Hyper-hemispherical Silicon Lens Research Articles

Terahertz quantum cascade laser array with spatially-separated beams

A lens-coupled terahertz (THz) quantum cascade laser (QCL) array with multiple spatially-separated beams is presented. The beam divergence and focusing efficiency after lens are both investigated by simulations and experiments. By integrating an appropriate hyper-hemispherical silicon lens with a well-designed six-element distributed feedback (DFB) QCL array inside the cryostat window, excellent emission characteristics of small divergence angles from 4.1°×3.5° to 7.2°×5.8° and multi-wavelength spectra are realized at 77 K. The deflection angle between adjacent beams is ~ 10°. The output intensity after lens is 113 to 25 times that of original beams with a focusing efficiency ranging from ~ 58% to ~ 37% for lasers at different positions. This method provides a simple solution to form a multi-beam THz source with high intensity and low far-field divergence.

Radiation pattern of planar optoelectronic antennas for broadband continuous-wave terahertz emission.

In future wireless communication networks at terahertz frequencies, the directivity and the beam profile of the emitters are highly relevant since no additional beam forming optics can be placed in free-space between the emitter and receiver. We investigated the radiation pattern and the polarization of broadband continuous-wave (cw) terahertz emitters experimentally and by numerical simulations between 100 GHz and 500 GHz. The emitters are indium phosphide (InP) photodiodes with attached planar antenna, mounted on a hyper-hemispherical silicon lens and integrated into a fiber-pigtailed module. As both packaging and material of the emitter was identical for all devices, similarities and differences can be directly linked to the antenna structure. We found that the feeding point structure that connects photodiode and antenna has a large influence on the radiation pattern. By optimizing the feeding point, we could reduce side lobes from -2 dB to -13 dB and narrow the 6dB beam angle from ±14° to ±9° at 300 GHz.

Vivaldi End-Fire Antenna for THz Photomixers

We propose a broadband end-fire antenna for continuous-wave terahertz (THz) photomixing–based devices working in the frequency range of 0.5–1 THz. A compact Vivaldi antenna is presented that does not require any hyper-hemispherical silicon lens to collect and collimate THz radiation unlike the conventionally used broadside antennas. The antenna is tailored to radiate THz into or receive radiation from a dielectric waveguide placed in close vicinity of it. The antenna is fabricated on an indium phosphide (InP) substrate. A silicon (Si) superstrate is used to improve the directionality of the radiated beam. THz power coupled into Si waveguides is measured using two different techniques between 0.1 and 1.15 THz. Firstly, the waveguide is placed in the optical path of a 1550 nm based continuous-wave THz setup with a commercial broadside emitter, focusing optics, and a detector fabricated on the InP substrate with log-periodic broadside antenna. Secondly, the waveguide is placed in direct contact with the designed Vivaldi antenna–based THz receiver and using the commercial broadside emitter as a source. It is observed that the direct coupling technique using the Vivaldi end-fire antenna outperforms the optically coupled approach at frequencies higher than 668 GHz. Efficient THz photoconductive sources and receivers based on the designed compact Vivaldi end-fire antenna will be suitable for launching THz power into on-chip THz circuitry and for compact THz systems.

Experimental Comparison of UTC- and PIN-Photodiodes for Continuous-Wave Terahertz Generation

We carried out an experimental comparison study of the two most established optoelectronic emitters for continuous-wave (cw) terahertz generation: a uni-traveling-carrier photodiode (UTC-PD) and a pin-photodiode (PIN-PD). Both diodes are commercially available and feature a similar package (fiber-pigtailed housings with a hyper-hemispherical silicon lens). We measured the terahertz output as a function of optical illumination power and bias voltage from 50 GHz up to 1 THz, using a precisely calibrated terahertz power detector. We found that both emitters were comparable in their spectral power under the operating conditions specified by the manufacturers. While the PIN-PD turned out to be more robust against varying operating parameters, the UTC-PD showed no saturation of the emitted terahertz power even for 50 mW optical input power. In addition, we compared the terahertz transmission and infrared (IR) blocking ratio of four different filter materials. These filters are a prerequisite for correct measurements of the absolute terahertz power with thermal detectors.

Compact and sensitive heterodyne receiver at 2.7 THz exploiting a quasi-optical HEB-QCL coupling scheme

We demonstrate a sensitive and compact terahertz heterodyne detection system based on a quantum cascade laser (QCL) as a local oscillator and a hot electron bolometer (HEB) as a mixer. It relies on an original optical coupling scheme where the terahertz (THz) signal to be detected and the local oscillator (LO) signal are coupled to the HEB from both sides of the integrated lens/antenna mixer. The THz signal of interest impinges on the front side through the silicon lens while the LO onto the rear (air) side. This concept allows us to remove the beam splitter usually employed in terahertz heterodyne receivers. The mixer consists of a Niobium Nitride HEB with a log-spiral planar antenna mounted on the flat side of a hyperhemispherical silicon lens. The local oscillator of the heterodyne detector is a low power consumption and low beam divergence 3rd-order distributed feedback laser with single mode emission at the target frequency of 2.7 THz. The coupling between the QCL and the HEB has been further optimized, using a dielectric hollow waveguide that reliably increases the laser beam directivity and permits us to pump the HEB into its most sensitive state through the air side of the planar antenna. We have measured a noncorrected double sideband receiver noise temperature of 880 K at 2.7 THz.

Waveguide coupler for resonant‐tunnelling diode oscillator at 420 GHz

The authors proposed a waveguide coupler that extracts the radiation from a resonant-tunnelling diode (RTD) oscillator at 420 GHz as the fundamental mode of a rectangular waveguide compatible to the WR-2.2 standard. In the proposed structure, an RTD oscillator chip is embedded inside a hyper hemispherical silicon lens to reshape the radiation from the tiny chip into a collimated beam, which is subsequently converted into the waveguide mode using a linear horn. The authors estimate the transmission efficiency of the coupler to be -3 dB. Integrated in one package, this approach enables handy use of RTD oscillators as a platform for various terahertz applications.

Broadband Terahertz Computed Tomography Using a 5k-pixel Real-time THz Camera

We present a novel THz computed tomography system that enables fast 3-dimensional imaging and spectroscopy in the 0.6-1.2 THz band. The system is based on a new real-time broadband THz camera that enables rapid acquisition of multiple cross-sectional images required in computed tomography. Tomographic reconstruction is achieved using digital images from the densely-packed large-format (80×64) focal plane array sensor located behind a hyper-hemispherical silicon lens. Each pixel of the sensor array consists of an 85 μm × 92 μm lithographically fabricated wideband dual-slot antenna, monolithically integrated with an ultra-fast diode tuned to operate in the 0.6-1.2 THz regime. Concurrently, optimum impedance matching was implemented for maximum pixel sensitivity, enabling 5 frames-per-second image acquisition speed. As such, the THz computed tomography system generates diffraction-limited resolution cross-section images as well as the three-dimensional models of various opaque and partially transparent objects. As an example, an over-the-counter vitamin supplement pill is imaged and its material composition is reconstructed. The new THz camera enables, for the first time, a practical application of THz computed tomography for non-destructive evaluation and biomedical imaging.

Hollow metallic waveguides integrated with terahertz quantum cascade lasers.

We present the realization of a compact, monolithically integrated arrangement of terahertz quantum cascade lasers with hollow metallic cylindrical waveguides. By directly mounting a copper pipe to the end facet of a double metal waveguide, it was possible to significantly improve the far field emission from such a sub-wavelength plasmonic mode, while preserving the characteristic performance of the laser. Careful alignment of the quantum cascade laser and the hollow waveguide is required in order to prevent the excitation of higher order/mixed modes as predicted with a high degree of accuracy by a theoretical model. Finally, this approach proved to be a superior method of beam shaping when compared to other in situ arrangements, such as a silicon hyper-hemispherical lens glued to the facet, which are presented.

A Terahertz Detector Array in a SiGe HBT Technology

This paper presents HBT terahertz power detectors implemented in an experimental 0.25- μm SiGe process technology with a peak fT/fmax of 280/435 GHz. Based on the nonlinearity of the HBT base-emitter junction, the detector operates as a square-law down converter, mixing terahertz frequencies directly to dc. Fifteen detectors have been arranged in a 3 × 5-pixel array with differential on-chip ring antennas. The array has been assembled with a hyper-hemispherical silicon lens. Referred to the collecting aperture of the lens, a maximum optical current responsivity RI of 1 A/W and a minimum noise equivalent power (NEP) of about 50 pW/√{Hz} have been measured at 0.7 THz with a 125-kHz chopping frequency. The detectors have been characterized from 0.65 to 1 THz.

A 288-GHz Lens-Integrated Balanced Triple-Push Source in a 65-nm CMOS Technology

A 288-GHz lens-integrated high-power source implemented in a 65-nm CMOS technology is presented. The source consists of two free-running triple-push ring oscillators locked out-of phase by magnetic coupling. The oscillators drive a differential on-chip ring antenna, which illuminates a hyper-hemispherical silicon lens through the backside of the die. An on-wafer breakout of the oscillators core achieves a peak output power of -1.5 dBm with a 275-mW DC power consumption. The radiated power of the packaged source is -4.1 dBm, which is the highest reported radiated power of a single CMOS source beyond 200 GHz. The source die including the antenna occupies only 500 x 570 μ m2.

Characterization of Metal-Semiconductor Schottky Diodes and Application on THz Detection

This paper presents a kind of air-bridged GaAs Schottky diodes which offer ultra low parasitic capacitance and series resistance in millimeter and THz wavelength. The Schottky barrier diodes have several advantages when used as millimeter wave and terahertz video, or power detectors. These include their fast time response, room temperature operation, simple structure and low cost. This paper describes the characterization of the metal-semiconductor Schottky diodes including principle, diode structure, non-linear voltage-current characteristic and signal-rectifying performance. For application, a quasi-optical THz detector was made by using the proposed Schottky diodes. It utilized a hyper hemispherical silicon lens to coupleand THz radiation to the diodes by integrating on a broadband planar bow-tie antenna. The measurement results of the Schottky diode based detector show a good room temperature performance.

Nb5N6 microbolometer arrays for terahertz detection

A novel room-temperature microbolometer array chip consisting of an Nb5N6 thin film microbridge and a dipole planar antenna, which is used as a terahertz (THz) detector, is described in this paper. Due to the high-temperature coefficient of the resistance of the Nb5N6 thin film, which is as high as −0.7% K−1, such an antenna-coupled microbolometer is ideal for detecting signals in a frequency range from 0.22 THz to 0.33 THz. The dc responsivity, calculated from the measured I–V curve of the Nb5N6 microbolometer, is about −760 V/W at a bias current of 0.19 mA. A typical noise voltage as low as 10 nV/Hz1/2 yields a low noise equivalent power (NEP) of 1.3 × 10−11 W/Hz1/2 at a modulation frequency above 4 kHz, and the best RF responsivity, characterized using an infrared device measuring method, is about 580 V/W, with the corresponding NEP being 1.7 × 10−11 W/Hz1/2. In order to further test the performance of the Nb5N6 microbolometer, we construct a quasi-optical type receiver by attaching it to a hyperhemispherical silicon lens, and the result is that the best responsivity of the receiver is up to 320 V/W. This work could offer another way to develop a large scale focal-plane array in silicon using simple techniques and at low cost.

A Multichannel THz Detector Using Integrated Bow-Tie Antennas

This paper presents a kind of a multichannel THz detector using lens-based bow-tie array. A hyperhemispherical silicon lens is employed to provide a focal plane; 8 bow-tie elements are arranged on the focal plane with careful design to show a performance of broadband, high gain, well compact, and easy assembling. These characteristics of the detector are preferred for detecting weak THz signal. Measured far field shows that the radiation pattern of each element is shifted angularly, by <i>&#x2248;</i>9&#xb0;, which can be used for THz imaging. Tested responsivity of the detector shows a good spectral performance from 260 to 400&#x2009;GHz: respective values were &#x2265;220&#x2009;V/W, and the best NEP is achieved at about 60&#x2009;pW/<svg style="vertical-align:-0.99644pt;width:33.099998px;" id="M1" height="17.125" version="1.1" viewBox="0 0 33.099998 17.125" width="33.099998" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(.017,-0,0,-.017,.062,12.862)"><path id="x221A" d="M782 703l-309 -950l-231 505l-91 -47l-18 35l147 78l194 -431h2l264 810h42z" /></g> <g transform="matrix(1.25,0,0,-1.25,90,90)"> <path d="m -61.56,70.93 15.99,0" style="fill:none;stroke:#000000;stroke-width:0.68000001;stroke-linecap:butt;stroke-linejoin:miter;stroke-miterlimit:10;stroke-opacity:1;stroke-dasharray:none" /> </g> <g transform="matrix(.017,-0,0,-.017,13.05,15.825)"><path id="x48" d="M729 650v-28q-62 -5 -76 -19.5t-14 -75.5v-404q0 -62 13.5 -76t75.5 -19v-28h-268v28q65 5 79.5 19t14.5 76v199h-341v-199q0 -62 14 -76t76 -19v-28h-263v28q61 5 74.5 19t13.5 76v404q0 62 -14 76t-76 19v28h259v-28q-58 -5 -71 -19t-13 -76v-162h341v162&#xA;q0 61 -13.5 75.5t-72.5 19.5v28h261z" /></g><g transform="matrix(.017,-0,0,-.017,25.901,15.825)"><path id="x7A" d="M387 138l26 -13l-46 -125h-329l-7 15q109 161 249 388h-89q-52 0 -71.5 -13.5t-41.5 -57.5l-14 -27l-25 9q20 70 34 144h21q8 -21 55 -21h230l6 -14l-244 -390h99q54 0 78.5 16.5t55.5 64.5z" /></g> </svg>. Besides that, the proposed antenna has advantages of simple structure, easy fabrication, and low cost.

Enhanced performance of resonant sub-terahertz detection in a plasmonic cavity

A multi-gate high electron mobility transistor coupled to a log-periodic antenna was engineered to detect sub-terahertz radiation through resonant excitation of plasmon modes in the channel. The device was integrated with a silicon hyper-hemispherical lens in order to enhance radiation collection and eliminate parasitic substrate modes. The continuous detector response spectrum from 185 GHz to 380 GHz indicates the presence of distinct collective plasmonic cavity modes resulting from the quantization of the plasmon wavevector. In a bolometric detection mode, a noise equivalent power of less than 50 pW/Hz1/2 and a responsivity exceeding 100 kV/W have been measured at 11.5 K.

Hybrid Spectral-Domain Ray Tracing Method for Fast Analysis of Millimeter-Wave and Terahertz-Integrated Antennas

In this paper, we present a computationally efficient hybrid spectral ray tracing (HSRT) method that requires only one spectral domain integration step for each observation point. The HSRT method is capable of modeling arbitrary three-dimensional dielectric and metallic structures. We compare and validate various versions of the HSRT method including MoM-SRT, and MLFMM-SRT, with measurements and commercial software FEKO for method of moments (MoM), multi-level fast multipole method (MLFMM) and physical optics (PO) via simulation of a bow-tie terahertz antenna backed by hyper-hemispherical silicon lens and an on-chip dipole antenna attached to lens. It is shown that the MoM-SRT is more accurate than MoM-PO and comparable in speed. The HSRT algorithm is applied to simulation of on-chip dipole antenna backed by Silicon lens and integrated with a 180-GHz VCO and compared with measurements. In addition, it is shown that the matrix formulation of SRT and HSRT is a promising approach for solving complex electrically large problems with high accuracy.

Beam-Profiling and Wavefront-Sensing of THz Pulses at the Focus of a Substrate-Lens

We report combined wavefront detection and beam profiling of single-cycle terahertz (THz) pulses. In our system, the electric field is recorded highly resolved in two spatial and one temporal dimension before and after propagation through an optical component. Using this approach, we examine the imaging properties of a hyperhemispherical silicon lens as it is commonly used in THz dipole antennas. We observe an asymmetric spatiotemporal field dynamic in the focus, which can be attributed to distortion of the incident wavefront in combination with the image properties of the lens. Diffraction on the lens aperture influences the spectral beam profile at the focus. The frequency dependence of the Airy pattern indicates a rapidly degrading Strehl ratio with increasing frequency.

A far-infrared Fourier transform spectrometer with an antenna-coupled niobiumbolometer

We have designed and constructed a custom far-infrared Fourier transform spectrometerusing an antenna-coupled bolometer as a detector. The active element of the detector is asuperconducting niobium microbridge, and the far-infrared signal is coupled to themicrobridge via a planar antenna mounted on a hyperhemispherical silicon lens. Thespectrometer uses a broadband blackbody source with frequency-independent opticalcomponents, and thus the system bandwidth is set by the detector antenna. We havefabricated devices with two different antenna types, the double dipole and the log spiral,and have characterized the spectral response of each. This spectrometer can utilizethe fast response of the niobium bolometer to perform time-resolved far-infraredspectroscopy on nanosecond to millisecond timescales. These timescales are toofast for standard commercial bolometers and too long for a typical optical delayline.

High-power and high-temperature THz quantum-cascade lasers based on lens-coupled metal-metal waveguides

A metal-metal waveguide quantum cascade laser with an abutted silicon hyperhemispherical lens is demonstrated at ~4.1 THz. The device produced 145 mW of peak pulsed power at 5 K with a wall-plug power efficiency of 0.7%, lasing up to a maximum operating temperature of 160 K. The far-field beam pattern has a full width at half-maximum value of ~4.8 degrees in the H plane. The same device produced ~26 mW of peak power using a Winston cone instead of a lens, lasing up to 165 K. The large increase in output power is mainly attributed to an increase in collection efficiency.

Sensitivity of high temperature superconducting Josephson detectors at millimetre-wave band

Sensitivity of high temperature superconducting (HTS) Josephson detectors at millimetre-wave band has been studied in detail. YBCO grain boundary Josephson junctions with the normal-state resistance (RN) around 1 Ω and ICRN product of less than 2 mV have been fabricated for this study, where IC is the critical current of the junction. A Gunn oscillator at the frequency of 98 GHz together with a set of calibrated attenuators with a total attenuation of 90 dB are used as a signal source. A quasi-optical system, which consists of a wideband planar log-periodic antenna encompassing the junction, parabolic mirrors and a hyperhemispherical silicon lens, has been used as an effective coupling system. Sensitivity increases with ICRN. Using a junction with ICRN of 300 µV, a sensitivity of about 9000 V W−1 has been obtained, which is higher than that of the Schottky diode detectors at the same frequency band and demonstrates that the HTS Josephson detectors are very sensitive at millimetre-wave band.

FDTD analysis of propagation characteristics of terahertz electromagnetic pulses

A three dimensional finite difference time domain (FDTD) technique is applied to analyze the propagation characteristics of terahertz (THz) antenna whose dimension is smaller than the wavelength. A diffraction theory is also utilized with the FDTD technique for observing the far-field dispersion and attenuation of radiation from the THz antenna with a hyper-hemispherical silicon lens which is much larger than the wavelength. The signal degradation effects due to misalignment between the antenna and silicon lens are also characterized.