Optical modulation, the modification of the properties of propagating electromagnetic radiation, is an important field in many applications as optical communications, security and biosensing since it allows to encode an information in a propagating electromagnetic wave. In particular, it is a crucial step for optical interconnections aimed at replacing electrical one given the higher speed and lower dissipative phenomena involving photons with respect to electrons. The most common modulation mechanisms rely on the modification of the amplitude or the phase of radiation. This is done by modifying the refractive index of the material where light is propagating by the application of an external stimulus. For instance, this can be an electric field, another optical wave or a piezoelectric signal. Semiconductors, polymers and dielectric crystals have been studied for the fabrication of wafer scale optical modulators but all of them present some drawbacks. A promising approach is the integration of layered materials onto other photonic platforms, such as the silicon one. In this way, the advantage of silicon microfabrication is combined with the optical properties of layered materials. Among them, transition metal dichalcogenides (TMDs) represent a promising candidate since the optical absorption is ruled by excitons, whose oscillator strength can be modified by the application of an external electric field. Indeed, the induced charge carriers result in a reduction or increase of exciton’s oscillator strength which causes a modification of the TMD’s refractive index. This work aimed at studying the electro-optic response of WSe2, a member of the TMD class which has been less explored in the literature. This was done by the fabrication of a free-space electro-optic modulator working in reflection condition. The device was fabricated on top of a silicon wafer. A thin metallic layer acts as mirror and back electrode. On top of it, a silicon nitride layer was grown in order to act as resonant cavity and as dielectric layer in a capacitor. Then metallic contacts were shaped on top of the dielectric. Finally, single layer WSe2 was transferred on top of the metallic contacts. The first objective of this thesis work is the optimisation of the parameters of the device. In particular, the thickness of the dielectric layer is crucial as it can be designed in order to achieve larger modulation efficiency. In order to optimise the device parameters, finite difference time domain and transfer matrix method simulations of the reflection from the device were performed with the aid of the software Lumerical. A thickness of 630nm was chosen in order to achieve the strongest modulation. In addition to that, it was chosen to fabricate other samples with 700nm-thick SiN in order to allow the study of modulation over a broader spectral range. The second objective of the thesis is the simulation of the reflection coefficient upon gate voltage application. Models from the literature were used to predict the effect of gate voltage on WSe2 refractive index. Results of these simulations showed strong modulation of reflection coefficient in the visible range, especially close to excitonic transition at 752nm. Then, in this work a free-space electro-optic modulator working in reflection condition was fabricated. The transferred WSe2 layer was characterised by Raman and photoluminescence spectroscopy. The results of these measurements demonstrated that the transferred flake is a WSe2 monolayer. Finally, the actual device characterisation was carried out. I-V curves in dark and under illumination were obtained and confirmed the presence of electrical contact between the WSe2 flake and the metal. After that, reflection measurements at applied gate voltage were performed using a lock-in amplifier. The applied voltage is 5V with a frequency of 90Hz. These measurements proved that the device acts as a modulator, as for an incident wavelength of 642nm a photovoltage equal to -80nV is achieved in presence of the lock-in modulating signal and a photovoltage of -20nV is observed in absence of lock-in signal.
La modulazione ottica, ovvero la modifica delle proprietà della radiazione elettromagnetica, è un importante campo in molte applicazioni, tra cui soprattutto la comunicazione ottica, la sicurezza e il biosensing in quanto permette di codificare un’informazione in un onda elettromagnetica. In particolare, è uno step cruciale per le interconnessioni ottiche volte a sostituire quelle elettrice in quanto l’utilizzo di fotoni al posto di elettroni garantisce maggiore velocità e meno fenomeni dissipativi. I più comuni meccanismi di modulazione si basano sulla modifica dell’ampiezza o della fase della radiazione elettromagnetica. Questo viene generalmente ottenuto modificando l’indice di rifrazione del materiale dove la radiazione elettromagnetica propaga, mediante uno stimolo esterno. Per esempio, questo può essere ottenuto mediante un campo elettrico, un’altra onda elettromagnetica o uno stimolo piezoelettrico. Semiconduttori, polimeri e cristalli dielettrici sono stati studiati come materiali per la fabbricazione di modulatori ottici su un wafer. Tuttavia, questi materiali presentano delle limitazioni. Un approccio promettente si basa sull’integrazione di layered materials su un’altra piattaforma fotonica, come ad esempio la piattaforma in silicio. In questo modo, il vantaggio del processo di microfabbricazione del silicio è unito alle proprietà ottiche dei layered materials. Tra questa classe di materiali, i dicalcogenuri dei metalli di transizione (TMDs) rappresentano un promettente candidato visto che l’assorbimento della radiazione elettromagnetica è influenzata dagli eccitoni, la cui "oscillator strength" può essere modificata mediante l’applicazione di un campo elettrico esterno. Infatti, le cariche che vengono accumulate nel materiale portano ad una riduzione o incremento dell’"oscillator strength" dell’eccitone che causa una modifica dell’indice di rifrazione del TMD. Questo lavoro è volto allo studio della risposta elettro-ottica del WSe2, diseleniuro di tungsteno, un membro della classe dei TMD che è stato meno studiato dalla letteratura. Questo è stato fatto mediante la fabbricazione di un modulatore elettro-ottico free-space che opera in condizione di riflessione. Il dispositivo è stato fabbricato su un wafer di silicio. Un sottile strato di metallo è stato depositato al fine di operare come specchio ed elettrodo. Su di esso, uno strato di nitruro di silicio è stato depositato al fine di operare come cavità e strato dielettrico in un condensatore. I contatti metallici sono stati definiti secondo un pattern, sopra lo strato di nitruro di silicio. Infine, un singolo strato di WSe2 è stato trasferito al di sopra dei contatti metallici. Il primo obiettivo della tesi è l’ottimizzazione dei parametri del dispositivo. In particolare, lo spessore dello strato dielettrico è importante in quanto uno sua ottimizzazione permette di ottenere un maggiore effetto di modulazione. Al fine di ottimizzare i parametri del dispositivo, il software Lumerical è stato utilizzato al fine di fare simulazioni della risposta ottica del dispositivo utilizzando i metodi finite-difference time-domain e transfer matrix method. Uno spessore di 630nm è stato scelto al fine di ottenere un’elevata modulazione. In aggiunta, è stato deciso di fabbricare anche un altro dispositivo con uno spessore di nitruro di silicio pari a 700nm in quanto questo permette di studiare la risposta del dispositivo su un più vasto range spettrale. Il secondo obiettivo della tesi è la simulazione del coefficiente di riflessione al variare del gate voltage applicato. Dalla letteratura sono stati considerati modelli per determinare l’effetto del gate voltage sull’indice di rifrazione del WSe2. I risultati di queste simulazioni hanno dimostrato un’elevata modulazione del coefficiente di riflessione nel visibile, e in particolare in corrispondenza della transizione eccitonica a 752nm. Infine, in questo lavoro un modulatore elettro-ottico free-space che opera in condizione di riflessione è stato fabbricato. Lo strato di WSe2 è stato caratterizzato mediante spettroscopia Raman e fotoluminescenza. Questa analisi ha dimostrato come il flake trasferito sia un monostrato di WSe2. Infine, è stata svolta la caratterizzazione del dispositivo. Sono state ottenute le curve I-V, sia in condizione di illuminazione che non, e queste hanno dimostrato la presenza di contatto elettrico tra il WSe2 e gli elettrodi metallici. Successivamente, sono state fatte misure di riflessione al variare del gate voltage applicato usando un lock-in amplifier. Queste misure hanno dimostrato come il dispositivo funzioni da modulatore, in quanto per una lunghezza d’onda incidente di 642nm un fotovoltaggio di -80nV in presenza del segnale modulante di lock-in è stato misurato, mentre un fotovoltaggio di -20nV è stato misurato in assenza di potenziale applicato.
WSe2 electro-optic modulators
Ferretti, Carlo
2019/2020
Abstract
Optical modulation, the modification of the properties of propagating electromagnetic radiation, is an important field in many applications as optical communications, security and biosensing since it allows to encode an information in a propagating electromagnetic wave. In particular, it is a crucial step for optical interconnections aimed at replacing electrical one given the higher speed and lower dissipative phenomena involving photons with respect to electrons. The most common modulation mechanisms rely on the modification of the amplitude or the phase of radiation. This is done by modifying the refractive index of the material where light is propagating by the application of an external stimulus. For instance, this can be an electric field, another optical wave or a piezoelectric signal. Semiconductors, polymers and dielectric crystals have been studied for the fabrication of wafer scale optical modulators but all of them present some drawbacks. A promising approach is the integration of layered materials onto other photonic platforms, such as the silicon one. In this way, the advantage of silicon microfabrication is combined with the optical properties of layered materials. Among them, transition metal dichalcogenides (TMDs) represent a promising candidate since the optical absorption is ruled by excitons, whose oscillator strength can be modified by the application of an external electric field. Indeed, the induced charge carriers result in a reduction or increase of exciton’s oscillator strength which causes a modification of the TMD’s refractive index. This work aimed at studying the electro-optic response of WSe2, a member of the TMD class which has been less explored in the literature. This was done by the fabrication of a free-space electro-optic modulator working in reflection condition. The device was fabricated on top of a silicon wafer. A thin metallic layer acts as mirror and back electrode. On top of it, a silicon nitride layer was grown in order to act as resonant cavity and as dielectric layer in a capacitor. Then metallic contacts were shaped on top of the dielectric. Finally, single layer WSe2 was transferred on top of the metallic contacts. The first objective of this thesis work is the optimisation of the parameters of the device. In particular, the thickness of the dielectric layer is crucial as it can be designed in order to achieve larger modulation efficiency. In order to optimise the device parameters, finite difference time domain and transfer matrix method simulations of the reflection from the device were performed with the aid of the software Lumerical. A thickness of 630nm was chosen in order to achieve the strongest modulation. In addition to that, it was chosen to fabricate other samples with 700nm-thick SiN in order to allow the study of modulation over a broader spectral range. The second objective of the thesis is the simulation of the reflection coefficient upon gate voltage application. Models from the literature were used to predict the effect of gate voltage on WSe2 refractive index. Results of these simulations showed strong modulation of reflection coefficient in the visible range, especially close to excitonic transition at 752nm. Then, in this work a free-space electro-optic modulator working in reflection condition was fabricated. The transferred WSe2 layer was characterised by Raman and photoluminescence spectroscopy. The results of these measurements demonstrated that the transferred flake is a WSe2 monolayer. Finally, the actual device characterisation was carried out. I-V curves in dark and under illumination were obtained and confirmed the presence of electrical contact between the WSe2 flake and the metal. After that, reflection measurements at applied gate voltage were performed using a lock-in amplifier. The applied voltage is 5V with a frequency of 90Hz. These measurements proved that the device acts as a modulator, as for an incident wavelength of 642nm a photovoltage equal to -80nV is achieved in presence of the lock-in modulating signal and a photovoltage of -20nV is observed in absence of lock-in signal.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/164750