Session L27: Experimental Optical/Laser Devices and Applications

Highly Efficient Vertical SnS2 Nanoflake Photodetector Decorated with PbS Colloidal Quantum Dots

Tin disulfide (SnS₂) is a 2D material with excellent optoelectronic properties suitable for various solar conversion and sensing applications. SnS₂ has been used in photodetectors due to its high carrier mobility and bandgap of 2.2eV. In applications where the detection of longer wavelengths of light is desired, other materials such as lead sulfide quantum dots (PbS QDs) with tunable band gaps are used together with SnS₂ to maximize light absorption and charge collection. Although several reports have demonstrated high sensitivity and responsivity of few-layer-SnS₂/PbS QDs photodetectors, their large scale production has not been addressed. These devices are often made by mechanically exfoliating bulk SnS₂ and depositing narrow metal contacts using e-beam lithography, which are both not scalable. In this work, we have fabricated photodetectors from vertical SnS₂ nanoflakes grown by a scalable close space sublimation and decorated by colloidal PbS QDs. The SnS₂/PbS QD device is fabricated on SiO₂/Si substrates with gold contacts patterned using standard photolithography. These devices exhibit excellent responsivity, on-off ratios and transient response critical for a photodetector. Our work presents a scalable method of fabricating photodetectors from SnS₂ nanoflakes and PbS QDs.   

Fundamental limits on the performance of electromagnetic devices

Formulating the design of electromagnetic devices as an optimization problem has opened up the possibility of achieving performance metrics that have eluded common design techniques. However, since these optimization problems can only be approximately solved, it is not clear what performance metrics (e.g. transmission and cross-talk) are achievable for an optical device, subject to constraints on device footprint, refractive index contrast, and feature size. In this work, we show how to efficiently calculate provable bounds on performance metrics that are expressible as a quadratic form in the electric field for a general class of linear-optical devices using Lagrangian duality and convex relaxations. We illustrate these techniques by calculating bounds on some representative 1D and 2D electromagnetic design problems: the reflectivity of a 1D mirror and the focusing efficiency of a 2D lens.   

Optical sensor development for toxic elements detection in liquids

Laser induced breakdown spectroscopy (LIBS) based sensor methodology was developed for the detection of three toxic elements; As, S, and Se in liquid. This concept will be used to develop a sensor for the in-situ analysis of coal power plant wastewater. Real time probe for the monitoring of the EPA regulated species is needed to control effluent streams in the power plants. Single and double pulse LIBS systems with 1064 nm Nd:YAG lasers were utilized to generate micro plasma from the surface of liquid jet containing these elements in ppm levels. Plasma emission was collected at 45 degree with the laser beam for As and Se emission lines detection in UV range while that was collected in collinear mode with laser beam for S line detection in near IR. Atomic emission lines at wavelengths 274.4 nm, 278.0 nm for As, 921 nm for S, and 196.09 nm, 203.9 nm, and 206.32 nm for Se were detected in LIBS spectra.   

Cryogenic Operation of Silicon Photonic Electro-Optic Modulators based on DC Kerr Effect

Scalable photonic integrated circuits operating at cryogenic temperatures are essential for quantum information processing and supercomputing. The silicon-on-insulator platform is highly promising for its compactness and CMOS compatibility. However, efficient electro-optic modulation in silicon at cryogenic temperatures remains an outstanding challenge, owing to carrier freeze-out at cryogenic temperatures in conventional plasma-dispersion-based modulators [1]. The generation of an induced second-order nonlinearity in silicon with an applied DC electric field has been demonstrated at room temperature [2]. In this work, we demonstrate DC Kerr-based modulation in silicon at a temperature of 5K at GHz speeds, showing the potential of DC Kerr modulators for use in large-scale silicon photonic integrated circuits for cryogenic computing.

[1] Gehl, M., et al. Optica 4.3, 374-382 (2017)
[2] Timurdogan, E., et al. Nature Photonics 11.3, 200 (2017)   

Pump-probe experiment in a non-linear chaotic photonic cavity : modulation of a Physically Unclonable Function

How to design a PUF, physically unclonable function[1], is a recurrent problem in information theory and cryptography. In our group, we use a non-linear truncated disk of silicon in a silicon dioxide background as a semi-chaotic cavity[2]. Silicon waveguides show non-linear effect during the propagation of high-amplitude pulses, which further complexifies the underlying physics of our device. From known results and performances of actual devices[3], we will first show here how one can enhance the photon lifetime in such a cavity to increase the information content in our operating bandwidth, and in a second part use a co-propagating laser pump in order to locally modulate the refractive index, allowing us to control the cavity's response.

[1] M. Roel, and I. Verbauwhede. Towards Hardware-Intrinsic Security. Springer, Berlin, Heidelberg, 2010. 3-37.
[2]Grubel, Brian C., et al. Optics express 25.11 (2017): 12710-12721.
[3] Grubel, Brian C., et al. arXiv preprint arXiv:1711.02222 (2017).   

Large emission enhancement and emergence of strong coupling with plasmons in nanoassemblies:Role of quantum interactions and finite emitter size.

Next-generation photonic devices, optical quantum communication, and information processing will rely on generating Quantum Emitter assemblies with high photonic efficiencies, that can be coupled to sources of localized radiation typically enabled by plasmons in ultrasmall metal nanoparticles . The Purcell effect has been the basis for several decades in understanding enhancement of photonic efficiency and decay rates of emitters through their coupling to cavity modes and metal nanostructures. However, it is not clear whether this regime of radiative enhancements can be extended to ultrasmall nanoparticle sizes or interparticle distances. Here we report large radiative enhancements of quantum dot assemblies with extremely small metal nanoparticles and emitter-particle separations R of a few nanometers, where Purcell effect would lead to either no enhancements or quenching. We invoke a new regime of radiative enhancements to explain the experimental data and also correctly predict the emergence of strong coupling below certain R, as observed in experiments. In addition, we show that the widely used point emitter approximations diverge from actual observations in the case of finite size emitters at such small separations. REF- PHYSICAL REVIEW B 100, 155413 (2019)   

Multiscale electrothermal simulation of quantum cascade lasers

Quantum cascade lasers (QCLs) are high-power, coherent light sources that emit at midinfrared and terahertz frequencies. The active core of these devices is a periodic multiple-quantum-well heterostructure where the electronic and lattice systems exist far from equilibrium and are strongly coupled. Heat generated in the active core (the part responsible for light emission) diffuses throughout the rest of the device. Coupled charge and heat transport in QCLs present a multiscale and multiphysics problem that governs device properties observed in experiment, such as the current-voltage characteristics, gain, and internal quantum efficiency. We present a strategy for simulating coupled heat and charge transport in QCLs, which occur on drastically different spatial scales, that employs a coupled ensemble Monte Carlo for electrons and phonons in a stage-level simulation of the active core and couples to a device-level heat diffusion simulation.   

Selectively launching propagating surface plasmon from designed symmetric plasmonic structures

The next generation signal processing technology demands high speed and energy efficient information carriers. Surface plasmon polaritons (SPPs) can be one of the most promising candidates since they propagate at almost the speed of light, and can be confined to sub-wavelength dimensions using designed nanostructures. In our studies, we show the intensity of propagating SPPs launched from opposing edges of a symmetric trench structure can be controlled by the linear polarization of the optical field. Through the FDTD simulations, we reveal that the coupling efficiency of the propagating SPPs is inversely proportional to that of the localized surface plasmon excited at the trench edges. We also explored the generation of propagating SPPs from a protruded silver cap structure with s-polarized femtosecond laser excitation. Surprisingly, our results show the SPPs propagate with a bifurcated spatial structure with an antisymmetric mirror plane and may be regarded as two spatially distinct, temporally phase-locked wave packets. Our findings can facilitate the design of plasmonic devices/components in nanophotonic circuits.   

Fundamental thermal noise limits for high-Q/V optical microcavities

Thermo-refractive noise – refractive index perturbations driven by fundamental thermal fluctuations – have recently been identified as a limiting noise source in various optical microcavities. Here, we present a theoretical and experimental characterization of this thermal noise in high quality factor (Q), small mode volume (V) photonic crystal cavities. The theory reveals a mode volume-dependent maximum “effective” Q-factor due to thermal dephasing. We quantify the implications for quantum optical devices operating in the qubit limit of cavity nonlinear optics and compare these results to calibrated frequency stability measurements of common 2D silicon photonic crystal cavities. The results highlight the importance of considering thermal noise in state-of-the-art high-Q/V optical resonators, but also inform design choices which minimize its impact on device performance.   

Cryogenic electro-optic interconnect for superconducting circuits

Encoding information onto optical fields is the backbone of modern telecommunication networks. Optical fibers offer low loss transport and vast bandwidth compared to electrical cables, and are currently also replacing copper cables for short-range communications. Optical fibers also exhibit significantly lower thermal conductivity, making optical interconnects attractive for interfacing with superconducting circuits and devices. Yet little is known about modulation at cryogenic temperatures. Here we demonstrate a proof-of-principle experiment, showing that currently employed Ti-doped LiNbO modulators maintain the Pockels coefficient at 3K---a base temperature for classical microwave amplifier circuitry. We realize electro-optical read-out of a superconducting electromechanical circuit to perform both coherent spectroscopy, measuring optomechanically-induced transparency, and incoherent thermometry, encoding the thermomechanical sidebands in an optical signal. Although the achieved noise figures are high, approaches that match the lower-bandwidth microwave signals, use integrated devices or materials with higher EO coefficient, should achieve added noise similar to current HEMT amplifiers, providing a route to parallel readout for emerging quantum or classical computing platforms.   

Photon Mediation of Electron Transitions in Quantum Cascade Lasers

Quantum cascade lasers (QCLs) are unipolar coherent light sources emitting in the terahertz and midinfrared portion of the electromagnetic spectrum. The active region of a QCL consist of periodic stacks of alternating semiconductor materials forming quantum wells which confine electrons in space and ensure they have desired discrete energy levels. Lasing is achieved by electron transition between these energy levels. In QCLs with the so-called diagonal design, this transition involves quantum tunneling between states with a small spatial overlap, separated by a potential barrier. This electron tunneling is mediated by photons present in the active region of the QCL and is referred to as photon-assisted (PA) tunneling. So far, theoretical studies dedicated to PA tunneling in QCLs have been limited to simple rate equations accompanied by empirical or phenomenological scattering rates. In this work, we present a quantum-transport study of PA tunneling in QCLs based on a Markovian master equation for the density matrix and investigate the effect of PA tunneling on electron dynamics and state lifetime.   

Extreme laser background suppression for resonance fluorescence in semiconductor nanostructures

Semiconductor nanostrustures are promising candidates for developing single photon technologies. Relevant demonstrations in this field has been carried out by resonantly coupling a laser beam to a quantum emitter. Such challenging measurements require the suppression of laser background by several order of magnitudes. One way to do that is to use cross polarization confocal microscopy. Normally, high quality commercial crossed polarizers allows a laser suppression down to 5 to 6 orders of magnitudes. Surprisingly, when used in combination with a confocal microscope, the extinction ratio is boosted up to 9 order of magnitudes. This unexpected but very welcome enhancement finds its origin in the Imbert-Fedorov effect, now commonly referred to as Spin Hall effect of light, which manifests itself in the reflectivity of a Gaussian laser beam off a mirror. In this presentation, we will discuss in details the physics and optics of such a remarkable effect, which we mapped in details for the first time.
  

Quantized Microwave Faraday Rotation

The phenomenon of rotation of polarization in the presence of static magnetic field known as Faraday rotation [1] is very well known. Here we present the quantitative observation of microwave Faraday rotation conducted with GaAs/AlGaAs semiconductor heterostructure. The microwave Faraday rotation observed in high mobility two-dimensional electron gas arises as a result of cyclotron motion of charge carriers. The Faraday rotation induced can be understood by Fresnel analysis for the transmission of right and left handed circularly polarized microwaves. As with the Hall effect, a continuous classical as well as quantized Faraday rotation is observed. In the quantum Hall regime, the Faraday rotation is quantized in units of fine structure constant. The dielectric response of the semiconductor host, and the modification of the wave impedance and field distribution by a wave guide [2] will lead to a modification of the quantized Faraday rotation away from the vacuum fine structure constant, α ≈ 1/137. The effect of frequency dependent electromagnetic confinement can be accounted with an effective fine structure constant α*.

[1] M. Faraday, Phil. Trans. Roy. Soc. London 136, 1 (1846).
[2] D. L. Sounas and C. Caloz, IEEE Trans. Microwave Theory Tech. 60, 901 (2012).   

In-chip laser nano-structuring inside silicon with spatially modulated beams

Recently, we demonstrated a laser-writing method that exploits nonlinear interactions to form subsurface (in-chip) modifications, created deep inside silicon [1]. This single-step, maskless method introduced a new capability, i.e., controlled 3D microfabrication capability at 1-µm resolution inside Si [1]. Here, we expand the technique for demonstrating, to the best of our knowledge, the first nanofabrication capability in Si, realized without damaging the wafer surface. In order to achieve this, we exploit the ‘non-diffracting’ nature of Bessel beams, which offer better spatial control in comparison to Gaussian beams. The laser pulses of ~5 ns, 1.55 µm and a few-microjoules energy are modulated with a spatial light modulator, before directed with a lens system, forming Bessel pattern inside the sample. The crystal structure is disrupted in the form of rod-like structures that have high aspect-ratios > 500, with structure-widths that are on the order of 250 nm, and of roughness ~30 nm. These in-chip nano-structures can potentially lead to novel infrared photonic elements for phase and polarization control at the nanoscale.

[1] Tokel et al, “In-chip microstructures and photonic devices fabricated by nonlinear laser lithography deep inside silicon,” Nature Photonics, 11, 639, 2017.   

Visible quantum dot light-emitting diodes with simultaneous high brightness and efficiency

Quantum dot light-emitting diodes are promising light sources for applications in displays. However, to date, there have been no reports of devices that simultaneously offer both high brightness and high external quantum efficiency. Here, we report red, green and blue quantum dot light-emitting diodes based on CdSe/ZnSe core/shell structures that have these attributes¹. We demonstrate devices with maximum external quantum efficiencies of 21.6%, 22.9% and 8.05% for red, green and blue colours with corresponding brightness of 13,300 cd m^–2, 52,500 cd m^–2 and 10,100 cd m^–2. The devices also offer peak luminance of 356,000 cd m^–2, 614,000 cd m^–2 and 62,600 cd m^–2, respectively. We postulate that this high performance is due to the use of Se throughout the core/shell regions and the existence of alloyed bridging layers at the core/shell interfaces. This study suggests that in the future visible quantum dot light-emitting diodes will also be suitable for lighting applications.

1. H. B. Shen, Q. Gao, Y. B. Zhang, Y. Lin, Q. L. Lin, Z. H. Li, L. Chen, Z. P. Zeng, X. G. Li, Y. Jia, S. J. Wang, Z. L. Du, L. S. Li, and Z. Y. Zhang, Nature Photon. 13, 192 (2019).