Jinxin Hu · PhD in photonics · University of Toronto
Every photon has a life. Here is one.
It is born, learns to travel, falls in love with a ring, changes colour, and finally lets go. My PhD research follows those moments on a chip: light–matter interaction, nonlinear optics and quantum optics.
An electron meets a hole inside a thin layer of InGaN and gives up its energy as a photon of light. Spontaneous emission: any direction, its own phase. My master's thesis at EPFL asked why some of these meetings end in heat instead of light.
Act II · Coherence
Then it learns to sing in unison.
Between two mirrors, one photon stimulates an excited atom to release a twin: same colour, same phase, same direction. The twins multiply until a beam slips out through the far mirror. A laser.
Act III · Duality
A wave in flight, a particle on arrival.
Send it at two slits and it passes through both, as a wave, and interferes with itself. Yet it lands on the screen at one single point, as a particle. One photon at a time, the bright and dark fringes fill in.
Act IV · Flight
It crosses oceans inside a thread of glass.
In an optical fibre the glass is densest along the axis, so light drifting toward the edge is bent gently back to the centre, again and again, and never escapes. At 1550 nm it loses only about 0.2 dB per kilometre.
Act V · Resonance
It meets a ring and goes around, and around.
On a chip, a waveguide carries it past a microring. At resonance it couples in and circulates many times, and the light inside builds far brighter than the light that came in.
Act VI · Union
Two become one, in a new colour.
Bright enough, the material answers nonlinearly: two photons at ω merge into one at 2ω, half the wavelength. In my photonics minor we wrote the χ⁽²⁾ grating into silicon-rich nitride with light itself, by all-optical poling.
Act VII · Entanglement
Or one becomes two, bound for good.
Run the process backwards and a single photon splits into a pair whose properties stay linked however far apart they fly. That is where quantum optics begins.
Act VIII · Superposition
Until someone looks, both at once.
At a half-silvered mirror the photon takes both paths. Schrödinger imagined a cat sealed in a box with such a quantum trigger, alive and dead together until the lid is lifted. Measure, and only one answer remains.
Act IX · Return
And it ends as it began, as an electron.
In a photodetector the photon is absorbed and lifts an electron across the band gap. A small current flows, and that is how we know it was ever there.
Prologue
Birth
Coherence
Duality
Flight
Resonance
Union
Entanglement
Superposition
Return
Ch. 1 · The Journey
From Lac Léman to Lake Ontario.
Scroll to fly it: 6,404 km on the great circle from Lausanne to Toronto. Then the two cities up close.
Lausanne · 46.52°N 6.57°E
Where the light began: EPFL, on the shore of Lac Léman.
Over the Atlantic · September 2024
One way, along the great circle, the shortest path a curved world allows.
Toronto · 43.66°N 79.40°W
Where it is shaped now: the University of Toronto, by Lake Ontario.
0 km
The two cities, up close
Signed, between two lakes
Ch. 2 · The Work
Everything light touches, in order.
Research projects and teaching, oldest at the top.
Each visit to this page lands on the map as a point of light. Counted by ClustrMaps, which keeps only an approximate location, never who you are.
The visitor map is on its way
Curriculum vitae · Jinxin (Jacob) Hu
One event at a time, a pattern appears.
Photons pass a slit one by one and land almost at random, until the diffraction pattern shows. A career builds up the same way. Education, research, experience and skills below.
Seeded second-harmonic generation in silicon-rich nitride
Writing a second-order nonlinearity into glass, with light.
Project poster
Summary
Demonstrating large second-order nonlinear effects in integrated platforms has been a widely pursued aim. All-optical poling of silicon-nitride (SiN) waveguides is a technique to induce second-order nonlinearities. Here, the pump and its phase-locked second harmonic generate a periodic photocurrent that satisfies the quasi-phase-matching condition, giving rise to a DC field and therefore a second-order susceptibility.
While generating photocurrent, these fields also increase the photoconductivity, leading to a complex interplay that governs the efficiency of these devices. The material constants had previously been extracted for stoichiometric silicon nitride. This project applied all-optical poling to silicon-rich silicon nitride waveguides; by the end of the project, the material properties were extracted and compared with stoichiometric silicon nitride.
Main objectives
Demonstration of all-optical poling in silicon-rich nitride
Fitting of optical poling traces to the governing equation
Extraction of the phase of the photocurrent, the photogalvanic coefficient and the conductivity
Finding the limits of the conversion efficiency and comparing with other materials
Conversion efficiency η₂ω versus pump and seed power
Supervisor
Prof. Camille-Sophie Brès, Photonic Systems Laboratory (PHOSL), EPFL
Master's thesis · Oct 2023 – Feb 2024
Efficiency of InGaN/GaN quantum wells
Investigation of non-radiative centres as revealed by deep-level optical spectroscopy (DLOS).
InGaN/GaN multi-quantum-well structure
Summary
It is established that the efficiency of InGaN/GaN quantum wells (QWs) is mainly governed by the presence of point defects, whose origin is still unknown.
This project aimed to elucidate the origin of these deep traps by varying different parameters and combining characterisation techniques such as temperature- and power-dependent photoluminescence and deep-level spectroscopy.
Determining the energy of the traps relies on modelling the energy states of the quantum well.
Supervisor
Prof. Nicolas Grandjean, Laboratory of Advanced Semiconductors for Photonics and Electronics (LASPE), EPFL
Semester project · 2022
Lab-on-a-chip biosensors using mid-infrared metasurfaces
Taking metasurface infrared sensing out of the lab and toward the point of care.
Mid-infrared metasurface biosensing
Background
Demand for sensitive, fast characterisation of biological samples in healthcare and environmental monitoring is expected to grow. Photonic metasurfaces, two-dimensional metamaterials made of subwavelength nanoantennas, can focus mid-infrared light into nanoscale volumes, where the fields interact strongly with molecules and amplify their chemically specific infrared absorption. This label-free method is surface-enhanced infrared absorption spectroscopy. The next challenge is to remove the need for expensive, bulky instruments that do not suit on-site use.
Summary
The project aimed to bring metasurface-based mid-IR sensors closer to real-world use with a lab-on-a-chip scheme built on low-cost, compact and readily available broadband mid-IR sources and detectors. Building the setup meant comparing optical configurations and components for the best overall performance, followed by sensing of relevant biological samples.
Experimental setup
Supervisors
Prof. Hatice Altug, Felix Richter and Aleksandrs Leitis, Bionanophotonic Systems Laboratory (BIOS), EPFL
Semester project · Sept 2021 – Feb 2022
Annular multi-mode fibres for computing
Light that carries a twist, used to process information at high speed.
OAM fibre profile
Summary
The objective was to build an optical platform to couple an orbital-angular-momentum (OAM) beam into an OAM fibre (Gen5A3) provided by Boston University. The project was motivated by the work of Uğur Teğin et al., who used fibres to process optical information at high speed instead of relying on heavy neuromorphic computing.
Tracking mouse head direction with deep learning in an optogenetics study.
The behaviour box, seen from above
Summary
Optogenetics, developed early this century, has become one of the biggest breakthroughs in neuroscience. In this project the researchers studied light activation of head-direction cells in mice: a viral vector made head-direction cells in the anterior dorsal thalamic nucleus express a light-sensitive ion channel (channelrhodopsin-2), so the cells could be activated by light while the mice were filmed.
We used the deep-learning software DeepLabCut, with MATLAB, to track specific body parts and follow head direction over time, and so determine how activating these cells with light changes the mouse's behaviour.
Determining the crystal structure of pyrite (FeS₂) by powder X-ray diffraction.
Diffraction analysis
Summary
Radio-crystallography determines the structure of materials at the atomic scale: the lattice, the space group and the crystal structure, with a precision that depends on the material and the method. In this experiment we first worked out how to mount a mineral crystal in a powder XRD system, then determined the structure of pyrite from its diffraction pattern. The approach generalises to crystals other than pyrite.
An automatic classification chain based on logistic regression.
Two classes and the learned boundary
Summary
The aim was to implement an automatic classification chain based on logistic regression: inference of the model, learning of its parameters, experimentation and extension.
