arXiv subjects

Giacomo Scalari

Publications and source records attributed to Giacomo Scalari.

At least 55 records · Page 3Linked to original sources

arXiv2021

THz intersubband electroluminescence from n-type Ge/SiGe quantum cascade structures

We report electroluminescence originating from L-valley transitions in n-type Ge/Si$_{0.15}$Ge$_{0.85}$ quantum cascade structures centered at 3.4 and 4.9 THz with a line broadening of $Δf/f \approx 0.2$. Three strain-compensated heterostructures, grown on a Si substrate by ultrahigh vacuum chemical vapor deposition, have been investigated. The design is based on a single quantum well active region employing a vertical optical transition and the observed spectral features are well described by non-equilibrium Green's function calculations. The presence of two peaks highlights a suboptimal injection in the upper state of the radiative transition. Comparison of the electroluminescence spectra with similar GaAs/AlGaAs structure yields one order of magnitude lower emission efficiency.

physics.optics↗

arXiv2020

Non-Invasive Near-field Spectroscopy of Single Sub-Wavelength Complementary Resonators

Subwavelength metallic resonators provide a route to achieving strong light-matter coupling by means of tight confinement of resonant electromagnetic fields. Investigation of such resonators however often presents experimental difficulties, particularly at terahertz (THz) frequencies. A single subwavelength resonator weakly interacts with THz beams, making it difficult to probe it using far-field methods; whereas arrays of resonators exhibit inter-resonator coupling, which affects the resonator spectral signature and field confinement. Here, traditional far-field THz spectroscopy is systematically compared with aperture-type THz near-field microscopy for investigating complementary THz resonators. Whilst the far-field method proves impractical for measuring single resonators, the near-field technique gives high signal-to-noise spectral information, only achievable in the far-field with resonator arrays. At the same time, the near-field technique allows us to analyze single resonators - free from inter-resonator coupling present in arrays - without significant interaction with the near-filed probe. Furthermore, the near-field technique allows highly confined fields and surface waves to be mapped in space and time. This information gives invaluable insight into resonator spectral response in arrays. This near-field microscopy and spectroscopy method enables investigations of strong light-matter coupling at THz frequencies in the single-resonator regime.

physics.optics↗

arXiv2020

Electric field correlation measurements on the electromagnetic vacuum state

Quantum mechanics ascribes to the ground state of the electromagnetic radiation zero-point electric field fluctuations that permeate empty space at all frequencies. No energy can be extracted from the ground state of a system and, therefore, these fluctuations cannot be measured directly with an intensity detector. The experimental proof of their existence came thus from more indirect evidence, such as the Lamb shift, the Casimir force between close conductors or spontaneous emission. A direct method to determine the spectral characteristics of vacuum field fluctuations has been missing so far. In this work, we perform a direct measurement of the field correlation on these fluctuations in the terahertz frequency range using electro-optic detection in a non-linear crystal placed in a cryogenic environment. We investigate their temporal and spatial coherence, which, at zero time delay and spatial distance, has a peak value of $6.2\cdot 10^{-2}~V^2/m^2$, corresponding to a fluctuating vacuum field of 0.25 V/m. With this measurement, we determine the spectral composition of the ground state of electromagnetic radiation which lies within the bandwidth of electro-optic detection.

quant-ph↗

arXiv2020

An antipodal Vivaldi antenna for improved far-field properties and polarization manipulation of broadband terahertz quantum cascade lasers

We present an antipodal Vivaldi antenna for broadband double metal waveguide terahertz quantum cascade lasers and frequency combs. Its exponentially curved flare profile results in an adiabatic in-plane mode expansion, producing an improved far-field with a single-lobed beam of (23°x19°) full width half maximum with an octave-spanning bandwidth. The antenna also acts as a wave retarder, rotating the polarization from vertical toward horizontal polarization by a frequency-dependent angle. The laser's emission spectrum and current-voltage characteristics are not affected, as well as frequency comb operation. Measurements agree well with numerical simulations, and the proposed antenna covers a broad spectral range (1.5-4.5 THz).

physics.optics↗

arXiv2020

Topological charge of finite-size photonic crystal modes

Topological charges are the winding numbers of polarization vectors around the vortex centers of far-field radiation. In this work, the topological charge of photonic crystal modes is theoretically analyzed using an envelope function approach. A group of modes is discovered with unique polarization properties dictated by their non-trivial envelope functions. Experimentally, lasing operation on such mode is demonstrated in an electrically pumped mid-infrared photonic crystal surface-emitting laser with high slope efficiency. The topological charge is directly observed from the polarization properties of single-mode laser emission.

physics.optics↗

arXiv2019

Landau polaritons in highly non-parabolic 2D gases in the ultra-strong coupling regime

We probe ultra-strong light matter coupling between metallic terahertz metasurfaces and Landau-level transitions in high mobility 2D electron and hole gases. We utilize heavy-hole cyclotron resonances in strained Ge and electron cyclotron resonances in InSb quantum wells, both within highly non-parabolic bands, and compare our results to well known parabolic AlGaAs/GaAs quantum well (QW) systems. Tuning the coupling strength of the system by two methods, lithographically and by optical pumping, we observe a novel behavior clearly deviating from the standard Hopfield model previously verified in cavity quantum electrodynamics: an opening of a lower polaritonic gap.

cond-mat.mes-hall↗

arXiv2019

Photon-driven broadband emission and RF injection locking of THz quantum cascade laser frequency combs

We present homogeneous quantum cascade lasers (QCLs) emitting around 3 THz which display bandwidths up to 950 GHz with a single stable beatnote. Devices are spontaneously operating in a harmonic comb state, and when in a dense mode regime they can be injection locked at the cavity roundtrip frequency with very small RF powers down to -55 dBm. When operated in the electrically unstable region of negative differential resistance, the device displays ultra-broadband operation exceeding 1.83 THz ($Δf/f=50\%$) with high phase noise, exhibiting self-sustained, periodic voltage oscillations. The low CW threshold (115 A$\cdot$ cm$^{-2}$) and broadband comb operation ($Δf/f=25\%$) make these sources extremely appealing for on-chip frequency comb applications.

physics.optics↗

arXiv2019

Thermoelectrically cooled THz quantum cascade laser operating up to 210 K

We present a \MF{terahertz} quantum cascade laser operating on a thermoelectric cooler up to a record-high temperature of 210.5 K. The active region design is based on only two quantum wells and achieves high temperature operation thanks to a systematic optimization by means of a nonequilibrium Green's function model. Laser spectra were measured with a room temperature detector, making the whole setup cryogenic free. At low temperatures ($\sim 40 K), a maximum output power of 200 mW was measured.

physics.optics↗

arXiv2019

Superradiantly limited linewidth in complementary THz metamaterials on Si-membranes

We study complementary double split ring THz resonators fabricated on a 10 mum thin Si-membrane. The linewidths of the fundamental LC-mode and dipolar mode are drastically narrowing with increased resonator spacing. The extracted decay rate of the LC-mode as a function of the resonator density shows a linear dependence, evidencing a collective superradiant effect of the resonator array. Furthermore, we show that a metamaterial can be designed for a low superradiant broadening of the resonance at high resonator densities, i.e. in the metamaterial condition. The use of a thin membrane as a substrate is crucial, since it shifts the THz surface plasmon polaritons modes to much higher frequencies, preventing them to couple to the LC mode and unveiling the superradiant broadening mechanism for a large range of lattice spacings. At higher frequencies, not interfering with the high Q LC-mode, other additional modes, which we ascribe to photonic crystal modes, form in the Si-membrane. We map the angle dependent band structure and show corresponding simulated electric field distributions.

physics.optics↗

arXiv2019

On-chip mid-infrared and THz frequency combs for spectroscopy

Frequency combs have revolutionized time and frequency metrology and in recent years, new frequency comb lasers that are highly compact or even on-chip have been demonstrated in the mid-infrared and THz regions of the electromagnetic spectrum. The emerging technologies include electrically pumped quantum and interband cascade semiconductor devices, as well as high-quality factor microresonators. In this guest editorial, the authors summarize recent advances in the field, the potential for rapid broadband spectroscopy, as well as the challenges and prospects for use in molecular gas sensing.

physics.optics↗

arXiv2019

Retrieval of phase relation and emission profile of quantum cascade laser frequency combs

The major development recently undergone by quantum cascade lasers has effectively extended frequency comb emission to longer-wavelength spectral regions, i.e. the mid and far infrared. Unlike classical pulsed frequency combs, their mode-locking mechanism relies on four-wave mixing nonlinear processes, with a temporal intensity profile different from conventional short-pulses trains. Measuring the absolute phase pattern of the modes in these combs enables a thorough characterization of the onset of mode-locking in absence of short-pulses emission, as well as of the coherence properties. Here, by combining dual-comb multi-heterodyne detection with Fourier-transform analysis, we show how to simultaneously acquire and monitor over a wide range of timescales the phase pattern of a generic frequency comb. The technique is applied to characterize a mid-infrared and a terahertz quantum cascade laser frequency comb, conclusively proving the high degree of coherence and the remarkable long-term stability of these sources. Moreover, the technique allows also the reconstruction of electric field, intensity profile and instantaneous frequency of the emission.

physics.optics↗

arXiv2018

Room temperature surface emission on large-area photonic crystal quantum cascade lasers

We design and fabricate large-area (1.1 mm $\times$ 1.1 mm) photonic crystal quantum cascade lasers, enabling single-mode (wavelength $\sim$ 8.5 $μ$m) surface emission at room temperature, with a maximum peak power up to 176 mW. The beam divergence is $< 1 ^{\circ}$ and without side-lobes. Moreover, by introducing asymmetry into the photonic crystal pillar shape, a single-lobed far-field is realized. The photonic band structure is measured with high spectral (0.72 cm$^{-1}$) and angular (0.1 $^{\circ}$) resolution by using the photonic crystal quantum cascade laser itself as a detector.

physics.optics↗

arXiv2018

Coexisting frequency combs spaced by an octave in a monolithic quantum cascade laser

Quantum cascade lasers are proving to be instrumental in the development of compact frequency comb sources at mid-infrared and terahertz frequencies. Here we demonstrate a heterogeneous terahertz quantum cascade laser with two active regions spaced exactly by one octave. Both active regions are based on a four-quantum well laser design and they emit a combined 3~mW peak power at 15~K in pulsed mode. The two central frequencies are 2.3~THz (bandwidth 300~GHz) and 4.6~THz (bandwidth 270~GHz). The structure is engineered in a way that allows simultaneous operation of the two active regions in the comb regime, serving as a double comb source as well as a test bench structure for all waveguide internal self-referencing techniques. Narrow RF beatnotes ($\sim$ 15~kHz) are recorded showing the simultaneous operation of the two combs, whose free-running coherence properties are investigated by means of beatnote spectroscopy performed both with an external detector and via self-mixing. Comb operation in a highly dispersive region (4.6~THz) relying only on gain bandwidth engineering shows the potential for broad spectral coverage with compact comb sources.

physics.optics↗

arXiv2018

Magneto-transport controlled by Landau polariton states

Hybrid excitations, called polaritons, emerge in systems with strong light-matter coupling. Usually, they dominate the linear and nonlinear optical properties with applications in quantum optics. Here, we show the crucial role of the electronic component of polaritons in the magneto-transport of a cavity-embedded 2D electron gas in the ultrastrong coupling regime. We show that the linear dc resistivity is significantly modified by the coupling to the cavity even without external irradiation. Our observations confirm recent predictions of vacuum-induced modification of the resistivity. Furthermore, photo-assisted transport in presence of a weak irradiation field at sub-THz frequencies highlights the different roles of localized and delocalized states.

quant-ph↗

arXiv2017

Broadband monolithic extractor for metal-metal waveguide based terahertz quantum cascade laser frequency combs

We present a monolithic solution to extract efficiently light from terahertz quantum cascade lasers with metal-metal waveguides suitable for broadband frequency comb applications. The design is optimized for a bandwidth of 500 GHz around a center frequency of 2.6 THz. A five-fold increase in total output power is observed compared to standard metal-metal waveguides. The extractor features a single-lobed far-field pattern and increases the frequency comb dynamical range to cover more than 50 % of the laser dynamic range. Frequency comb operation up to a spectral bandwidth of 670 GHz is achieved.

physics.optics↗

arXiv2017

Intensity autocorrelation measurements of quantum cascade laser frequency combs in the Terahertz range

We report on the first direct measurement of the emission character of quantum cascade laser based frequency combs, using intensity autocorrelation. The correlation technique is based on fast electro-optic sampling, with a bandwidth optimized to match the emission spectra of the comb laser, centered around 2.5~THz. We find the output light to be amplitude and frequency modulated at the same time, with intensity modulation depth as high as 90\%, occuring on timescales as short as few picoseconds. We compare these findings with intensity autocorrelation measurements of pulsed Terahertz light originating from a photoconductive antenna and find very different results. We observe no significant difference for the comb and dispersed regime. The technique will be of significant importance in future for the measurement of ultra-short pulses from quantum cascade lasers.

physics.ins-det↗

arXiv2016

Ultra-broadband quantum cascade laser operating from 1.88 to 3.82 THz

We report on a heterogeneous active region design for terahertz quantum cascade laser based frequency combs. Dynamic range, spectral bandwidth as well as output power have been significantly improved with respect to previous designs. When operating individually the lasers act as a frequency comb up to a spectral bandwidth of 1.1 THz, while in a dispersed regime a bandwidth up to 1.94 THz at a center frequency of 3 THz can be reached. A self-detected dual-comb setup has been used to verify the frequency comb nature of the lasers.

physics.optics↗

arXiv2016

Measuring intensity correlations of a THz quantum cascade laser around its threshold at sub-cycle timescales

The quantum nature of photonic systems is reflected in the photon statistics of the light they emit. Therefore, the development of quantum optics tools with single photon sensitivity and excellent temporal resolution is paramount to the development of exotic sources, and is particularly challenging in the THz range where photon energies approach kbT at T=300 K. Here, we report on the first room temperature measurement of field g1(τ) and intensity correlations g2(τ) in the THz range with sub-cycle temporal resolution (146 fs) over the bandwidth 0.3-3 THz, based on electro-optic sampling. With this system, we are able to measure the photon statistics at threshold of a THz Quantum Cascade Laser.

physics.optics↗