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This paper investigates fully integrated Terahertz (THz) Schottky detectors using silver (Ag) metallic nanowires (NWs) with 120 nm diameter as bridge contacts for zero-bias operating THz detectors based on highly doped Gallium Arsenide (GaAs) and Indium Gallium Arsenide (InGaAs) layers. The combination of InGaAs and metallic NWs shows improved performance at zero-bias than a GaAs based detector with a simulated capacitance of 0.5 fF and a series resistance of 29.7 $\Omega $ . Thus, the calculated maximum cut-off frequency of 2.6 THz was obtained for a NW contacted vertical InGaAs THz detector. Initial THz measurements were carried out using a common THz setup for frequencies up to 1.2 THz. A responsivity of 0.81 A/W and a low noise-equivalent power (NEP) value of 7 pW/ $ {~\sqrt {Hz}}$ at 1 THz were estimated using the measured IV-characteristics of the zero-bias NW-InGaAs based THz Schottky detector.

Optical interferometric sensors have acquired significant importance in metrology and information technology, especially in terms of their potential application in launching size, selectivity, sensitivity, resolution, spectral tuning ranges, efficiency, and cost. However, these demands are often contradictory and counteract one another, and are thus difficult to simultaneously fulfill during their interaction. This review focuses on a detailed comparison of seven different strongly miniaturized sensor concepts investigating the limits of these demands. For the visible and near-infrared spectral range, seven optical sensors were reviewed based on the following methodologies: classical optical transmission and reflection gratings, arrayed waveguide gratings, static Fabry–Pérot (FP) filter arrays, MEMS tunable FP interferometers, MEMS tunable photonic crystals, plasmonic filters, and fiber tip sensors. The comparison between the selected concepts concentrates on (i) the minimum space required for a particular spectral range, (ii) resolution, (iii) the integration in optical fiber technology, (iv) tunability to save space, (v) efficiency in using available light, (vi) multiplexing, (vii) miniaturization limits, and (viii) the potential of nanoimprint for cost reduction. Technologies for enhancing efficiency to obtain more available light and their applicability to the different methodologies were studied.

New CW THz detectors based on graphene nanoelectrodes photomixer are completely investigated. The graphene nanoelectrodes are placed on low-temperature-grown (LTG) GaAs as photoconductive material. Due to the high transparency and high conductivity of graphene, increasing carrier densities were obtained which leads to the high performance detection of THz induced photocurrent being one order of magnitude higher than for conventional metallic interdigitated photomixer. The THz measurement was performed in the range of 50 GHz to 1.1 THz.

The authors would like to acknowledge the financial support from Singapore National Research Foundation under the Program of NRF-NRFI2016-08, the Competitive Research Program NRF-CRP14-2014-03 and Singapore Ministry of Education AcRF Tier-1 grant (MOE2019-T1-002-087). H.V.D is also grateful to acknowledge additional financial support from the TUBA. Transition metal doped colloidal nanomaterials (TMDCNMs) have recently attracted attention as promising nano-emitters due to dopant-induced properties. However, despite ample investigations on the steady-state and dynamic spectroscopy of TMDCNMs, experimental understandings of their performance in stimulated emission regimes are still elusive. Here, the optical gain properties of copper-doped CdSe colloidal quantum wells (CQWs) are systemically studied with a wide range of dopant concentration for the first time. This work demonstrates that the amplified spontaneous emission (ASE) threshold in copper-doped CQWs is a competing result between the biexciton formation, which is preferred to achieve population inversion, and the hole trapping which stymies the population inversion. An optimum amount of copper dopants enables the lowest ASE threshold of approximate to 7 mu J cm(-2), about 8-fold reduction from that in undoped CQWs (approximate to 58 mu J cm(-2)) under sub-nanosecond pulse excitation. Finally, a copper-doped CQW film embedded in a vertical cavity surface-emitting laser (VCSEL) structure yields an ultralow lasing threshold of 4.1 mu J cm(-2). Exploiting optical gain from TMDCNMs may help to further boost the performance of colloidal-based lasers. National Research Foundation, Singapore NRF-NRFI2016-08 NRF-CRP14-2014-03 Ministry of Education, Singapore MOE2019-T1-002-087 Turkish Academy of Sciences European Commission

This contribution deals with the investigation and characterisation of fully integrated broadband Terahertz (THz) Schottky diodes using silver (Ag) metallic nanowires (NWs) with 120 nm diameter as bridge contacts for zero-bias operating THz detectors based on highly doped Gallium Arsenide (GaAs) and Indium Gallium Arsenide (InGaAs) layers. The combination of InGaAs and metallic NWs shows almost 7 times higher forward current than the metallic NW with GaAs at 0.27 V with a capacitance of 0.5 fF and a series resistance of $29\ \Omega$ . The highest calculated cut-off frequency of 10.7 THz was obtained for a NW contacted vertical InGaAs diode.

In this paper, we report the fabrication and characterization of a THz Gunn source based on Gallium Nitride (GaN) with side-contact and field-plate technologies. This source works stable at operating voltages up to 20 V due to good passivation layer and good heat sink to the GaN substrate. The GaN-based Gunn diode has relatively high forward current of 0.9 A and a high current drop of about 50 mA for higher frequency and output power.

Ever growing optical data transmission networks and miniaturization of spectroscopic equipment demand for small, energy efficient and mass-produced optical components. Here we present distributed Bragg reflector (DBR) mirror based widely tunable optical Fabry-Perot filters comprising a novel material combination namely, silicon carbide (SiC) and silicon dioxide (SiO 2 ) layers with refractive indices of 2.46 and 1.45, respectively. The resulting refractive index step of 1, provides high reflectivites (>99.5%) with wide stopbands of more than 300 nm [1]. Hence, large wavelength tuning range and a narrow full width at half maximum (FWHM) filtered signals can be produced. Structuring the dielectric layers by surface micromachining creates a free standing movable mirror supported by four beams. Due to inherent stress created during low-temperature plasma enhanced chemical vapor depositions (PECVD) we obtain a tunable stable plane concave resonator. In the end the cross section of a filter shows a fixed bottom DBR deposited on a silicon substrate, a tunable airgap and an electrothermally actuated top MEMS DBR with a gold electrode for heating current I MEMS . On the substrate backside where the filtered signal is exiting an anti-reflection coating is deposited.

The effect of using graphene as 2D nanoelectrode for continuous wave (CW) terahertz (THz) photomixers were investigated. The multilayer graphene (6–8 and 3–5 MLG) sheets were used as six-finger interdigital nanoelectrodes on a low- temperature-grown (LTG) GaAs photoconductor. The MLG nanoelectrode shows unique properties with high short-circuit current capabilities and reliable high photocurrents. The number of graphene layers play a big role on the electrical and the optical properties of graphene nanoelectrode. In one hand, a thinner graphene increases the transparency of the nanoelectrodes to improve the power conversion efficiency, and on the other hand, a thicker graphene is required to carry high current densities and enhance the photomixer performance.

Fluorescence spectra and optical constants in planar waveguides of spiro-quaterphenyl and two of its derivatives are investigated. Threshold of amplified spontaneous emission (ASE) and optical gain are determined. The thickness dependence of ASE spectral position, ASE threshold, and optical gain are investigated as well. Additionally, photodegradation in air and in vacuum for spiro-quaterphenyl and methoxy-spiro-quaterphenyl is compared. Films with various thicknesses (30–220 nm) have been deposited by vacuum vapor deposition. A measurement setup based on the variable stripe length method was used to determine the threshold and the optical gain using ASE. A threshold of 22±5.7 µJ/cm 2 , 51±2.1 µJ/cm 2 and 70±10 µJ/cm 2 was measured for spiro-quaterphenyl, methoxy-spiro-quaterphenyl and phenoxy-spiro-quaterphenyl respectively. At pump energy fluence of 3000 µJ/cm 2 , maximum gain of 107.5±7 cm −1 , 104±6 cm −1 and 184±1.2 cm −1 was achieved for spiro-quaterphenyl, methoxy-spiro-quaterphenyl and phenoxy-spiro-quaterphenyl respectively.

Three indicators for the determination of the saturation length limiting the validity of the variable stripe length method are presented. Depending on the maximum stripe length being used for the nonlinear fit, the resulting modal gain, its confidence interval, and the spectral full width at half maximum vary and give clear indications on the saturation length. Exemplarily, a blend of organic semiconducting spiro-compounds has been measured to present the importance of this length and demonstrate its determination. Subsequently, the optical modal gain is calculated with consideration of the saturation length. Both the saturation length and modal gain are shown to be dependent on the pump energy but result in a constant gain-length product of 2.2 ± 0.2, independent of the pump energy. Additionally, spectral gain profiles for different pump energies are presented with a maximum net modal gain of ( 128 ± 4 ) cm − 1 found at ( 2.98 ± 0.09 ) mJ / cm 2.

Deep vertical etching of a 10@mm thick SiO"2/Si multilayer stack is presented. The stack was deposited by ion beam sputtering. To etch the stack, an octafluorocyclobutane (C"4F"8)-oxygen (O"2) based inductively coupled plasma - reactive ion etching (ICP-RIE) process was developed. The recipe shows very similar etching rates for silicon dioxide (SiO"2) and silicon (Si); thus it can be implemented as a simple one step etching process. In order to protect the stack properly during the etching, a durable hard mask is required. Investigations of the deposition of such a mask, using chromium (Cr) deposited by electron beam physical vapour deposition (EBPVD), are also presented.

We present a fabrication approach for photonic crystals and similar nanophotonic structures in InP using focused ion beam milling and plasma etching. The high quality of ion milling lithography in a dielectric hardmask is combined with reactive ion etching to obtain simultaneously fast processing speeds as well as smooth and vertical sidewalls. Different hardmask materials have been investigated yielding very good results for SiO"2 and Si"xN"y. Parameters for the optimization of lithography and plasma etching are given. Finally, results of a released InP membrane with a photonic crystal structure featuring elliptical base elements are presented.

We report on the optical properties of highly doped Gallium Nitride (GaN:Si) layers in regard to plasmonic applications. Samples with different carrier concentrations were grown by PA-MBE and characterized by spectroscopic ellipsometry. Functions for the isotropic and anisotropic complex refractive indices are presented and discussed in the context of the possible excitation of plasmonic states.

In this paper we present a novel approach for a hybrid solid-state laser with alternating passive inorganic and active organic materials. This distribution of the active material promises to result in lasers with a lower threshold compared to a conventional VCSEL with a micro cavity. A passive air-gap structure is fabricated and subsequently infiltrated with the organic active material which represents the high refractive index material of the distributed Bragg reflectors. First experimental results of this fabrication method are presented and discussed.

This paper presents a narrowband optical thin-film filter based on a multiple-layer stack with layer thicknesses varying in the direction of light propagation (termed as chirping). By chirping, we distribute the cavity of a Fabry–Perot resonator over several periods, causing a wider spatial distribution of the electric field of the resonant modes. Better control of the mode distribution may improve the performance of VCSEL devices. These chirped filters are investigated by theoretical model calculations with respect to important optical design parameters such as chirp functions, chirp distribution, chirp volume, total number of periods and the refractive index contrast. Additionally, the chirped filters are fabricated and their optical characterization results are compared with the numerical investigations.

In the present work we investigate the electromagnetic properties of a photonic crystal micro-opto-electro-mechanical system. It consists of vertically arranged semiconductor membranes with air gaps forming a Fabry–Perot cavity with distributed Bragg reflectors on each side. In the top membrane, circular holes are etched introducing a photonic crystal. We investigate the optical transmission behavior of this system by detailed simulation. It includes an analysis of the three-dimensional eigenmodes and calculation of the transmittivity from the source problem. We find that in addition to the vertical Fabry–Perot cavity modes additional modes exist. The origin of these modes can be traced to resonances occurring in the top photonic crystal membrane. It is shown that the photonic crystal modes have a strong impact on the transmittivity of the filter system.

In this paper we report on narrowband optical filters with chirped thin films. The continuous variation of the layer thickness represents the cavity of a Fabry-Perot resonator that has been distributed over the device. New design parameters are accessible for tailoring filter properties and the option to obtain a more even distribution of resonant modes may improve the performance of VCSEL devices. Results of the numerical investigation of these chirped filters as well as our experimental results of their fabrication and characterization are presented and discussed.

This paper presents the dry etching characteristics of Nb2O5/SiO2 multilayer stacks deposited using ion beam sputtering. Dry etching is achieved by using a combination of CHF3 and Ar gases in an inductively coupled plasma (ICP)-reactive ion etching system. The effect of the etching parameters such as the gas ratio (CHF3/Ar), radio-frequency power, ICP power, chamber pressure, and temperature is investigated. An etching recipe is optimized to achieve a high etch rate, good selectivity, and optimum etch profile. The optimized etching recipe provides a single step etch solution for both dielectric materials. Furthermore, the deposition and structuring process of a chromium based etch mask is investigated.

Dielectric materials are of great interest for optical applications since they are transparent in the UV, visible and IR spectral range. That makes them very suitable for optical filters, polarizer, waveguides or even reflectors. When dielectrics are processed with conventional techniques based on electron or ion bombardments, they suffer from severe charging effects. For this reason, we present temporally shaped femtosecond laser pulses as a novel direct patterning method of wide band gap materials with very high precision to create photonic crystal structures in dielectrics. Material processing with temporally shaped femtosecond laser pulses overcomes the charging problems. Fabrication of structures well below the diffraction limit is feasible with temporally shaped asymmetric pulse trains due to nonlinear ionisation effects like multiphoton ionisation and avalanche ionisation. For the implementation as optical filters, a thin-film waveguide with a 2D periodic pattern of photonic crystals with circular base elements is investigated. The wave guiding layer consists of a material with a higher refractive index than the surrounding materials, in our case SiO2. Although the refractive index contrast is low, numerical design results prove that light with normal incidence to the plane of periodicity couples to waveguide modes and Fano resonances are excited. This makes the device extremely interesting as a compact narrow-band optical filter.

We present high aspect ratio Fano-resonance structures for optical filter application along with their fabrication technique, design and numerical simulation. The structuring of photonic crystal elements using temporally shaped femtosecond laser pulses is described. Performance considerations for the presented device due to angular spread of a real optical beam are discussed in addition.

We present a direct patterning method of dielectric materials via temporally shaped femtosecond laser pulses. A thinfilm waveguide with a 2D periodic pattern of photonic crystals with circular base elements is investigated. We use dielectrics since they are transparent especially in the visible spectral range, but also in UV and near infrared range. Thus, they are very suitable as optical filters in the very same spectral region. Since structuring of non-conductive dielectric materials suffers from charging, the implementation of laser processing as patterning method instead of conventional processing techniques like electron beam lithography or focused ion beams is a very attractive alternative. Despite a low refractive index contrast, we show by numerical results that normal incident of light to the plane of periodicity couples to a waveguide mode and can excite Fano resonances. That makes the device extremely interesting as narrow-band optical filter. Applications of optical filters in the visible and UV range require fabrication of photonic crystal structures in the sub-100 nm range. Temporally shaped femtosecond laser pulses are applied as a novel method for very high precision laser processing of wide band gap materials to create photonic crystal structures in dielectrics. Shaping temporally asymmetric pulse trains enable the production of structures well below the diffraction limit. 1 We combine this process with deposition of a high refractive index layer to achieve the targeted resonant waveguide structure. Additionally, we focus on the rim formation arising by laser processing since this is an important issue for fabrication of photonic crystal arrays with small lattice constants.

We present nanophotonic structures consisting of a dielectric slab waveguide with low index contrast and photonic crystals. The guided-mode resonance effect is used to implement elements with selective reflection properties. The design procedure, including numerical simulations, as well as a fabrication approach employing a focused ion beam system is described. Finally, a device layout with DBR mirrors to reduce lateral losses is discussed.

Our goal in this work is to achieve a tunable photonic crystal micro-opto-electro-mechanical system which excites two resonances with opposite polarization. In order to get a polarization selectivity we use two-dimensional photonic crystals with periodically arranged elliptical holes. We investigate the tunability and make statements about parameters that play an important role in optimization of this system. A final design is found for our goal. For the computation we use a three-dimensional finite element solver which solves the inhomogeneous Helmholtz equation.

We present 2D Photonic Crystals fabricated in an InP membrane to provide polarization selectivity to InP/Air-gap Fabry-Perot filters. The characterization setup and preliminary results of the polarization properties of the photonic crystal structure are reported.

Narrow band Fabry-Perot MEMS filters are investigated for the integration in optical fiber sensor networks. We present a technological process to implement a 2D photonic crystal (PhC) structure on an InP membrane, in order to provide an InP/air-gap Fabry-Perot filter with polarization selectivity. The capabilities and limitations of Focused Ion Beam (FIB) lithography combined with Reactive Ion Etching (RIE) are investigated to fabricate the PhC structure. Special focus is given to the etching rate of the required hard mask, utilized in the FIB lithography, to determine the optimal thickness to produce high quality structures. Additionally, the mechanical properties of the PhC membrane and the influence on the basic filter structure are investigated.

We present a tunable photodetector MEMS device consisting of an InP/air-gap Fabry-Perot filter combined with a pin-photodiode structure. Epitaxial growth and advanced surface micromachining are used for the monolithic integration of both devices. Special process steps regarding detector geometry, photo current and signal quality have been investigated to assure high performance.

InP based tunable optical MEMS devices, such as Fabry-Perot filters, VCSELs, photodiodes, consist of two distributed Bragg reflectors (DBRs) and a cavity. Tuning of the filter wavelength is achieved by electrostatic actuation of the two DBRs which are p-doped and n-doped, respectively, and reversely biased. The cavity and the DBRs consist of a stress compensated InP/airgap structure which is fabricated by sacrificial layer removal, using FeCl3 wet etching of InGaAs layers. In this work we investigated the influence of p-and n-type conductivity on the etching process. We observed that the sacrificial layer etch rate of n-InGaAs is 7 times slower than the p-InGaAs. This influences the stress in the n-DBR section of the tunable device. Based on these results novel wavelength tunable optical devices with multiple InP membrane/airgap structures will be designed.

A detailed investigation on the control of residual stresses, especially gradient stress, in InP/air-gap structures is presented. Free standing cantilevers with stress compensation by Ga0.03In0.97P layers of various thicknesses and by a variation of V/III flux ratio in growth direction were fabricated, leading to controlled stress properties. White light interferometry was used for evaluation of the gradient stress values

A new integrated lens-antenna is designed and implemented for a nanocontact based terahertz (THz) photomixer. The new design replaces the standard conventional bulky silicon lens, which normally no THz device can avoid. The Fresnel Zone Plate (FZP) is used to design the new lens-antenna and is simulated on the MIT open-source tool called Meep. The final design showed, with only two simple fabrication technology processing steps (normal optical lithography) that the lens-antenna can be implemented including the THz photomixer. The simulation and the fabrication results showed promising results towards the THz measurements, compared with the conventional bulky silicon lens.

New types of continuous-wave (CW) THz photomixers were fabricated using a silver nanowire (Ag-NW) as nanocontact on low-temperature-grown (LTG) and nitrogen-ion-implanted GaAs, respectively. The Ag-NW nanoelectrodes produced a large reduction of the device capacitance and a significant enhancement of the photocurrent as compared to conventional photomixers. The combination of very low capacitance and high photocurrent are shown reliable and efficient THz generation in the upper part of the broad spectral THz range. The main enhancement is shown from the wave patterns of surface plasmon polaritons (SPPs) on the Ag-NW which are measured using a scattering-type Scanning Near-field Optical Microscope (s-SNOM)