Andrei Gorodetsky

Application of Terahertz Pulse Time-Domain Holography for Phase Imaging

Terahertz pulse time-domain holography (THz PTDH) is the powerful technique for high-resolution amplitude and phase THz imaging that allows mapping spectroscopic information across the imaged object. In this paper, we consider most sought after applications of phase imaging provided by this technique and experimentally demonstrate the ability of the method to reconstruct smooth and stepped relief features of an object that is transparent in THz region. Unlike the amplitude distribution, which does not contain any significant information in this case, phase distribution not only reveals the object qualitatively, but also allows the reconstruction of the object thicknesses pattern, even in low signal-to-noise registration conditions. Main limitations of the proposed method, such as transverse resolution and low signal-to-noise environment are carefully studied and mitigated.

Enhancement of terahertz photoconductive antenna operation by optical nanoantennas

Photoconductive antennas and photomixers are very promising sources of terahertz radiation that is widely used for spectroscopy, characterisation and imaging of biological objects, deep space studies, scanning of surfaces and detection of potentially hazardous substances. These antennas are compact and allow generation of both ultrabroadband pulse and tunable continuous wave terahertz signal at room temperatures, without a need of high-power optical sources. However, such antennas have relatively low energy conversion efficiency of femtosecond laser pulses or two close pump wavelengths (photomixer) into the pulsed and continuous terahertz radiation, correspondingly. Recently, an approach to solving this problem has been proposed, that involves known methods of nanophotonics applied to terahertz photoconductive antennas and photomixers. This approach comprises the use of optical nanoantennas for enhancing the efficiency of pump laser radiation absorption in the antenna gap, reducing the lifetime of photoexcited carriers, and improving antenna thermal efficiency. This Review is intended to systematise all the results obtained by researchers in this promising field of hybrid optical-to-terahertz photoconductive antennas and photomixers.

Boosting Terahertz Photoconductive Antenna Performance with Optimised Plasmonic Nanostructures

Advanced nanophotonics penetrates into other areas of science and technology, ranging from applied physics to biology, which results in many fascinating cross-disciplinary applications. It has been recently demonstrated that suitably engineered light-matter interactions at the nanoscale can overcome the limitations of today’s terahertz (THz) photoconductive antennas, making them one step closer to many practical implications. Here, we push forward this concept by comprehensive numerical optimization and experimental investigation of a log-periodic THz photoconductive antenna coupled to a silver nanoantenna array. We shed light on the operation principles of the resulting hybrid THz antenna, providing an approach to boost its performance. By tailoring the size of silver nanoantennas and their arrangement, we obtain an enhancement of optical-to-THz conversion efficiency 2-fold larger compared with previously reported results for similar structures, and the strongest enhancement is around 1 THz, a frequency range barely achievable by other compact THz sources. We also propose a cost-effective fabrication procedure to realize such hybrid THz antennas with optimized plasmonic nanostructures via thermal dewetting process, which does not require any post processing and makes the proposed solution very attractive for applications.

Optimization of nanoantenna-enhanced terahertz emission from photoconductive antennas

We present the results of hybrid photoconductive antenna THz emission enhanced by silver nanoantenna arrays. By varying the size of nanoantennas and the distance between them, we obtain the greatest value of optical-to-THz conversion efficiency reached so far. The results of experimental investigations are in a good agreement with numerical simulations. The conversion efficiency reveals over 5-fold improvement at certain frequencies, if compared with similar photoconductive antenna without silver nanoparticles, while previous results for this type of antenna barely exceeded 2-fold conversion efficiency gain.

Quantum-dot based ultrafast photoconductive antennae for efficient THz radiation

Here we overview our work on quantum dot based THz photoconductive antennae, capable of being pumped at very high optical intensities of higher than 1W optical mean power, i.e. about 50 times higher than the conventional LT-GaAs based antennae. Apart from high thermal tolerance, defect-free GaAs crystal layers in an InAs:GaAs quantum dot structure allow high carrier mobility and ultra-short photo carrier lifetimes simultaneously. Thus, they combine the advantages and lacking the disadvantages of GaAs and LT-GaAs, which are the most popular materials so far, and thus can be used for both CW and pulsed THz generation. By changing quantum dot size, composition, density of dots and number of quantum dot layers, the optoelectronic properties of the overall structure can be set over a reasonable range-compact semiconductor pump lasers that operate at wavelengths in the region of 1.0 μm to 1.3 μm can be used. InAs:GaAs quantum dot-based antennae samples show no saturation in pulsed THz generation for all average pump powers up to 1W focused into 30 μm spot. Generated THz power is super-linearly proportional to laser pump power. The generated THz spectrum depends on antenna design and can cover from 150 GHz up to 1.5 THz.

Solution for reconstruction enhancement of pulsed broadband THz holograms recorded with small area detectors

In this proceeding, we discuss the method that allows for field of view and reconstruction quality enhancement of pulsed THz holograms recorded by matrix detectors that do not exceed the the object transverse dimensions, at distances, that are comparable with the object size. The method comprises the use of random phase mask situated between the object and the hologram, at the hologram registration process. The introduced phase variation levels out the input from closer and further (to the hologram pixel) points of the object, and thus improves overall reconstruction quality. Here, we study numerically this approach and demonstrate 4 times increase of the properly reconstructed object area, if compared to the undisturbed hologram recording, and consecutive increase of the correlation between the reconstructed and actual object from 0.34 to 0.82.

Ultrafast Intraband Spectroscopy of Hot-Carrier Cooling in Lead-Halide Perovskites

The rapid relaxation of above-band-gap “hot” carriers (HCs) imposes the key efficiency limit in lead-halide perovskite (LHP) solar cells. Recent studies have indicated that HC cooling in these systems may be sensitive to materials composition, as well as the energy and density of excited states. However, the key parameters underpinning the cooling mechanism are currently under debate. Here we use a sequence of ultrafast optical pulses (visible pump–infrared push–infrared probe) to directly compare the intraband cooling dynamics in five common LHPs: FAPbI3, FAPbBr3, MAPbI3, MAPbBr3, and CsPbBr3. We observe ∼100–900 fs cooling times, with slower cooling at higher HC densities. This effect is strongest in the all-inorganic Cs-based system, compared to the hybrid analogues with organic cations. These observations, together with band structure calculations, allow us to quantify the origin of the “hot-phonon bottleneck” in LHPs and assert the thermodynamic contribution of a symmetry-breaking organic cation toward rapid HC cooling.

Tunable CW THz generation from an all-quantum-dot-based system

The development of a compact, continuous wave (CW), room-temperature, broadly-tunable terahertz (THz) laser source is of great interest for a number of applications ranging from biomedical imaging and indoor communication to spectroscopy, security and defence. Among the usually used laser sources, direct quantum cascade lasers (QCLs) and semiconductor mixers of mid-IR QCLs are regarded as most popular sources offering the highest-to-date wall-plug efficiency. However, such lasers require a cryogenic cooling and suffer from the drawbacks in terms of the production complexity, the lack of broad tunability and the limitation of the generated frequencies range to above 2 THz. On the other hand, THz emitters based on photomixing in semiconductor photoconductive antennae are very promising for the development of practical, compact, sufficiently powerful and reasonably-cheap, room-temperature THz sources. In this respect, quantum-dot (QD) semiconductor materials, InAs/GaAs QDs in particular, are of great interest for the use in both the pump laser and the photoconductive antenna for the development of a compact, room-temperature, widely-tunable THz laser source. Such QD-based lasers can offer broad wavelength coverage [1] together with the ability to generate two tunable longitudinal modes simultaneously [2], and InAs/GaAs QD-based antennae are capable of being pumped at high optical intensities of more than 1W [3]. Furthermore, the use of similar InAs/GaAs QD structures in both the pump laser and the antenna is beneficial for the generation of efficient THz radiation, particularly for the pump of the antenna in the vicinity of the QD excited states [3].

Photoconductivity of an InAs/GaAs self-assembled quantum dot photoconductive THz antenna

A broadband terahertz (THz) source is desirable for applications such as imaging, spectroscopy and security. Towards this, an InAs/GaAs quantum dot (QD) based photoconductive antenna (PCA) is a promising and compact solution for THz generation. Coherent THz radiation in the pulsed and the CW regime has been generated with a QD PCA under a resonant and off-resonant pumps [1, 2]. While photoconductivity of QD materials in mid- and far-IR at lower temperatures has been studied for cryogenic sensors and attributed to interlevel transitions, near-IE interband photoconductivity needs further investigation [3, 4]. In this work, we report on the photoconductive properties of an InAs/GaAs QD PCA pumped by a broadly-tunable InAs/GaAs QD external-cavity diode laser.

Dark soliton generation from semiconductor optical amplifier gain medium in ring fiber configuration

We have investigated the mode-lock operation from a semiconductor optical amplifier (SOA) gain chip in the ring fibre configuration. At lower pump currents, the laser generates dark soliton pulses both at the fundamental repetition rate of 39 MHz and supports up to the 6th harmonic order corresponding to 234-MHz repetition rate with an output power of ~2.1 mW. At higher pump currents, the laser can be switched between the bright, dark and concurrent bright and dark soliton generation regimes.