Adding the option to remove negative frequencies in GaussianPulse

tidy3d/components/source/time.py

@@ -140,17 +140,37 @@ class GaussianPulse(Pulse):
         "pulse spectrum which can have a nonzero DC component.",
     )
 
+    remove_negative_frequencies: bool = pydantic.Field(
+        False,
+        title="Remove negative frequency components",
+        description="If ``True``, apply a Hilbert transform to the Gaussian pulse spectrum. This "
+        "should have no effect unless the pulse is broad enough to extend into negative "
+        "frequencies and is useful when the source time is used in combination with a complex "
+        "amplitude, e.g. in a ``CustomCurrentSource``, as it will ensure that the injected "
+        "amplitudes at the postive frequencies are what is specified by the source. Otherwise, "
+        "negative frequencies interfere due to the fact that only the real part of the source "
+        "signal is injected. The transform can however add slowly decaying tails to the time "
+        "signal, so this option should be used with caution as it might lead to fields not "
+        "decaying past the shutoff value, and a larger ``offset`` may be needed. The issue is "
+        "exacerbated by DC-component in the original signal, which is why it is only allowed in "
+        "combination with ``remove_dc_component=True``.",
+    )
+
     def amp_time(self, time: float) -> complex:
         """Complex-valued source amplitude as a function of time."""
 
+        if self.remove_negative_frequencies:
+            # Double the time grid by mirroring around the first point
+            time = np.concatenate((2 * time[0] - time[:0:-1], time))
+
         omega0 = 2 * np.pi * self.freq0
         time_shifted = time - self.offset * self.twidth
 
         offset = np.exp(1j * self.phase)
         oscillation = np.exp(-1j * omega0 * time)
-        amp = np.exp(-(time_shifted**2) / 2 / self.twidth**2) * self.amplitude
+        amp = np.exp(-(time_shifted**2) / 2 / self.twidth**2)
 
-        pulse_amp = offset * oscillation * amp
+        pulse_amp = oscillation * amp
 
         # subtract out DC component
         if self.remove_dc_component:
@@ -159,7 +179,13 @@ def amp_time(self, time: float) -> complex:
             # 1j to make it agree in large omega0 limit
             pulse_amp = pulse_amp * 1j
 
-        return pulse_amp
+        if self.remove_negative_frequencies:
+            import scipy.signal as sps
+
+            pulse_amp = sps.hilbert(np.real(pulse_amp)).conj()
+            pulse_amp = pulse_amp[pulse_amp.size // 2 :]
+
+        return pulse_amp * self.amplitude * offset
 
     def end_time(self) -> Optional[float]:
         """Time after which the source is effectively turned off / close to zero amplitude."""

tidy3d/components/time.py

@@ -48,6 +48,7 @@ def spectrum(
         times: ArrayFloat1D,
         freqs: ArrayFloat1D,
         dt: float,
+        use_real: bool = True,
     ) -> complex:
         """Complex-valued spectrum as a function of frequency.
         Note: Only the real part of the time signal is used.
@@ -62,6 +63,9 @@ def spectrum(
         dt : float or np.ndarray
             Time step to weight FT integral with.
             If array, use to weigh each of the time intervals in ``times``.
+        use_real : bool
+            Whether to use the real part of the time signal only, which is default as the real part
+            is injected in an FDTD simulation.
 
         Returns
         -------
@@ -71,7 +75,9 @@ def spectrum(
 
         times = np.array(times)
         freqs = np.array(freqs)
-        time_amps = np.real(self.amp_time(times))
+        time_amps = self.amp_time(times)
+        if use_real:
+            time_amps = np.real(time_amps)
 
         # if all time amplitudes are zero, just return (complex-valued) zeros for spectrum
         if np.all(np.equal(time_amps, 0.0)):