Strong Second-harmonic Signal Research Articles

Gallium Phosphide Nanowires in a Free-Standing, Flexible, and Semitransparent Membrane for Large-Scale Infrared-to-Visible Light Conversion.

Engineering of nonlinear optical response in nanostructures is one of the key topics in nanophotonics, as it allows for broad frequency conversion at the nanoscale. Nevertheless, the application of the developed designs is limited by either high cost of their manufacturing or low conversion efficiencies. This paper reports on the efficient second-harmonic generation in a free-standing GaP nanowire array encapsulated in a polymer membrane. Light coupling with optical resonances and field confinement in the nanowires together with high nonlinearity of GaP material yield a strong second-harmonic signal and efficient near-infrared (800-1200 nm) to visible upconversion. The fabricated nanowire-based membranes demonstrate high flexibility and semitransparency for the incident infrared radiation, allowing utilizing them for infrared imaging, which can be easily integrated into different optical schemes without disturbing the visualized beam.

Infrared laser locking to a rubidium saturated absorption spectrum via a photonic chip frequency doubler.

To extend the coherence of quantum transitions for laser locking, as well as increase the compactness and stability of the experimental setup, we propose to utilize photonic integrated resonators with high second-harmonic (SH) generation efficiencies as reliable frequency doublers that link the desired frequencies with the frequency references. In this Letter, a sufficiently strong SH signal up to microwatts was generated by a photonic integrated frequency doubler using a milliwatt infrared (IR) laser source. Furthermore, an increased SH generation bandwidth covering Rb85 and Rb87D2 transition lines, as well as saturated absorption spectroscopy, was demonstrated by tuning the pump power and chip temperature. Here we present, to the best of our knowledge, the first successful locking of an IR laser to Rb saturated absorption lines via a photonic chip frequency doubler.

Plasmonic enhancement of second-harmonic generation of dielectric layer embedded in metal-dielectric-metal structure

The enhancement of second-harmonic generation from a dielectric layer embedded in a metal-dielectric-metal structure upon excitation of surface plasmon polaritons is demonstrated experimentally. The metal-dielectric-metal structure consisting of a Gex(SiO2)1–x layer sandwiched by two Ag layers was prepared, and the surface plasmon polaritons were excited in an attenuated total reflection geometry. The measured attenuated total reflection spectra exhibited two reflection dips corresponding to the excitation of two different surface plasmon polariton modes. Strong second-harmonic signals were observed under the excitation of these surface plasmon polariton modes. The results demonstrate that the second-harmonic intensity of the Gex(SiO2)1–x layer is highly enhanced relative to that of the single layer deposited on a substrate. Under the excitation of one of the two surface plasmon polariton modes, the estimated enhancement factor falls in a range between 39.9 and 171, while under the excitation of the other surface plasmon polariton mode, it falls in a range between 3.96 and 84.6.

Second-harmonic generation of ZnO nanoparticles synthesized by laser ablation of solids in liquids

We report the synthesis of small zinc oxide nanoparticles (ZnO NPs) based colloidal suspensions and the study of second-harmonic generation from aggregated ZnO NPs deposited on glass substrates. The colloidal suspensions were obtained using the laser ablation of solids in liquids technique, ablating a Zn solid target immersed in acetone as the liquid medium, with ns-laser pulses (1064nm) of a Nd-YAG laser. The per pulse laser fluence, the laser repetition rate frequency and the ablation time were kept constant. The absorption evolution of the obtained suspensions was optically characterized through absorption spectroscopy until stabilization. Raman spectroscopy, SEM and HRTEM were used to provide evidence of the ZnO NPs structure. HRTEM results showed that 5–8nm spheroids ZnO NPs were obtained. Strong second-harmonic signal is obtained from random ZnO monocrystalline NPs and from aggregated ZnO NPs, suggesting that the high efficiency of the nonlinear process may not depend on the NPs size or aggregation state.

Three-dimensional current collapse imaging of AlGaN/GaN high electron mobility transistors by electric field-induced optical second-harmonic generation

Three-dimensional (3D) current collapse imaging of Schottky gate AlGaN/GaN high electron mobility transistor devices was achieved by a combination of two-dimensional (2D) and depth directional electric field-induced optical second-harmonic generation (EFISHG) measurements. EFISHG can detect the electric field produced by trapped carriers, which causes the current collapse. In the 2D measurement, the strong second-harmonic (SH) signals appeared within 1 μm from the gate edge on the drain side at 0.8 μs after the transition from the off- to no bias- state in both unpassivated and passivated samples. In the depth measurement, the SH signals were generated mainly from the AlGaN surface region of the unpassivated sample due to the presence of high-density trap sites in the AlGaN layer, and SH signals from bulk GaN region were also detected at 50 μs after the transition from the off- to no bias- state in the passivated sample. The origin of the traps is presumably the nitrogen vacancies in the GaN buffer layer.

Nonlinear Plasmonic Sensing with Nanographene.

Plasmons provide excellent sensitivity to detect analyte molecules through their strong interaction with the dielectric environment. Plasmonic sensors based on noble metals are, however, limited by the spectral broadening of these excitations. Here we identify a new mechanism that reveals the presence of individual molecules through the radical changes that they produce in the plasmons of graphene nanoislands. An elementary charge or a weak permanent dipole carried by the molecule are shown to be sufficient to trigger observable modifications in the linear absorption spectra and the nonlinear response of the nanoislands. In particular, a strong second-harmonic signal, forbidden by symmetry in the unexposed graphene nanostructure, emerges due to a redistribution of conduction electrons produced by interaction with the molecule. These results pave the way toward ultrasensitive nonlinear detection of dipolar molecules and molecular radicals that is made possible by the extraordinary optoelectronic properties of graphene.

Degradation analysis and current collapse imaging of AlGaN/GaN HEMTs by measurement of electric field-induced optical second-harmonic generation

On the basis of electric field-induced optical second-harmonic generation (EFISHG) measurements, the degradation of AlGaN/GaN high electron mobility transistor (HEMT) devices was analyzed and the current collapse phenomenon was imaged. Strong second-harmonic (SH) signals were observed at the gate edge at the site of a small defect during the application of a high drain stress voltage (300V), thus indicating that this technique can identify degradation of HEMTs. The EFISHG technique also generated images of the trapped carriers that lead to current collapse in these devices during the on-state. Immediately following the completion of drain-stress voltage application in the off-state (that is, 1μs), SH signals were observed at the gate edge on the drain electrode. This signal location is in good agreement with the well-known virtual gate model of current collapse. The EFISHG measurement technique is therefore capable of detecting pre-breakdown phenomena and producing images of current collapse in AlGaN/GaN HEMT devices.

Second-order optical nonlinearities from χ(2) gratings induced by holographic all-optical poling

The paper presents the second-order optical nonlinearities from χ (2) gratings induced by holographic all-optical poling for azobenzene polymer. Second harmonic (SH) signal along the directions with two different vectors was measured. One is strong SH signal diffracted in the same direction as 2ω writing beam with wave vector k 2ω and the other is weak SH signal diffracted in the direction of wave vector of 4 k ω – k 2ω + Δ k where k ω is wave vector of ω beam and Δk is the wave vector mismatch whose vector is parallel to k ω. The latter signal was used as a tool to monitor the formation of holographic χ (2) gratings in real-time because it has off-axis wave vector different from both k ω and k 2ω. The increase of 2ω intensity on poling process led to the large increase of second-order optical nonlinearity. The real-time monitoring showed that it also gave the large relaxation of second-order optical nonlinearity on poling process. The increase of 2ω (532 nm) energy enhanced the increase of local heating, which led to easier alignment of azobenzene chromophore and also larger relaxation of aligned chromophore.

Second-harmonic generation from complementary split-ring resonators

We present experiments on second-harmonic generation from arrays of magnetic split-ring resonators and arrays of complementary split-ring resonators. In both cases, the fundamental resonance is excited by the incident femtosecond laser pulses under normal incidence, leading to comparably strong second-harmonic signals. These findings are discussed in terms of Babinet's principle and in terms of a recently developed microscopic classical theory that leads to good agreement regarding the relative and the absolute nonlinear signal strengths. The hydrodynamic convective contribution is found to be the dominant source of second-harmonic generation--in contrast to a previous assignment [Science 313, 502 (2006)].

Second harmonic generation from patterned GaAs inside a subwavelength metallic hole array

By extending GaAs dielectric posts with a large second-order nonlinear susceptibility through the holes of a subwavelength metallic hole array coupled to the metal surface-plasma wave, strong second harmonic (SH) signal is observed. The SH signal is strengthened as a result of the enhanced electromagnetic fields inside the hole apertures.

Simulation study of second-harmonic microscopic imaging signals through tissue-like turbid media

We establish, for the first time, a simulation model for dealing with the second-harmonic signals under a microscope through a tissue-like turbid medium, based on the Monte Carlo method. With this model, the angle-resolved distribution and the signal level eta of second-harmonic light through a slab of the turbid medium are demonstrated and the effects of the thickness (d) of the turbid medium, the numerical aperture (NA) of the objective as well as the size (rho) of the scatterers forming the turbid medium are explored. Simulation results reveal that the use of a small objective NA results in a narrow angle distribution but strong second-harmonic signals. A turbid medium consisting of large scattering particles has a strong influence on the angle distribution and the signal level eta, which results in a low penetration limit for second-harmonic signals made up of ballistic photons. It is approximately 30 microm in our situation.

Optical second harmonic imaging of zinc oxide thin films grown by metal organic chemical vapour deposition (MOCVD)

We demonstrate optical second harmonic (SH) imaging using femtosecond laser pulses (80 fs, 1.59 eV, 80 MHz) as a suitable probe to spatially investigate the structural homogeneity of zinc oxide (ZnO) thin films grown by metal organic chemical vapour deposition (MOCVD). We find that, despite obvious differences in crystalline structure as well as lattice mismatch, ZnO films grown on the three different substrates Si(100), GaAs(100) and amorphous glass under equal growth conditions show a very similar crystalline structure, which appears to be of polycrystalline nature. Our study on films of different thickness reveals that initially micro-crystallites are deposited randomly on the substrate and possibly act as seeds for the further growth process. Furthermore, SH images of ZnO films grown on GaAs show sample regions (20–50 μm in size), where the SH response vanishes, indicating that the strong SH signal generated in the underlying substrate material is suppressed. We attribute these regions to presumably amorphous particles deposited during the growth process, which have light absorbing properties and suppress the SH generation process. Hence SH imaging is an applicable method to obtain detailed information about some aspects of the film deposition process. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spontaneous Ordering of Nonlinear Optical Groups in a Polymer Thin Film Through Cross-Linking

Polymer thin films with nonlinear optical groups were prepared by spin-coating poly-DR1-(anthranilic acid) on glass substrates. These films were then heated to different temperatures and cooled down to obtain samples with various degrees of cross-linking. The visible absorption decreased for samples annealed at high temperatures. Strong second-harmonic signal generated from the polymer film indicated the nonlinear side chains of the polymer were aligned macroscopically through the heat treatment without external field.

Second-harmonic generation from high-lying P-states of potassium

A strong second-harmonic signal has been measured by two-photon excitation of opposite parity states 4s2S → np2P0 of potassium (n=9 to 34) using a tunable dye laser. Polarization effects have been investigated. Comparison is made with the predictions of the ionization and collisional theoretical models.

Determining metal-semiconductor interface structure by optical second-harmonic generation

Conventional surface science techniques provide little information about the structure of the buried metal-semiconductor interface, where many layers of metal have been deposited on the semiconductor surface. The author shows that optical second-harmonic generation can be used successfully to probe this structure for metal-silicon systems. Second-harmonic studies of Si(111)-Au and Si(100)-Au have been carried out in a conventional ultra-high vacuum system equipped with low-energy electron diffraction and Auger electron spectroscopy. A strong second-harmonic signal is obtained which is shown to originate at the buried interface and to persist through at least 30 AA of deposited gold.