Despite their wide use in terahertz (THz) research and technology, the application spectra of photoconductive antenna (PCA) THz detectors are severely limited due to the relatively high optical gating power requirement. This originates from poor conversion efficiency of optical gate beam photons to photocurrent in materials with subpicosecond carrier lifetimes. Here we show that using an ultra-thin (160 nm), perfectly absorbing low-temperature grown GaAs metasurface as the photoconductive channel drastically improves the efficiency of THz PCA detectors. This is achieved through perfect absorption of the gate beam in a significantly reduced photoconductive volume, enabled by the metasurface. This Letter demonstrates that sensitive THz PCA detection is possible using optical gate powers as low as 5 μW-three orders of magnitude lower than gating powers used for conventionalPCAdetectors.We show that significantly higher optical gate powers are not necessary for optimal operation, as they do not improve the sensitivity to the THz field. This class of efficient PCA THz detectors opens doors for THz applications with low gate power requirements.
We analyze experimentally and theoretically the transport spectra of a gated lateral GaAs double quantum dot containing two holes. The strong spin-orbit interaction present in the hole subband lifts the Pauli spin blockade and allows to map out the complete spectra of the two-hole system. By performing measurements in both source-drain voltage directions, at different detunings and magnetic fields, we carry out quantitative fitting to a Hubbard two-site model accounting for the tunnel coupling to the leads and the spin-flip relaxation process. We extract the singlet-triplet gap and the magnetic field corresponding to the singlet-triplet transition in the double-hole ground state. Additionally, at the singlet-triplet transition we find a resonant enhancement (in the blockaded direction) and suppression of current (in the conduction direction). The current enhancement stems from the multiple resonance of two-hole levels, opening several conduction channels at once. The current suppression arises from the quantum interference of spin-conserving and spin-flipping tunneling processes.
We report a detailed study of the tunnel barriers within a single-hole GaAs/AlGaAs double quantum dot device (DQD). For quantum information applications as well as fundamental studies, careful tuning and reliable measurements of the barriers are important requirements. In order to tune a DQD device adequately into the single-hole electric dipole spin resonance regime, one has to employ a variety of techniques to cover the extended range of tunnel couplings. In this work, we demonstrate four separate techniques, based upon charge sensing, quantum transport, time-resolved pulsing, and electron dipole spin resonance spectroscopy to determine the couplings as a function of relevant gate voltages and magnetic field. Measurements were performed under conditions of both symmetric and asymmetric tunnel couplings to the leads. Good agreement was observed between different techniques when measured under the same conditions. The results indicate that even in this relatively simple circuit, the requirement to tune multiple gates and the consequences of real potential profiles result in non-intuitive dependencies of the couplings as a function of the plunger gate voltage and the magnetic field.
There is rapidly expanding interest in exploiting the spin of valence-band holes rather than conduction-band electrons for spin qubit semiconductor circuits composed of coupled quantum dots. The hole platform offers stronger spin-orbit interaction (SOI), large difference between in-dot-plane and out-of-dot-plane g-factors, i.e. g-factor anisotropy, and a significantly reduced hyperfine coupling to nuclei in the host material. These attributes collectively can deliver fast all-electric coherent spin manipulation, efficient spin-flip inter-dot tunneling channels, a voltage tunable effective g-factor, a g-factor adjustable to nearly zero in an appropriately oriented external magnetic field, and long spin relaxation and coherence times. Here, we review our recent work on the physics of heavy holes confined in a planar GaAs/AlGaAs double quantum dot system with strong SOI. For a single-hole, we have performed resonant tunneling magneto-spectroscopy to extract spin-flip and spin-conserving tunneling strengths, implemented spin-flip Landau-Zener-Stückelberg-Majorana (LZSM) interferometry, determined the spin relaxation time T 1 as a function of magnetic field using a fast single-shot latched charge technique, electrically tuned the effective g-factor revealed by electric dipole spin resonance, and found signatures of the hyperfine interaction and dynamic nuclear polarization with holes. For two-holes, we have measured the energy spectrum in the presence of strong SOI (and so not limited by Pauli spin blockade), quantified the heavy-hole (HH) g-factor anisotropy on tilting the magnetic field, described a scheme to employ HHs whose g-factor is tunable to nearly zero for an in-plane magnetic field for a coherent photon-to-spin interface, and observed a well-defined LZSM interference pattern at small magnetic fields on pulsing through the singlet-triplet anti-crossing.
Addamane, Sadhvikas J.; Laurain, Alexandre; Baker, Caleb W.; Rotter, Thomas J.; Watt, John; Reno, John L.; Balakrishnan, Ganesh; Moloney, Jerome V.
Semiconductor saturable absorber mirrors (SESAMs) enable passive modelocking of several ultrafast solid-state lasers. Conventionally, SESAMs in the 1-µm wavelength range have employed InGaAs quantum wells (QWs) as absorbers. Here we demonstrate a SESAM based on InAs/GaAs submonolayer quantum dots (SML QDs) capable of generating femtosecond pulses by passively modelocking a vertical-external-cavity surface-emitting laser (VECSEL). Structural measurements are carried out to verify the quality and composition of the QDs. Modelocking experiments with a VECSEL and the QD SESAM in a ring cavity configuration yield pulses as short as 185 fs at 1025 nm. Compared to a traditional QW absorber, SML QD SESAMs exhibit ~ 25% faster recovery times. This also translates to slower power degradation rates or higher damage thresholds in SML QD SESAMs.
We present in this paper the results from a recent study on the stability of the quantum Hall skyrmions state at a Landau level filling factor (ν) close to ν = 1 in a narrow GaAs quantum well. Consistent with previous work, a resonant behavior is observed in the resistively detected NMR measurements. In the subsequent current-voltage (I-V) measurements to examine its breakdown behavior under radio frequency radiations, we observe that the critical current assumes the largest value right at the 75As nuclear resonant frequency. We discuss possible origin for this unexpectedly enhanced stability.
Not long ago, it was shown that a discrete time crystal can be realized if a quantum system is periodically driven to a non-equilibrium state. Proof-of-concept experiments are reported by two groups using trapped ions and nitrogen-vacancy centers in diamond, respectively. The concept of discrete time crystals vividly demonstrates that the coherence time of a quantum system may be enhanced by driving the system out of equilibrium. In this project, we want to test this novel concept in another canonical quantum system, the quantum Hall system in a two-dimensional electron gas (2DEG). Compared to other systems, quantum Hall magnetism (QHM) in high quality, industry-compatible GaAs/AlGaAs heterostructures allows for detailed and quantitative studies in a particularly simple and clean environment. This detailed knowledge should help achieve longer coherence times in a driven QHM system. This report will detail the results from a recent study on the stability of the quantum Hall skyrmions (QHS) state at a Landau level filling close to ν = 1 by measuring its current-voltage (I-V) breakdown characteristics under radio-frequency (RF) radiations. We observe that the critical current increases visibly when the RF frequency is right at the Larmor frequency of 75As nuclei, where the hyperfine interaction between electron and nuclear spins perturbs the QHS state most significantly. We believe that this observation is consistent with the novel concept that the coherence time of a quantum system may be enhanced by driving the system out of equilibrium.
Curwen, Christopher A.; Reno, John L.; Williams, Benjamin S.
We report a terahertz quantum-cascade vertical-external-cavity surface-emitting laser (QC-VECSEL) based upon a metasurface consisting of an array of gain-loaded resonant patch antennas. Compared with the typical ridge-based metasurfaces previously used for QC-VECSELs, the patch antenna surface can be designed with a much sparser fill factor of gain material, which allows for reduced heat dissipation and improved thermal performance. It also exhibits larger amplification thanks to enhanced interaction between the incident radiation and the QC-gain material. We demonstrate devices that produce several milliwatts of continuous-wave power in a single mode at ∼4.6 THz and dissipate less than 1 W of pump power. Use of different output couplers demonstrates the ability to optimize device performance for either high power or high operating temperature. Maximum demonstrated power is 6.7 mW at 4 K (0.67% wall-plug efficiency, WPE) and 0.8 mW at 77 K (0.06% WPE). Directive output beams are measured throughout with divergence angles of ∼5°.
Terahertz laser frequency combs based on quantum cascade lasers provide coherent, broadband, electrically pumped, THz radiation sources for use in future spectroscopic applications. Here, we explore the feasibility of such lasers in a dual-comb spectroscopy configuration for the detection of multiple molecular samples in the gas phase. The lasers span approximately 180 GHz of optical bandwidth, centered at 3.4 THz, with submilliwatt total optical power. One of the main advantages of dual-comb spectroscopy is its high speed, which opens up the possibility for direct observations of chemical reaction dynamics in the terahertz spectral region. As a proof-of-concept, we recorded continuously evolving spectra from gas mixtures with 1 ms temporal resolution.
In this work, we investigate cascaded third harmonic generation in a dielectric metasurface by exploiting high quality factor Fano resonances obtained using broken symmetry unit cells.
We design a resonant metasurface that uses Mie quadrupole modes to suppress the-1 diffraction order. We show that this suppression can be spectrally tuned using optical pumping on a picosecond timescale.
Terahertz semiconductor quantum-cascade lasers (QCLs) are widely implemented with metallic cavities that support low-loss plasmonic optical modes at long wavelengths. However, resonant optical modes in such cavities suffer from poor radiative characteristics due to their subwavelength transverse dimensions. Consequently, single-mode terahertz QCLs with metallic cavities and large (> 100 mW) output power have only been realized in the surface-emitting configuration that affords a large radiating surface. Here, we demonstrate a method to enhance radiative outcoupling from such plasmonic lasers for high-power emission in the edge-emitting (end-fire or longitudinal) direction. Single-sided plasmon waves propagating in vacuum are resonantly excited in surrounding medium of metallic cavities with the QCL semiconductor medium. The vacuum guided plasmon waves with a large wavefront phase-lock multiple metallic cavities longitudinally, which leads to intense radiation in multiple directions, including that in the longitudinal direction in a narrow single-lobed beam. The multicavity array radiates predominantly in a single spectral mode. A peak-power output of 260 mW and a slope efficiency of 303 mW/A are measured for the end-fire beam from a 3.3 THz QCL operating at 54 K in a Stirling cooler. Single-mode operation and lithographic tuning across a bandwidth of ∼ 150 GHz are demonstrated.
Scattering due to interface-roughness (IR) and longitudinal-optical (LO) phonons are primary transport mechanisms in terahertz quantum-cascade lasers (QCLs). By choosing GaAs/Al0.10Ga0.90As heterostructures with short-barriers, the effect of IR scattering is mitigated, leading to low operating current-densities. A series of resonant-phonon terahertz QCLs developed over time, achieving some of the lowest threshold and peak current-densities among published terahertz QCLs with maximum operating temperatures above 100 K. The best result is obtained for a three-well 3.1 THz QCL with threshold and peak current-densities of 134 A/cm2 and 208 A/cm2 respectively at 53 K, and a maximum lasing temperature of 135 K. Another three-well QCL designed for broadband bidirectional operation achieved lasing in a combined frequency range of 3.1-3.7 THz operating under both positive and negative polarities, with an operating current-density range of 167-322 A/cm2 at 53 K and maximum lasing temperature of 141 K or 121 K depending on the polarity of the applied bias. By showing results from QCLs developed over a period of time, here we show conclusively that short-barrier terahertz QCLs are effective in achieving low current-density operation at the cost of a reduction in peak temperature performance.
Hole spins have recently emerged as attractive candidates for solid-state qubits for quantum computing. Their state can be manipulated electrically by taking advantage of the strong spin-orbit interaction (SOI). Crucially, these systems promise longer spin coherence lifetimes owing to their weak interactions with nuclear spins as compared to electron spin qubits. Here we measure the spin relaxation time T1 of a single hole in a GaAs gated lateral double quantum dot device. We propose a protocol converting the spin state into long-lived charge configurations by the SOI-assisted spin-flip tunneling between dots. By interrogating the system with a charge detector we extract the magnetic-field dependence of T1 ∝ B−5 for fields larger than B = 0.5 T, suggesting the phonon-assisted Dresselhaus SOI as the relaxation channel. This coupling limits the measured values of T1 from ~400 ns at B = 1.5 T up to ~60 μs at B = 0.5 T.
Curwen, Christopher A.; Reno, John L.; Williams, Benjamin S.
Changing the length of a laser cavity is a simple technique for continuously tuning the wavelength of a laser but is rarely used for broad fractional tuning, with a notable exception of the vertical-cavity surface-emitting laser (VCSEL)1,2. This is because, to avoid mode hopping, the cavity must be kept optically short to ensure a large free spectral range compared to the gain bandwidth of the amplifying material. Terahertz quantum-cascade lasers are ideal candidates for such a short cavity scheme as they demonstrate exceptional gain bandwidths (up to octave spanning)3 and can be integrated with broadband amplifying metasurfaces4. We present such a quantum-cascade metasurface-based vertical-external-cavity surface-emitting laser (VECSEL) that exhibits over 20% continuous fractional tuning of a single laser mode. Such tuning is possible because the metasurface has subwavelength thickness, which allows lasing on low-order Fabry–Pérot cavity modes. Good beam quality and high output power are simultaneously obtained.
A mechanism to electrically tune the frequency of terahertz quantum cascade lasers (QCLs) is developed that allows for tuning, while the QCL is operated close to its peak bias and temperature. Two optically coupled but electrically isolated cavities are used in which the bias of a control cavity tunes the resonant-mode of the coupled QCL cavity independent of the QCL's operating bias. Approximately 4 GHz electrical tuning is realized for a 3.6 THz distributed-feedback QCL operating in pulsed mode at 58 K in a Stirling cooler. The single-mode QCL emits near-constant peak-power in the range of 5 - 5.3 mW through the tuning range and radiates in a narrow single-lobed beam with a far-field divergence of ∼ 4 ° × 11 °. The superlattice structure of the QCL is designed to implement a low-voltage intersubband absorption transition that is detuned from that of its gain transition, the strength of which could be controlled sensitively with applied voltage utilizing resonant-tunneling injection of electrons in the absorption subband. The tuning is realized by the application of small bias voltages (∼ 6 - 7 V) and requires a narrow bias range (∼ 1 V, ∼ 40 A / cm 2) to traverse across the entire tuning range, and the method should be generally applicable to all intersubband lasers including mid-infrared QCLs.
Hyperspectral imaging is a spectroscopic imaging technique that allows for the creation of images with pixels containing information from multiple spectral bands. At terahertz wavelengths, it has emerged as a prominent tool for a number of applications, ranging from nonionizing cancer diagnosis and pharmaceutical characterization to nondestructive artifact testing. Contemporary terahertz imaging systems typically rely on nonlinear optical downconversion of a fiber-based near-infrared femtosecond laser, requiring complex optical systems. Here, we demonstrate hyperspectral imaging with chip-scale frequency combs based on terahertz quantum cascade lasers. The dual combs are freerunning and emit coherent terahertz radiation that covers a bandwidth of 220 GHz at 3.4 THz with ~10 µW per line. The combination of the fast acquisition rate of dual-comb spectroscopy with the monolithic design, scalability, and chip-scale size of the combs is highly appealing for future imaging applications in biomedicine and the pharmaceutical industry.
Albo, Asaf; Flores, Yuri V.; Hu, Qing; Reno, John L.
We present a so-called "split-well direct-phonon" active region design for terahertz quantum cascade lasers (THz-QCLs). Lasers based on this scheme profit from both elimination of high-lying parasitic bound states and resonant-depopulation of the lower laser level. Negative differential resistance is observed at room temperature, which indicates that each module behaves as a clean 3-level system. We further use this design to investigate the impact of temperature on the dephasing time of GaAs/AlGaAs THz-QCLs.
Terahertz (THz) photoconductive devices are used for generation, detection, and modulation of THz waves, and they rely on the ability to switch electrical conductivity on a subpicosecond time scale using optical pulses. However, fast and efficient conductivity switching with high contrast has been a challenge, because the majority of photoexcited charge carriers in the switch do not contribute to the photocurrent due to fast recombination. Here, we improve efficiency of electrical conductivity switching using a network of electrically connected nanoscale GaAs resonators, which form a perfectly absorbing photoconductive metasurface. We achieve perfect absorption without incorporating metallic elements, by breaking the symmetry of cubic Mie resonators. As a result, the metasurface can be switched between conductive and resistive states with extremely high contrast using an unprecedentedly low level of optical excitation. We integrate this metasurface with a THz antenna to produce an efficient photoconductive THz detector. The perfectly absorbing photoconductive metasurface opens paths for developing a wide range of efficient optoelectronic devices, where required optical and electronic properties are achieved through nanostructuring the resonator network.
We use GaAs metasurfaces with (111) crystal orientation to channel the second harmonic generation (SHG) into the zero-diffraction order that is suppressed for SHG obtained from GaAs metasurfaces with (100) orientation.
We demonstrate the use of custom high electron mobility transistors (HEMTs) fabricated in GaAs/AlGaAs heterostructures to amplify current from quantum dot devices. The amplifier circuit is located adjacent to the quantum dot device, at sub-Kelvin temperatures, in order to reduce the impact of cable capacitance and environmental noise. Using this circuit, we show a current gain of 380 for 0.56 μW of power dissipation, with a bandwidth of 2.7 MHz and current noise referred to the input of 24 fA/Hz 1/2 for frequencies of 0.1-1 MHz. The power consumption required for similar gain is reduced by more than a factor of 20 compared to a previous demonstration using a commercial off-the-shelf HEMT. We also demonstrate integration of a HEMT amplifier circuit on-chip with a quantum dot device, which has the potential to reduce parasitics and should allow for more complex circuits with reduced footprints.
We experimentally demonstrate simultaneous generation of second-, third-, fourthharmonic, sum-frequency, four-wave mixing and six-wave mixing processes in III-V semiconductor metasurfaces and show how to tailor second harmonic generation to zerodiffraction order via crystal orientation.
The term photonic wire laser is now widely used for lasers with transverse dimensions much smaller than the wavelength. As a result, a large fraction of the mode propagates outside the solid core. Here, we propose and demonstrate a scheme to form a coupled cavity by taking advantage of this unique feature of photonic wire lasers. In this scheme, we used quantum cascade lasers with antenna-coupled third-order distributed feedback grating as the platform. Inspired by the chemistry of hybridization, our scheme phase-locks multiple such lasers by π coupling. With the coupled-cavity laser, we demonstrated several performance metrics that are important for various applications in sensing and imaging: a continuous electrical tuning of ~10 GHz at ~3.8 THz (fractional tuning of ~0.26%), a good level of output power (~50–90 mW of continuous-wave power) and tight beam patterns (~100 of beam divergence).
Performance of terahertz (THz) photoconductive devices, including detectors and emitters, has been improved recently by means of plasmonic nanoantennae and gratings. However, plasmonic nanostructures introduce Ohmic losses, which limit gains in device performance. In this presentation, we discuss an alternative approach, which eliminates the problem of Ohmic losses. We use all-dielectric photoconductive metasurfaces as the active region in THz switches to improve their efficiency. In particular, we discuss two approaches to realize perfect optical absorption in a thin photoconductive layer without introducing metallic elements. In addition to providing perfect optical absorption, the photoconductive channel based on all-dielectric metasurface allows us to engineer desired electrical properties, specifically, fast and efficient conductivity switching with very high contrast. This approach thus promises a new generation of sensitive and efficient THz photoconductive detectors. Here we demonstrate and discuss performance of two practical THz photoconductive detectors with integrated all-dielectric metasurfaces.
The term photonic wire laser is now widely used for lasers with transverse dimensions much smaller than the wavelength. As a result, a large fraction of the mode propagates outside the solid core. Here, we propose and demonstrate a scheme to form a coupled cavity by taking advantage of this unique feature of photonic wire lasers. In this scheme, we used quantum cascade lasers with antenna-coupled third-order distributed feedback grating as the platform. Inspired by the chemistry of hybridization, our scheme phase-locks multiple such lasers by π coupling. Alongside the coupled-cavity laser, we demonstrated several performance metrics that are important for various applications in sensing and imaging: a continuous electrical tuning of ~10 GHz at ~3.8 THz (fractional tuning of ~0.26%), a good level of output power (~50–90 mW of continuous-wave power) and tight beam patterns (~100 of beam divergence).
A surface-emitting distributed feedback (DFB) laser with second-order gratings typically excites an antisymmetric mode that has low radiative efficiency and a double-lobed far-field beam. The radiative efficiency could be increased by using curved and chirped gratings for infrared diode lasers, plasmon-assisted mode selection for mid-infrared quantum cascade lasers (QCLs), and graded photonic structures for terahertz QCLs. Here, we demonstrate a new hybrid grating scheme that uses a superposition of second and fourth-order Bragg gratings that excite a symmetric mode with much greater radiative efficiency. The scheme is implemented for terahertz QCLs with metallic waveguides. Peak power output of 170 mW with a slope-efficiency of 993 mW A-1 is detected with robust single-mode single-lobed emission for a 3.4 THz QCL operating at 62 K. The hybrid grating scheme is arguably simpler to implement than aforementioned DFB schemes and could be used to increase power output for surface-emitting DFB lasers at any wavelength.
A frequency mixer is a nonlinear device that combines electromagnetic waves to create waves at new frequencies. Mixers are ubiquitous components in modern radio-frequency technology and microwave signal processing. The development of versatile frequency mixers for optical frequencies remains challenging: such devices generally rely on weak nonlinear optical processes and, thus, must satisfy phase-matching conditions. Here we utilize a GaAs-based dielectric metasurface to demonstrate an optical frequency mixer that concurrently generates eleven new frequencies spanning the ultraviolet to near-infrared. The even and odd order nonlinearities of GaAs enable our observation of second-harmonic, third-harmonic, and fourth-harmonic generation, sum-frequency generation, two-photon absorption-induced photoluminescence, four-wave mixing and six-wave mixing. The simultaneous occurrence of these seven nonlinear processes is assisted by the combined effects of strong intrinsic material nonlinearities, enhanced electromagnetic fields, and relaxed phase-matching requirements. Such ultracompact optical mixers may enable a plethora of applications in biology, chemistry, sensing, communications, and quantum optics.
Wang, Xiaowei; Cui, Xiaorui; Bhat, Abhishek; Savage, Donald E.; Reno, John L.; Lagally, Max G.; Paiella, Roberto
Single-crystal semiconductor nanomembranes provide unique opportunities for basic studies and device applications of strain engineering by virtue of mechanical properties analogous to those of flexible polymeric materials. Here, we investigate the radiative properties of nanomembranes based on InGaAs (one of the standard active materials for infrared diode lasers) under external mechanical stress. Photoluminescence measurements show that, by varying the applied stress, the InGaAs bandgap energy can be red-shifted by over 250 nm, leading to efficient strain-tunable light emission across the same spectral range. These mechanically stressed nanomembranes could therefore form the basis for actively tunable semiconductor lasers featuring ultrawide tunability of the output wavelength.
We demonstrate ultra-low power cryogenic high electron mobility transistor (HEMT) amplifiers for measurement of quantum devices. The low power consumption (few uWs) allows the amplifier to be located near the device, at the coldest cryostat stage (typically less than 100 mK). Such placement minimizes parasitic capacitance and reduces the impact of environmental noise (e.g. triboelectric noise in cabling), allowing for improvements in measurement gain, bandwidth and noise. We use custom high electron mobility transistors (HEMTs) in GaAs/A1GaAs heterostructures. These HEMTs are known to have excellent performance specifically at mK temperatures, with electron mobilities that can exceed 106 cm2 /Vs, allowing for large gain with low power consumption. Low temperature measurements of custom HEMT amplifiers at T = 4 K show a current sensitivity of 50 pA at 1 MHz bandwidth for 5 mW power dissipation, which is an improvement upon performance of amplifiers using off-the-shelf HEMTs.
Broadband terahertz radiation potentially has extensive applications, ranging from personal health care to industrial quality control and security screening. While traditional methods for broadband terahertz generation rely on bulky and expensive mode-locked lasers, frequency combs based on quantum cascade lasers (QCLs) can provide an alternative compact, high power, wideband terahertz source. QCL frequency combs incorporating a heterogeneous gain medium design can obtain even greater spectral range by having multiple lasing transitions at different frequencies. However, despite their greater spectral coverage, the comparatively low gain from such gain media lowers the maximum operating temperature and power. Lateral heterogeneous integration offers the ability to cover an extensive spectral range while maintaining the competitive performance offered from each homogeneous gain media. Here, we present the first lateral heterogeneous design for broadband terahertz generation: by combining two different homogeneous gain media, we have achieved a two-color frequency comb spaced by 1.5 THz.
Curtis, Jeremy A.; Burch, Ashlyn D.; Barman, Biplob; Linn, A.G.; Mcclintock, Luke M.; O'Beirne, A.L.; Stiles, M.J.; Reno, John L.; Mcgill, S.A.; Karaiskal, D.; Hilton, D.J.
In this paper, we describe the development of a broadband (0.3–10 THz) optical pump-terahertz probe spectrometer with an unprecedented combination of temporal resolution (≤200 fs) operating in external magnetic fields as high as 25 T using the new Split Florida-Helix magnet system. Finally, using this new instrument, we measure the transient dynamics in a gallium arsenide four-quantum well sample after photoexcitation at 800 nm.
Curwen, Christopher A.; Reno, John L.; Williams, Benjamin S.
We report a terahertz quantum-cascade vertical-external-cavity surface-emitting laser (QC-VECSEL) whose output power is scaled up to watt-level by using an amplifying metasurface designed for increased power density. The metasurface is composed of a subwavelength array of metal-metal waveguide antenna-coupled sub-cavities loaded with a terahertz quantum-cascade gain material. Unlike previously demonstrated THz QC-VECSELs, the sub-cavities operate on their third-order lateral modal resonance (TM03), instead of their first-order (TM01) resonance. This results in a metasurface with a higher spatial density of the gain material, leading to an increased output power per metasurface area. In pulsed mode operation, peak THz output powers up to 830 mW at 77 K and 1.35 W at 6 K are observed, while a single-mode spectrum and a low divergence beam pattern are maintained. In addition, piezoelectric control of the cavity length allows approximately 50 GHz of continuous, single-mode tuning without a significant effect on output power or beam quality.
We perform Landau-Zener-Stückelberg-Majorana (LZSM) spectroscopy on a system with strong spin-orbit interaction (SOI), realized as a single hole confined in a gated double quantum dot. Analogous to electron systems, at a magnetic field B=0 and high modulation frequencies, we observe photon-assisted tunneling between dots, which smoothly evolves into the typical LZSM funnel-shaped interference pattern as the frequency is decreased. In contrast to electrons, the SOI enables an additional, efficient spin-flip interdot tunneling channel, introducing a distinct interference pattern at finite B. Magnetotransport spectra at low-frequency LZSM driving show the two channels to be equally coherent. High-frequency LZSM driving reveals complex photon-assisted tunneling pathways, both spin conserving and spin flip, which form closed loops at critical magnetic fields. In one such loop, an arbitrary hole spin state is inverted, opening the way toward its all-electrical manipulation.
In this work, we experimentally demonstrate simultaneous occurrence of second-,third-, fourth-harmonic generation, sum-frequency generation, four-wave mixing and six-wave mixing processes in III-V semiconductor metasurfaces with spectra spanning from the UV to the near-IR.
We present an 8-beam local oscillator (LO) for the astronomically significant [OI] line at 4.7 THz. The beams are generated using a quantum cascade laser (QCL) in combination with a Fourier phase grating. The grating is fully characterized using a third order distributed feedback (DFB) QCL with a single mode emission at 4.7 THz as the input. The measured diffraction efficiency of 74.3% is in an excellent agreement with the calculated result of 75.4% using a 3D simulation. We show that the power distribution among the diffracted beams is uniform enough for pumping an array receiver. To validate the grating bandwidth, we apply a far-infrared (FIR) gas laser emission at 5.3 THz as the input and find a very similar performance in terms of efficiency, power distribution, and spatial configuration of the diffracted beams. Both results represent the highest operating frequencies of THz phase gratings reported in the literature. By injecting one of the eight diffracted 4.7 THz beams into a superconducting hot electron bolometer (HEB) mixer, we find that the coupled power, taking the optical loss into account, is in consistency with the QCL power value.
Karl, Nicholas; Heimbeck, Martin S.; Everitt, Henry O.; Chen, Hou T.; Taylor, Antoinette J.; Brener, Igal; Benz, Alexander; Reno, John L.; Mendis, Rajind; Mittleman, Daniel M.
Switchable metasurfaces fabricated on a doped epi-layer have become an important platform for developing techniques to control terahertz (THz) radiation, as a DC bias can modulate the transmission characteristics of the metasurface. To model and understand this performance in new device configurations accurately, a quantitative understanding of the bias-dependent surface characteristics is required. We perform THz variable angle spectroscopic ellipsometry on a switchable metasurface as a function of DC bias. By comparing these data with numerical simulations, we extract a model for the response of the metasurface at any bias value. Using this model, we predict a giant bias-induced phase modulation in a guided wave configuration. These predictions are in qualitative agreement with our measurements, offering a route to efficient modulation of THz signals.
Yang, Yang; Burghoff, David; Reno, John L.; Hu, Qing
Frequency combs based on quantum cascade lasers (QCLs) are finding promising applications in high-speed broadband spectroscopy in the terahertz regime, where many molecules have their “fingerprints.” To form stable combs in QCLs, an effective control of group velocity dispersion plays a critical role. The dispersion of the QCL cavity has two main parts: a static part from the material and a dynamic part from the intersubband transitions. Unlike the gain, which is clamped to a fixed value above the lasing threshold, dispersion associated with the intersubband transitions changes with bias, even above the threshold, and this reduces the dynamic range of comb formation. Here, by incorporating tunability into the dispersion compensator, we demonstrate a QCL device exhibiting comb operation from Ith to Imax, which greatly expands the operation range of the frequency combs.
We present a study of an electrically modulated nonlinear metamaterial consisting of an array of split-ring resonators fabricated on n-type gallium arsenide. The resonant metamaterial nonlinearity appears as an intensity-dependent transmission minimum at terahertz frequencies and arises from the interaction between local electric fields in the split-ring resonator (SRR) capacitive gaps and charge carriers in the n-type substrate. We investigate the active tuning range of the metamaterial device as the incident terahertz field intensity is increased and conversely the effect of an applied DC bias on the terahertz field-induced nonlinear modulation of the metamaterial response. Applying a DC bias to the metamaterial sample alters the nonlinear response and reduces the net nonlinear modulation. Similarly, increasing the incident terahertz field intensity decreases the net modulation induced by an applied DC bias. We interpret these results in terms of DC and terahertz-field-assisted carrier acceleration, scattering, and multiplication processes, highlighting the unique nature of this DC-field modulated terahertz nonlinearity.
Albo, Asaf; Flores, Yuri V.; Hu, Qing; Reno, John L.
The mechanisms that limit the temperature performance of diagonal GaAs/Al0.15GaAs0.85-based terahertz quantum cascade lasers (THz-QCLs) have been identified as thermally activated leakage of charge carriers through excited states into the continuum. THz-QCLs with energetically higher-laying excited states supported by sufficiently high barriers aim to eliminate these leakage mechanisms and lead to improved temperature performance. Although suppression of thermally activated carrier leakage was realized in a three-well THz-QCL based on a resonant-phonon scheme, no improvement in the temperature performance was reported thus far. Here, we report a major improvement in the temperature performance of a two-quantum-well direct-phonon THz-QCL structure. We show that the improved laser performance is due to the suppression of the thermally activated carrier leakage into the continuum with the increase in the injection barrier height. Moreover, we demonstrate that high-barrier two-well structures can support a clean three-level laser system at elevated temperatures, which opens the opportunity to achieve temperature performance beyond the state-of-the-art.
Xu, Luyao; Curwen, Christopher A.; Reno, John L.; Williams, Benjamin S.
A terahertz quantum-cascade (QC) vertical-external-cavity surface-emitting-laser (VECSEL) is demonstrated with over 5 mW power in continuous-wave and single-mode operation above 77 K, in combination with a near-Gaussian beam pattern with a full-width half-max divergence as narrow as ∼5° × 5°, with no evidence of thermal lensing. This is realized by creating an intra-cryostat VECSEL cavity to reduce the cavity loss and designing an active focusing metasurface reflector with low power dissipation for efficient heat removal. Also, the intra-cryostat configuration allows the evaluation of QC-VECSEL operation vs. temperature, showing a maximum pulsed mode operating temperature of 129 K. While the threshold current density in the QC-VECSEL is higher compared to that in a conventional edge-emitting metal-metal waveguide QC-laser, the beam quality, slope efficiency, maximum power, and thermal resistance are all significantly improved.
Khalatpour, Ali; Reno, John L.; Kherani, Nazir P.; Hu, Qing
Photonic wire lasers are a new genre of lasers that have a transverse dimension much smaller than the wavelength. Unidirectional emission is highly desirable as most of the laser power will be in the desired direction. Owing to their small lateral dimension relative to the wavelength, however, the mode mostly propagates outside the solid core. Consequently, conventional approaches to attach a highly reflective element to the rear facet, whether a thin film or a distributed Bragg reflector, are not applicable. Here we propose a simple and effective technique to achieve unidirectionality. Terahertz quantum-cascade lasers with distributed feedback (DFB) were chosen as the platform of the photonic wire lasers. Unidirectionality is achieved with a power ratio of the forward/backward of about eight, and the power of the forward-emitting laser is increased by a factor of 1.8 compared with a reference bidirectional DFB laser. Furthermore, we achieved a wall plug power efficiency of â 1/41%.
Raichev, O.E.; Hatke, A.T.; Zudov, M.A.; Reno, John L.
We report on nonlinear transport measurements in a two-dimensional electron gas hosted in GaAs/AlGaAs heterostructures. Upon application of direct current, the low-temperature differential resistivity acquires a positive correction, which exhibits a pronounced maximum followed by a plateau. With increasing temperature, the nonlinearity diminishes and disappears. These observations can be understood in terms of a crossover from the Bloch-Grüneisen regime to the quasielastic scattering regime as the electrons are heated by direct current. Calculations considering the interaction of electrons with acoustic phonons provide a reasonable description of our experimental findings.
Amplifiers of free-space radiation are quite useful, especially in spectral ranges where the radiation is weak and sensitive detectors are hard to come by. A preamplification of the said weak radiation signal will significantly boost the S/N ratio in remote sensing and imaging applications. This is especially true in the terahertz (THz) range where the radiation signal is often weak and sensitive detectors require the cooling of liquid helium. Although quantum cascade structures are promising for providing amplification in the terahertz band from 2 to 5 THz, a THz amplifier has been demonstrated in an integrated form, in which the source is in close proximity to the amplifier, which will not be suitable for the aforementioned applications. Here we demonstrate what we believe is a novel approach to achieve significant amplification of free-space THz radiation using an array of short-cavity, surface-emitting THz quantum cascade lasers operating marginally below the lasing threshold as a Fabry–Perot amplifier. This free-space “slow light” amplifier provides 7.5 dB(×5.6) overall gain at ∼3.1 THz. The proposed devices are suitable for low-noise pre-amplifiers in heterodyne detection systems and for THz imaging systems. With the sub-wavelength pixel size of the array, the reflective amplifier can also be categorized as active metasurface, with the ability to amplify or absorb specific frequency components of the input THz signal.
Dielectric metasurfaces that exploit the different Mie resonances of nanoscale dielectric resonators are a powerful platform for manipulating electromagnetic fields and can provide novel optical behavior. In this work, we experimentally demonstrate independent tuning of the magnetic dipole resonances relative to the electric dipole resonances of split dielectric resonators (SDRs). By increasing the split dimension, we observe a blue shift of the magnetic dipole resonance toward the electric dipole resonance. Therefore, SDRs provide the ability to directly control the interaction between the two dipole resonances within the same resonator. For example, we achieve the first Kerker condition by spectrally overlapping the electric and magnetic dipole resonances and observe significantly suppressed backward scattering. Moreover, we show that a single SDR can be used as an optical nanoantenna that provides strong unidirectional emission from an electric dipole source.
We have performed two-dimensional Fourier transform spectroscopy on intrinsic and modulation doped quantum wells in external magnetic fields up to 10 T. In the undoped sample, the strong Coulomb interactions and the increasing separations of the electron and hole charge distributions with increasing magnetic fields lead to a nontrivial in-plane dispersion of the magneto-excitons. Thus, the discrete and degenerate Landau levels are coupled to a continuum. The signature of this continuum is the emergence of elongated spectral line shapes at the Landau level energies, which are exposed by the multidimensional nature of our technique. Surprisingly, the elongation of the peaks is completely absent in the lowest Landau level spectra obtained from the modulation doped quantum well at high fields.
Xu, Luyao; Chen, Daguan; Curwen, Christopher A.; Memarian, Mohammad; Reno, John L.; Itoh, Tatsuo; Williams, Benjamin S.
Dynamic control of a laser’s output polarization state is desirable for applications in polarization sensitive imaging, spectroscopy, and ellipsometry. Using external elements to control the polarization state is a common approach. Less common and more challenging is directly switching the polarization state of a laser, which, however, has the potential to provide high switching speeds, compactness, and power efficiency. Here, we demonstrate a new approach to achieve direct and electrically controlled polarization switching of a semiconductor laser. This is enabled by integrating a polarization-sensitive metasurface with a semiconductor gain medium to selectively amplify a cavity mode with the designed polarization state, therefore leading to an output in the designed polarization. Here, the demonstration is for a terahertz quantum-cascade laser, which exhibits electrically controlled switching between two linear polarizations separated by 80°, while maintaining an excellent beam with a narrow divergence of ∼3° × 3° and a singlemode operation fixed at ∼3.4 THz, combined with a peak power as high as 93 mW at a temperature of 77 K. The polarization-sensitive metasurface is composed of two interleaved arrays of surface-emitting antennas, all of which are loaded with quantum-cascade gain materials. Each array is designed to resonantly interact with one specific polarization; when electrical bias is selectively applied to the gain material in one array, selective amplification of one polarization occurs. The amplifying metasurface is used along with an output coupler reflector to build a vertical-external-cavity surface-emitting laser whose output polarization state can be switched solely electrically. This work demonstrates the potential of exploiting amplifying polarization-sensitive metasurfaces to create lasers with desirable polarization states—a concept which is applicable beyond the terahertz and can potentially be applied to shorter wavelengths.
Hole transport experiments were performed on a gated double quantum dot device defined in a p-GaAs/AlGaAs heterostructure with a single hole occupancy in each dot. The charging diagram of the device was mapped out using charge detection confirming that the single hole limit is reached. In that limit, a detailed study of the two-hole spin system was performed using high bias magnetotransport spectroscopy. In contrast to electron systems, the hole spin was found not to be conserved during interdot resonant tunneling. This allows one to fully map out the two-hole energy spectrum as a function of the magnitude and the direction of the external magnetic field. The heavy-hole g factor was extracted and shown to be strongly anisotropic, with a value of 1.45 for a perpendicular field and close to zero for an in-plane field as required for hybridizing schemes between spin and photonic quantum platforms.
IEEE Journal of Selected Topics in Quantum Electronics
Xu, Luyao; Curwen, Christopher; Chen, Daguan; Reno, John L.; Itoh, Tatsuo; Williams, Benjamin
A longstanding challenge for terahertz quantum-cascade (QC) lasers is achieving both a high power and high-quality beam pattern, this is due in part due to their use of sub-wavelength metallic waveguides. Recently, the vertical-external-cavity surface-emitting laser (VECSEL) concept was demonstrated for the first time in the terahertz range and for a QC-laser. This is enabled by the development of an amplifying metasurface reflector capable of coupling incident free-space THz radiation to the QC-laser material such that it is amplified and re-radiated. The THz metasurface QC-VECSEL initiates a new approach for making QC-lasers with high power and excellent beam pattern. Furthermore, the ability to engineer the electromagnetic phase, amplitude, and polarization response of the metasurface enables lasers with new functionality. Our article provides an overview of the fundamental theory, design considerations, and recent results for high-performance THz QC-VECSELs.
Terahertz (THz) double-metal plasmonic resonators enable enhanced light-matter coupling by exploiting strong field confinement. The double-metal design however restricts access to the internal fields. We propose and demonstrate a method for spatial mapping and spectroscopic analysis of the internal electromagnetic fields in double-metal plasmonic resonators. We use the concept of image charges and aperture-type scanning near-field THz time-domain microscopy to probe the fields confined within the closed resonator. The experimental method opens doors to studies of light-matter coupling in deeply sub-wavelength volumes at THz frequencies.
The efficiency of photoconductive (PC) devices, including terahertz detectors, is constrained by the bulk optical constants of PC materials. Here, we show that optical absorption in a PC layer can be modified substantially within a hybrid cavity containing nanoantennas and a Distributed Bragg Reflector. We find that a hybrid cavity, consisting of a GaAs PC layer of just 50 nm, can be used to absorb >75% of incident photons by trapping the light within the cavity. We provide an intuitive model, which describes the dependence of the optimum operation wavelength on the cavity thickness. We also find that the nanoantenna size is a critical parameter, small variations of which lead to both wavelength shifting and reduced absorption in the cavity, suggesting that impedance matching is key for achieving efficient absorption in the optically thin hybrid cavities.
We demonstrate the ultrafast formation of terahertz (THz) metasurfaces through all-optical creation of spatially modulated carrier density profiles in a deep-subwavelength GaAs film. The switch-on of the transient plasmon mode, governed by the GaAs effective electron mass and electron− phonon interactions, is revealed by structured-optical pump THz probe spectroscopy, on a time scale of 500 fs. By modulating the carrier density using different pump fluences, we observe a wide tuning of the electric dipole resonance of the transient GaAs metasurface from 0.5 THz to 1.7 THz. Furthermore, we numerically demonstrate that the metasurface presented here can be generalized to more complex architectures for realizing functionalities such as perfect absorption, leading to a 30 dB modulation depth. The platform also provides a pathway to achieve ultrafast manipulation of infrared beams in the linear and, potentially, nonlinear regime.
Using III-V dielectric metasurfaces, we experimentally demonstrate resonantly enhanced harmonic generations up to the 4th order. Moreover, we observe large enhancements and spectral tailoring of the photoluminescence of quantum dots embedded inside dielectric metasurfaces.
International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz
Li, Xinwei; Zhang, Qi; Lou, Minhan; Reno, John L.; Pan, Wei; Watson, John D.; Manfra, Michael J.; Kono, Junichiro
We have integrated an ultrahigh mobility twodimensional electron gas with a high-quality-factor terahertz photonic cavity. With a quantizing magnetic field and at low temperatures, we demonstrated collective nonperturbative coupling of the electron cyclotron resonance with terahertz cavity photons with a high cooperativity. Due to the suppression of superradiance-induced broadening of cyclotron resonance by the high-quality-factor cavity, our hybrid quantum system exhibited unprecedentedly sharp polariton lines and a large vacuum Rabi splitting simultaneously.
Proceedings of SPIE - The International Society for Optical Engineering
Jin, Yuan; Wu, Chongzhao; Reno, John L.; Kumar, Sushil
Plasmonic lasers generate coherent long-range or localized surface-plasmon-polaritons (SPPs), where the SPP mode exists at the interface of the metal (or a metallic nanoparticle) and a dielectric. Metallic-cavities sup- porting SPP modes are also utilized for terahertz quantum-cascade lasers (QCLs). Due to subwavelength apertures, plasmonic lasers have highly divergent radiation patterns. Recently, we theoretically and experimentally demonstrated a new technique for implementing distributed-feedback (DFB), which is termed as an antenna- feedback scheme, to establish a hybrid SPP mode in the surrounding medium of a plasmonic laser's cavity with a large wavefront. This technique allows such lasers to radiate in narrow beams without requirement of any specific design considerations for phase-matching. Experimental demonstration is done for terahertz QCLs that show beam-divergence as small as 4-degrees. The antenna-feedback scheme has a characteristic feature in that refractive-index of the laser's surrounding medium affects its radiative frequency in the same vein as refractive- index of the cavity. Hence, any perturbations in the refractive-index of the surrounding medium could lead to large modulation in the laser's emission frequency. Along this line, we report ∼57 GHz reversible, continuous, and mode-hop-free tuning of such QCLs operating at 78 K based on post-process deposition/etching of a dielectric on an already mounted QCL chip. This is the largest tuning range achieved for terahertz QCLs when operating much above the temperature of liquid-Helium. We review the aforementioned experimental results and discuss methods to increase optical power output from terahertz QCLs with antenna-feedback. Peak power output of ∼13 mW is realized for a 3.3 THz QCL operating in a Stirling cooler at 54 K. A new dual-slit photonic structure based on antenna-feedback scheme is proposed to further improve output power as well as provide enhanced tunability.
