Compact and efficient coupler to interface hybrid dielectric-loaded plasmonic waveguide with silicon photonic slab waveguide

A coupler is proposed to interface a hybrid dielectric-loaded plasmonic waveguide (HDLPW) with a silicon photonic slab waveguide. The HDLPW is firstly designed and optimized to attain the best tradeoff between the mode confinement and the propagation distance. The designed coupler is inspired from the taper configuration and numerically modeled through finite-difference time-domain (FDTD) simulation. The results demonstrate that a high confinement and low loss of the energy is achieved from a silicon photonic slab waveguide into the dielectric slot of area 50×200 nm² in the HDLPW. The transmission attained through the coupler with a compact size of 400 nm is found to be as high as 80% (1 dB). Further, the planar nature of taper configuration makes the coupler easy to fabricate using the state-of-the-art CMOS facilities. The proposed coupler is useful in enabling the integration between photonic and hybrid plasmonic waveguides and thus realizing on-chip hybrid integrated circuits.     

Cross-shaped metal–semiconductor–metal plasmonic crystal for terahertz modulator

The transmission and tuning properties of a cross-shaped plasmonic crystal based on periodic metal–semiconductor–metal (MSM) structures have been investigated in the terahertz (THz) regime. According to the mode analysis, we find that the different resonance modes in the plasmonic crystal show the different changes when this device is actively controlled by the carrier injection of the MSM structures. The longitudinal modes disappear, while the horizontal mode moves to a higher frequency. The former leads to an intensity modulation at 0.5 THz and 1.1 THz when the groove depth h = 60 μm, and the later leads to a band blue-shift from 1.325 THz to 1.38 THz. These results will be applied to THz modulation and tunable filtering.     

A terahertz single-polarization single-mode photonic crystal fiber with a rectangular array of micro-holes in the core region

A single-polarization single-mode (SPSM) photonic crystal fiber with a rectangular array of micro-holes in the core region was designed in the terahertz frequency range near 1 THz. Based on the asymmetric arrangement of the micro-holes, birefringence between the fundamental x-polarized and y-polarized modes is introduced. A SPSM operation of the terahertz fiber can be supported due to the different mode indices of the x-polarized and y-polarized modes. The SPSM operation band is about 320 GHz with a central frequency of 1 THz. In addition, the proposed terahertz fiber also shows a good property of reduced propagation loss comparing with the dielectric absorption.     

Vertical coupling characteristics between hybrid plasmonic slot waveguide and Si waveguide

For development of complementary metal–oxide–semiconductor (CMOS)-compatible integrated optical circuits, vertical directional coupling between a hybrid plasmonic slot waveguide and a Si waveguide is theoretically investigated in detail. To determine the vertical separation gap and efficient coupling length, we investigate the characteristics of the even and odd supermodes at a wavelength of 1.55 μm. The vertical coupler transfers 90% of the power carried by the Si waveguide to the hybrid plasmonic slot waveguide after normalizing to reference waveguides when the gap is 60 nm and the coupling length is 2.6 μm. Because of the lossy hybrid guided mode in the plasmonic waveguide, the transmitted power exhibits damped sinusoidal behavior depending on the overlapping length. The proposed vertical coupler shows more efficient light coupling between a dielectric and plasmonic waveguide in comparison to the other types of hybrid coupler, and can be exploited further for on-chip integrated opto-electronic circuits.     

Design and VNA-measurement of coplanar waveguide (CPW) on benzocyclobutene (BCB) at THz frequencies

The low permittivity and the low loss tangent of the benzocyclobutene polymer (BCB) offers to coplanar waveguides (CPW) a low dispersive propagation properties at THz frequency. These transmission lines have been designed, modeled with a three dimensional (3D) solver of Maxwell equations based on finite element method (FEM) from 20 to 1000 GHz at various characteristic impedances (Z_c). Their dispersion and losses (radiation, conduction and dielectric) have been investigated separately versus the waveguide size, the nature of the substrate (dielectric or semiconductor) to optimize the THz signal propagation. Monomode CPW on BCB numerically designed for various Z_c were realized and measured with vector network analyzer (VNA). S-parameters of CPW are de-embedded by optimization of the accesses’ model. A good agreement is found between experimental and numerical results with low attenuation constants of 2.7 dB/mm and 3.5 dB/mm at 400 GHz and 500 GHz, respectively.     

Micromachined silicon lenses for terahertz applications

Silicon microlenses are a very important tool for coupling terahertz (THz) radiation into antennas and detectors in integrated circuits. They can be used in a large array structures at this frequency range reducing considerably the crosstalk between the pixels. Drops of photoresist have been deposited and their shape transferred into the silicon by means of a Reactive Ion Etching (RIE) process. Large silicon lenses with a few mm diameter (between 1.5 and 4.5 mm) and hundreds of μm height (between 50 and 350 μm) have been fabricated. The surface of such lenses has been characterized using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM), resulting in a surface roughness of about ∼3 μm, good enough for any THz application. The beam profile at the focal plane of such lenses has been measured at a wavelength of 10.6 μm using a tomographic knife-edge technique and a CO₂ laser.     

Magnetic field effects on THz quantum cascade laser: A comparative analysis of three and four quantum well based active region design

We consider the influence of additional carrier confinement, achieved by application of strong perpendicular magnetic field, on inter Landau levels electron relaxation rates and the optical gain, of two different GaAs quantum cascade laser structures operating in the terahertz spectral range. Breaking of the in-plane energy dispersion and the formation of discrete energy levels is an efficient mechanism for eventual quenching of optical phonon emission and obtaining very long electronic lifetime in the relevant laser state. We employ our detailed model for calculating the electron relaxation rates (due to interface roughness and electron–longitudinal optical phonon scattering), and solve a full set of rate equations to evaluate the carrier distribution over Landau levels. The numerical simulations are performed for three- and four-well (per period) based structures that operate at 3.9 THz and 1.9 THz, respectively, both implemented in GaAs/Al_0.15Ga_0.85As. Numerical results are presented for magnetic field values from 1.5 T up to 20 T, while the band nonparabolicity is accounted for.     

Improvement of polarization extinction in silicon waveguide devices

The SOI based waveguide devices are found to be highly polarization sensitive. Unwanted polarization excitations can be attenuated by integrating a TE- or TM-pass polarizer. A large attenuation of TM-polarized light has been observed when a thin film of metal is coated on the top of silicon rib waveguide, while TE-polarized light remains almost unaffected. The attenuation of TM-polarized light is attributed to the plasmonic absorption of the evanescent field in the metal cladding. Typically, with an Al cladding of thickness ～ 100 nm and a length of 1 mm on top of a single-mode (λ ～ 1550 nm) SOI rib waveguide structure, TE vs TM extinction ratio of ～ 15 dB has been obtained. Integrating such waveguide polarizers in a directional coupler and MZI based DWDM channel interleaver, we have also achieved an improvement in polarization extinction by ～ 15 dB.     

A waveguide high-pass filter system for measuring the spectrum of pulsed terahertz sources

We propose a system for measuring spectra of terahertz (THz) pulses, including single pulses, which is based on high-pass filters (HPFs). The system consists of channels for measuring amplitudes of pulses (initial pulses and those transmitted via HPFs with different cutoff frequencies) and an algorithm for processing of the obtained data. The pulse spectrum is restored by using the iteration method or the amplitude–frequency method. The iteration method of spectrum restoration is applicable in the range of THz pulse durations from 10⁻⁹ s to 10⁻⁷ s. The amplitude–frequency method is applicable to THz pulses with durations exceeding 10⁻⁸ s. The system for measuring of THz pulse spectra was simulated by using the characteristics of specially developed waveguide HPFs. The relative simulation error of determining the central frequency by the amplitude–frequency method is equal to 2 · 10⁻⁶ for THz pulse durations of 10⁻⁵ s and longer.     

Thickness dependence of the soft ferroelectric mode in SrTiO3 thin films deposited on MgO

We have measured the complex dielectric constants of SrTiO₃ thin films deposited on MgO substrate, by using the broadband terahertz time-domain spectroscopy. The dielectric dispersion of SrTiO₃ thin films with thickness of 1046, 460 and 55 nm has been observed in the frequency range from 0.1 to 8 THz. The dispersion mainly consists of the TO1 ferroelectric soft mode with a slight absorption of the TO2 phonon mode. From the analysis of the obtained dispersion, we found that the soft mode frequency hardens as the thickness of the film becomes thinner, and is extremely large compared with the bulk crystals. The damping of the soft mode is also larger than that of bulk SrTiO₃, which suggests the extrinsic nature of the broadening of the soft mode dispersion. In the thinnest film of 55 nm, even the shape of the dielectric dispersion changes, which may be related to integrated defects near the interface.     

Wide-field imaging using a tunable terahertz free electron laser and a thermal image plate

The appearance of intense terahertz sources such as quantum cascade laser and free electron laser opens up new opportunities for 2D imaging. Though microbolometer and pyroelectric arrays are promising recorders, they are of small size and cannot be used when wide-field imaging in the longwave region is required. We applied for terahertz imaging 3″ × 3″ and 6″ × 6″ Macken Instruments Inc. “thermal image plates”, a set of thermal sensitive phosphor screens operating in a room temperature environment. The Novosibirsk free electron laser was used as a source of radiation. We have found that the response of thermal image plate is linear until the relative quenching is less than 60% of the initial luminescence intensity. The response curve follows the Seitz–Mott law. The threshold sensitivity was found to be 100 mW/cm² at 1.5 THz and 40 mW/cm² at 2.3 THz. Interferograms, holograms, and terahertz beam spatial distributions recorded in the spectral range of 1.2–2.5 THz are given as examples.     

Identifying Radix Curcumae by using terahertz spectroscopy

The absorption spectra from 0.2 THz to 1.6 THz of four kinds of similar Chinese herbs, including huangyujin, lvyujin, guiyujin and wenyujin, have been investigated by terahertz time-domain spectroscopy (THz-TDS). Furthermore, by using support vector machines (SVM) method, the linear kernel function, the polynomial kernel function, and the radial basis kernal function are employed for separating four kinds of Radix Curcumae. The calculated results show that the accuracy of discrimination for these four kinds of Chinese herbs is 100%.     

Terahertz conductivity measurement of FeSe0.5Te0.5 and Co-doped BaFe2As2 thin films

The terahertz (THz) conductivity of FeSe_0.5Te_0.5 (‘11’-type) and Co-doped BaFe₂As₂ (‘122’-type) thin films are investigated. For ‘11’-type, the frequency dependence of the complex conductivity can be understood as that of BCS-type superconductor near the superconducting gap energy, and we estimated the superconducting gap energy to be 0.6 meV. For ‘122’-type, we estimated the superconducting gap energy to be 2.8 meV, which is considered to be the superconducting gap opened at the electron-type Fermi surface near the M point.     

Terahertz emission dependence on the intensity ratio of 400–800 nm in generating terahertz waves from two-color laser-induced gas plasma

A transient photocurrent model is used to explain terahertz emission from gas plasma irradiated by a laser pulse and the second harmonic. By introducing the second harmonic, 400 nm, the corresponding terahertz emission is greatly enhanced. The exact dependence of terahertz emission on the intensity ratio of 400–800 nm is studied for the case with total intensity of 5.00 × 10¹⁴ W/cm². Results show the emission reaches the maximum at about the case for energy distribution of I_ω = 4.00 × 10¹⁴ W/cm², I_2ω = 1.00 × 10¹⁴ W/cm².     

Cascaded continuous-wave singly resonant optical parametric oscillator pumped by a single-frequency fiber laser

We present a cascaded continuous-wave singly resonant optical parametric oscillator (SRO) delivering idler output in mid-IR and terahertz frequency range. The SRO was pumped by an ytterbium-doped fiber laser with 27 W linear polarization pump powers, and based on periodically poled MgO:LiNbO₃ crystal (PPMgLN) in two-mirror linear cavity. The PPMgLN is 50 mm long with 29.5 μm period. The idler power output at 3811 nm was obtained 2.6 W. The additional spectral components that have been attributed to cascaded optical parametric processes are described at increasing pump levels. Besides the initial signal component at about 1476.8 nm, further generated wavelengths with frequency shifts about 47 cm^?1, 94 cm^?1 and 104 cm^?1 were observed. It was speculated that the idler waves lie in the terahertz (THz) domain from the observed results.     

一种基于不平坦基底结构的混合等离子体波导

提出了一种工作于太赫兹波段的、基于半圆柱形"不平坦"InSb基底结构的混合等离子体波导.分析了半导体材料InSb的相对介电常量随工作频率的变化特性,重点研究了该混合波导以及两种不同形变结构的有效模场面积、传播长度、品质因数、能量分布等随波导工作频率、波导尺寸的变化情况.结果表明,本文提出的"不平坦"基底结构波导可以达到与传统结构几乎相同的传播长度,有效模场面积较传统结构减小了一个数量级以上,具有非常强的模场约束性,适合于太赫兹频段高密度集成电路中的应用.     

Ultrafast and low-power terahertz wave modulator based on organic photonic crystal

We theoretically investigate the modulation efficiency, response time, and pump power of a terahertz-beam intensity modulator by using an organic photonic crystal slab structure with high quality factor “defect” cavity. The basic operation of an ultrafast low-power terahertz wave modulator actuated by the dynamical shifts of the defect mode induced by pump intensity is discussed in detail. The finite-difference time-domain method is used to verify and analyze the characteristics of the terahertz wave modulator. The device exhibited extinction ratio of 47.15 dB and insertion loss of 3.2 dB at frequency of 1.062 THz with ultrafast response times on the order of several picoseconds.     

A new design of low-loss and ultra-flat zero dispersion photonic crystal fiber using hollow ring defect

New hollow ring defect structure is introduced in photonic crystal fiber design for ultra- flat zero dispersion with very low waveguide losses. The hollow ring defect consisted of a central hole surrounded by a doped silica ring provides highly flexible defect engineering capabilities in photonic crystal fibers to achieve precise control of dispersion value and dispersion slope while independently maintaining low waveguide losses, which was not attainable in previous designs. A nearly flat zero dispersion of D=0±0.51 ps/nm km was obtained in the wavelength range of 1.44–1.61 μm with the maximum slope of ?2.7×10^?2 ps/nm² km. The confinement loss was less than 5.75×10^?8 dB/m along with the bending loss of 2.8×10^?6 dB/m for the radius of 10 mm, and splice loss of less than 1.57 dB to conventional single mode fiber at 1.55 μm.     

CMOS monolithic optoelectronic integrated circuit for on-chip optical interconnection

A monolithic silicon CMOS optoelectronic integrated circuit (OEIC) is designed and fabricated using standard 0.35-μm CMOS technology. This OEIC monolithically integrates light emitting diode (LED), silicon dioxide waveguide, photodetector and receiver circuit on a single silicon chip. The silicon LED operates in reverse breakdown mode and can emit light at 8.5 V. The output optical power is 31.2 nW under 9.8 V reverse bias. The measured spectrum of LED showed two peaks at 760 nm and 810 nm, respectively. The waveguide is composed of silicon dioxide/metal multiple layers. The responsivity of the n-well/p-substrate diode photodetector is 0.42 A/W and the dark current is 7.8 pA. The LED-emitted light transmits through the waveguide and can be detected by the photodetector. Experimental results show that on-chip optical interconnects are achieved by standard CMOS technology successfully.     

Analytical method for studying terahertz vibrations in a stearic acid single crystal

The anisotropy of terahertz absorption was investigated for a B-form single crystal of stearic acid. A polarized terahertz spectrometer was used to obtain absorption spectra over the range of 1–5 THz. Seven vibrational modes were observed; four exhibited significant angular dependence. Additionally, we demonstrated a simple approach for estimating the angle of the dipole moment for each vibrational mode.