WO2016026481A1

WO2016026481A1 - Chalcogenide glass-silicon hybrid waveguide and resonators for wavelength conversion

Info

Publication number: WO2016026481A1
Application number: PCT/DE2015/000427
Authority: WO
Inventors: Peter Nolte; Jörg SCHILLING; Nicolai GRANZOW; Markus Schmidt
Original assignee: Martin-Luther-Universität Halle-Wittenberg
Priority date: 2014-08-18
Filing date: 2015-08-17
Publication date: 2016-02-25

Abstract

The aim of the invention is to increase the efficiency of degenerate four-wave mixing in SOI waveguides. For this purpose, slotted silicon (Si) waveguides which lie on a silicon oxide layer (SiO₂) and which are infiltrated with and covered by chalcogenide glasses (e.g. As²S³) are used. In order to increase efficiency, said waveguides were shaped to annular or racetrack resonators. Chalcogenide glasses were chosen due to their high optical third-order nonlinearity (X (³)).

Description

Chalcogenide glass / silicon hybrid waveguides and resonators for wavelength conversion

For the forwarding and distribution of information between and on electronic computer chips are known to date electrical currents in electrical

Tracks used. Especially at high clock frequencies, this leads to an increase in energy consumption and heat production. There is also another

Acceleration of computing speed limited in this conventional way.

As a new variant of information distribution, optical information processing will become more and more important in the future and will be further developed. For this purpose, similar to the established fiber-optic network, the information is to be coded in light pulses, which are forwarded to an optical chip and processed. As a platform for such optical chips, the SOI system (silicon insulator) has been established in recent years. SOI consists of a few 100 nm thin silicon layer, which is applied to a few micrometers thick silicon oxide layer. Since silicon is transparent in the infrared spectral range and has a significantly higher refractive index than silicon oxide, the near-infrared light pulses used for information processing can propagate in the silicon layer. To control the propagation of light in the thin silicon layer, narrow strips are removed from the silicon layer, which then act as waveguides and guide the infrared light.

In order to be able to transport high information densities in one and the same waveguide, different wavelengths of light are used (WDM wavelength division multiplexing method (wavelength dispersive multiplexing)). Each carrier wavelength corresponds to an information channel, via which the information is transmitted coded as light pulses. In order to achieve high information densities, so the generation of many sharply limited carrier frequencies is necessary, which can propagate through the silicon waveguides. To develop efficient and inexpensive optical chips, an important objective is the monolithic union of light source, light modulation, light pipe, and light detection on a substrate. As a light source, because of the required high light intensity usually only one laser in question. To generate many different wavelengths of light (WDM) from the single intense wavelength the laser provides, nonlinear optical processes such as difference or sum frequency generation or four-wave mixing can be used. For this, it is necessary that the intense wavelength emitted by the laser (hereinafter referred to as the pump wavelength) pass through a nonlinear optical material having second- or third-order susceptibility (χ ⁽²⁾ , χ ⁽³⁾ ). Of particular interest for the generation of different spectrally closely lying wavelengths (so-called "frequency combs") is the four-wave mixing or simplifies in particular the degenerate four-wave mixing.

The use of slotted waveguides and ring resonators is known, which, however, are aimed at the majority of the construction of optical modulators, wherein the refractive index of a nonlinear optical material is varied due to high intensity or an applied voltage. As infiltrated materials in this case organic polymers are used (US 8,340,486 Bl; US2007 / 0035800 AI).

The invention was based on the problem to increase the efficiency of degenerate four-wave mixing in SOI waveguides and by exploiting a high χ ⁽³⁾ more

To produce wavelengths of light, which can serve as further channels for the transmission of information. Furthermore, the invention should make it possible to transmit a signal from one wavelength to another.

The problem was solved according to the invention by means of a combination of silicon (Si) waveguides, which are located on a silicon oxide layer (SiO 2), with chalcogenide glasses (eg As ₂ S ₃ ) which have a high third order optical nonlinearity (χ ⁽³⁾ ) , Using the established silicon technology, the Si waveguides are made to allow the light pipe. The optical non-linearity of the chalcogenide glass is used to generate new wavelengths of light via the nonlinear optical four-wave mixing process.

According to the invention, the technical solution consists of a ring or racetrack resonator, which is constructed of a slotted waveguide and the slot of a

Chalcogenide glass with high χ ⁽³⁾ and high nonlinear optical "figure of merit" (FOM = Re [% ⁽³⁾ ] / lm [x ⁽³⁾ ]) is infiltrated (FIG. 1). As ₂ S ₃ is one such material. In this case, the slit waveguide consists of two separated narrower Si strips which have a spacing of a few 10 to a few 100 nm ("the slit".) A straight classic Si strip waveguide extends in the immediate vicinity of the resonator slotted waveguide on one side of the resonator. This acts as a bus waveguide for coupling and decoupling the infrared light into and out of the resonator The slotted waveguide resonator and the bus waveguide in the region of the resonator are completely covered with the chalcogenide glass (eg As ^). The chalcogenide glass completely fills the slit of the slit waveguide (Embodiment 1) As a basis for the non-linear optical light generation, the degenerate four-wave mixing which is particularly efficient in the slit waveguides and resonators is used an intense pump wavelength and a (generally) black Cheren signal wavelength, a third, different from the first two Idlerwellenlänge. The following relationship between the

Be satisfied with light frequencies:

Because of this relationship between the light frequencies, all 3 involved

Light frequencies spectrally closely adjacent e.g. occur in the near infrared.

To exploit this effect in the proposed structure, the intensive

Pump frequency and the preferably also intense signal frequency in the

Bus waveguide coupled. The waves reach the area of the ring or

Racetrack resonators can inject pump and signal wave via the evanescent portions of the mode in the adjacent resonator. In the slit waveguide of the resonator, the electric fields of the TE polarized modes concentrate in the slot (Figure 2). This effect is particularly strong when the refractive index of the infiltrated material is low.

This field concentration in the slot, which is infiltrated with the chalcogenide glass (eg As ₂ S ₃ ), leads to increased nonlinear optical processes and efficient degenerate four-wave mixing, from which the idler frequency already considered as new Light frequency is generated. Due to the closed ring structure, there is also an amplification of the light fields due to the resonator effect. For light with certain frequencies (resonance frequencies), there is a constructive interference of the rotating waveguide mode, so that their field increases greatly. These

Resonant frequencies of an As2S ₃ -infiltrierten Racetrackresonators were in a

Transmission spectrum, which was measured on a bus waveguide, already observed.

For Q factors in the range of 100,000, this means that the field strength of the light mode in the slotted waveguide resonator is 100,000 times stronger than the coupled-in light. This benefits the degenerate four-wave mixing as a nonlinear optical process in particular. Important for the function is therefore that all 3 light frequency (pump, signal and idler) on

Resonances fall. Only then can they be amplified in the resonator. However, since the resonances occur in the entire spectral range and apart from dispersion effects always have the same spectral distance (FSR), a fulfillment of the frequency condition of formula (1) is possible. After theoretical calculations, the geometry shown in embodiment 2 is also suitable for acting as a parametric oscillator. For this purpose, only a single very intense pump wavelength is radiated into a resonant frequency. The function of the signal wavelength then takes over the ever-present electromagnetic noise. Once a signal (noise) photon "reacts" with 2 pump photons through the process of degenerate four-wave mixing, an idler photon and another signal photon are produced, and if the resonator losses are low enough, these two signal photons can continue to process of the degenerate four-wave mixing, so that ultimately intense signal and idler waves arise, a necessary condition being that the nonlinear optical figure of merit (FOM = Re [I3] / lm [l3]) for the As ₂ S3-infiltrated slotted waveguide is greater than 1. Calculations based on the mode profile have shown that an optimum slit waveguide infiltrated with As ₂ S _{3 has} a FOM = 1.3 and thus can fulfill this necessary condition. Decisive for the function of the oscillator, however, is also that the further losses can be minimized eg by scattering and ultimately lead to even higher factors. The structure according to the invention has the following advantages in comparison with the previously known structures and methods for optical information processing: 1) The use of a slotted waveguide results in field concentration in the slit where the nonlinear optically active material is placed, which supports efficient four-wave mixing.

2) The use of a high Q-factor resonator (about 100,000) results in multiple field enhancement in the resonator, which also makes four-wave mixing efficient when using low power continuous wave lasers in the mW range as sources become.

3) The "division of tasks" in this hybrid structure allows optimal utilization of the respective material advantages: The high refractive index of Si and the advanced fabrication technology for Si causes the waveguides to be formed, whereas the nonlinear optical process mainly occurs in the infiltrated chalcogenide glass.

4) The chalcogenide glasses have high x ⁽³⁾ values and are therefore suitable materials for four-shaft mixing processes. In addition, unlike organic polymers, they are still relatively stable even at higher light intensities. 5) In particular, the chalcogenide glass As ₂ S ₃ is optimal for functioning as a nonlinear optical material in this structure and in the near-infrared spectral range around 1500 nm. The linear absorption of As ₂ S ₃ starts at λ <700 nm. This means that As ₂ S3 has no two-photon absorption for λ> 1400 nm. So it has to Si in this

Spectral range a clear advantage, which is also in a hundred times higher

expresses nonlinear FOM for AS2S3 versus Si in the spectral range around 1500 nm. As a result, As ₂ S _{3 is} significantly better suited for nonlinear optical processes in the near IR than Si.

embodiments

1. The silicon waveguides, including the ring resonator or racetrack resonator, are fabricated from silicon-on-insulator (SOI) substrate by making the slit waveguide forming the ring or racetrack resonator and the adjacent one

Strip waveguide medium defined by e-beam lithography and etched out of the thin Si-top layer (SOI device layer) by means of a plasma etching process. Then the chalcogenide glass (in particular As ₂ S ₃ ) on the prepared Si waveguides deposited. This can be done by various processes such as sol-gel spin coating, pulsed laser deposition (PLD) and in particular thermal evaporation. This achieves an As ₂ S ₃ coverage of the waveguides. The narrow slot in the slotted waveguide or the small distance between the ring resonator and bus waveguide is but in

Generally not completely filled. In order to ensure the necessary complete infiltration of the chalcogenide glass into the narrow slot, the structure is tempered after the chalcogenide glass deposition under an inert gas atmosphere at temperatures between 200 ° C and 400 ° C. In particular, in this process, an argon atmosphere with a pressure of 50 bar and a process temperature of about 300 ° C has proven. In this case, the chalcogenide glass which is flowable at a higher temperature is pressed into the slot at this high pressure. After cooling, the chalcogenide glass remains in the slot and the fully infiltrated structure is completed. This process corresponds to an adaptation of the injection molding process to chalcogenide glasses and nanostructured geometries and can therefore be referred to as nano-injection molding ("nano-injection molding").

The mode of operation for the efficient generation of further light frequencies

(Idlerfrequenzen) due to degenerate four-wave mixing in the

Slotted waveguide racetrack resonators could be detected experimentally. For this purpose, 2 continuous wave laser sources were tuned to the resonance wavelengths 1549.7 nm and 1550.9 nm and fed via the bus waveguide into the resonator. At the output of the bus waveguide, the spectrum of Fig. 4 was detected, containing 2 newly formed idler wavelengths at 1548.5 nm and at 1552.1 nm. Since the two injected frequencies have nearly the same intensity, they can interchange the roles of pump wavelength. Thus, in the first case, when the pump wavelength is 1549.7 nm and the signal wavelength is assumed to be 1550.9 nm, the idler wavelength at 1548.5 nm arises

Idler wavelength at 1552.1 nm, on the other hand, arises when the pump wavelength is assumed to be 1550.9 nm and the signal wavelength at 1549.7 nm. Since both processes are possible, both idler wavelengths are created. The so far illustrated operation corresponds to a parametric amplifier. In addition to the newly formed Idlerwellenlänge the signal wavelength is further amplified.

Explanations to the figures

FIG. 1:

Ring or racetrack resonator of As ₂ S ₃ infiltrated and covered Si slotted waveguide adjacent to the straight Si strip waveguide.

The left-hand detail shows the waveguide profile (cross section) of the ring or racetrack resonator.

The right-hand detail drawing shows the waveguide profile in the coupling region between solid straight Si strip waveguide and slit waveguide of the ring or racetrack resonator.

FIG. 2:

Distribution of the electric field of the TE mode in a As ₂ S ₃ -infiltrated slotted waveguide. The concentration of the field in the infiltrated slot can be clearly seen. Therefore, the nonlinear optical process of four-wave mixing also proceeds particularly efficiently there.

FIG. 3 a:

Section of a transmission spectrum measured on a bus waveguide next to an As ₂ S ₃ -infiltrierten Racetrackresonator.

Two resonances of the racetrack resonator are clearly visible. They are spectrally separated from each other by the "free spectral ranks" (FSR).

FIG. 3 b:

Section of a transmission spectrum measured on a bus waveguide next to an As2S ₃ -infiltrierten Racetrackresonator.

The measurement of the individual resonances results in a Q-factor of approximately 100,000.

FIG. 4:

Detection of degenerate four-wave mixing in the As ₂ S ₃ -infiltrated slotted waveguide resonator. The two idler wavelengths at 1548.5 nm and 1552.1 nm respectively result from the fact that once the intense wavelength at 1549.7 nm acts as pump and once as signal wavelength, while the wavelength at 1550.9 nm then either the signal or pump wavelength represents.

Claims

claims

1. Integrated optical structure consisting of chalcogenide glass / Si hybrid waveguides and resonators for wavelength conversion.

2. Integrated-optical structure according to claim 1 for the efficient coherent generation of light with different wavelengths in the near-infrared spectral range of intense light with one or two different wavelengths in the

near-infrared spectral range.

3. integrated optical structure according to claim 1 consisting of a Si slot waveguide, which has been formed into a ring or Racetrack resonator.

4. Integrated optical structure according to claim 1 and 3 for generating high

Field enhancements and concentrations.

5. Integrated-optical structure according to claim 1 and 3, characterized in that the structure has been covered with a chalcogenide glass and / or infiltrated.

6. Integrated-optical structure according to claim 5, characterized in that the chalcogenide glass As ₂ S3 is used.

7. A process for producing an integrated optical structure according to claim 1 and 3, characterized in that a) the deposition of the chalcogenide glass, in particular As ₂ S ₃ on the

Slotted waveguide resonator structure takes place;

b) the deposition of the chalcogenide glass takes place in particular from the gas phase; c) the deposition takes place via evaporation of the chalcogenide glass

and

d) the heating of the structure to temperatures in and around the

Glass transition temperature of the chalcogenide glass under inert gas atmosphere with high Pressing and pressing the deposited chalcogenide glass into the

nanoscopic slots of the slotted waveguide takes place;

e) the heating of the chalcogenide structure As ₂ S ₃ to temperatures in the range 200 to 400 ° C in an argon atmosphere under a pressure greater than 1 bar and takes place

f) the cooling process under an inert gas atmosphere at high pressure

is carried out.