Field-induced Second Harmonic Generation Research Articles

Characterizing electric fields in semiconductor devices: effect of second-harmonic light interference.

Characterizing electric fields in semiconductor devices using electric field-induced second-harmonic generation (EFISHG) has opened new opportunities for an advanced device design. However, this new technique still has challenges due to the interference between background second-harmonic generation (SHG) and EFISHG generated light. We demonstrate that interference effects can effectively be eliminated during EFISHG measurements by focusing the laser from the transparent substrate side of a GaN PN diode, enabling straightforward quantitative electric field analysis, in contrast to PN junction interface side measurements. A model based on wave generation and propagation is proposed and highlights the incoherence between background SHG and EFISHG light. This incoherence may be attributed to the depth of focus of the incident laser and phase mismatch between incident and SHG light.

Measurement of inhomogeneous electric field based on electric field-induced second-harmonic generation

The electric field-induced second-harmonic generation (E-FISH) method can offer high spatiotemporal resolution in electric field measurement, but faces challenge in reconstructing the inhomogeneous electric field along the probing laser. This paper presents an inversion method to reconstruct the electric field profile by using E-FISH signals measured from multiple points along the probing laser. The relationship between the condition number of the inversion coefficient matrix and the parameters of E-FISH measurement system is clarified. The inversion method is validated through experimental investigations conducted in three different electrode configurations, namely the plate-to-plate gap, the rod-to-rod gap, and the rod-plate gap. With the inhomogeneity coefficient varies from 1 to 10.7, the maximum deviation between the measured electric field and the finite element simulation is less than 6%. This method does not rely on priori assumptions and electrode configurations, and exhibits good robustness in resisting the effect of measurement noise on the inversion accuracy.

Optical Setup for Investigating the Hyperpolarizability of Organic Nanostructure Materials in Solutions Using Electric Field Induced Second Harmonic Generation

Abstract: In this paper, an optical setup for electric field-induced second-harmonic generation (EFISH) at 1.9 μm experiment was arranged and used to investigate and determine the hyperpolarizability of some nanostructure organic components in two different solutions. First, a laser beam at 1.9 μm was generated by pumping a hydrogen Raman cell pressurized to 55 bar with 10 Hz Q-switched Nd: YAG nanosecond laser operating at λ = 1.064 μm. Next, the generated laser beam was aligned with all optical components within the assembly. The final step was to optimize the beam’s power and polarization at the center of the EFSH cell. Different nanostructure organic samples in solutions were prepared with nearly the same standard concentration of about 5 mmol/L to be investigated under the optimized system. Two solvents were used in this work, dichloromethane, or DCM (CH2Cl2), and chloroform, or trichloromethane (CHCl3). First, the harmonic order hyperpolarizability of five organic molecules in solutions with different chemical components such as quinolinium groups and organic boron complexes (supplied by the Chemistry Department at Catania University, Italy) were experimentally investigated experimentally. Only component (2-(2-[5′-(N,N-dimethylamino)-(2,2′-bithiophen)-5-yl]vinyl)-1-methyquinolin-1-ium iodide) in chloroform showed a significant difference in Maker fringes amplitude of the applied electrical field in comparison with fringes of its pure solvent. The value ofμfor this component has been calculated as 1320 x 10-48 esu. This value indicates that the component is a suitable candidate for use in second-harmonic generation imaging for biological applications. Keywords: Electric field-induced second-harmonic generation (EFISH), Harmonic light, Organic materials, hyperpolarizability. PACS: Nonlinear optics, 42.65.-k, Electric fields, instrumentation for measurement, 07.50.-e, Organic-inorganic hybrid nanostructures, 81.07.Pr, Organic materials optical materials, 42.70.Jk

Gapless detection of broadband terahertz pulses using a metal surface in air based on field-induced second-harmonic generation

We investigate second-harmonic generation (SHG) light from a Pt surface in atmosphere under terahertz (THz) pulses. THz pulse-modulated SHG intensity, ΔI2ω, shows a clear time profile of the THz field, which is similar to that of the conventional electro-optic sampling. The result can be explained by interference between THz field-induced second-harmonic light from air molecules in an optical path and a local oscillator from a Pt surface, whereby heterodyne detection of the THz waveform can be achieved. Using numerical calculations of a wave equation, we discuss the contribution of the Gouy phases of all the pulses, including near-infrared, SHG, and THz pulses, and identification of effective nonlinear susceptibility of the Pt surface. Our method, simply using a polished metal surface and air molecules, does not suffer from phonons or phase matching in solid-state optics and does not require any power supply, bias voltage, or fabrication process, but it offers a simple and gapless sampling method for broadband THz pulses. Here, we demonstrate the gapless detection of a broadband THz pulse in the region of 0.2–20 THz using this method.

Field-induced second-harmonic generation induced by distorted soft chiral crystal

The nonlinear optical response of soft chiral crystal system-blue-phase (BP) liquid crystals was studied experimentally using a second-harmonic-generation (SHG) microscope. With the aid of the SHG microscope (SHM), the internal coupling between the polarization and structural deformation was visualized in a short time. In this study, a fringing field, formed at the electrode edges, causes lattice deformation of the cubic BPs, which contributes to the flexoelectric-optic response and field-induced SHG at low frequencies. Using the SHM, we can observe the spatial distribution of the induced polarization in the BPs, and the mean SHG intensity of the cubic BP depends quadratically on the strength of the electric field at a lower value. As the applied electric field increases, the structure of the BPs transfers to the chiral nematic phase (N*), and then the SHG intensity remains constant. Compared to the mean intensities of the SHG signal in N* and the different BPs in the low electric field, the SHG signal caused by the lattice deformation in BPs is weaker in N* and depends on the cubic structure of the BPs. The experimental results demonstrate that through the SHM, the influence of the inhomogeneous electric field on the BPs can be exhibited clearly because the response of the SHG signal in BPs is sensitive to field-induced lattice deformation and phase transitions between the BPs and chiral nematic. This will help us elucidate the mechanism of the secondary electro-optical response in BPs and for further improvement and development of high-performance photonic devices using BPs.

Efficient photo-induced second harmonic generation in silicon nitride photonics.

Silicon photonics lacks a second-order nonlinear optical () response in general because the typical constituent materials are centro-symmetric and lack inversion symmetry, which prohibits nonlinear processes such as second harmonic generation (SHG). Here, we realize record-high SHG efficiency in silicon photonics by combining a photo-induced effective nonlinearity with resonant enhancement and perfect-phase matching. We show a conversion efficiency of (2,500 ± 100) %/W, which is 2 to 4 orders of magnitude larger than previous field-induced SHG works. In particular, our devices realize milliwatt-level SHG output powers with up to (22 ± 1) % power conversion efficiency. This demonstration is a major breakthrough in realizing efficient processes in silicon photonics, and paves the way for further integration of self-referenced frequency combs and optical frequency references.

Iris-assisted terahertz field-induced second-harmonic generation in air.

Terahertz field-induced second-harmonic generation (TFISH) is a technique for optical detection of broadband THz fields. We show that by placing an iris at the interaction volume of the THz and optical fields, the TFISH signal increases by several tenfold in atmospheric air. The iris-assisted TFISH amplification is characterized at varying air pressures and probe intensities and provides an elegant platform for studying nonlinear phase matching in the gas phase.

Optical second harmonic generation in a ferromagnetic liquid crystal.

A comparative experimental investigation of the dependence of second harmonic generation (SHG) on an applied external voltage between a standard nematic liquid crystalline material and an analogue ferromagnetic nematic liquid crystalline material was performed by using a fundamental optical beam at an 800 nm wavelength. For the ferromagnetic material, the dependence of SHG on an applied magnetic field was also examined. Three different polarization combinations of the fundamental and the second harmonic radiation were analysed. The SHG signal observed in the former material is attributed to a combination of electric field-induced SHG (EFISHG) and flexoelectric deformation-induced SHG, while the SHG signal observed in the latter material is attributed solely to flexoelectric deformation-induced SHG. The obtained dependences of the SHG signal on the associated optical retardation show that, in the most favourable polarization combination, the two contributions generate about the same effective nonlinear optical susceptibility.

Strong-field optoelectronics in solids

Perturbative optical nonlinearities induced by static electric fields1 have proven useful in visualizing dynamical function in systems including operating circuits2,3, electric and magnetic domain walls4, and biological matter5, and in controlling light for applications in silicon photonics6. Here, we extend field-induced second-harmonic generation to the non-perturbative regime. We demonstrate that static or transient fields up to terahertz (THz) frequencies applied to silicon and ZnO crystals generate even-order high harmonics. Images of the even harmonics confirm that static fields delivered with conventional electronics control the spatial properties of the high-harmonic emission. Extending our methodology to higher-harmonic photon energies7,8 paves the way for realizing active optics in the extreme ultraviolet and will allow imaging of operating electronic circuits9, of Si-photonic devices10 and of other functional materials11,12, with higher spatio-temporal resolution than perturbative methods. For THz spectroscopy, our method has the bandwidth to allow measurement of attosecond transients imprinted on THz waveforms. Near-infrared femtosecond laser pulses are sent to a Si or ZnO crystal to generate high-harmonic waves via static or transient field-induced optical nonlinearities. The beam profile of the high-harmonic emission is controlled by electronic methods.

Experimental and theoretical study of free induction decay of water molecules induced by terahertz laser pulses

We experimentally investigate the free induction decay in the asymmetric top molecule ${\mathrm{H}}_{2}\mathrm{O}$ after interaction with short terahertz pulses. Two different experimental techniques are used to perform measurements in the gas phase under different experimental conditions: electro-optical detection and terahertz field-induced second-harmonic generation. Simulations of the free induction decay signals are performed with a theoretical approach in which the absorption and dispersion coefficients are evaluated from spectroscopic data pertaining to the water molecule. Experimental and theoretical signals are compared directly in the time domain in two limiting cases. For a short propagation length of $\ensuremath{\sim}40$ cm, both signals show a slightly altered electric field consistent with the approximation of weak absorption and dispersion. For a longer propagation length of $\ensuremath{\sim}300$ cm, the electric field is heavily distorted. A good agreement between experimental and calculated signals is obtained in both cases. Finally, we discuss the advantages and/or disadvantages of the two experimental techniques.

Nonlinear-optical study of magnetoelectric interactions in multilayer structures

ABSTRACTTemperature dependences of electric field-induced and magnetic field-induced optical second harmonic generation (SHG) were used to study La0.7Ca0.25MnO3/BaTiO3 (LCMO/BTO) superlattice. Ferroic properties of the superlattice are reveal by magnetic field-induced and electric field-induced SHG hysteresis loops (“magnetic” and “electric” SHG loops). However, direct modifications of the SHG hysteresis loops by the other field (electric field for the “magnetic loop” and vice versa) were not found. Strong modifications of the “electric” SHG hysteresis loops occur at temperatures close to the paramagnetic-ferromagnetic and the insulator-metal phase transitions in LCMO. In this way, the magnetoelectric interaction in an LCMO/BTO superlattice was established indirectly.

(Invited) Non-Destructive Characterization of Dielectric - Semiconductor Interfaces by Second Harmonic Generation

One of the key tasks in terms of characterization of microelectronic devices is the non-destructive measurement of electrical properties of the interfaces between dielectric materials and semiconductors. For example, ITRS 2013 [1] mentions that the measurement techniques of local interfacial electrical properties are among the difficult challenges in terms of metrology. This is not only critical in microelectronics, but also in other fields: for example in photovoltaics, the electrical quality of the passivation layer of the semiconductor is crucial in order to obtain high yield solar cells. In this context, a characterization tool based on the second harmonic generation (SHG) [2], answers positively to most of the metrology requirements: it is contactless, non-destructive, sensitive and can provide a high spatial resolution down to micrometer size. The working principle of the method is to illuminate a sample with a high intensity laser beam of wavelength λ and to measure the generated second harmonic signal (at wavelength λ/2). If an internal electrical field is present in the material, the SHG signal is partially given by the electrical field-induced SHG (EFISH) [3]. For centrosymmetric media (as Si, SiO2 etc.), the interest of the SHG lays in its extremely high sensitivity to the dielectric-semiconductor interface, since the electric field in such a stack of materials relates to the defects in the oxide and at the interface itself. For instance the SHG measurements proved to be interesting to detect radiation induced defects in the buried oxide of silicon-on-insulator wafers [4], or metal contamination on such wafers [5]. The presence of traps induced at the interface Si/SiO2 due to boron were also probed by SHG [6]. Other studies show charging dynamics at Al2O3 –Si interfaces measured by SHG [7]. We show our recent results on the characterization of dielectric materials deposited (or grown) on silicon. The SHG measurement tool used is the new inspection system Harmonic F1x from FemtoMetrix [8]. A pulsed femtosecond laser of 780nm wavelength is shined upon the sample. The second harmonic generated beam from the sample is detected at 390nm by a photomultiplier, coupled with a photon counter. We have used this equipment on various dielectric materials deposited (or grown) on silicon wafers. Figure 1a shows an example of the SHG evolution in time obtained on Al2O3 thin films grown by Atomic Layer deposition on Si substrates, as-deposited and after an annealing step. The annealing is known to increase the density of negative fixed charges in the alumina layer. The electric field at the interface between the alumina layer and the Si is reinforced and gives a higher SHG signal after annealing. Additionally, we can map samples in order to locally estimate spatial variations of the SHG signal (see an example in Figure 1b: Al2O3 on Si (100) after annealing). Different materials, as well as thermal processes will be discussed and compared in the full paper. The SHG results will be validated by comparison with other well-known techniques, such as the photoconductance decay for lifetime measurements. [1] http://www.itrs.net/Links/2013ITRS/Home2013.htm. (2013). [2] J. J. H. Gielis, P. M. Gevers, I. M. P. Aarts, M. C. M. van de Sanden, and W. M. M. Kessels, "Optical second-harmonic generation in thin film systems," Journal of Vacuum Science & Technology A, vol. 26, pp. 1519-1537, 2008. [3] P. N. Butcher and D. Cotter, The Elements of Nonlinear Optics: Cambridge University Press, 1991. [4] B. Jun, R. D. Schrimpf, D. M. Fleetwood, Y. V. White, R. Pasternak, S. N. Rashkeev, et al., "Charge trapping in irradiated SOI wafers measured by second harmonic generation," IEEE Transactions on Nuclear Science, vol. 51, pp. 3231-3237, 2004. [5] M. L. Alles, R. Pasternak, X. Lu, N. H. Tolk, R. D. Schrimpf, D. M. Fleetwood, et al., "Second Harmonic Generation for Noninvasive Metrology of Silicon-on-Insulator Wafers," Semiconductor Manufacturing, IEEE Transactions on, vol. 20, pp. 107-113, 2007. [6] H. Park, J. Qi, Y. Xu, K. Varga, S. M. Weiss, B. R. Rogers, et al., "Boron induced charge traps near the interface of Si/SiO2 probed by second harmonic generation," Physica Status Solidi (b), vol. 247, pp. 1997-2001, 2010. [7] J. J. H. Gielis, B. Hoex, M. C. M. van de Sanden, and W. M. M. Kessels, "Negative charge and charging dynamics in Al2O3 films on Si characterized by second-harmonic generation," Journal of Applied Physics, vol. 104, p. 073701, 2008. [8] http://femtometrix.com. Figure 1

Current collapse imaging of Schottky gate AlGaN/GaN high electron mobility transistors by electric field-induced optical second-harmonic generation measurement

Two-dimensional current collapse imaging of a Schottky gate AlGaN/GaN high electron mobility transistor device was achieved by optical electric field-induced second-harmonic generation (EFISHG) measurements. EFISHG measurements can detect the electric field produced by carriers trapped in the on-state of the device, which leads to current collapse. Immediately after (e.g., 1, 100, or 800 μs) the completion of drain-stress voltage (200 V) in the off-state, the second-harmonic (SH) signals appeared within 2 μm from the gate edge on the drain electrode. The SH signal intensity became weak with time, which suggests that the trapped carriers are emitted from the trap sites. The SH signal location supports the well-known virtual gate model for current collapse.

Optical second-harmonic generation induced by electric current in graphene on Si and SiC substrates

We find that the flow of direct electric current (dc) through graphene on substrate enhances surface optical second-harmonic generation (SHG) from the graphene/substrate system. The current can enhance surface SHG by about 300% for a chemical-vapor-deposition (CVD) graphene monolayer on a SiO${}_{2}/\mathrm{Si}(001)$ substrate, and by about 25% for an epitaxial four-layer-graphene film on a 3.5\ifmmode^\circ\else\textdegree\fi{}-miscut vicinal SiC(0001) substrate. The enhancement in both the CVD and epitaxial graphene samples is due to electric field-induced SHG, which is produced by the current-associated vertical electric field at the SiO${}_{2}/$Si interface or at the graphene/SiC interface. Measurements of rotational-anisotropy SHG (RA-SH) from both samples revealed that the current-induced SHG varies strongly with the measurement location along the current flow direction. By measuring RA-SH from the vicinal SiC(0001) substrate, we determined all three second-order susceptibility tensor elements (${d}_{33}$ = \ensuremath{-}52.0 pm/V, ${d}_{15}$ = 20.0 pm/V, and ${d}_{31}$ = 18.7 pm/V) that characterize the SHG response of hexagonal SiC at the fundamental wavelength of 740 nm. We further determined the three effective susceptibility tensor elements (${d}_{33}$ = \ensuremath{-}135.8 pm/V, ${d}_{15}$ = 18.5 pm/V, and ${d}_{31}$ = 14.6 pm/V) that characterize the surface SHG from the graphene/vicinal-SiC(0001) sample and finally showed that the current-dependent tensor element ${d}_{33}$ can be enhanced to a large value of ${d}_{33}$ = \ensuremath{-}199.0 pm/V by electric current in epitaxial graphene.

Enhanced Optical Second-Harmonic Generation from the Current-Biased Graphene/SiO2/Si(001) Structure

We find that optical second-harmonic generation (SHG) in reflection from a chemical-vapor-deposition graphene monolayer transferred onto a SiO2/Si(001) substrate is enhanced about 3 times by the flow of direct current electric current in graphene. Measurements of rotational-anisotropy SHG revealed that the current-induced SHG from the current-biased graphene/SiO2/Si(001) structure undergoes a phase inversion as the measurement location on graphene is shifted laterally along the current flow direction. The enhancement is due to current-associated charge trapping at the graphene/SiO2 interface, which introduces a vertical electric field across the SiO2/Si interface that produces electric field-induced SHG. The phase inversion is due to the positive-to-negative polarity switch in the current direction of the trapped charges at the current-biased graphene/SiO2 interface.

Phase characterization in broadband THz wave detection through field-induced second harmonic generation

We present a theoretical and experimental investigation of the THz pulse phase measured by a broadband heterodyne detection method via field-induced second-harmonic generation in ambient air. The dependence of the detected THz phase spectra on the positions of the wire shaped electrodes scanning along the detection plasma is discussed. An additional phase shift around the beam focus is observed. Theoretical deductions reveal that it is caused by the Gouy shift of the optical probe beam and THz beam during the heterodyne detection process.

Charge trapping defects in Si/SiO2/Hf(1−x)SixO2 film stacks characterized by spectroscopic second-harmonic generation

Hf-based dielectric films are susceptible to formation of charge trapping, performance-limiting point defects, making improved characterization of these defects a high priority of research on high-k dielectrics. Here we report noninvasive optical characterization of defect spectra and charge trapping kinetics in Si/SiO2/Hf1−xSixO2 film stacks using internal multiphoton photoemission and time-dependent electrostatic field-induced second-harmonic (TD-EFISH) generation. Oxygen vacancy defects unique to as-deposited HfO2 films are identified by resonant two-photon ionization of the 3.24 eV transition from their occupied mid-gap ground states to conduction-band-edge excited states and subsequent tunneling of the photoelectrons to the Si substrate. These defects are found to be located within the HfO2 bulk, and to be quenched upon annealing or silication of the high-k dielectric layer. Charge trapping in Hf-silicate samples is found to be dominated by traps at the oxide surface. Trapping rate is nearly independent of excitation wavelength and alloy composition. By contrast, trap lifetimes are found to be as much as two orders of magnitude shorter in Hf-silicate surfaces than at HfO2 surfaces.

Correlation between second-order optical response and structure in thermally poled sodium niobium-germanate glass

We have carried out Raman and second harmonic generation (SHG) measurements to probe thermal poling-induced phenomena in glass 20Na2O-80[0.35Nb2O5-0.65GeO2]. A SHG response of 0.6 pm/V was measured after poling in a ∼3 μm thick layer under the anode and found to deviate from the widely used electric field-induced SHG model. This effect was associated with complex structural rearrangements in the subanode layer involving destruction of nonbridging oxygen atoms, formation of molecular oxygen, and enhancement of cross-linking in the glass network.

Hot carrier injection from nanometer-thick silicon-on-insulator films measured by optical second-harmonic generation

Time-dependent electrostatic field-induced second-harmonic (TD-EFISH) generation is used to probe optically excited hot carrier injection (HCI) from silicon-on-insulator (SOI) films as thin as 2 nm into both native oxide and buried oxide (BOX), without device fabrication. The two HCI processes induce TD-EFISH signals of opposite sign, at different rates, whereby they are distinguished straightforwardly. HCI at the SOI/BOX interface is dominated by two-photon injection into HF defect induced traps created during SOI thinning. The results demonstrate that SHG can noninvasively and quantitatively characterize HCI, a key determinant of SOI device reliability.

Rattling the cage: Micro- to mesoscopic structure in liquids as simple as argon and as complicated as water

The water molecule has the convenient property that its molecular polarizability tensor is nearly isotropic while its dipole moment is large. As a result, the low-frequency anisotropic Raman spectrum of liquid water is mostly collision-induced and therefore reports primarily translational motions while the far-infrared (terahertz) and dielectric spectrum is dominated by rotational modes. Atomic and globular–molecular liquids have a zero dipole moment as well as an isotropic polarizability tensor. These spectrum-simplifying properties were exploited in a study of a number of liquids and solutions using ultrafast optical Kerr-effect (OKE) spectroscopy combined with dielectric relaxation spectroscopy (DRS), terahertz time-domain spectroscopy (THz-TDS), and terahertz field-induced second-harmonic generation (TFISH) spectroscopy. For room-temperature ionic liquids (RTILs), liquid water, aqueous salt solutions, noble gas liquids, and globular–molecular liquids it was found that, in each case, surprising structure and/or inhomogeneity is observed, ranging from mesoscopic clustering in RTILs to stretched-exponential dynamics in the noble gas liquids. For aqueous electrolyte solutions it is shown that the viscosity, normally described by the Jones–Dole expression, can be explained in terms of a jamming transition, a concept borrowed from soft condensed matter studies of glass transitions in colloidal suspensions.