Broadband Amplifier Research Articles

Beyond the ordinary acoustoelectric effect: Superluminal phenomena in the acoustic realm and phonon-mediated Bloch gain

It has been shown that coherent phonons can be used as a potent tool for controlling and enhancing optoelectronic and transport properties of nanostructured materials. Recent studies revealed that interaction of acoustic phonons and fast-moving carriers in semiconductor heterostructures can be accompanied by electron-phonon instabilities that cause ordinary and induced Cherenkov effects. However, the development of such instabilities is still poorly understood. Our study shows that other supersonic phenomena, beyond the Cherenkov instability, are possible for nonequilibrium charge transport in the miniband semiconductor superlattices (SLs) driven by an acoustic plane wave. Using semiclassical nonperturbative methods and elements of the bifurcation theory, we find the conditions for the onset of dynamical instabilities (bifurcations) which are caused by the emission of specific SL phonons by supersonic electrons, and their back action on the electrons. Notably, the underlying radiation mechanism is connected either to normal or anomalous Doppler effects in full accordance with the Ginzburg-Frank-Tamm theory. The appearance of induced Doppler effects is also discussed in relation to the formation of electron bunches propagating through the spatially periodic structure of the SL. When the amplitude of the acoustic wave exceeds a certain threshold, the dynamical instabilities developed in the system are manifested as drift velocity reversals, resonances in sound attenuation, and absolute negative mobility. We demonstrate that the discovered superluminal Doppler phenomena can be utilized for tunable broadband amplification and generation of GHz-THz electromagnetic waves, which creates a ground for the development of novel phononic devices.

Design of a Low-Cost UWB Time-Domain Radar System for Subcentimeter Image Resolution

The design and development of a hybrid Gaussian pulse radar transceiver, including transmitter and receiver, is presented. The transmitter is realized using a transistor-based square pulse generator, a tunable step recovery diode (SRD) Gaussian pulse-shaping network, and a differentiator. The SRD circuit design is modified using a high-pass filter to have a low ringing and narrow pulsewidth. The output pulse with a broadband amplifier has a power of 500 mW, at a pulse repetition frequency (PRF) of 10 MHz, and tunable pulsewidth ranges from 56 to 300 ps. A 90° coupler is configured to convert the Gaussian pulse into a monopulse. Then, a transmitter antenna is used to radiate the pulse, and a separate receiving antenna is used to receive the reflected pulse. The receiver is comprised of a local Gaussian pulse generator, a radio frequency mixer (RF-mixer) accompanied by a low-pass filter (LPF), a low-frequency amplifier (LFA), and an analog-to-digital converter (ADC). The RF-mixer is used for expanding the receiving pulse from the receiving antenna by a factor of 100000. The LPF and LFA are used to detect the envelope of the output of the RF-mixer and amplify it. The ADC is interposed between the LFA and a processor to digitize the expanded pulse for data processing. The validity and reliability of the digitized expanded pulse compared to those received using a sampling oscilloscope are 93% in agreement, which is remarkable. A version of the system is also developed for a single-antenna operation by utilizing an ultrawideband (UWB) coupler as a semicirculator. Both the systems are compared, and the benefits of each are illustrated. The flexibility and performance of the designs are analyzed. The proposed system demonstrates subcentimeter imaging capabilities.

Er3+-doped barium sodium borate glasses development for 1.54 µm broadband amplifier and optical laser

Er3+-doped BaNaB glasses with 0.5, 1.0, 2.0, 3.0 and 4.0 mol% of Er3+ were prepared by melt-quenching technique for study their luminescence properties in NIR and visible regions. The absorption spectra showed the absorption bands at 381, 406, 451, 488, 522, 545, 653, 804, 977 and 1532 nm in UV-Vis-NIR region. The NIR emission spectra, excited with 977 nm, show the broad band centered at 1540 nm in the range of 1460–1625 nm covers the S, C and L bands that have low transmission loss region in fiber-optic communication. The glass sample with 3.0 mol% of Er3+ provides the highest intensity of NIR emission. From the Judd-Ofelt analysis and McCumber theory, the glass sample with 4.0 mol% of Er3+ show the best performance for use as a broadband amplifier with the high gain bandwidth at 633.64 × 10−28 cm3 and the high emission cross section at 7.51 × 10−21 cm2. For the emission in visible region, when excited with 349 nm, the glass sample with 3.0 mol% of Er3+ provides the most intense emission in yellow-green region, when excited with 402 nm, the glass sample with 4.0 mol% of Er3+ provides the most intense emission in green region. The results from the McCumber theory and gain efficiency show good agreement of the glass sample with 4.0 mol% of Er3+ for use as broadband amplifier in C and L communication windows when the population inversion is more than 50%.

Design a Broad-band High flatness Gain Low Noise Amplifier for radar Applications

Design a Broad-band High flatness Gain Low Noise Amplifier for radar Applications

Broadband Amplification as Tinnitus Treatment.

This study investigated the effect of broadband amplification (125 Hz to 10 kHz) as tinnitus treatment for participants with high-frequency hearing loss and compared these effects with an active placebo condition using band-limited amplification (125 Hz to 3–4 kHz). A double-blinded crossover study. Twenty-three participants with high-frequency (≥3 kHz) hearing loss and chronic tinnitus were included in the study, and 17 completed the full treatment protocol. Two different hearing aid treatments were provided for 3 months each: Broadband amplification provided gain in the frequency range from 125 Hz to 10 kHz and band-limited amplification only provided gain in the low-frequency range (≤3–4 kHz). The effect of the two treatments on tinnitus distress was evaluated with the Tinnitus Handicap Inventory (THI) and the Tinnitus Functional Index (TFI) questionnaires. The effect of the treatment on tinnitus loudness was evaluated with a visual analog scale (VAS) for loudness and a psychoacoustic loudness measure. Furthermore, the tinnitus annoyance was evaluated with a VAS for annoyance. The tinnitus pitch was evaluated based on the tinnitus likeness spectrum. A statistically significant difference was found between the two treatment groups (broadband vs. band-limited amplification) for the treatment-related change in THI and TFI with respect to the baseline. Furthermore, a statistically significant difference was found between the two treatment conditions for the annoyance measure. Regarding the loudness measure, no statistically significant differences were found between the treatments, although there was a trend towards a lower VAS-based loudness measure resulting from the broadband amplification. No changes were observed in the tinnitus pitch between the different conditions. Overall, the results from the present study suggest that tinnitus patients with high-frequency hearing loss can experience a decrease in tinnitus-related distress and annoyance from high-frequency amplification.

Physical properties and luminescence of highly stable erbium-doped antimony glasses for NIR broadband amplification

Glasses with compositions (0.8-x)Sb2O3−0.2Na2O-(x)WO3-(0.0025) Er2O3, (0 ≤ x ≤ 0.4 mol) has been synthesized by the melt-quenching technique. Physical, optical characteristics and spectroscopy have been identified and investigated. As the glass composition changes, definite variations in physio-optical properties can be observed as well as the temperature of the glass transition (Tg), the density (ρ), molar volume (Vm), bandgap energy (Eg), and the refractive index (n). The Judd-Ofelt theory has been performed using a UV–VIS-NIR absorption spectrum to examine the compositional effect on Er3+ spectroscopic properties. Some basic spectroscopic parameters including electric dipole line strengths, Judd-Ofelt parameters, the rate of radiative probability, and the lifetime of radiative levels, are determined. A high NIR emission at 1.53 μm was observed when exciting glasses at 980 nm. The FWHM, the effective line-width (Δλeff), the emission cross-section (σem), and the gain bandwidth values were calculated using the theory of Füchtbauer–Ladenburg, and the findings were critically analyzed. the decay of fluorescence spectra for the 4I13/2 level was registered, and the lifetime for the studied glasses was calculated. Upconversion luminescence was observed in the green and red regions under 980 nm excitation, with a relative increase in emission intensity due to the added tungsten content. Based on the results obtained, the studied glasses are favorable materials for broadband amplifiers in WDM systems and optical devices.

Floquet-Mode Traveling-Wave Parametric Amplifiers

Simultaneous ideal quantum measurements of multiple single-photon-level signals would advance applications in quantum information processing, metrology, and astronomy, but require the first amplifier to be simultaneously broadband, quantum limited, and directional. However, conventional traveling-wave parametric amplifiers support broadband amplification at the cost of increased added noise and are not genuinely directional due to non-negligible nonlinear backward wave generation. In this work, we introduce a new class of amplifiers which encode the information in the Floquet modes of the system. Such Floquet mode amplifiers prevent information leakage and overcome the trade-off between quantum efficiency (QE) and bandwidth. Crucially, Floquet mode amplifiers strongly suppress the nonlinear forward-backward wave coupling and are therefore genuinely directional and readily integrable with qubits, clearing another major obstacle towards broadband ideal quantum measurements. Furthermore, Floquet mode amplifiers are insensitive to out-of-band impedance mismatch, which otherwise may lead to gain ripples, parametric oscillations, and instability in conventional traveling-wave parametric amplifiers. Finally, we show that a Floquet mode Josephson traveling-wave parametric amplifier implementation can simultaneously achieve $>\!20\,$dB gain and a QE of $\eta/\eta_{\mathrm{ideal}}\!> 99.9\%$ of the quantum limit over more than an octave of bandwidth. The proposed Floquet scheme is also widely applicable to other platforms, such as kinetic inductance traveling-wave amplifiers and optical parametric amplifiers.

Broadband optical parametric amplification using PPLN waveguide pumped by detuned second harmonic.

Optical parametric amplification in the range of 1.3-1.8 µm was demonstrated by using a periodically poled LiNbO3 (PPLN) waveguide as a nonlinear medium by varying the detuning of the pump wavelength. A wide range of detuning was enabled by using a multiple-quasi-phase-matched (M-QPM) LiNbO3 waveguide for pump generation through second harmonic generation (SHG) and temperature control of the PPLN waveguide. Broadband optical amplification and wavelength conversion through difference frequency generation (DFG) are considered useful for widening the bandwidth of optical communication.

A Broadband Amplifier With Flat Bandwidth for Modulator and Measurement Driver Circuits

This paper presents a broadband amplifier for high-speed modulators and measurement driver circuits using distributed amplifiers configuration. The flat low-frequency performance of the conventional distributed amplifiers (DAs) is limited by termination impedance. This paper shows on-chip low frequency extension can be achieved by optimizing the capacitively coupled circuit and impedance termination. Although capacitively coupled circuit has been introduced in previous studies to extend the high-frequency gain, sufficient analysis has not been studied. Furthermore, a novel broadband equalizer termination applying for distributed configuration is introduced that can be employed to balance the rising gain. This DA features the lowest cut-off frequency among broadband amplifiers using the on-chip circuit. The amplifier provides a comparable low-frequency performance of MHz with greater than 15 dB gain and benchmarks the first demonstration of a flat MHz performance monolithic microwave integrated circuit (MMIC) using on-chip passive components. The proposed amplifier achieves 17 dBm output 1dB compression point (OP1dB) covering eight-octave bandwidth verified using a 0.15-μm GaAs pseudomorphic High Electron Mobility Transistor (pHEMT) technology. The new design architecture combined with the broadband equalizer characteristics and gate series network can provide performance benefits for optic modulators, instrumentations, and advanced broadband transmitters.

Immunity Testing of Mixed Signal Electronics Against Power Supply Disturbances in the Frequency Range From 10 kHz To 5 MHz

A method is presented to test the immunity of mixed signal (digital and analog) electronics to power supply disturbances in the frequency range between 10 kHz and 5 MHz, as those originating from switched mode power supplies. An example of application of the method to a Serializer/Deserializer (SerDes) integrated circuit is illustrated. For these electronic devices the power supply noise can critically affect performance. The SerDes is hosted by an evaluation board supplied by an external Power Module. An ad-hoc disturbance source and Coupling/Decoupling Network (CDN) have been designed to couple a disturbance of significant amplitude to the power supply of the evaluation board while decoupling it from the Power Module. The radiofrequency (RF) impedance of the bypass network of the evaluation board has been considered for the design of both the disturbance source and the CDN. Details about the architecture and operation of the high-current, broadband and linear power amplifier used for disturbance generation are provided, along with component selection and verification of the CDN. The practical implementation of the test, including a feedback control loop capable of generating the specified disturbance level over the frequency range of interest, is described. Finally, test results are reported in terms of the SerDes bit error rate (BER) degradation as a function of disturbance amplitude and frequency.

High-energy broadband Nd: Glass regenerative amplifier with coated birefringent filter for the high-power laser facilities

The regenerative amplifier is an important component of the high-power laser facility. A high-energy broadband Nd: glass regenerative amplifier with an energy of 34.1 mJ and a bandwidth of 4.3 nm was demonstrated using an intra-cavity specially coated birefringent filter (CBRF). This letter proposes a design scheme for a CBRF that can control the flat-top area of gain spectrum of the regenerative amplifier, which has been studied theoretically and experimentally. The transmittance of the specially coated BRF was tested in this experiment. The amplifier output spectrum had a flexible control ability and long-term stability. It can be applied to support high-energy broadband amplification in inertial confinement fusion (ICF) facilities, and can also be extended to other broadband amplification system to support high-energy picosecond petawatt laser facilities.

Cyclotron Resonance Maser With Zigzag Quasi-Optical Transmission Line: Concept and Modeling

A microwave system in the form of a quasi-optical (QO) transmission line is proposed as an interaction circuit of an amplifier or an oscillator based on the cyclotron resonance maser (CRM) instability. The circuit consists of periodically spaced along the horizontal <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${z}$ </tex-math></inline-formula> -axis focusing mirrors, which transport the Gaussian wave beam along a zigzag path with vertical and inclined segments. A beam of electrons having sufficiently high perpendicular velocities is guided by a static B-field along the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${z}$ </tex-math></inline-formula> -axis so that the e-beam periodically crosses the wave beam. The CRM instability occurs in regions where an electromagnetic wave having a Bz-component, as in a gyrotron, propagates strictly across the e-beam resulting in the interaction with minimal sensitivity to particle velocity spread. Three-dimensional Particle-In-Cell (3-D PIC) simulations demonstrate the attractiveness of this “zigzag” CRM for the implementation of relatively broadband amplifiers or oscillators with unique (more than an octave bandwidth) frequency tunability at wavelengths of 1–2 mm.

Investigation of spectroscopic and photoluminescence properties of Erbium doped phosphate (P2O5-K2O3-Al2O3) glasses

Erbium (Er) doped P2O5-K2O3-Al2O3:Er2O3 glasses (KPEr) with different contents of Er3+ ions were fabricated by famous melt-quenching technique. The fabricated samples were characterized by using physical, X-ray photoelectron spectroscopy (XPS), photoluminescence, absorption, and J-O analysis techniques. The XRF analyses were performed in order to know the change in elemental composition of fabricated glasses, while using the various concentrations of Er3+ ions. Three emission peaks were observed, two peaks in visible regions of 526 nm (2H11/2→4I15/2) and 550 nm (4S3/2→4I15/2) and one peak in near infrared (NIR) region of 1545 nm (4I13/2→4I15/2). Nine peaks were observed in absorption spectra in the range of 390–1800 nm. The trend of JO parameters for the present KPEr6 glass sample is Ω2> Ω4> Ω6. Higher values of stimulated emission cross-section σemi(λ), full wave half maxima FWHM and (FWHM x σemi (λ)), for KPEr6 indicate that the present glass has potential candidate to be used for broadband amplification in telecommunication. The McCumber theory is used to study absorption and emission cross-section for 4I13/2→4I15/2 transition. The luminescence colors of these samples were almost entirely fall in the yellow-green (Y-G) region (x = 0.33, y = 0.64) in the CIE 1931 diagram. The calculated CCT values for our prepared glasses were found to be 5554 K. The present study suggest that KPEr glass samples have potential candidate for broadband amplification in telecommunication, green LED and display devices.

Broadband parametric amplification for micro-ring gyroscopes

Given the fact that MEM gyroscopes can provide a better solution for inertial sensors in terms of size and cost, increasing their performance will remain an important issue in the development of that field. Researchers seek enhancing different performance measures, mainly the bias stability, Angle Random Walk and sensitivity. Nevertheless, different amplification techniques unfortunately decrease the signal to noise ratio (SNR) and thereby decrease the gyroscope’s precision. In this context, parametric excitation methods emerged to have positive effects on both the gyroscope’s sensitivity and SNR, this is based on the fact that the amplification is achieved mechanically.However, parametric amplification is conventionally limited to the resonant frequencies. This limitation affects the efficiency of the control methods used to tune the excitation frequency. This can be, however, overcome by introducing a broadband parametric amplification method. This parametric excitation method is described in this paper in detail. It is based on inducing a modal-coupled phase-shifted parametric excitation in multi-degree of freedom dynamic systems. Using this excitation method the system can be subjected to destabilization on a broad frequency band, i.e. not only at resonant frequencies. In that way the parametric excitation can then offer a negative-damping effect thus mechanically increasing the amplitude and the device’s sensitivity.In this paper, a micro-ring gyroscope is subjected to a bimodal coupled phase-shifted parametric excitation. The system’s differential equations were derived using Hamilton’s principle and discretized using Galerkin’s method. Afterwards, the aforementioned excitation method is applied through a suggested control circuit. The whole system is analyzed by a semi-analytical method, namely, the method of normal forms and verified numerically using the Floquet method. Using the data of a realistic micro-ring gyroscope we could then achieve broadband amplification under certain conditions.

Characterization of Diamond and Silicon Carbide Detectors With Fission Fragments

Experimental fission studies for reaction physics or nuclear spectroscopy can profit from fast, efficient, and radiation-resistant fission fragment (FF) detectors. When such experiments are performed in-beam in intense thermal neutron beams, additional constraints arise in terms of target-detector interface, beam-induced background, etc. Therefore, wide gap semi-conductor detectors were tested with the aim of developing innovative instrumentation for such applications. The detector characterization was performed with mass- and energy-separated fission fragment beams at the ILL (Institut Laue Langevin) LOHENGRIN spectrometer. Two single crystal diamonds, three polycrystalline and one diamond-on-iridium as well as a silicon carbide detector were characterized as solid state ionization chamber for FF detection. Timing measurements were performed with a 500-µm thick single crystal diamond detector read out by a broadband amplifier. A timing resolution of ∼10.2 ps RMS was obtained for FF with mass A = 98 at 90 MeV kinetic energy. Using a spectroscopic preamplifier developed at INFN-Milano, the energy resolution measured for the same FF was found to be slightly better for a ∼50-µm thin single crystal diamond detector (∼1.4% RMS) than for the 500-µm thick one (∼1.6% RMS), while a value of 3.4% RMS was obtained with the 400-µm silicon carbide detector. The Pulse Height Defect (PHD), which is significant in silicon detectors, was also investigated with the two single crystal diamond detectors. The comparison with results from α and triton measurements enabled us to conclude that PHD leads to ∼50% loss of the initial generated charge carriers for FF. In view of these results, a possible detector configuration and integration for in-beam experiments has been discussed.

АНАЛИЗ УПРАВЛЯЕМОСТИ НЕКОТОРЫХ ЦИФРОВЫХ ФИЛЬТРОВ С КОНЕЧНОЙ ИМПУЛЬСНОЙ ХАРАКТЕРИСТИКОЙ

This overview article covers finite impulse response filters and filter banks. The use of these filtersfor hearing aids is considered. Ways to compensate for hearing loss and ways to increase loudnessusing broadband amplification are considered. A schematic diagram of a method for digitalsignal processing using a bank of filters, as well as a technique for synthesizing interpolation filterswith low computational complexity, is presented. Also, the application of the MATLAB system for thesynthesis of narrow-band non-recursive FIR filters, their design procedure, methodology and examplesare considered. Finite Impulse Response (FIR) filters and filter banks have specific propertiesthat guarantee stability. Therefore, they are popular in many applications such as communicationsystems, audio signal processing, biomedical instruments, and so on. Unfortunately, due to the longerwavelength, the cost of implementing an FIR filter is usually not higher than an infinite impulse response(IIR) filter that meets the same requirements. It is well known that the length of an FIR filter isinversely proportional to its transition bandwidth. Therefore, the disadvantage becomes acute when agiven filter has a narrow transition band. The main goal is to consider computationally efficientmethods for designing FIR filters and filter banks. The masking method (FRM) results in significantsavings in the number of multipliers. Next, a 16-band, low group delay, non-equal-spacing digitalFIR filter bank is considered. Overall latency is significantly reduced as a result of a new filter structurethat reduces the interpolation factor for prototype filters. Masking filter may be an interpolatedfinite impulse response (IFIR) filter that helps reduce complexity.

Design and implementation of a broad-band high gain low noise amplifier for 3G/4G applications

The main aim of the paper is designing and implementing a broadband low-noise-amplifier (LNA) based on compensated matching network techniquein order to get high stable gain, low noise figure, low cost and smaller sizefor 3G/4G communication system applications at 2 GHz with bandwidth 600MHz. The Advanced Design System simulates the proposed circuit (ADS).The implementation was done with a class A bias circuit and a low noise transistor BFU 730F with a lower Noise Figure (NFmin) 0.62 dB. Collectorcurrent is measured to be 5.8mA and base current is 19.1μA with a supply voltage of 2.25V. The new design proposed a (NFmin) of 0.62 dB with a 17.8dB high stable amplifier gain. The microstrip lines (MSL) and compensated matching network techniques were used to improve the LNA’s stability and achieve a good result. The LNA board is implemented and assembled on the FR4 botton layer material. The results are virtually non existence equivalent between the simulated and the measured results.

Quantum point contact galvanically coupled to planar superconducting resonator: a shot-noise-limited broad-band electrical amplifier

Probing single charge dynamics in solids can give insights into various quantum transport phenomena, most of which are fragile and short-time-scaled. Detection of these events in real-time requires a mesoscopic electrical amplifier with unprecedented sensitivity and operational bandwidth. In this work, we explore a hybrid electrical amplifier consisting of a semiconducting quantum point contact galvanically coupled to a superconducting λ/2 transmission-line resonator for ultra-fast and ultra-sensitive charge amplification. The resonator, made of aluminium with a coplanar waveguide geometry, is designed to operate at its first harmonic resonant mode ∼3.4 GHz, where the reflected power from the resonator is amplitude-modulated by the conductance changes in the quantum point contact channel. From the sidebands of the amplitude modulated reflected signal we extract a conductance sensitivity of ). This sensitivity translates to a unit signal-to-noise measurement time ∼1.62 ns for a variation of 0.01 ( in the conductance. From the analysis of the noise characteristics of the device, we find that up to a few MHz of signal frequency the sensitivity is limited by the photon assisted shot-noise associated with the electron tunnelling in the QPC channel. The optimization of various operational parameters of the device reveals a bandwidth of ∼155 MHz which corresponds to a rise-time ∼2.2 ns. Both the sensitivity and bandwidth that we obtain are greater by an order compared to the existing reports. In addition, the device also exhibits very good sensitivities up to a measurable frequency of 240 MHz. The extremely high sensitivity, shot-noise limited detection, ultra-fast operation reaching the nanosecond time-scales, and the circuit QED architecture makes this scheme an attractive choice for single charge detection and counting experiments for spin-qubit readout and quantum electrical metrology.

A 56–161 GHz Common-Emitter Amplifier with 16.5 dB Gain Based on InP DHBT Process

This paper presents a broadband amplifier MMIC based on 0.5 µm InP double-heterojunction bipolar transistor (DHBT) technology. The proposed common-emitter amplifier contains five stages, and bias circuits are used in the matching network to obtain stable high gain in a broadband range. The measurement results demonstrate a peak gain of 19.5 dB at 146 GHz and a 3 dB bandwidth of 56–161 GHz (relative bandwidth of 96.8%). The saturation output power achieves 5.9 and 6.5 dBm at 94 and 140 GHz, respectively. The 1 dB compression output power is −4.7 dBm with an input power of −23 dBm at 94 GHz. The proposed amplifier has a compact chip size of 1.2 × 0.7 mm2, including DC and RF pads.

Synchronizing Bloch-Oscillating Free Carriers in Moiré Flat Bands.

Achieving Bloch oscillations of free carriers under a direct current, a long-sought-after collective many-body behavior, has been challenging due to stringent constraints on the band properties. We argue that the flat bands in moiré graphene fulfill the basic requirements for observing Bloch oscillations, offering an appealing alternative to the stacked quantum wells used in previous work aiming to access this regime. Bloch-oscillating moiré superlattices emit a comblike spectrum of incommensurate frequencies, a property of interest for converting direct currents into high-frequency currents and developing broadband amplifiers in terahertz domain. The oscillations can be synchronized through coupling to an oscillator mode in a photonic or plasmonic resonator. Phase-coherent collective oscillations in the resonant regime provide a realization of current-pumped terahertz lasing.