order nonlinear processes in ring resonators and layered structures.

Partial angular coherence and the angular Schmidt spectrum of entangled two-photon fields

We study partially coherent fields that have a coherent-mode representation in the orbital-angular-momentum-mode basis. For such fields, we introduce the concepts of the angular coherence function and the coherence angle. Such fields are naturally produced by the process of parametric down-conversion---a second-order nonlinear optical process in which a pump photon breaks up into two entangled photons, known as the signal and idler photons. We show that the angular coherence functions of the signal and idler fields are directly related to the angular Schmidt (spiral) spectrum of the down-converted two-photon field and thus that the angular Schmidt spectrum can be measured directly by measuring the angular coherence function of either the signal or the idler field, without requiring coincidence detection.

Nonlinear, dispersive, and phase-matching properties of the new chalcopyrite CdSiP_2 [Invited

We compare the nonlinear and dispersive properties of the recently discovered mid-infrared nonlinear crystal CdSiP2 with other chalcopyrite materials to establish its potential for super-continuum generation through a second-order nonlinear process.

Percolation-enhanced generation of terahertz pulses by optical rectification on ultrathin gold films

Emission of pulses of electromagnetic radiation in the terahertz range is observed when ultrathin gold films on glass are illuminated with femtosecond near-IR laser pulses. A distinct maximum is observed in the emitted terahertz amplitude from films of average thickness just above the percolation threshold. Our measurements suggest that the emission is through a second-order nonlinear optical rectification process, enhanced by the excitation of localized surface plasmon hot spots on the percolated metal film.

Determination of the Optimum Resonance Length Leading to the Global Maximum Intensity of a Wave in a Resonator

A theoretical study is proposed to determine an optimum resonance length that leads to a global maximum of the wave intensity among a large number of local maxima of the intensity corresponding to local resonance lengths satisfying a resonant condition. From resonating a second-harmonic wave under the established condition of the resonant feedback and the no-resonant feedback, the intensity distributions of a resonant-harmonic wave in the second-harmonic generation and a converted wave in the cascaded dierence frequency generation are analyzed. In the second-order nonlinear processes in the resonant structure, each of the distributions shows that a global maximum intensity exists at the optimum resonance length due to the balance between the accumulation gain and the propagation loss and that the optimum resonance length shortens as the wave intensity increases in the resonator.

Broadband third-harmonic generation in two-dimensional short-range ordered nonlinear photonic structure

We study parametric frequency conversion in two-dimensional nonlinear photonic crystal with short-range ordered ferroelectric domains. We demonstrate that the short-range order enables broadband third-harmonic generation via cascading of two second-order quasi-phase matched nonlinear processes. We obtain conversion efficiencies that are much higher than those achieved from a randomized medium, owing to the unique distribution of reciprocal lattice vectors in the short-range ordered structure.

Polarization-free second-order nonlinear frequency conversion using the optical superlattice

We propose a universal and practical scheme to realize polarization-free second-order nonlinear frequency-conversion processes, including sum-frequency generation, difference-frequency generation, and optical parametric amplification. This scheme is based on the optical superlattice with a noncritical phase-matching condition, which is suitable for optical integration. A chirped dual-period structure is proposed to ensure the efficiency and stability of the conversions.

Wavelength tuning of femtosecond pulses generated in nonlinear crystals by using diffractive lenses

We demonstrate that diffractive lenses (DLs) can be used as a simple method to tune the central wavelength of femtosecond pulses generated from second-order nonlinear optical processes in birefringent crystals. The wavelength tunability is achieved by changing the relative distance between the nonlinear crystal and the DL, which acts in a focusing configuration. Besides the many practical applications of the so-generated pulses, the proposed method might be extended to other wavelength ranges by demonstrated similar effects on other nonlinear processes, such as high-order harmonic generation.